MILNET: Nuclear Weapons Delivery
Of equal importance in the delivery field, is the safety precautions for shipment of weapons not yet required to be armed. This enters into the discussion of topics such as interlocks, accidental detonation, and other safety features. We will discuss this aspect first, and will refer to other information available on MILNET which speaks to the nuclear accident issue.
Public knowlege of interlocks on nuclear weapons became quite visible after several nuclear weapons accidents where the weapons were either lost or accidently separated from their carrying aircraft. A famous incident was the loss of several nuclear gravity bombs from a B-52 in Europe in the mid 1960s.
The U.S. methods for ensuring a weapon is NOT armed during this period of time were called Permissive Action Link or PAL. The name describes more than is actually discussed in the public documentation and we will not provide supposition. What is known is that there were at least two types during this era.
Also used on all weapons of the period was a simple ARM/SAFE switch which was required to be in the armed position prior to PAL activation. During all manipulations of the weapons prior to takeoff or launch of a nuclear armed aircraft or rocket, the switch would be in its normal state of safe, which prevented not only PAL activation, but physical maninpulation of nuclear components which could cause detonation. It is thought that later weapons (i.e. compression technology weapons which did not allow removal of the "nuclear pit") were not as physically restrained.
Some early warheads did not even have PAL interlocks, using only the ARM/SAFE method. Usually, these were devices that had a "sealed pit", taht is, the weapon was essentially ready to become critical through some physical means, and the components were not removable. Thus some physical safing means was used to block against accidental high order detonation.
Another safety feature on many warheads in use during this period were based upon actual operational parameters. For instance, the Genie nuclear tipped Air to Air Missile had a warhead which, through environmental sensors, was required to detect the immense acceleration to Mach 3 from launch and then decelleration as it neared the target (presuambly after rocket engine burnout). Similarly, some gravity bombs required a minimum altitude to have been achieved prior to final arming being possible. In almost all aircraft borne weapons, the final arming came from the cockpit just before release of the weapon. The types of triggering were ground burst, contact (or proximity), or air burst. Typically the pilot could make these decisions, however some early weapons would have to be selected physically on the weapon by the arming technician on the ground.
PAL-A
PAL A was a simple, multiple combination lock, with unlimited retries. Not much else is available publicly. The concept was simple, and usually performe on the ground by a technician (usually an enlisted man trained specifically for this task, sometimes called the monitor or warden. His job was to be in charge of at most, two weapons at any one time, the weapons never being further apart than 100 feet. The monitor was responsible for confirming the warheads were in a SAFE condition before movement, uploading or downloading, or before closing the door on them in secure storage facility. As per the regulations, the monitor was accompanied at all times by one other person, the two man rule in effect for the lifetime of the weapon.
Once the weapon was uploaded onto the aircraft (or embedded into a ground launched system), the ARM/SAFE switch was moved to ARM, and the PAL was encoded. This in earlier models was all that was necessary to arm the weapon for release, however in aircraft the weapon also received a command from the cockpit. Later PALs required the correct PAL code to be sent, thus matching that configured on the ground, before release, thereby becoming the final arm order. Also a pin on the weapon would be pulled by the dropping action from the bomb rack, establishing the bomb was away and truly on its way. Some rockets (again the Genie serves as a good example) also had embedded within a means to detect acceleration, then decceleration as well as some period of steady flight in order to truly prepare the weapon for detonation. Gravity bombs may also have been set to air burst, which meant a minimum exceeded altitude sensor may also have been necessary to finally arm the device, and then of course the burst altitude would then set the device off.
PAL - B
We actually know more about the PAL B interlock. This was a four digit code, which in early weapons, could be entered multiple times. Later devices however, required the code to be set correctly within a certain number of tries or the device was locked out. For instance, the multiple try might allow an unauthorized user to attempt all combinations in an effort to discover the correct one. This would not be all that hard, since four digits (assuming the combo wheels had 0 through 9 for each digit...possibly an invalid assumption), would yield only 10 to the 4th (10,000) combinations. While this could take some considerable time, it did mean that an unauthorized person could discover the correct combination given that time...not unreasonable for a stolen weapon.
Later PAL versions did in fact require the combination to be set within a certain number of trys, however, the early versions assumed weapons to remain in relatively secure hands...thus the PAL sequence was used only to prohibit unauthorized use on a whim, where the device was to be placed on an aircraft thus with little time for disovery of an unknown combination.
Ferry Operation
Weapons being ferried were configured in the safe condition, that is, any combinaiton of the most safe condition possible. For instance, for weapons with removable pits, the pits were removed. For weapons with detonators surrounding a compression core, the detonators were removed. However HE explosives were typically left in place as they had been configured at the assembly plant.
If the weapon had a ARM/SAFE switch, this was set to SAFE and in many cases wired in that condition for transport. Only upon storage where movement could not be cause for accidental setting of the switch, was the safety wire removed.
Readiness Tests
Weapons were at times uploaded onto aircraft during readiness tests. In this case, the weapon might be cycled into the arm state using the ARM/SAFE switch only to ensure the proper procedure were known and could be followed in the correct sequence.
Armed Weapons On Mission Means Storage on the Ground
During the early 1960s, numerous weapons were carried aboard aircraft on combat ready missions. These missions were intended to act as a deterent to other nations (primarily the former Soviet Union), thus the weapons carried were "Hot", that is, not training rounds. The implication is of course, that each combat flight must have the prerequisite number of weapons in storage at their home base, meaning SAC (Strategic Air Command) and TAC (Tactical Air Command) or TAC's predecessor ADC (Air Defense Command) bases were outfitted with secure storage facilities. In many cases, ADC shared facilities with SAC, the two tasks, bombers and fighters being co-located on the same Air Force Base. For example, at one time Castle AFB near Merced, California, not only home based a full air wing of B-52s loaded with nuclear bombs, but also the 456th Fighter Interceptor Squadron featuring the F-106 Delta Dart carrying Genie nuclear tipped AAMs (Air to Air Missiles). It is presumed the nuclear warheads for the Genie were assembled in full up rounds and stored with the nuclear gravity bombs for the B-52s in facilities shared with the SAC.
As mentioned earlier, the weapon was usually connected to the aircraft so that it could be armed with the final arming command (in later scenarios this required the arming personnel (pilot or bombadier) to reenter the correct PAL sequence, matching the one input on the ground for the device. Other means for preventing entry of this code may also have been in place as well, however official, confirmable information was not discovered in our research.
Bombers, typically were tasked to drop the weapon from high altitude and proceed in a straight line away from the drop area, the drop distance before detonation providing for ample time to move away. In some cases, however, a parachute or other means of slowing the bomb's descent (retardation) was used to provide a better safety margin for the crew.
In the case of the Genie missile, which was most likely armed with a 2KT warhead, the missile had to be close enough to the target to do the necessary damage, therefore the fighter had to be quite close to the blast. Perhaps this is the reason for choosing the relatively low yield weapon...i.e. a larger warhead might be possible to carry in the missile, but would not allow the fighter time to flee from the intercept and escape the radiation from the explosion.
ICBMs, of course, did not have these considerations, since the tasking was to explode over the territory of the enemy, thus only damaging the enemy and the delivery vehicle, which had no humans aboard. ICBM delivery was also considered a more reliable means of delivery, since ICBMs were difficult to intercept, and in any case, interception at the final phases of the attack might mean lessening but not eliminating the impact of damage. This was especially true for EMP (Electro-Magnetic Pulse) attacks which did not necessarily need to be all that close to the ground (actual details of the energy released require analysis of yield versus atmospheric attenuation, which can be calculated, however, is not publically published officially).
Clandestine delivery of nuclear weapons was a consideration during the Cold War and is a major concern in today's terrorist rich environments. Both the U.S. and the Soviets produced small weapons capable of being delivered by artillery shells.
Moreover, the primary device which is used to detonate a larger, thermo nuclear (i.e. fusion bomb) does not have to be all that large. Typically, a primary can be as small as 1 or 2 KT. The size of that weapon need only be a small compression style device with a plutonium core. Small shaped charges symetrically compress this core to criticality. Thus the primary is relatively safe...a non-symetrical compression will cause some radiation leakage, followed by heat and dispersal of toxic nuclear material, however will not cause a high order explosion (i.e. an HE explosion, not a fission or fusion explosion. Yield of a malfunctionning, non symetrical explosion would be on the order of the HE explosive itself...perhaps equivalent to a few pounds of C4. While still quite deadly, not at all like that of 2,000 tons of TNT.
Unfortunately, it is clear that such a primary device can be carried in a standard adult backpack like that worn by high school or college students all over the world. Thus this type of device is easily transported, and physically able to be carried a great distance on the human form, and is for all intents and purposes, difficult to discover outside the range of highly sensitive instruments. They cannot, however, be transported through airport metal detectors or other baggage scanning equipment.
The easiest scenario for a terrorist transportation is overland. For instance Europe is a wide open problem for transportation of small nuclear weapons, with many borders only having quite passive inspection and detection gear. Similarly, movement across the U.S. border with Mexico provides an excellent opportunity for terrorists targeting U.S. cities. Once into the U.S., there is little to stop overland transportation of a small nuclear weapon.
Thus delivery of a small nuclear weapon could be via backpack, car trunk, truck body, or private aircraft. Shipment might be problematic on public carriers however, as some precautions do exist, while spotty. Also, some explosive residue from the HE used in a nuclear weapon might attract attention of explosive sniffing animals, however, this would be rare for a nuclear weapon.
Larger nuclear weapons could be constructed using the small device as the primary, however, larger weapons are more easily detected with radiation measurement devices, and the sheer weight of MT scale weapons (lots of U235 or U238 for instance...Uranium is heavier than lead) would mean truck or rail transport. Again, today's public carriers do conduct spot checks and a terrorist might not want to subject his precious cargo to that risk.
A clandestine attack could be conducted by a national level asset, however. For instance, a Libyan freighter could easily steam into New York harbor and detonate a multi-megaton weapon, assuming the weapon could be gotten past "outside-the-harbor" customs agents. However, it is not clear if inspections would occur far enough outside the port to eleviate much of the damage of a megaton scale weapon. And of course dockside inspections would be far too late to stop a terrorist attack using a foreign flagged vessel as the delivery vehicle.
Private aircraft are also a possibility, since once in the U.S., a large weapon could be assembled and then transported to the target area. For instance, a nuclear weapon could be flown on a private plane to the White House, as already witnessed by a real world stunt at the White House in this decade, and the Kremlin in the last decade.
With a 500kt weapon, the terrorist need only get within "warning" distance before being able to destroy both the White House and the National Mall (Congress and all the musuems and archives at ground level).
A similar attack could just as easily be carried out upon the Pentagon in the U.S., or Buckingham Palace in the U.K..
Canadian Nuclear Weapons, Dr John Clearwater, Copyright 1998, John Clearwater, published by the Dundurn Press, 8 Market Street, Suite 200, Toronto, Ontario, Canada, M5E 1M6, ISBN 1-55002-299-7.
