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GUN ASSEMBLY

The basic problem of a fission weapon is to assemble two subcritical fragments of U-235 in the shortest time possible. The gun assembly is the most intuitive way to do this. The fragments have to be assembled quickly since there is the possibility of the weapon blowing up way before a chain reaction has started.

The way to do this is to machine the fragments into a pac-man shape, and a wedge. You fit these into a cylindrical tube, and bring them together using an explosive charge.

This sort of technique won't work with Pu-239 since it is extremely difficult to separate large quantities of Pu-239 from Pu-240 and other contaminants. These unwanted materials emit neutrons through spontaneous fission, all before super- criticality. They would give a premature start to any chain reaction before you could achieve a full assembly.

You can have gun assemblies small enough to fit into artillery shells. (See the gifs for an example of this.) merlin@neuro.usc.edu supplies the examples:

M454 155mm Nuclear Projectile:
2'10" long, 155mm diameter, 119.5 lbs weight,
W-48 nuclear warhead, 1-2kt yield, 8.75 mile range.

and

XM785 155mm Nuclear Projectile:
2'10.3"  long, 155mm diameter, 96 lbs weight,
W-82-1 fission warhead, 1-2kt yield, 18.5 mile range using
rocket assistance.
                           [Gravity Bomb Model]
                       ----------------------------
                      -> Cutaway Sections Visible <-
                                    /\
                                   /  \ <---------------------------[1]
                                  /    \
                _________________/______\_________________
               | :      ||:      ~      ~               : |
   [2]-------> | :      ||:                             : |
               | :      ||:                             : |
               | :      ||:                             : |
               | :      ||:                             : |
               | :      ||:                             : |
               | :      ||:                             : |
               | :      ||:                             : |
               | :      ||:                             : |
               | :      ||:                             : |
               | :      ||:                             : |
               | :      ||:                             : |
               | :______||:_____________________________: |
               |/_______||/______________________________\|
                \       ~\       |              |         /
                 \       |\      |              |        /
                  \      | \     |              |       /
                   \     |  \    |              |      /
                    \    |___\   |______________|     /
                     \  |     \ |~               \   /
                      \|_______\|_________________\_/
                      |_____________________________|
                      /                             \
                     /       _________________       \
                    /      _/                 \_      \
                   /    __/                     \__    \
                  /    /                           \    \
                 /__ _/                             \_ __\
   [3]_______________________________                 \ _|
                 / /                 \                 \ \
                / /                  \/                 \ \
               / /              ___________              \ \
              | /            __/___________\__            \ |
              | |_  ___     /=================\     ___  _| |
   [4]---------> _||___|====|[[[[[[[|||]]]]]]]|====|___||_ <--------[4]
              | |           |-----------------|           | |
              | |           |o=o=o=o=o=o=o=o=o| <-------------------[5]
              | |            \_______________/            | |
              | |__                |: :|                __| |
              | |  \______________ |: :| ______________/  | |
              | | ________________\|: :|/________________ | |
              | |/            |::::|: :|::::|            \| |
   [6]----------------------> |::::|: :|::::| <---------------------[6]
              | |             |::::|: :|::::|             | |
              | |             |::==|: :|== <------------------------[9]
              | |             |::__\: :/__::|             | |
              | |             |::  ~: :~  ::|             | |
   [7]----------------------------> \_/   ::|             | |
              | |~\________/~\|::    ~    ::|/~\________/~| |
              | |            ||::         <-------------------------[8]
              | |_/~~~~~~~~\_/|::_ _ _ _ _::|\_/~~~~~~~~\_| |
   [9]-------------------------->_=_=_=_=_::|             | |
              | |             :::._______.:::             | |
              | |            .:::|       |:::..           | |
              | |        ..:::::'|       |`:::::..        | |
   [6]---------------->.::::::' ||       || `::::::.<---------------[6]
              | |    .::::::' | ||       || | `::::::.    | |
             /| |  .::::::'   | ||       || |   `::::::.  | |
            | | | .:::::'     | ||    <-----------------------------[10]
            | | |.:::::'      | ||       || |      `:::::.| |
            | | ||::::'       | |`.     .'| |       `::::|| |
  [11]___________________________  ``~''  __________________________[11]
            : | | \::            \       /            ::/ | |
           |  | |  \:_________|_|\/__ __\/|_|_________:/  | |
           /  | |   |  __________~___:___~__________  |   | |
          ||  | |   | |          |:::::::|          | |   | |
  [12]   /|:  | |   | |          |:::::::|          | |   | |
|~~~~~  / |:  | |   | |          |:::::::|          | |   | |
|----> / /|:  | |   | |          |:::::::|        <-----------------[10]
|     / / |:  | |   | |          |:::::::|          | |   | |
|      /  |:  | |   | |          |::::<-----------------------------[13]
|     /  /|:  | |   | |          |:::::::|          | |   | |
|    /  / |:  | |   | |          `:::::::'          | |   | |
|  _/  / /:~: | |   | `:           ``~''           :' |   | |
|  |  / / ~.. | |   |: `:                         :' :|   | |
|->| / /   :  | |   :::  `.                     .' <----------------[11]
|  |/ / ^   ~\|  \  ::::.  `.                 .'  .::::  /  |
|  ~   /|\    |   \_::::::.  `.             .'  .::::::_/   |
|_______|     |      \::::::.  `.         .'  .:::<-----------------[6]
              |_________\:::::.. `~.....~' ..:::::/_________|
              |          \::::::::.......::::::::/          |
              |           ~~~~~~~~~~~~~~~~~~~~~~~           |
              `.                                           .'
               `.                                         .'
                `.                                       .'
                 `:.                                   .:'
                  `::.                               .::'
                    `::..                         ..::'
                      `:::..                   ..:::'
                        `::::::...        ..::::::'
  [14]------------------> `:____:::::::::::____:' <-----------------[14]
                            ```::::_____::::'''
                                   ~~~~~
  1. Diagram Outline -
  2. ——————–
                      [1] - Tail Cone
                      [2] - Stabilizing Tail Fins
                      [3] - Air Pressure Detonator
                      [4] - Air Inlet Tube(s)
                      [5] - Altimeter/Pressure Sensors
                      [6] - Lead Shield Container
                      [7] - Detonating Head
                      [8] - Conventional Explosive Charge
                      [9] - Packing
                     [10] - Uranium (U-235) [Plutonium (See other diagram)]
                     [11] - Neutron Deflector (U-238)
                     [12] - Telemetry Monitoring Probes
                     [13] - Receptacle for U-235 upon detonation
                            to facilitate supercritical mass.
                     [14] - Fuses (inserted to arm bomb)

[5] Altimeter

  1. ——–
   An ordinary aircraft altimeter uses a type of Aneroid Barometer which

measures the changes in air pressure at different heights. However, changes in air pressure due to the weather can adversely affect the altimeter's readings. It is far more favorable to use a radar (or radio) altimeter for enhanced accuracy when the bomb reaches Ground Zero.

   While Frequency Modulated-Continuous Wave (FM CW) is more complicated,

the accuracy of it far surpasses any other type of altimeter. Like simple pulse systems, signals are emitted from a radar aerial (the bomb), bounced off the ground and received back at the bomb's altimeter. This pulse system applies to the more advanced altimeter system, only the signal is continuous and centered around a high frequency such as 4200 MHz. This signal is arranged to steadily increase at 200 MHz per interval before dropping back to its original frequency.

   As the descent of the bomb begins, the altimeter transmitter will send

out a pulse starting at 4200 MHz. By the time that pulse has returned, the altimeter transmitter will be emitting a higher frequency. The difference depends on how long the pulse has taken to do the return journey. When these two frequencies are mixed electronically, a new frequency (the difference between the two) emerges. The value of this new frequency is measured by the built-in microchips. This value is directly proportional to the distance travelled by the original pulse, so it can be used to give the actual height.

   In practice, a typical FM CW radar today would sweep 120 times per

second. Its range would be up to 10,000 feet (3000 m) over land and 20,000 feet (6000 m) over sea, since sound reflections from water surfaces are clearer.

   The accuracy of these altimeters is within 5 feet (1.5 m) for the higher

ranges. Being that the ideal airburst for the atomic bomb is usually set for 1,980 feet, this error factor is not of enormous concern.

   The high cost of these radar-type altimeters has prevented their use in

commercial applications, but the decreasing cost of electronic components should make them competitive with barometric types before too long.

[3] Air Pressure Detonator

  1. ———————
   The air pressure detonator can be a very complex mechanism, but for all

practical purposes, a simpler model can be used. At high altitudes, the air is of lesser pressure. As the altitude drops, the air pressure increases. A simple piece of very thin magnetized metal can be used as an air pressure detonator. All that is needed is for the strip of metal to have a bubble of extremely thin metal forged in the center and have it placed directly underneath the electrical contact which will trigger the conventional explosive detonation. Before setting the strip in place, push the bubble in so that it will be inverted.

   Once the air pressure has achieved the desired level, the magnetic bubble

will snap back into its original position and strike the contact, thus completing the circuit and setting off the explosive(s).

[7] Detonating Head

  1. ————–
   The detonating head (or heads, depending on whether a Uranium or

Plutonium bomb is being used as a model) that is seated in the conventional explosive charge(s) is similar to the standard-issue blasting cap. It merely serves as a catalyst to bring about a greater explosion. Calibration of this device is essential. Too small of a detonating head will only cause a colossal dud that will be doubly dangerous since someone's got to disarm and re-fit the bomb with another detonating head. (an added measure of discomfort comes from the knowledge that the conventional explosive may have detonated with insufficient force to weld the radioactive metals. This will cause a supercritical mass that could go off at any time.) The detonating head will receive an electric charge from the either the air pressure detonator or the radar altimeter's coordinating detonator, depending on what type of system is used.

[8] Conventional Explosive Charge(s)

  1. ——————————-
   This explosive is used to introduce (and weld) the lesser amount of

Uranium to the greater amount within the bomb's housing.

   Plastic explosives work best in this situation since they can be

manipulated to enable both a Uranium bomb and a Plutonium bomb to detonate.

[11] Neutron Deflector

  1. —————-
   The neutron deflector is comprised solely of Uranium-238.  Not only is

U-238 non-fissionable, it also has the unique ability to reflect neutrons back to their source.

   The U-238 neutron deflector can serve 2 purposes.  In a Uranium bomb, the

neutron deflector serves as a safeguard to keep an accidental supercritical mass from occurring by bouncing the stray neutrons from the `bullet' counterpart of the Uranium mass away from the greater mass below it (and vice- versa). The neutron deflector in a Plutonium bomb actually helps the wedges of Plutonium retain their neutrons by `reflecting' the stray particles back into the center of the assembly.

[6] Lead Shield

  1. ———-
   The lead shield's only purpose is to prevent the inherent radioactivity

of the bomb's payload from interfering with the other mechanisms of the bomb. The neutron flux of the bomb's payload is strong enough to short circuit the internal circuitry and cause an accidental or premature detonation.

[14] Fuses

  1. —-
   The fuses are implemented as another safeguard to prevent an accidental

detonation of both the conventional explosives and the nuclear payload. These fuses are set near the surface of the `nose' of the bomb so that they can be installed easily when the bomb is ready to be launched. The fuses should be installed only shortly before the bomb is launched. To affix them before it is time could result in an accident of catastrophic proportions.

1994

/data/webs/external/dokuwiki/data/pages/archive/science/gun.txt · Last modified: 2001/11/04 03:42 by 127.0.0.1

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