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
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
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.
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