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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.
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[Gravity Bomb Model]
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-> Cutaway Sections Visible <-
/\
/ \ <---------------------------[1]
/ \
_________________/______\_________________
| : ||: ~ ~ : |
[2]-------> | : ||: : |
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| :______||:_____________________________: |
|/_______||/______________________________\|
\ ~\ | | /
\ |\ | | /
\ | \ | | /
\ | \ | | /
\ |___\ |______________| /
\ | \ |~ \ /
\|_______\|_________________\_/
|_____________________________|
/ \
/ _________________ \
/ _/ \_ \
/ __/ \__ \
/ / \ \
/__ _/ \_ __\
[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]
```::::_____::::'''
~~~~~
============================================================================
- Diagram Outline -
---------------------
[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
---------
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
----------------------
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
---------------
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)
--------------------------------
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
-----------------
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
-----------
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
-----
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.
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1994