1 September 2003: Description of nuclear weapons permissive action links (PAL) and role of encryption in the technology:


31 August 2003.


Source: The Swords of Armageddon: U.S. Nuclear Weapons Development since 1945, Chuck Hansen, September 4, 1995, vol. VIII, pp. 3-45.



The arming and fuzing of nuclear weapons present two distinct and diametrically-opposed problems: on one hand, due to their tremendously powerful combination of explosive, incendiary, and biologically destructive effects, nuclear warheads must never be detonated accidentally.

On the other hand, the arming and fuzing mechanisms must be designed and engineered so that once a weapon is released and armed it will fire. An intact weapon in unfriendly hands can be turned against attackers or disabled or disassembled and copied. For these reasons, arming and fuzing systems must be absolutely foolproof.

By way of definition, the arming system of a nuclear weapon is that portion of the weapon which originates the signals required to arm, safe, or re-safe the firing and fuzing systems, and to actuate the nuclear safing system.

The fuzing system is that portion of the weapon that originates the signal which triggers the firing system. The fuzing system normally consists of radars, baro switches, timers, impact crystals, antennas, and baro sensing elements.

As with all other subsystems in modern U.S. nuclear weapons, the fuzing mechanisms are complex and backed-up by, and interlocked with, other systems. As an example, even the primitive LITTLE BOY and FAT MAN bombs of 1945 had three separate and interleaved parts to their fuzing systems. The main component was a modified U.S. Army Air Corps APS-13 fighter tail warning radar, nicknamed "Archie."

As early as July 1943, possible fuses being considered for the bombs included time clocks, barometric devices, and radio (radar) fuses. An arrangement of two clocks in parallel wired in series with a second parallel pair appeared to satisfy requirements for certainty of operation and safety against inadvertent early arming.

The original radar proximity fuses were to have been built by the University of Michigan and made with commercially-available radio tubes at a frequency of about 100 megacycles.

The prototype arming and fuzing system was to be tested in 14-inch diameter bombs lengthened to 12 feet and equipped with telemetry equipment and smoke puffs fired at pre-set altitudes when the arming and firing system had operated as intended.1


1 Memorandum dated July 19, 1943 to Captain W. S. Parsons from R. B. Brode, subject: Fuze Group Program.

By the spring of 1944, the APS-13 "tail warning device" was under study for use as a radar fuse.2 Originally designed to warn a pilot of another aircraft approaching the rear of his plane, the "Archie" had an effective range of about 2,000 to 2,500 feet.3


2 Memorandum dated 15 April 1944 to Capt. W. S. Parsons from R. B. Brode, subject: Report on Trip, March 23 to April 3, to Ann Arbor, New York, Camden, Washington, Tulsa, and Delavan, Wisconsin.

3 Letter dated 7 June 1944 to Maj. Gen. L. R. Groves from Robert B. Brode.

The APS-13 was designed and manufactured by RCA (Radio Corporation of America)4; it operated at frequencies of 410 to 420 megahertz (MHz).5 Those fitted to the MK I and MK III bombs were altered to close a relay at about 1,800 feet above the ground.


4 HISTORY OF MODERN PHYSICS, Vol. II, pp. 120, 121, 195.

5 THE HISTORY OF U.S. ELECTRONIC WARFARE, Alfred Price, Association of Old Crows, Alexandria, Virginia, 1984, p. 232; memorandum dated 24 August 1944 to Major General L. R. Groves from R. N. Brode, subject: Personnel with Radar Experience. According to the latter, the APS-13 was originally called the "Charlie" and those units to be adapted to atomic bombs were to be rigged to operate at selectable frequencies between 250 and 500 megahertz. By 1 January 1945, the "Archies," with a "natural" range of 405 to 435 MHz, could be tuned between 325 and 485 MHz. ("Interim Report Archie Frequency Range," Lt. Leo Gross, 1 January 1945.)

Four "Archies" were used on each bomb, with a network of relays so that when any two of the units signaled, stored current was sent into the detonators (one on the MK I and 32 on the MK III). The antennas for the "Archies" were attached to the sides of the bomb casings.

A barometric pressure-sensing switch closed when the bomb passed 7,000 feet; the bombs were dropped from altitudes between 31,000 and 32,000 feet. A bank of clock-operated timer switches, started by arming wires pulled out when the bomb was dropped, closed 15 seconds after release to prevent the "Archies" from being triggered, in case of baroswitch short-circuit, by radar signals reflected from the delivery aircraft.

The arming and firing sequence for the first two atomic bombs was (1) 15 seconds after release, when the weapon had fallen 3,600 feet, the timer switches closed part of the firing circuit; (2) at an altitude of 7,000 feet, the barometric switch closed another part of the firing circuit and allowed electrical current from batteries in the bomb to charge a number of capacitors and turn on the radar fuses; (3) at an altitude of about 1,800 feet, radar signals emanating from the "Archies" and reflecting from the ground completed the last part of the firing circuit and triggered the detonation signal.

In the MK I LITTLE BOY, the firing signal went directly to the explosive primer that ignited the propellant to fire the uranium-235 projectile into the "target" uranium-235 assembly in the nose of the casing. In the MK III FAT MAN, the firing signal released stored electrical energy from capacitors into detonators, firing the high explosive "lenses" that imploded the plutonium-239 core.

By May 1945, a number of burst heights were under consideration at Los Alamos. The LITTLE BOY might be fuzed for either 1,550 feet or 2,400 feet, depending upon its expected yield of 5 or 15 kilotons. Since delay circuits in the FAT MAN introduced a 400 foot delay in detonation, its fuses were to be set to detonate at either 980, 1,440, or 1,950 feet, corresponding to expected yields of 700, 2,000 or 5,000 tons.

Fuses were re-designed for four different altitudes: 1,000, 1,400, 2,000 and 2,400 feet. These could be used in either type of bomb (LITTLE BOY or FAT MAN); the 1,400 foot fuse looked like the most likely candidate for both weapons.6


6 Memorandum dated 12 May 1945 for Major General L. R. Groves from Major J. A. Derry and Dr. N. F. Ramsey, Subject: Summary of Target Committee Meetings on 10 and 11 May 1945.

On May 14, 1945, Dr. J. Robert Oppenheimer recommended that the LITTLE BOY be fuzed for detonation at 1,700 or 2,500 feet, and the FAT MAN for 1,100, 1,500, or 2,100 feet; of these, the 1,700 and 1,500 foot heights were most likely to be used, with FAT MAN burst height depending on TRINITY results.7


7 Memorandum dated May 14, 1945 to R. B. Brode from J. R. Oppenheimer, Subject: Fuse Setting.

Two weeks later, five height-of-burst settings, between 1,100 and 2,500 feet were under consideration, with planned detonations between 700 and 2,100 feet.8


8 Minutes of Third Target Committee Meeting, Washington, 28 May 1945.

By mid-July, results of the successful TRINITY test of the FAT MAN caused the height of burst for the LITTLE BOY to be raised to 1,850 feet.9 Actual burst heights over Japan for the LITTLE BOY and FAT MAN were both around 1,850 feet.


9 Memoranda dated 18 and 21 July 1945 to R. B. Brode from J. R. Oppenheimer; memorandum for Brigadier General T. F. Farrell and Captain W. S. Parsons, USN, dated 23 July 1945 from J. R. Oppenheimer.

Contemporary warhead fuzing systems are much more varied than those of the first two weapons. These systems include contact (impact) fuses; time fuses (interval timers); hydrostatic (water pressure) fuses; radar proximity fuses for airburst warheads on bombs, AFAPs, and air-to-air missiles; and barometric (air pressure) fuses.

More than one type of fuse is usually employed in each warhead; for example, a contact fuse often doubles as a backup to a radar fuse. This use of a contact fuse to back-up a radar or barometric fuse is called "salvage fuzing." Most atomic weapons have more than one type of fuzing built into them.10


10 BASIC NUCLEAR EMERGENCY COURSE (BNEC), Headquarters, Defense Atomic Support Agency, Washington, D.C., August 1962, p. 23.

The arming and firing systems evolved and improved as did other nuclear weapon subsystems; early insertable plugs were replaced by larger, more-sophisticated mechanisms that offered better reliability and safety.

Later weapons were armed with complex electronic and electromechanical devices. Bombs and warheads are now carried unarmed on their delivery vehicles and a series of electronic arming switches must be activated in a particular sequence, often by more than one person to insure only authorized use.

These switches have evolved from early bulky and heavy vacuum-tube devices to contemporary lightweight and compact solid-state integrated circuits.

Numerous techniques and a range of equipment are employed to insure weapon safety. A major step toward improved weapon safety was the in-flight insertion and extraction of fissile components, so that bombs or warheads were armed only after takeoff or launch, minimizing the chance of an accidental nuclear explosion in event of a delivery vehicle crash. The first in-flight insertion was manual and required access to unpressurized aircraft bomb bays.

Safing devices include pullout switches, safe-separation devices, and safing relays.11


11 BASIC NUCLEAR EMERGENCY COURSE (BNEC), Headquarters, Defense Atomic Support Agency, Washington, D.C., August 1962, p. 26.

Currently, as many as six different types of safety devices are used together in a single warhead to prevent inadvertent nuclear detonation. Some safety devices, such as inertial switches or accelerometers, will permit arming only when subjected to a specific acceleration for a specified period of time. Arming may also be accomplished by a single highenergy electrical pulse generator when a weapon is released  from its delivery vehicle.

A trajectory switch, another common arming device, functions only at a prescribed value determined by the ratio of  longitudinal to lateral acceleration. These switches are collectively known as ESDs, or environmental sensing devices, which are activated by an environment unique to a weapon's flight or trajectory. Contemporary ESDs are tiny microcomputers that are programmed to be activated by conditions unique to the latter stages of a weapon's stockpile-to-target sequence.

Other arming mechanisms include pull-out switches, in the form of arming plugs or wires whereby the force applied by weapon release opens two normally closed-contact circuits and closes two other normally open-circuit contacts, providing electrical paths for weapon arming; the barometric switch described above which closes electrical contacts at a preselected altitude; motor-driven rotary safing switches which isolate power sources in a weapon from the firing components; sequential timers as described above to delay arming for periods from 10 to 75 seconds; and an arm-safe switch which establishes an "exclusion zone" or electrical break in the firing circuit.

With the arm/safe switch, the weapon can be "safed" either manually or electrically, but the switch can only be rotated electrically to the "armed" position.

For ground handling, ready/safe switches are used to provide "safing" (disarming) by opening critical circuits. Arm/safe plugs, when removed, open electrical arming circuits and when inserted, close the circuits.

A modern arming device, called a Permissive Action Link or PAL, was developed in the mid-1960s. The first PALs were  manually-operated multiple-digit combination electromechanical locks which had to be opened before weapons could be armed. In some cases, arming codes were not carried on board aircraft but instead were to be transmitted from base after an attack order had been given.

Another means of "safing" weapons is to make them and their high explosive components as resistant as possible to accidental ignition during a fire or high-speed impact. A standard series of tests for new weapon designs include setting the weapon in the middle of a pool of burning jet aircraft fuel; detonating explosives near the weapon; drops from various heights; and rocket-driven impacts at speeds up to 1,500 MPH into unyielding concrete targets. All these tortures are intended to make the weapon as safe as possible against accidental detonation.

One new development is the fire-resistant pit, or FRP. The W-87 was the first warhead specifically designed around an FRP. The B 83 and W-84 pits have limited fire-resistance by means of their construction, although this was not a design goal. The canceled W-89 and W-91 were also to have included FRPs.12


12 Transcript of questions & answers between Representative Fazio and W. Graham Claytor, Assistant Secretary for Defense Programs, DOE, undated (received May 1991).

Within an FRP, the plutonium-oralloy nuclear explosives are encased within a ductile, high-melting point metal shell that can withstand prolonged exposure to a jet fuel fire with a temperature around 1,800o F without melting or being holed by the corrosive action of molten plutonium which melts at about 1,186 oF. Although the plutonium itself may melt, it will remain contained within the encasing shell and not be dispersed into the atmosphere.13


13 Kidder, UCRL-LR-107454, p. 2.

The FRP will lessen, but not prevent, plutonium dispersal during a fire following an aircraft crash if the container is punctured or ruptured by the impact of the crash. It also will not prevent plutonium dispersion if the HE around the pit detonates.

Because a rocket propellant fire burns about twice as hot as an aircraft fuel fire, such a fire would easily melt the FRP (despite this, W-87 warheads on the PEACEKEEPER ICBM are equipped with FRPs).14


14 Kidder, UCRL-LR-107454, pp. 5, 6.

An additional path to weapon safety is to make the high explosive in the warhead as shock-insensitive as possible so that it does not detonate accidentally and only burns when accidentally ignited, rather than exploding. Great strides have been made in U.S. nuclear weapons explosives safety since 1945. Shock-insensitivity is frequently gained at the cost of explosive power and a requirement for more-powerful detonators.

A recent development to improve electrical safety of nuclear weapons against accidental detonation is the modern Enhanced Nuclear Detonation System (ENDS, also known as EEI, or Enhanced Electrical Isolation) designed and fabricated by Sandia National Laboratory in 1972 and introduced to stockpiled weapons starting with the Air Force's MK/B 61 Mod 5 bomb in 1977.15


15 Kidder, UCRL-LR-107454, p. 2.

The system includes as an integral part of the firing circuit two "strong links" and one "weak link" that are located within an "exclusion region" in the weapon. ENDS is relatively simple and inexpensive.

For the weapon to arm, both "strong links" have to be closed electrically, one by specific operator-coded information and the other by an environmental sensing device. The "weak link," on the other hand, is broken as part of the arming process, which prevents arming if the weapon were exposed to an abnormally high temperature, such as a fire.

The ENDS system is designed to prevent premature arming of nuclear weapons subjected to abnormal environments. The basic idea of ENDS is the physical isolation of critical electrical elements in the arming system into an "exclusion region" that is physically defined by structural casings and barriers that isolate the region from all sources of unintended energy.

The only access point into the "exclusion region" for normal arming and firing power is through "strong links" that cover small openings in the exclusion barrier; the "strong links" are designed to resist energy from abnormal environments, such as fires, electrical shocks, or impacts or crushing.

The "weak link" is a functional electrical element, such as a capacitor, which fails or becomes irreversibly inoperative at lower energy levels, e.g., small electrical currents or moderate temperatures, than those that might cause failure of the "strong links."

At the beginning of 1990, slightly more than half (52%) of the weapons in the U.S. nuclear stockpile were equipped with ENDS. The remaining weapons are awaiting either scheduled retirement or modernization under the Stockpile Improvement Program.16


16 Drell-Foster-Townes Report, pp. 13, 25, 26.

The last of the older warheads without ENDS will not be retired until after the year 2000.17 All systems designed after 1975 have the "strong link/weak link" concept built into them; these include the upgraded MK 53 bombs, the MK/B 61 Mod 5, W-76, W-78, W-79, W-80 Mods 0 and 1, B 83, W-84, W-85, W-87, and W-88.18


17 "Officials Respond to Warhead Safety Report," ARMS CONTROL TODAY, Vol. 21 No. 3, April 1991, p. 26.

18 Transcript of questions & answers between Representative Fazio and W. Graham Claytor, Assistant Secretary for Defense Programs, DOE, undated (received May 1991).

Within the primary stage, mechanical arming devices, such as those used on the W-47 and W-56 warheads, can be employed to prevent accidental nuclear detonation. By 1991, mechanical safing had been used successfully in U.S. nuclear weapons for more than 20 years.19


19 Kidder, UCRL-LR-107454, p. 2.

The latest mechanical safety device is the MSAD, or Mechanical Safing and Arming Device detonator, which holds a small charge of sensitive high explosive out of alignment from the main charge of insensitive high explosive; only after the warhead is armed via unique mechanical and electrical signals does the MSAD bring the detonator high explosive into position with the main charge.20


20 RDA-TR-138522-001, pp. 8, 9, 10; 12; attachment to letter dated February 27, 1987 from George Miller, Associate Director for Defense Systems, LLNL, to Senator Edward M. Kennedy, p. 2; "The Weaponization Program," William H. Hubbell, Jr., ENERGY & TECHNOLOGY REVIEW, September 1986, Lawrence Livermore National Laboratory, pp. 27, 31.

The MSAD, integral with the primary implosion system, assures the safety of the warhead from arbitrary electrical inputs such as lightning, and is especially designed for use with TATB HE systems. A typical MSAD is armed via as many as 35 unique electronic "push/pull" signals; a wrong pulse irreversibly locks the mechanism and precludes detonation.21


21 "National Defense," ENERGY & TECHNOLOGY REVIEW, UCRL-52000-83-7, July 1983, pp. 5, 42.

Between 1950 and 1980, there were many "Broken Arrow," “Bent Spear,” and “Dull Sword” accidents involving U.S. nuclear weapons (see Appendix 3). Several have involved detonations of  HE components and fires and at least two dispersed uranium and plutonium, but there never has been an accidental nuclear explosion of an operational U.S. weapon.

Since the end of World War II, U.S. nuclear weapons have used a variety of fuzing systems, including radiating (radar) types, barofuses, powder-train pyrotechnic time fuses, and mechanical, impact, inertia, and hydrostatic types. Details of a few of these systems follow.

Early Radar Fuses

By late January 1945, LASL was seeking improved radar fuses for its atomic bombs. A natural place to seek new designs was the Massachusetts Institute of Technology's Radiation Laboratory in Cambridge, Massachusetts. The MIT Rad Lab was deeply involved in the research, design, and testing of early U.S. radars and radar countermeasures equipment.

The Rad Lab suggested that LASL use a variant of the gunlaying ARO, Automatic Range Only, radar which was being produced for bomber defense. Other systems of interest might include the APG 4, APG 5, and APG 6 fire-control radars.

However, it would probably be most advisable if the lab were to develop an entirely new radar fuse specifically tailored to LASL's requirements. Such a new fuse would probably take eight months to a year to produce.22


22 Memorandum dated 27 January 1945 to R. B. Brode from N. F. Ramsey, subject: Radiation Laboratory Production of Fuses.

In February 1945, LASL had requested the MIT Radiation Laboratory to design and construct an X-band proximity fuse with a 30" diameter parabolic reflector located in the nose of the bomb. The proposed fuse was to be interchangeable with the "Archie" as far as power supplies and operation of arming circuits were concerned.23


23 Memorandum dated 7 February 1945 to J. R. Oppenheimer from R. B. Brode, subject: X-Band Proximity Fuze.

The proposed X-band radar fuse was to have a high degree of reliability, with a chance of dudding only once in 100,000 operations and one chance in 10,000 for premature operation. The radar fuse, characterized as "an X-band tail warner," was to have a fixed detonation altitude setting of either 2,000, 1,000, 500, or 250 feet; two fuses were to be wired in parallel in each bomb, receiving signals from a parabolic reflector in the nose of the bomb. The entire system was to run off a 24 to 30 volt battery.

These radar fuses were considered as one of the most "top secret" aspects of the atomic bomb program since knowledge of their operation and frequencies might allow an enemy to devise countermeasures that might either prevent bomb detonation altogether or force premature detonation at too high or too low an altitude:24

This Archie unit is the primary fusing device and, as a consequence, security of the information concerning its operation and application is of the utmost importance. It must be pointed out that detailed information regarding the nature and specific application of these units would be perhaps the most valuable single piece of information the enemy could learn about this project.25


24 Memorandum dated 17 February 1945 to J. R. Oppenheimer from R. B. Brode, subject: Fuze Program; memorandum dated 15 March 1945 to Dr. I. I. Rabi from R. B. Brode, subject: Radar Fuze.

25 Memorandum dated 10 May 1945 to Captain Jones from E. B. Doll, subject: Outside Modification of Archies.

When the MIT Radiation Laboratory initially refused to work on the X-band fuse, on the grounds that the lab would not undertake a project in which the whole facility did not have access to complete information about the device, LASL suggested that the Bell Telephone Laboratories, which was also working on wartime radars, be used instead to develop the X-band system. In contrast to the policies of the MIT Rad Lab, Bell Labs followed a policy of strictly compartmentalized research.26


26 Memorandum dated 29 March 1945 to W. S. Parsons from R. B. Brode, subject: X-Band Fuse.

The X-band fuse offered several distinct advantages over the "Archie." The "Archie" could not be used at low altitudes, which meant that if the armed services were to ask for low-yield airburst warheads, the fuse would not be adaptable to these weapons.

In addition, the relatively-low operating frequencies of the "Archie" made it much more vulnerable to jamming than a higher-frequency radar fuse, such as the X-band fuse. The X-band would also offer an opportunity to design a new fuse specifically for an airburst bomb, which would not have to be adapted from a radar developed for an entirely different use, as the "Archie" tail warning device had been.27


27 Memorandum dated April 17, 1945 to Major General L. R. Groves from J. R. Oppenheimer, subject: X-Band Fuses.

On May 5, 1945, the National Defense Research Committee (NDRC) formally requested the MIT Radiation Laboratory to begin work, under the utmost secrecy possible, on the X-band radar fuse. No reference to the work was to be made in Rad Lab project reports and the program was not to be discussed in any laboratory division or staff meeting.

To camouflage the fuse project, its funding was to be hidden in an existing radar development contract.28 General Leslie  Groves authorized on May 16 the development of the X-band radar fuse.29


28 Letter dated May 5, 1945 to Dr. Alfred L. Loomis, Chief, Division 14, NDRC, Radiation Laboratory, MIT, from Edward L. Moreland, Executive Officer, NDRC; letter dated May 5, 1945 to Major General L. R. Groves from Edward L. Moreland, Executive Officer, NDRC.

29 Letter dated 16 May 1945 to Dr. J. R. Oppenheimer from Major General L. R. Groves, USA.

An early alternate to the "Archie" radar fuse was the PMR (Proximity Measuring Radar) or "Amos," a device developed at the University of Michigan. "Amos" had a lower firing range than the "Archie" and was retained in stand-by status in case a lower-altitude detonation was desired. "Amos" was tested at LASL before the end of World War II and found satisfactory.30



These early fuzing devices were among the most sensitive of LASL's secrets during the war, although paradoxically, nothing was supposed to be done to call attention to their secrecy! A February 1945 memo noted that:

While all parts of fuzing program are secret, the primary components, Archie and Amos (PMR), are to be considered of a higher order of secrecy. They may be classified as SECRET RESTRICTED and should not under any circumstances be discussed with or in the presence of anyone outside Group O-3 (Author's note: the LASL bomb fuzing group).

Frequencies on which the above units operate must not be stated verbally or in writing in such a way that they may come to the attention of anyone outside the Group.

Antennas must be kept concealed as much as is practical. When being carried through the halls they must be completely covered.


Visitors should be discouraged, without arousing their curiosity, in any room where Archie or Amos are on display.

Nothing should be done to call the attention of others to the secrecy connected with Archie and Amos.31


31 Memorandum dated 20 February 1945 to All O-3 Personnel from R. B. Brode, subject: Archie and PMR.

How visitors might be discouraged from asking questions about draped equipment was not explained in this memo.

Another early radar fuse design was called the "Andy."32 At the end of August 1945, LASL requested that the University of  Michigan prepare and package for overseas shipment 100 "Andy" radar fuses for stockpile storage at Sandia Base in Albuquerque, New Mexico, and to continue its research on an  extremely low-altitude fuse for use between ten and 50 feet above ground.33


32 Report on Contract W-22-075-eng-30, Period from June 1 to July 1, 1945.

33 Letter dated 27 August 1945 to Professor H. R. Crane from R. B. Brode.

By this time, experimental fuses had been tested at altitudes down to 60 feet. Measurements of the sensitivity of the PMR over various types of terrain and ground cover, including rolling hills, forests, flat ground, and factories and residential areas, were made by carrying a radar fuse on a helicopter. Jamming experiments had also been conducted to determine how much radiative power would be required by a transmitter strong enough to cause either a dud or premature detonation.

By the beginning of September, 25 "Andy" units had already been shipped to Sandia Base, and 75 others were being adjusted and flight-tested. All manufacturing of radar fuses had stopped.34


34 "Contract W-22-075-eng-30, August 1 to September 1, 1945."

At the beginning of October 1945, full-scale jamming experiments had been conducted on both the "Amos" and "Andy" units by operating jammers on the ground while the PMRs were carried aboard an aircraft at various altitudes.35


35 "Report on Contract W-22-075-eng-30, Period from September 1 to October 1, 1945."

Although the arming-fuzing-firing systems of the MK I and MK III were far from perfect, no changes were made for several years after the war. As the range of yields available from early postwar weapons widened, so did the corresponding range in the height of burst; the latter required a different kind of fuse, although the arming and firing systems of earlier weapons could still be retained. The wartime "Archies" could be set for detonation heights only between 700 and 2,000 feet.36


36 Memorandum dated 29 May 1945 to Commanding General, XXI Bomber Command from Brig. Gen. Lauris Norstad, USA, Chief of Staff, subject: 509th Composite Group; Special Functions.

The Los Alamos technical research program for 1949, as stated in the fall of 1948, called for "continued experimentation in fuzing techniques to decrease the possibility of jamming, increase the reliability and accuracy, increase the ease of calibration and field setting, and to determine the  possibility and advisability of after take-off height setting," and "investigation of other fuzing techniques including timing and barometric devices ..."37 (Jamming of the radar fuses was a problem considered as early as February 1945.)38


37 Letter dated 7 September 1948 from N. E. Bradbury, Director, Los Alamos Scientific Laboratory, to Carroll L. Tyler, Manager, Office of Santa Fe Directed Operations, U.S. Atomic Energy Commission, p. 8.

38 Memorandum dated 7 February 1945 to Captain W. S. Parsons from R. B. Brode, subject: Jamming.

The relatively high cost of fissionable material compared to the cost of ordinary high explosives made it particularly important to have reliable fuzing systems in atomic weapons. For sake of redundancy, two or more radars were usually used to guarantee proper burst height.

In-flight change to the height of burst was desirable so that different targets at different elevations could be attacked, in case an aircraft could not reach its primary target and had to go to its secondary target. Manual adjustment to baroswitches was first considered; this was abandoned in favor of resetting the new radar fuses.39


39  Furman, p. 272.

In anticipation of the MK 4, MK 5, and MK 6 bombs, Sandia began the development of a new radar fuse in 1947. Need for improvement was dictated by increasing weapon yields and by U.S. - Soviet cold war tensions that nearly broke into open warfare during the Berlin blockade in 1948.

The new fuse was called the "Abee;" it was turned over to a contractor for engineering in 1949. It had better circuitry and packaging than the "Archie" but it could not be reset in-flight to change the height of burst, and although it had better discrimination than the "Archie" against chaff, balloons, and corner reflectors, it was still believed to be overly susceptible to jamming.40


40 AF ATOMIC ENERGY PROGRAM, Vol. IV, pp. 80 - 83.

For several years after World War II, radar fuses were believed to be unduly susceptible to jamming which could cause either premature detonation of the weapon or no detonation at all:

At the present time, the weapon is provided with an essentially radar altimeter type fuse which causes the detonation of the weapon at a pre-set altitude above the ground. Such a fuse is inherently electronic and must be carefully safeguarded against failure, premature operation, and the possibility of enemy jamming. At the present time, other types of fuzing are under investigation which may increase the reliability, simplicity, and ease of adjustment of the fuzing device, and which can deal with the possibility that the enemy may refine his jamming technique to the point where a radiating device may have to be abandoned.

These problems are solvable in principle, and solutions to them may be expected at a rate proportional to the actual demand for them in competition with the demand for other developments.41


41 "The Potentialities of the Atomic Bomb," N. E. Bradbury, 4 January 1949, p. 12.

Later studies showed that a prohibitively large number of jammers would be required to be an effective countermeasure.42


42 SEMIANNUAL AFSWC HISTORY, 1 April - 31 December 1952, p. 83.

At this time, radar airburst fuses also had shortcomings in their accuracy:

Clearly, the nature of city attacked and the explosive equivalent of the weapon will determine the actual extent of structural damage. Given the average type of building it is desired to attack, the average class of damage it is desired to inflict, and the explosive equivalent of the weapon, it is possible to determine within a few hundred feet the most effective height for the detonation of the weapon.

Such calculations are neither precise nor critical and, for potential enemy cities, must necessarily be subject to wide error, since the desired pressures to cause structural damage are never precisely known. However, it is doubtful if the enemy could distinguish in total damage between two bombs detonated within three or four hundred feet of the optimum altitude.

Accordingly, the precision of radar fuzing is not completely necessary, but some technique which can fix the altitude of detonation within plus or minus 400 feet probably is.43


43 "The Potentialities of the Atomic Bomb," N. E. Bradbury, 4 January 1949, pp. 17, 18.

Other bomb fuzing approaches were suggested: a control wire like the Navy used on wire-guided torpedoes or a time-of-fall computer or baroswitch. In the meantime, the "Abee" fuze went into production in August 1949 (the same month that the Soviets exploded their first atomic bomb).

Fears of Soviet radar countermeasures led to the design of another radar fuse, the AR-10A "Albert." "Albert" had greater reliability and better countermeasures resistance than the "Abee" and could be adjusted in-flight to set height-of-burst.  

In the middle of "Albert" development, Dr. Klaus Fuchs was arrested in England for espionage; he soon confessed to spying in the U.S. for the Soviet Union between 1943 and 1946. Fuchs was well-informed on all aspects of American nuclear weapons technology, including radar fuses, during this period. Fuchs' treachery shocked and dismayed the Air Force and LASL; among many other things, Fuchs had compromised U.S. nuclear weapon radar fuse secrets.44


44 Fuchs, as a member of the wartime "British Mission" to LASL between August 1944 and June 1946, attended many technical seminars and colloquia at LASL. Most of the postwar U.S. fission weapon improvements described in Volume II (e.g., hollow cores, boosting, levitation, composition, U-233 cores, alternate tamper materials, external neutron generators, etc.) were all identified and studied or discussed to some extent at LASL during the war, as well as some early thermonuclear weapon concepts. (Drs. Fuchs and John von Neumann, at the end of May 1946, filed for a U.S. patent on a method to initiate burning of a deuterium-tritium mixture.) Fuchs undoubtedly passed most or all of this information along to the Soviets: the third Soviet fission test on 19 October 1951, just two years after their first test, used a composite, probably levitated uranium-plutonium core. (NWD 86-3, February 1986, revised 2 June 1988, pp. 8, 13.)

Eventually, early weapons such as the MK 7 carried both barometric and radar fuses to obviate jamming problems and to guarantee a greater likelihood of successful weapon detonation. A first step against jamming was the use of a radar receiver on the weapon that "sniffed" or listened for jamming signals; if jamming were detected, the radar fuse was disabled and the weapon was detonated by a time-of-fall fuse.

By January 1950, the Sandia Corporation and the Research Division of the Bendix Aviation Corporation were making progress on the development of two improved types of radar fuses which would be less susceptible to jamming.45 The new fuses were to be manufactured by the Motorola Corporation at a new laboratory and plant in Phoenix, Arizona.46


45 Memorandum for Dr. John H. Manley, Secretary, General Advisory Committee, from Brig. Gen. James McCormack Jr., Director of Military Application, USAEC, January 27, 1950, Subject: Items for GAC Meeting; Report of the Manager, Santa Fe Operations, U. S. Atomic Energy Commission, July 1947 to July 1950, Book Two, p. 52.

46 Report of the Manager, Santa Fe Operations, U. S. Atomic Energy Commission, July 1947 to July 1950, Book Two, p. 53.

By the fall of 1950, a jamming signal detector had been devised for the older "Archie" radar; this detector could be inserted into a weapon and prevented the fuzing system from being activated by jamming. If the jamming were continuous, the bomb would be fired by a delayed baro signal. New radar fuses for the MK 6 bomb were reasonably unjammable.47


47 Minutes of the Twenty-Third Meeting of the General Advisory Committee to the U.S. Atomic Energy Commission, October 30, 31, November 1, 1950, p. 9.

Three years later, radar fuses were being removed from stockpiled MK 5 and MK 6 bombs and experimental TX-13 bombs and were being replaced by barometric pressure-sensitive fuses coupled to contact fuses. While a bomb containing a malfunctioning radar fuse would probably detonate on impact with the ground, there might or might not be a nuclear explosion. Using a contact fuse as a "salvage" fuse offered a greater likelihood of a full-scale detonation.48 By this time, previous requirements for making radar fuzing systems highly resistant to jamming were no longer important.49


48 Letter dated November 4, 1953 to Richard G. Elliott, Director, Information Division, Santa Fe Operations Office, USAEC, from E. F. Cox, Manager, Weapons Effects Dept., Sandia Corporation, Albuquerque, Subject: Paper on Failure Probability of Air-Burst Fuzes; Minutes of the 41st Meeting of the General Advisory Committee to the USAEC, July 12-15, 1954, p. 4; "Failure Prediction of Air-Burst Bombs," 4 November 1953, p. 2, Attachment L in Attachments to Report of Committee to Study Nevada Proving Grounds, undated, but ca. January 1954.

49 Semiannual Historical Report, Headquarters, Field Command, Armed Services Special Weapons Command, Sandia Base, Albuquerque, New Mexico, Activities for the Period 1 January 1953 - 30 June 1953, p. 244.

At the end of 1953, flight testing had been conducted to determine the variation in radar return from various types of terrain at different altitudes, similar to the wartime work on early ground proximity radars. Pulse radars operating at 415 megahertz and 3,840 megahertz with a pulse rate of two kilocycles per second and a pulse width of 0.2 microseconds had been tested over smooth inland waters, the Pacific Ocean, sandy deserts, and wooded areas. The data was being analyzed by the Physical Science Laboratories of the New Mexico College of Agriculture and Mechanical Arts at Las Cruces, New Mexico. Similar future tests were planned for radars operating in the 1,600 megahertz region.50


50 Semiannual Historical Report, Headquarters, Field Command, Armed Services Special Weapons Command, Sandia Base, Albuquerque, New Mexico, Armed Services Special Weapons Command, Sandia Base, Albuquerque, New Mexico, Activities for the Period 1 July 1954 - 31 December 1953, p. 239.

Six months later, the MK 3 radar was capable of operation down to altitudes as low as 600 feet.51


51 Semiannual Historical Report, Headquarters, Field Command, Activities for the Period 1 January 1954 - 30 June 1954, p. 274.

Barometric & Other Fuses

A number of other warhead fuse types have been used on U.S. nuclear weapons since the end of World War II. These include barometric fuses, contact fuses, hydrostatic fuses, time fuses, and proximity fuses.

Fuses for early atomic bombs were radiating types, similar in principle and most respects to conventional radar proximity fuses widely employed in anti-aircraft and bombardment shells during World War II. By their nature, they were only marginally suitable for atomic warheads; complexity, sensitivity, fragility, and susceptibility to jamming detracted from their usefulness.

In June 1949, the Air Force began exploring alternate means of fuzing air burst weapons and by October had decided that a switch actuated by barometric pressure offered substantial advantages.

Adding to Air Force trepidation at this time was the revelation that Klaus Fuchs, an émigré German physicist brought to the U.S. in 1943 with the British Mission to the Manhattan Project, had turned over to the Soviets all U.S. radar fuse knowledge and technology up to mid-1946.

A new barometric fuse was suggested as an alternate to a possibly irredeemably-compromised radar fuse. The objection was immediately raised that barometric fuses were notoriously inaccurate, erring by as much as 850 feet. An inaccuracy of this magnitude could not be accepted for the relatively low-yield MK I and MK III bombs: a height-of-burst error of this size, while not producing optimum target damage, would not be so important with higher-yield postwar nuclear weapons.

Primarily for this reason, while a barometric fuse was accepted, it was always used in conjunction with a radar fuse.

On January 10, 1950, Field Command, Armed Forces Special Weapons Project requested Sandia to equip the MK 4 with a pressure-sensitive, non-radiating baroswitch. AFSWP wanted to both prevent jamming of a radar fuse, which might result in either no detonation or detonation at an undesirable altitude, and to reduce bomb weight by replacing the radar fuses and their power sources by a group of lightweight direct-acting barometric switches.

Sandia set up a task force to determine optimal detonation heights versus bomb yields; the task force concluded that improvements in bombing accuracy were more effective in causing blast damage than increases in bomb yield and optimum heights-of-burst.52


52 Furman, pp. 399-400.

By the fall of 1950, fuses for low altitude airbursts, from 10 to 300 feet, were nonexistent, and variable-time, sonic, or infrared devices were being examined by LASL.53


53 Minutes of the Twenty-Third Meeting of the General Advisory Committee to the U.S. Atomic Energy Commission, October 30, 31, November 1, 1950, p. 9.

Among tests conducted to determine effects of terrain and height-of-burst on blast phenomena was Operation TUMBLER in the spring of 1952 in Nevada. Objectives of TUMBLER included the collection of additional blast data to evaluate contemporary pressure-versus-height of burst curves, showing the relation of ground over-pressures for various weapons and burst heights.54


54 Memorandum dated March 17, 1952 for Mr. James S. Lay, Executive Secretary, National Security Council, from Gordon Dean, Chairman, USAEC, Subject: Request of Presidential Approval for TUMBLER-SNAPPER.

Knowledge of these effects was vital to weapons targeting and U.S. offensive war plans, and previous nuclear airburst tests, especially during Operation BUSTER in 1951,55 had revealed substantial errors in theoretical data in current height-of-burst versus pressure curves that was used by various military agencies for operational planning and by research and development groups to establish criteria for weapons development. Erroneous figures could have led to potentially disastrous wartime consequences:

The Joint Chiefs of Staff have informed me that preliminary results of recent tests (Operation BUSTER) have shown that the measured air blast pressures were 1/3 - 1/2 of those presently being used for planning purposes by the Armed Services. The data from these tests are still in the process of analysis but the general trend of the results has been confirmed by several sets of independent measurements.

The presently used values of air blast pressure and the optimum heights of detonation for the production of damage are based on theory and data from high explosive experiments. No previous pressure measurements have been made with atomic weapons detonated at operational altitudes over land.

If the BUSTER data are confirmed, the use of the presently planned optimum heights of burst might decrease the area of damage and the resultant effectiveness of the atomic weapons stockpile by a factor of 4 or more.

This loss could be partially remedied by new selections of height of burst based on revised data. It is not anticipated that BUSTER data will be adequate for establishment of new heights of burst even when analysis is complete.

In addition to affecting the selection of the optimum height of burst, the new data, if confirmed, will reduce the present estimates of the maximum possible damage area from a bomb of given yield and may indicate a variation of damage area with the nature of the target.

Therefore, new target analyses will be in order. Following such analyses, it may be necessary to alter the selected bomb yield, the number of bombs per target, and the fuzing. All such changes are of vital importance in national planning.56



55 "Importance of the Nevada Proving Grounds to the Department of Defense," 28 August 1953, p. 4, in Attachments to Report of Committee to Study Nevada Proving Grounds, undated, but ca. January 1954; letter dated January 8, 1952 to Brien McMahon, Chairman, JCAE, from Gordon Dean, Chairman, USAEC; letter dated March 19, to Brien McMahon, Chairman, JCAE, from Henry D. Smyth, Acting Chairman, USAEC. The latter states that "experimental data obtained from the BUSTER shots are at variance with presently-used values with respect to the effects of height of burst on the air blast pressures measured on the ground;" TUMBLER was to obtain additional overpressure data for a variety of operational target conditions.

56 Memorandum dated 16 January 1952 for The Executive Secretary, National Security Council, from Robert A. Lovett, Secretary of Defense, Subject: Test to Determine Air Blast Effects of Atomic Weapons.

Data from the first four TUMBLER shots enabled DOD to establish optimum heights-of-burst to maximize desired weapons effects, especially blast overpressures. TUMBLER was extremely successful in generating information on blast pressures that would be produced from atomic weapons of different yields detonated at various altitudes. The uncertainties resulting from Operation BUSTER were largely eliminated, and it became possible to produce a set of burst altitude versus blast pressure curves to use in future target planning.57


57 Letter dated July 9, 1952 to Senator Brien McMahon, Chairman, JCAE, from Gordon dean, Chairman, USAEC.

AFSWP urged the development of a new, more-accurate barometric fuse in 1951 and a new baro fuse was developed and ready for stockpiling with the MK 5 in the spring of 1952.58


58 AF ATOMIC ENERGY PROGRAM, Vol. IV, pp. 84 - 89.

This fuse was produced by the Red Bank Division of the Bendix Aviation Corporation, which had manufactured fuses for the MK III and MK 4 weapons.59


59 Report of the Manager, Santa Fe Operations, U. S. AtomicEnergy Commission, July 1947 to July 1950, Book Two, p. 52.

The new baro fuse ultimately replaced the proximity device as the leading candidate for guided missile application. The only major competitor was a time-of-fall switch dependent upon a radar altimeter combined with a radar range-finder which operated only once during the fall into the target; technical considerations made this arrangement unsuitable for guided missiles.

A baroswitch capable of detecting pressure differentials in its passage from upper to lower atmospheric levels seemed to be considerably more appropriate; nevertheless, timing devices and radiating fuses were also perfected and applied to guided missiles.60



By July 1954, in order to simplify field logistics, barometric fuzing had been substituted for earlier radar fuses in strategic weapons. To guarantee detonation, contact fuses were also used.61


61 Minutes of the 41st Meeting of the General Advisory Commission to the U.S. Atomic Energy Commission, July 12-15, 1954, p. 4.

The biggest problem with early barometric fuses and baroswitches was their large measurement deviation: the switch altitude reading could be in error much as several hundred feet; local air pressure determined by the target's altitude above sea level and weather in the target area could also affect the reading. This was the reason that a baroswitch, and not a barometric fuse, was used in the MK I and MK III arming systems.

Baroswitches were not as accurate as radar; pressure sensing variations and measurement errors worsened as the warhead reached transonic speeds. Early baro fuses were occasionally rendered almost inoperative by high speed air flowing past their sensing ports after the bomb was dropped; this high-velocity air caused lower-than-actual air pressure measurements. Many of these problems were resolved in later years by the design of new and ingenious pressure sensing systems.

Another concern over barometric fuses was their vulnerability to overpressures, possibly caused by exploding defensive nuclear warheads, that might result in premature detonation.

There were ways to protect against this. One method was low-altitude arming, another was to redesign the baroswitch so that it would not respond to a sharp blast or "pressure spike."62


62 Minutes of the Fiftieth Meeting of the General Advisory Committee to the USAEC, July 16-18, 1956, pp. 33, 34.

Contact fuses were usually based on crystals of barium titanate, which generated electrical signals when impacted or struck by sonic shock waves. (The tendency of a material to generate electricity in response to mechanical stress is called piezoelectricity.) By the middle of 1953, barium titanate contact fuses had been developed and tested for the TX-5-X1, TX-6-X4, TX-7-X1 (MK 7 Mod 1), TX-12, TX-13, XW-7/HONEST JOHN, XW-7/CORPORAL, XW-5/REGULUS, XW-5/MATADOR, XW-5/RASCAL, XW-5/RIGEL, and XW-5/HERMES A-3B-E1 warhead applications.

One problem associated with these contact fuses was premature activation caused by rain, hail, air shock waves, and vibration.63


63 Semiannual Historical Report, Headquarters, Field Command, Armed Services Special Weapons Command, Sandia Base, Albuquerque, New Mexico, Activities for the Period 1 January 1953 - 30 June 1953, pp. 208, 209.

Contact fuses for U.S. atomic bombs were developed starting in 1951 and they have often been employed as last-ditch backups to other types of warhead fuses.64 These fuses are also known as "salvage" fuses in that they can "salvage" the bomb and cause it to explode when all other fuses fail. These fuses must function after withstanding extreme deceleration forces and delivery vehicle deformation.65



65 Rosengren, RDA-TR-122100-001-Rev. 1, p. 102.

Some early U.S. fission weapons with contact fuses included the TX-5-X1, TX-6-X4, XW-5/REGULUS, XW-7/CORPORAL, and the TX-13.66


66 Semiannual Historical Report, Headquarters, Field Command, Armed Services Special Weapons Command, Sandia Base, Albuquerque, New Mexico, Activities for the Period 1 July 1953 - 31 December 1953, p. 241.

One problem encountered in attempts to fit contact fuses to early U.S. thermonuclear bombs was the apparent requirement for an external nose-spike that would detonate the bomb before it was buried underground upon impact. Unfortunately, the bomb bays of the first U.S. bombers that could carry these weapons were too short to accommodate these spikes.67


67 Minutes of the 41st Meeting of the General Advisory Commission to the U.S. Atomic Energy Commission, July 12-15, 1954, p. 6.

Hydrostatic fuses are used on depth bombs and set to detonate at a given depth where the water pressure activates the fuse. Time fuses have been used in two forms, electrical (now electronic solid-state) and pyrotechnic. Time fuses can be programmed to detonate airbursts, when the fuse is set to a time shorter than free-fall or parachute-retarded fall from a given altitude, or to set off a delayed surface burst following retarded "laydown" delivery. Pyrotechnic time-delay fuses have been used on penetrating subsurface-bursting weapons where a rugged, simple fuse that can survive a highspeed, multiple-"g" impact is required.

Proximity fuses are used on AFAPs, where they can be set for a given height-of-burst, and on air-to-air weapons where a target provides either a radar reflection, magnetic influence, acoustic pressure, or infrared signal to trigger the fuse. A proximity fuse may be active (i.e., it radiates its own energy and detects a reflected portion), semi-active (i.e., it detects reflected energy originating from another "friendly" source), or passive (i.e., it detects energy radiating from the target). Most proximity fuses use radio, radar, or infrared energy as their triggering stimulus.

In-flight Insertion & Extraction

In-flight insertion and extraction were early means of "safing" U.S. nuclear weapons by keeping the fissile core apart from the HE assembly before the weapon was armed. Since the advent of "sealed pit" weapons, wherein the core and HE are inseparable, all U.S. nuclear weapons have been delivered in a potentially armed condition (which is why one-point safety testing is so important) and elaborate electronic and mechanical steps have been taken to keep all stored electrical energy away from the detonators except when commanded.

The need for some method of "safing" nuclear weapons so that they would not detonated during servicing, loading, takeoff, or in flight, was apparent from the earliest days of the Manhattan Project and was underlined the night before the Nagasaki atomic bombing mission on August 9, 1945, when at least two conventionally-armed B-29s carrying full bomb loads crashed and burned during takeoff at Tinian Island.68


68 Furman, p. 127.

The LITTLE BOY could be safed by carrying the uranium projectile apart from the rest of the weapon, or by keeping the powder charge out of the weapon until nearly at the target, but the FAT MAN implosion bomb could not easily be "safed" in the same fashion. In September 1944, LASL director  J. Robert Oppenheimer commented on the problems of safing the FAT MAN, and sought guidance and suggestions from the Army Air Force:

With our FM designs, it is possible that a crash landing or a bullet may initiate the high explosive. Similarly, errors in assembly procedure before take-off might conceivably do this. Such an explosion involving about two tons of HE would probably hopelessly disperse the precious materials, and would be fatal to personnel and installations immediately around. In particular, it would wreck the plane if the explosion took place in the bomb bay.

Initiation of this kind is not likely to lead to the gadget's operating at anything like full effectiveness, but the possibility of a nuclear explosion cannot be excluded although it would seem to require rather freak conditions.

It is possible for us to safe the gadget against nuclear reaction so that even if the explosive were detonated ideally no nuclear reaction would take place. Such safing mechanisms could then be removed shortly before delivery as a special sort of arming.

In this way, one could be sure that no explosion of unparalleled violence would take place at the forward base and that the explosion, if occurring in the air, would not endanger the entire squadron.

Some of the mechanisms proposed for this safing are relatively complicated and the operations which must be carried out for arming are not quite trivial. It may be possible to provide safing which does not add essential complexity to design or operation, but we are not in a position to guarantee this and much further work would be needed to establish it.

Since the FM is already an extraordinarily complicated piece of machinery and the inevitable operations of assembly and delivery are far from routine, we should like to avoid any additions which are not absolutely necessary.

We should, therefore, like the opinion of qualified representatives of the AAF as to whether in view of the small risk of a nuclear initiation, the hazards of a large explosion are regarded as sufficiently serious to warrant our taking precautionary measures.

In particular, we should like to know whether the take-off can be arranged at such a location that the effects of a nuclear explosion would not be disastrous for the base and the squadron. The undesirability of further contraptions in the FM should be emphasized in presenting this question.

It is of course clear, but it should be restated, that we shall put great effort on elementary precautions to prevent the initiation of the HE during assembly, and to reduce the probability of such initiation from enemy fire. The precautions must be taken in any case.

At Captain Parson's suggestion, I should like to emphasize that the FM will be assembled at the point of take-off, and that before that time no accidental HE detonation can produce a nuclear reaction, since the active material will be separately, and safely, transported and stored.69


69 Letter dated September 20, 1944 to Lt. Col. John Lansdale, Jr. from J. R. Oppenheimer.

The use of an insertable "capsule" of fissionable materials also afforded a convenient means of handling and transporting the active material independently of an entire weapon. This eliminated the necessity of performing time-consuming mechanical assembly and nuclear measurements during weapon assembly.70


70 WT-102, p. 4.

The safety problem was still very real by the fall of 1946, following the CROSSROADS operation at Bikini. Many B-29s had been destroyed by fire following crashes during takeoffs from Pacific island bases during the latter part of World War II.

The Army Air Force had a requirement for only one chance in 150,000 of a crash on takeoff; however, theoretical calculations at LASL indicated that there was a possibility of an explosive yield of as much a 2,000 tons of TNT equivalent if a FAT MAN were involved in a B-29 crash.

The effects of such a blast, and the subsequent plutonium contamination, would be disastrous:

Disaster caused by a low order nuclear explosion, due to a plane accident on takeoff or landing with the bomb, causes a failure of the tactical mission, loss of definite proportion of the Country's (sic) strategic bombing force, incapacitates personnel who are particularly valuable due to their special training and capability to operate from advanced bases, and the abandonment of airport facilities with the consequent heavy loss in morale and prestige.

If correctly timed in the course of a war, such a disaster could be as catastrophic as Pearl Harbor. The long finger of public opinion will point to the Air Force before it points to the bomb.71


71 Memorandum dated 31 October 1946 to Colonel G. M. Dorland from R. S. Warner, Jr., subject: Problems of Disaster on the Ground.

In January 1949, LASL director Norris Bradbury emphasized the pressing need for some means of in-flight arming of nuclear weapons:

Up to the present, it has been required that an atomic weapon take off from its final base completely loaded with fissionable material. This requirement has been viewed with the greatest concern in the light of the ever-present possibility of a crash on take-off followed by a fire which would reach the bomb bay. Under these circumstances, it is probable that a small nuclear explosion would take place - possibly of the order of a few hundred tons of TNT equivalent.

A crash alone is much less likely to accomplish this, but an uncontrolled fire is almost certain to. Such an explosion, while probably not too serious from the point of view of physically damaging installations, can have a catastrophic effect from the point of view of contamination of the base with both fission products from the explosion and with unconsumed nuclear material.

It is not difficult to imagine situations in which downwind personnel might be subjected to serious psychological hazard and the radioactivity of the base such as to preclude further use for a period of years.

Fortunately, techniques can now be foreseen whereby the active material can be kept separate from the weapon on take-off and inserted in the weapon only after the takeoff has been successfully accomplished and the aircraft over either enemy territory or open areas, such as the ocean.72



72 "The Potentialities of the Atomic Bomb," N. E. Bradbury, 4 January 1949, pp. 11, 12.

During the early postwar years, new methods and equipment were developed to permit both manual and automatic motor-driven insertion and extraction of active material after the delivery vehicle was in flight.

Manual IFI/IFE

Manual IFI/IFE (In-flight Insertion and In-flight Extraction; called In-flight Arming when applied to gun-type weapons) often involved special tools or equipment and access to an unpressurized aircraft bomb bay, which meant that crew compartments had to be depressurized to permit weaponry egress and crewmen were required to wear oxygen masks. It also required special access plates or doors at the noses or tails of nuclear weapons and manual handling of radioactive fissile materials and sensitive high explosives and detonators.

Manual IFI/IFE was usually difficult and inconvenient, and sometimes dangerous. It necessitated special training and lengthy and delicate handiwork in cramped, cold, noisy, and vibrating airplane bomb bays. IFI could take anywhere from 10 to 30 minutes to complete.

The weaponeer of the 1945 LITTLE BOY chose, for safety reasons and at the suggestion of the LASL Weapons Committee, to complete weapon arming manually in flight by inserting the powder charge behind the internal U-235 projectile. There was some fear that if the delivery aircraft crashed and burned, and if the powder charge were in the bomb, then the fire might detonate the charge and cause a nuclear explosion which might "neutralize a considerable fraction of an important Army advance base."73


73 Memorandum dated 9 July 1945 to J. R. Oppenheimer from N. F.Ramsey, subject: Dangers from Accidental Detonations of Active Gadgets.

Because the bomb was not designed for IFI, the threads of the breech block and breech plug were coated with graphite for easy assembly and the corners and edges of the breech locking lugs had sharp edges. The weaponeer usually ended up with blackened and cut or scratched hands from placing the cordite bags in the gun breech.74


74 Rowe, p. 93; Knebel and Bailey, pp. 116, 124; Rhodes, TMAB, p. 699.

In a similar fashion, the FAT MAN HE charge assembly was also not easily adaptable to in-flight manipulation. Steps were taken to rectify this situation as early as May 1945, although not originally for the purpose of in-flight arming.

In order to improve safety while transporting the test FAT MAN gadget from Los Alamos to the test site at Alamogordo, New Mexico, the suggestion was made to create a "trap door" in the HE-pusher-tamper assembly through which the core and initiator could be inserted, followed by a pusher-tamper plug and an HE charge:

Kistiakowsky has suggested, and Oppenheimer concurred, that it would be very desirable if the FM gadget could be completely assembled at Site Y, and only the active material inserted at the destination.

GBK has suggested that this might be accomplished by the use of a "trap door" in the polar cap through which a dummy charge would be removed at destination, a plug in the tamper sphere unscrewed, the active material inserted, the tamper plug returned to position, and an HE charge inserted in place of the dummy.

The trap door would then be replaced and provided with positive pressure. It might be even more simple to remove the whole polar cap; a process which would then require no modification of the cap.

By copy of this memo, X-6 is requested to determine whether and under what circumstances (HE) blocks can be removed from an assembled MK III charge in such a manner as to permit the operations suggested above.

What is the largest opening that would then be available at the surface of the tamper?75


75 Memorandum dated 11 May 1945 to Lt. Schaffer and R. Warner from Comdr. N. E. Bradbury, subject: Trap Door Assembly. "GBK" was Dr. George B. Kistiakowsky, director of LASL's wartime "X" or Explosives Division.

As implemented on the FAT MAN, the sphere was placed in an upright position, the forward polar cap removed, and a vacuum cup was used to remove the top pentagonal HE lens and its inner HE charge block. A dummy brass plug was then removed from the tamper, and the plutonium core, with its initiator already inserted, was loaded into the hollow tamper, followed by a screw-in tamper plug and an aluminum plug. The two HE blocks were then re-inserted with the vacuum cup, and their detonators installed.76


76 Memorandum dated 4 August 1945 to Lt. Col. R. W. Lockridge from Captain Larkin, Subject: Trap Door FM Assemblies; memorandum dated October 24, 1945 to Capt. W. F. Schaffer from R. W. Henderson, subject: Disassembly Inspection of Overseas F.M.; Hoddeson, et al., p. 333. The aluminum plug comprised a portion of the aluminum "pusher" between the tamper and the HE.

While this procedure was relatively easy when the HE charges had not been in the sphere for long, considerable difficulty was encountered in removing the inner HE charge if the charges had been assembled and sealed for a long period. Forces of up to 1,000 pounds were sometimes required to dislodge the balky inner charge.77


77 Memorandum dated November 1, 1945 to Capt. W. F. Schaffer from R. A. Rice, subject: Disassembly of 1560 Sphere after Overseas Shipment; memorandum dated 8 November 1945 to G. B. Kistiakowsky from Capt. Schaffer, subject: Unit F-32 Disassembly, 25 October 1945. FAT MAN unit F-32 was to have been the next bomb dropped on Japan after August 9; the drop was canceled when the Japanese issued a cease-fire on August 14. Unit F-32 was originally assembled on July 25, 1945.

Manual IFI/IFE did offer one very important feature in the early days of the U.S. atomic weapons program: if a bomb had to be jettisoned during an emergency, the fissile capsule could be removed before the weapon was dropped. When the entire U.S. nuclear stockpile was composed of a few dozen nuclear cores, every one was worth more than its weight in gold, and extreme measures -- to the extent of providing special recoverable parachute-retarded secure containers for the capsules -- were taken to insure capsule safety and retention during and after in-flight emergencies. For example, if a nuclear capsule in its carrying case (called a "birdcage") were being transported over water, a self-inflating one-man dinghy was to be attached to it.78


78 HISTORY OF THE TACTICAL AIR COMMAND, 1 July through 31 December 1951, Volume VII, Special Weapons Activities, p. 56; White, SIEGELSBACH, pp. 148, 149.

A development program to produce equipment to facilitate manual IFI/IFE was first proposed in May 1948 and initial work began in October of that year. The Sandia Laboratory designed the equipment with Air Force assistance. Production orders were issued in November 1949 and by January 1950, the first 50 sets of the equipment had been produced at the Bendix Aviation facility at Kansas City, Missouri, following the manufacture of seven prototype sets there late in 1949. A lightweight "trap door" assembly was used in conjunction with this equipment.79


79 Memorandum for Dr. John H. Manley, Secretary, General Advisory Committee, from Brig. Gen. James McCormack Jr., Director of Military Application, USAEC, January 27, 1950, Subject: Items for GAC Meeting; MONTHLY PROGRESS REPORT OF SANDIA LABORATORY, September 18, 1949 to November 1, 1949, SL-97, Sandia Laboratory Branch of the Los Alamos Scientific Laboratory, University of California, Albuquerque, New  Mexico, pp. 46, 47; Monthly Status and Progress Reports for November 1949, U.S. Atomic Energy Commission, December 21, 1949, p. 24.

A Special Weapons Command report dated 6 December 1949 concluded that in-flight insertion gear was operationally suitable for aerial nuclear insertion in all types of atomic bomb-carrying aircraft then in service. No major aircraft modifications were required and no previous training was required for weaponeers.

Essentially, the manual in-flight insertion gear consisted of a vacuum pump which grasped and held the high explosive charges that had to be removed from the bomb sphere before manual insertion of the nuclear capsule (even as early as the TRINITY test in July 1945, a vacuum cup had been found to be extremely useful in handling HE charges),80 and two basket arrangements which stored the HE charges while the capsule was being inserted.


80 Bainbridge, LA-6300-H, p. 42.

This in-flight insertion process was necessary for both the MK 4 bomb and the MK 6 Mod 0 bomb. The Air Force procured 460 H-1 IFI kits during 1950 after receiving engineering approval. SAC got 133 sets of IFI equipment for immediate installation on its nuclear bombs.

At times, and especially at high altitudes, frost formed on the surfaces of the HE charges and prevented the vacuum cups from gripping the removable charges securely. To work around this problem, a special "cored" HE charge was developed by LASL that significantly eased nuclear arming operations in cramped and unpressurized aircraft weapon bays.81


81 Furman, p. 409. The MK 6 was one of the first weapons to use a "cored" pit.

Manual insertion of the atomic capsule during flight was never compatible with the Boeing B-47; it was too difficult under high-altitude conditions.

Introduction of the 60-point HE system in later mods of the MK 6 marked a great improvement in simplicity and reliability over the H-1 gear system. In the 60-point HE system, a revolver mechanism, installed in the nose of the bomb, held the HE charges during the insertion operation. There were some minor problems with charges sticking and requiring undue force to remove; in addition, because of the increase in  number of detonation points and corresponding increase in the number of HE charges, more had to be removed to insert the capsule.

The H-1 gear was no longer required after the MK 4 was retired and the MK 6 Mod 2 was stockpiled. MK 5 variants and  all later weapons had automatic insertion features. By January 1953, all H-1 gear had been retired.82



A successor to the H-1 gear was the H-2 manual IFI equipment set, which was in experimental production by the spring of 1951. The H-2 gear consisted of two slides which bolted to the nose of the bomb; a carriage, with a vacuum cup, rode on the slides. The carriage was placed in position with the vacuum cup against the face of the trap door, and vacuum was applied.

The trap door charge was withdrawn, the carriage removed from the slides, and the charge placed safely aside on a nonconducting mat. After both outer and inner charges were removed, the nuclear core was inserted. When insertion was completed, the charges were replaced, and the bomb was "buttoned up."83


83 WT-73, p. 9.

The idea of manually-inserted nuclear components resurfaced again during the late 1980s. At that time, DOD and DOE considered the concept of "insertable nuclear components" (INCs), wherein a number of tactical weapon systems such as submarine torpedoes and surface-to-air missiles could be made  "dual capable" by interchanging conventional high explosive warheads with nuclear "capsules" composed of fissile cores, high explosives, and detonators.84


84 "Administration Renews Study of 'Clip-In' Nuclear Warheads," San Jose Mercury News, June 15, 1986.

Reportedly, several "clip in" alternate nuclear warheads were developed for the Army's LANCE surface-to-surface missile and in 1972, a joint Los Alamos - Livermore lab study examined a "convertible" warhead for the Navy's MK 48 ASW torpedo. The conventional MK 48 has a void from which 20 of its 650 lbs. of PBXN-103 HE could be removed to provide room for a sub-kiloton-yield insertable nuclear component (SKINC) with an explosive force equivalent to 10 tons of TNT.

Insertable nuclear warheads have been fired during underground nuclear tests and can, according to DOE, be developed within five years of authorization.85 There have been unexpected results during nuclear tests of INCs.86 One of these INC devices was the LLNL WAXWING, which had the implosive HE stored as a paste in a missile body before it was transferred to its final location elsewhere in the missile.87


85 "The Navy's Vanishing Nuclear Arsenal," David C. Morrison, NATIONAL JOURNAL, September 13, 1986, p. 2185; "Nuclear Torpedoes," Norman Polmar and Donald M. Kerr, Naval Institute PROCEEDINGS, Vol. 112/8/1002, August 1986, pp. 66, 67; "The Implications of Sub-kiloton Nuclear Torpedoes," John L. Englehardt, Naval Institute PROCEEDINGS, Vol. 113/8/1014, August 1987, pp. 102 -104.

86 Rosengren, RDA-TR-122100-001-Rev. 1, p. 3.

87 DRAWING BACK THE CURTAIN OF SECRECY: RESTRICTED DATA DECLASSIFICATION POLICY, 1946 TO THE PRESENT, RDD-2, U.S. Department of Energy, Office of Declassification, Washington, D.C., January 1, 1995, p. 96.

A revival of in-flight insertion was also suggested in the early 1990s as a means of enhancing warhead safety by retaining the core in a hardened, fire-resistant container adjacent to the pit until just before detonation.88 By the end of 1991, the Lawrence Livermore National Laboratory expected to begin testing of such designs within the next two years.89 These tests have not yet been carried out, due to the current U.S. nuclear weapons testing moratorium; the last U.S. nuclear test was in September 1992.


88 "Test Ban Debate, Round Three: Warhead Safety," Frank von Hippel, BULLETIN OF THE ATOMIC SCIENTISTS, Vol. 47 No. 3, April 1991, p. 30.

89 "Defense Systems," ENERGY & TECHNOLOGY REVIEW, UCRL-52000-91-7/8, July/August 1991, Lawrence Livermore National Laboratory, Livermore, California, p. 4.

One unexpected benefit of keeping the fissile core apart from the remainder of the weapon was that damage or degradation to weapon components caused by the heat generated by the alpha decay of plutonium was obviated.90


90 "Explosive Properties of Various Types of Plutonium," paper presented by Richard L. Garwin at the NATO Advanced Research Workshop on Managing the Plutonium Surplus: Applications and Options, Chatham House, London, January 24, 1994, p. 12.

Automatic IFI/IFE

Around 1952, nuclear-armed missiles and bombs began to be carried externally on aircraft, requiring remotely-controlled mechanical capsule insertion in place of manual insertion. This was eventually accomplished by an electrically-operated reversible screw-jack to both insert and extract nuclear  capsules. The safety of this arrangement could be compromised by inadvertent or accidental IFI motor operation.91


91 Kidder, UCRL-LR-107454, p. 6.

Automatic remotely-controlled insertion of a complete or partial nuclear core also required special equipment that was not common to both gun-type and implosion-type weapons. The weapon pits had to modified to accept the core; in event of weapon jettison, the core could not be salvaged unless the entire weapon were salvaged or parachute-dropped over land in a "safe" condition. The addition of an electromechanical motor-driven mechanism added weight and raised the possibility of electrical or mechanical malfunction.

Automatic IFI (AIFI) involved boring a hole through the HE to accommodate the nuclear insertion equipment; a metal lining was inserted to allow a close-tolerance fit. The nuclear capsule, fitted to a section of HE in which a detonator was embedded, was stored in the IFI mechanism adjacent to the HE sphere, where it had been loaded manually before delivery vehicle takeoff or launch, until the mechanism was activated, moving the core into the center of the HE sphere.

The gear-driven screw-jack type IFI device was operated electrically by flight control equipment in an aircraft cockpit or by an arming control in a missile. In cases of manned aircraft with internal bomb bays, the automatic IFI/IFE mechanism could also be operated manually with a special wrench.

A prototype automatic IFI mechanism was designed in 1950.92 The MK/W-5 weapon and the MK 7 were the first bombs to include remotely-controlled, motor-driven mechanically-inserted capsules.93 Both missile warheads and externally-carried bombs required this type of equipment; insertion of the capsule usually did not occur until the weapon was armed as it neared its target.


92 MANAGING NUCLEAR OPERATIONS, Ashton B. Carter, John D. Steinbruner, and Charles A. Zraket, eds., The Brookings Institution, Washington, D.C., 1987, p. 27.

93 Furman, pp. 410-411.

For missiles, arming did not occur until the delivery vehicle crossed into enemy territory so that in case of accident or crash, an armed warhead would not land on friendly territory, nor could a warhead be recovered by the enemy.94


94 Furman, p. 498.

By the middle of 1953, the employment concepts of several guided missiles required that their nuclear warhead capsules be stored in the in-flight insertion mechanism so that the missile could be kept ready for instant firing for periods between 15 and 90 days. Such storage exposed the capsule to oxidation damage, and methods were devised to keep the capsule in an environmentally-controlled atmosphere nearly equivalent to that in a sealed carrying case.95 These methods required the use of dessicants and pressurized, humidity- and temperature-controlled capsule containers integral with the missile airframe.


95 Semiannual Historical Report, Headquarters, Field Command, Armed Services Special Weapons Command, Sandia Base, Albuquerque, New Mexico, Activities for the Period 1 January 1953 - 30 June 1953, p. 212.

Starting in 1957, AIFI was abandoned in favor of reasonably one-point-safe "sealed pit" weapons in which the core and HE were inseparable. Since that time, various approached have been tried to return to separable pits and HE assemblies.

Limited development (such as the INC concept) has been going on since the mid-1970s without, as yet, a practical result, and prospects of developing a practical separable component design do not yet appear promising.

All proposals to date have invariably involved substantial weight, size, and complexity penalties. For these and other reasons, "sealed pit" weapons remain the design of choice in the U.S. nuclear arsenal.96


96 Kidder, UCRL-LR-107454, pp. 6, 11.

Permissive Action Link Development

A fairly-recent development in U.S. nuclear weapons arming and fuzing systems is the Permissive Action Link, or PAL. A PAL is essentially an electronic or electromechanical combination lock (an electronic keypad is used to arm the latest weapons) that must be set in the proper order before the weapon can be armed and either launched or dropped over its target.

In addition, for some missile systems, more than one individual must co-arm the weapon with another person to avoid unintended use. As a further security precaution, PAL combinations are often changed at regular intervals.

To date, six different type of PALs have been developed by the U.S., as shown in Table 5-1.


Table 5-1: Permissive Action Links (PALs)
Category Description
(none) Mechanical combination lock
A Four-digit, 10-position electromechanical coded switch (most retired or replaced by 1987)
B Ground & airplane-operable 4-digit coded switch (later version with limited try followed by lockout until reset)
C Single-code 6-digit switch, limited try followed by lockout
D Multiple-code 6-digit switch, limited try followed by lockout
F Multiple-code 12-digit switch,  limited try followed by lockout


PAL Design & Function97


97 The following description and history of U.S. nuclear weapon PALs is drawn largely from ASSURING CONTROL OF NUCLEAR WEAPONS: The Evolution of Permissive Action Links, Center for Science and International Affairs Occasional Paper No. 2, Peter Stein and Peter Feaver, University Press of America, Lanham, Maryland, pp. 1-127; and Carter, et. al., MANAGING NUCLEAR OPERATIONS, pp. 46-51, 167, 168, and 449.

A PAL is a physical device, either a simple combination lock or a sophisticated electronic keypad-activated interlock, that guarantees that a nuclear warhead will not be armed either accidentally or without proper authority.

Like an ESD (Environmental Sensing Device), a PAL is intended to preclude warhead detonation unless a direct positive physical action has been taken. However, there are key differences between the design and intent of PALs and ESDs.

An ESD prohibits accidental detonation by not arming the weapon until a series of events or forces peculiar to the warhead's delivery vehicle trajectory occur in a prescribed sequence , such as the sequential acceleration, deceleration, and thermal heating of a missile RV; the high acceleration and spin of an AFAP; or the changing barometric or hydrostatic pressures of air-dropped bombs or ship-launched ASW weapons.

ESDs help prevent nuclear detonation of warheads involved in handling and transportation accidents or delivery vehicle crashes (insensitive high explosives, IHE, also avert accidental nuclear detonation).

A PAL is intended to preclude both accidental and unauthorized detonation. By locking out critical electrical paths -- regardless of any environmental effects -- the PAL virtually guarantees that a nuclear warhead will not explode without proper authority.

PALs are of two major types: simple combination locks and sophisticated electromechanical switches. A combination-lock PAL may occupy space within a weapon into which arming components must fit, or it may simply block access to arming and fuzing electrical controls.

In 1981, according to one estimate, about half of the U.S. nuclear weapons then deployed in Europe, mostly artillery shells, were equipped with combination locks. Recent combination locks include four-digit split-knowledge codes whereby no one person knows the entire code.

Electromechanical PALs are more advanced devices built into weapon casings and not removable without disassembly of the entire warhead. Many U.S. nuclear weapons are reportedly "booby-trapped" to destroy critical internal components if the casing is disassembled.

Four types of electromechanical PALs have been incorporated into U.S. nuclear warheads.

Category A PALs were Model 1541 switches installed on U.S. nuclear weapons during the early 1960s. They were built around a four-digit, 10-position switch, and originally retrofitted to MK/W-28, W-49, W-50, and W-52 weapons. The MK 28 bombs and W-50 PERSHING warheads were later re-equipped with newer PALs; all Category A PAL-equipped warheads have been retired.

Design of Category B PALs started in 1961. The PAL also contained a four-digit 10-position switch, but was controllable from an aircraft cockpit. Category B PALs were retrofitted to a number of gravity bombs and designed directly into early mods of the MK/B 61 bomb.

Category D PAL design was completed by the late 1970s. It includes a six-digit multiple-code switch with a "limited try" lockout feature. Category D PALs were retrofitted to W-70 LANCE missile warheads and designed into the W-80 for the ALCM, a number of gravity bombs, and the W-79 and W-82 AFAPs.

The Category F PAL is the latest design; it incorporates a multiple-code, 12-digit switch with lockout which disables the warhead after repeated attempts to enter codes. Category F PALs are used on the Mod 3, 4, 9, and 10 versions of the MK/B 61 gravity bomb and on W-84 GLCM and W-85 PERSHING II missile warheads. Weapons designed with Category F PALs, or weapons to which Category F PALs are added, must be tested in full-scale nuclear tests to assure warhead performance.98


98 "Toward A Comprehensive Nuclear Warhead Test Ban," Dr. Dan Fenstermacher, Dr. Nikolai Kapranov, and Frank von Hippel, The International Foundation, Washington, D.C., January 1991, p. 56.

History of PALs

Like all other aspects of U.S. nuclear weaponry, PAL evolution was a gradual process between 1945 and the early 1960s. Reasons for development and installation of PALs on American weapons were both political and technological.

Politically, by the late 1950s, the U.S. nuclear stockpile was growing rapidly and weapons were increasingly being deployed overseas where they were not always under direct physical control of American forces. While there was no particular reason to mistrust our allies, the possibility of illegal or irrational warhead launch had to be addressed. In addition, the maintenance of central U.S. control over warhead use would reassure nervous Soviet leaders who feared unauthorized or independent use of U.S. nuclear weapons.

Technologically, by the late 1950s U.S. "sealed pit" warheads had begun to resemble both conceptually and physically the long-sought holy grail of the so-called "wooden bomb" or "wooden round" which did not require elaborate and lengthy assembly or arming procedures. Some new form of control was required to further guarantee that this new generation of weapons could be kept under positive control from unauthorized use.

An important step in the development of PALs occurred in 1953 with the Missiles and Rockets Agreement between DOD and the nuclear weapons laboratories. This arrangement provided for a division of authority under which the labs, through the AEC, designed and manufactured the warheads and the U.S. armed services assumed arming and firing responsibilities.

In effect, the AEC pledged to deliver to DOD warheads that would explode if specified voltages were applied to appropriate pins. The labs were free to pursue their own research into integral weapon use-control devices so that when policy makers eventually requested implementation of controls, engineering and prototype development were already in progress.

Following this agreement, the weapons labs in the mid to late 1950s began to build ESDs into many new "sealed pit" warheads, including the W-25, W-30, W-49, W-54, and W-59. These ESDs provided control over weapon use without impinging on DOD arming and fuzing options.

The first suggestion of a mechanical lock to prevent unauthorized weapon use appeared in a 1958 RAND Corporation report on the avoidance of accidental nuclear war. Also at this time, UCRL and Sandia-Livermore weapons designers began to study improved warhead use-control, including an  electromechanical, remotely-controlled combination lock to be inserted into the warhead's arming circuitry.

Because atomic demolition munitions were (and still are) unsuitable to ESD installation (the sequence of events for unauthorized detonation is the same as that for proper use), and because of the possibility of theft, W-30 and W-31 ADMs in Europe were equipped expeditiously in 1959 with simple three-digit combination locks that blocked access to arming components.

Work on prototype PALs remained at a low level until 1960. In 1959, UCRL had begun working on a so-called "Golden Key," a conceptual arming device that could distinguish between wartime and peacetime. A year later, a prototype was ready for testing, and by late 1960, Sandia had developed prototypes of several new combination locks adaptable to a number of types of weapons.99


99 "Weapon Dispersal without Fear of Unauthorized Use," SANDIA LAB NEWS, Family Day Special Edition, Vol. 38 No. 20, October 10, 1986, p. 4.

Not much interest was aroused in the idea until an inspection trip to Europe in December 1960 by members of the congressional Joint Committee on Atomic Energy revealed that forward-positioned U.S. nuclear weapons in the possession of North Atlantic Treaty Organization (NATO) forces were under inadequate and possibly-illegal control (as defined by terms of the Atomic Energy Act of 1954).

The sight of fully-operational American weapons aboard NATO quick-reaction aircraft (QRA) manned by foreign pilots who were essentially free to "scramble" with or without proper authorization was enough to cause a renewed interest in a positive-control arming lock.

In January 1961, Sandia-Livermore sent a letter to the AEC's Division of Military Application proposing physical protection devices on nuclear weapons. At this time, NATO forces controlled W-49 warheads on JUPITER missiles and lowyield MK 7 and MK 12 gravity bombs. An electronic or electromechanical coded receiver, ready for production in nine to 12 months, was suggested. This device became the "Category A" PAL.

This proposal was given further impetus by a congressional committee report issued in February which called for either complete withdrawal of JUPITER missiles in Europe or improved security for their warheads. Several congressional hearings on the subject of weapon use-control were held in the spring of 1961; a prototype PAL, then called a "Proscribed Action Link," was demonstrated during one of the meetings.

Work continued on PALs for the next year; U.S. military forces were unenthusiastic, fearing loss of independence and device malfunction that would disable all nuclear warheads in time of crisis. In June 1962, the National Security Council issued National Security Action Memorandum 160 (NSAM 160) directing installation of PALs in all U.S. nuclear weapons in Europe. All other American weapons were at least temporarily excluded from the PAL requirement.

Sandia estimated that enough PALs could be built within six months. PAL installation in Europe on W-31s aboard NIKE HERCULES and HONEST JOHN rockets, W-45 and W-54 ADMs, W-54s on the Army's DAVY CROCKETT system, and W-33 and W-48 AFAPs, was completed by September 1962. The implementation of NSAM 160 cost $23 million.

The first operational PAL was a simple five-digit mechanical combination lock. This so-called "Category A" electromechanical PAL was later retrofitted to long-range theater weapon systems, including the MK 7 and MK 28 gravity bombs, W-49 warheads on THOR and JUPITER missiles, and W-28, W-50, and W-52 warheads on MACE, PERSHING, and SERGEANT missiles, respectively.

The new PAL had to meet two conflicting demands: be secure at all times, yet be enabled immediately by some method other than requiring an individual to walk up to the weapon, open a panel, and dial in the enabling code.

Sandia came up with a unique solution:

The design concept we settled on ... was brand new. It was to be the first of many electromechanical PALs. It would be buried within the weapon so that it couldn't be easily bypassed. It would be remotely operated so if an alert came, you wouldn't have to literally run out to a missile launch complex and plug in the authorizing code before the weapon could be used. It would be harder to pick than earlier PALs. And finally, all of the associated encoders and recoders would be new.

The result was a reversible DC motor-driven device with wheels that responded to electrical signals from a remote controller. If the proper code was inserted, the wheels would align and electrical switches would close, allowing arming signals to be transmitted inside the weapon.

As it turned out, we had less than 18 months from the issuance of the JCAE report to the time when the new PALs had to be delivered.

Within that tight period, we completed development of what came to be known as the MC 1541, and the design, testing, and manufacture of several hundred of these first-of-a-kind, remotely-operated use-control switches for the NATO weapons.100


100 "Of Extreme Importance and Urgency in the National Defense," Sandia National Laboratories booklet dated October 1991, p. 4.

At the time of NSAM 160, Navy and USAF SAC warheads were exempted from PAL control. These weapons were not being used (nor was their use likely) by NATO forces. SAC resisted individual PAL arming of its weapons because of the complexity of aircraft-to-bomb electrical connections.

An ongoing research program to improve PAL technology began in 1962. Guidelines were issued by the Secretary of Defense ordering that PALs not degrade weapon reliability or safety, that they be compatible with the weapon's operational concept, and that they not significantly lengthen the reaction time for the weapon system. Between 1962 and 1972, no fewer than seven types of PALs were developed; five of them were fitted to U.S. nuclear warheads. The Category B PAL was the first that could be operated from an aircraft cockpit.101


101 "Weapon Dispersal without Fear of Unauthorized Use," SANDIA LAB NEWS, Family Day Special Edition, Vol. 38 No. 20, October 10, 1986, p. 4.

Starting in 1961, scientists at LRL turned their attention toward a PAL that would permanently disable itself and require weapon disassembly to replace it when attempts were made to operate it by entering multiple codes. Such a device was an alternate to a pick-proof lock. By 1964, the Category B PAL featured this "lockout" provision in its design.

In 1967, PAL use was extended to the strategic weapons carried by SAC bombers. Provisions were made both within the aircraft and the bombs and missile warheads to allow arming of one or more weapons via cockpit-operable switches. To ensure added control, more than one switch had to be operated simultaneously by two physically-separated crewmen.

In much the same manner, PAL control was extended to SAC's ICBMs in the mid-1970s. Silo-based SAC ICBMs are now armed by crewmen in command centers and are to be launched only after a favorable multiple-site "vote." Naval SLBMs, because of the independent nature of the seaborne deterrent forces, and because deep-running submarines are more physically secure from outside intruders and far more likely to survive longer during a nuclear war, are armed by the joint actions of several key officers.102 However, by mid-1995, proposals were being advanced to apply PALs to SLBMs, and plans were underway to begin doing this in 1996.


102 In February 1990, the Navy said it had installed PAL devices on nuclear weapons stored ashore and or in shipment, as well as on "all newer Navy non-strategic nuclear weapons." The canceled B 90 strike/depth bomb was to have used a Category D PAL. ("U.S., Soviets Raise Concern Over Sea-Based Nuclear Security," DEFENSE NEWS, May 21, 1990, p. 6.)

At the present time, all American-owned non-naval nuclear weapons based outside the continental U.S. have some sort of PAL attached to them. Of all U.S. Army, Air Force, and Navy tactical weapons formerly based in Europe or the Far East, about half had non-electronic coded locks derived from the three-digit combination locks of the ADMs of the early 1960s.

All SAC weapons, including bombs and air- and surface-launched missile warheads, are equipped with either PALs or other use-control devices.

ICBM/IRBM Warhead Fuzing

The safing, arming, and fuzing subsystems of a long-range ballistic missile sense particular points in the re-entry vehicle (RV) trajectory and complete steps to arm, fuze, and finally fire the warhead. An on-board computer receives signals from various pressure and temperature-sensing transducers and thermocouples mounted on the RV exterior.

A baroswitch starts the arming sequence when the missile has risen to a pre-set altitude. A trajectory-sensing device  determines whether or not the missile is on a correct course and completes the next step of the arming sequence. If the missile is off-course, this sensing equipment will issue a self-destruct signal to destroy the missile, RV, and warheads.

If the missile is on course, the arming computer samples current environmental conditions to determine what to do next. On re-entry to the earth's atmosphere, deceleration forces of 20 to 50 times the force of gravity provide a signal to complete arming and firing of the warhead at a preselected burst altitude.103 In addition, re-entry temperatures on the order of several thousand degrees can also be used to trigger part of the arming process.



Once the warheads are armed, they can be individually fuzed by radar, barometric, or time fuses to detonate as airbursts, or can be fired by contact fuses on impact.