293 lines
14 KiB
Plaintext
293 lines
14 KiB
Plaintext
GUN ASSEMBLY
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The basic problem of a fission weapon is to assemble two subcritical fragments
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of U-235 in the shortest time possible. The gun assembly is the most
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intuitive way to do this. The fragments have to be assembled quickly since
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there is the possibility of the weapon blowing up way before a chain reaction
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has started.
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The way to do this is to machine the fragments into a pac-man shape, and a
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wedge. You fit these into a cylindrical tube, and bring them together using
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an explosive charge.
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This sort of technique
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won't work with Pu-239 since it is extremely difficult to separate
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large quantities of Pu-239 from Pu-240 and other contaminants. These unwanted
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materials emit neutrons through spontaneous fission, all before super-
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criticality. They would give a premature start to any chain
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reaction before you could achieve a full assembly.
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You can have gun assemblies small enough to fit into artillery shells.
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(See the gifs for an example of this.) <merlin@neuro.usc.edu> supplies
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the examples:
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M454 155mm Nuclear Projectile:
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2'10" long, 155mm diameter, 119.5 lbs weight,
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W-48 nuclear warhead, 1-2kt yield, 8.75 mile range.
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and
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XM785 155mm Nuclear Projectile:
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2'10.3" long, 155mm diameter, 96 lbs weight,
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W-82-1 fission warhead, 1-2kt yield, 18.5 mile range using
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rocket assistance.
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============================================================================
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[Gravity Bomb Model]
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----------------------------
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-> Cutaway Sections Visible <-
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/\
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/ \ <---------------------------[1]
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/ \
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_________________/______\_________________
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| : ||: ~ ~ : |
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[2]-------> | : ||: : |
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| : ||: : |
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| : ||: : |
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| : ||: : |
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| : ||: : |
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| : ||: : |
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| : ||: : |
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| : ||: : |
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| : ||: : |
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| : ||: : |
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| : ||: : |
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| :______||:_____________________________: |
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|/_______||/______________________________\|
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\ ~\ | | /
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\ |\ | | /
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\ | \ | | /
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\ | \ | | /
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\ |___\ |______________| /
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\ | \ |~ \ /
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\|_______\|_________________\_/
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|_____________________________|
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/ \
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/ _________________ \
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/ _/ \_ \
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/ __/ \__ \
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/ / \ \
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/__ _/ \_ __\
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[3]_______________________________ \ _|
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/ / \ \ \
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/ / \/ \ \
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/ / ___________ \ \
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| / __/___________\__ \ |
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| |_ ___ /=================\ ___ _| |
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[4]---------> _||___|====|[[[[[[[|||]]]]]]]|====|___||_ <--------[4]
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| | |-----------------| | |
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| | |o=o=o=o=o=o=o=o=o| <-------------------[5]
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| | \_______________/ | |
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| |__ |: :| __| |
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| | \______________ |: :| ______________/ | |
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| | ________________\|: :|/________________ | |
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| |/ |::::|: :|::::| \| |
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[6]----------------------> |::::|: :|::::| <---------------------[6]
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| | |::::|: :|::::| | |
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| | |::==|: :|== <------------------------[9]
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| | |::__\: :/__::| | |
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| | |:: ~: :~ ::| | |
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[7]----------------------------> \_/ ::| | |
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| |~\________/~\|:: ~ ::|/~\________/~| |
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| | ||:: <-------------------------[8]
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| |_/~~~~~~~~\_/|::_ _ _ _ _::|\_/~~~~~~~~\_| |
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[9]-------------------------->_=_=_=_=_::| | |
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| | :::._______.::: | |
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| | .:::| |:::.. | |
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| | ..:::::'| |`:::::.. | |
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[6]---------------->.::::::' || || `::::::.<---------------[6]
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| | .::::::' | || || | `::::::. | |
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/| | .::::::' | || || | `::::::. | |
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| | | .:::::' | || <-----------------------------[10]
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| | |.:::::' | || || | `:::::.| |
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| | ||::::' | |`. .'| | `::::|| |
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[11]___________________________ ``~'' __________________________[11]
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: | | \:: \ / ::/ | |
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| | | \:_________|_|\/__ __\/|_|_________:/ | |
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/ | | | __________~___:___~__________ | | |
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|| | | | | |:::::::| | | | |
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[12] /|: | | | | |:::::::| | | | |
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|~~~~~ / |: | | | | |:::::::| | | | |
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|----> / /|: | | | | |:::::::| <-----------------[10]
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| / / |: | | | | |:::::::| | | | |
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| / |: | | | | |::::<-----------------------------[13]
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| / /|: | | | | |:::::::| | | | |
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| / / |: | | | | `:::::::' | | | |
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| _/ / /:~: | | | `: ``~'' :' | | |
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| | / / ~.. | | |: `: :' :| | |
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|->| / / : | | ::: `. .' <----------------[11]
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| |/ / ^ ~\| \ ::::. `. .' .:::: / |
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| ~ /|\ | \_::::::. `. .' .::::::_/ |
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|_______| | \::::::. `. .' .:::<-----------------[6]
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|_________\:::::.. `~.....~' ..:::::/_________|
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| \::::::::.......::::::::/ |
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| ~~~~~~~~~~~~~~~~~~~~~~~ |
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`. .'
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`. .'
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`. .'
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`:. .:'
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`::. .::'
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`::.. ..::'
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`:::.. ..:::'
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`::::::... ..::::::'
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[14]------------------> `:____:::::::::::____:' <-----------------[14]
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```::::_____::::'''
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~~~~~
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============================================================================
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- Diagram Outline -
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---------------------
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[1] - Tail Cone
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[2] - Stabilizing Tail Fins
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[3] - Air Pressure Detonator
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[4] - Air Inlet Tube(s)
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[5] - Altimeter/Pressure Sensors
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[6] - Lead Shield Container
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[7] - Detonating Head
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[8] - Conventional Explosive Charge
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[9] - Packing
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[10] - Uranium (U-235) [Plutonium (See other diagram)]
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[11] - Neutron Deflector (U-238)
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[12] - Telemetry Monitoring Probes
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[13] - Receptacle for U-235 upon detonation
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to facilitate supercritical mass.
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[14] - Fuses (inserted to arm bomb)
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[5] Altimeter
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---------
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An ordinary aircraft altimeter uses a type of Aneroid Barometer which
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measures the changes in air pressure at different heights. However, changes
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in air pressure due to the weather can adversely affect the altimeter's
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readings. It is far more favorable to use a radar (or radio) altimeter for
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enhanced accuracy when the bomb reaches Ground Zero.
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While Frequency Modulated-Continuous Wave (FM CW) is more complicated,
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the accuracy of it far surpasses any other type of altimeter. Like simple
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pulse systems, signals are emitted from a radar aerial (the bomb), bounced off
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the ground and received back at the bomb's altimeter. This pulse system
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applies to the more advanced altimeter system, only the signal is continuous
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and centered around a high frequency such as 4200 MHz. This signal is
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arranged to steadily increase at 200 MHz per interval before dropping back to
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its original frequency.
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As the descent of the bomb begins, the altimeter transmitter will send
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out a pulse starting at 4200 MHz. By the time that pulse has returned, the
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altimeter transmitter will be emitting a higher frequency. The difference
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depends on how long the pulse has taken to do the return journey. When these
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two frequencies are mixed electronically, a new frequency (the difference
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between the two) emerges. The value of this new frequency is measured by the
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built-in microchips. This value is directly proportional to the distance
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travelled by the original pulse, so it can be used to give the actual height.
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In practice, a typical FM CW radar today would sweep 120 times per
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second. Its range would be up to 10,000 feet (3000 m) over land and 20,000
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feet (6000 m) over sea, since sound reflections from water surfaces are
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clearer.
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The accuracy of these altimeters is within 5 feet (1.5 m) for the higher
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ranges. Being that the ideal airburst for the atomic bomb is usually set for
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1,980 feet, this error factor is not of enormous concern.
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The high cost of these radar-type altimeters has prevented their use in
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commercial applications, but the decreasing cost of electronic components
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should make them competitive with barometric types before too long.
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[3] Air Pressure Detonator
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----------------------
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The air pressure detonator can be a very complex mechanism, but for all
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practical purposes, a simpler model can be used. At high altitudes, the air
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is of lesser pressure. As the altitude drops, the air pressure increases. A
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simple piece of very thin magnetized metal can be used as an air pressure
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detonator. All that is needed is for the strip of metal to have a bubble of
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extremely thin metal forged in the center and have it placed directly
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underneath the electrical contact which will trigger the conventional
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explosive detonation. Before setting the strip in place, push the bubble in
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so that it will be inverted.
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Once the air pressure has achieved the desired level, the magnetic bubble
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will snap back into its original position and strike the contact, thus
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completing the circuit and setting off the explosive(s).
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[7] Detonating Head
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---------------
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The detonating head (or heads, depending on whether a Uranium or
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Plutonium bomb is being used as a model) that is seated in the conventional
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explosive charge(s) is similar to the standard-issue blasting cap. It merely
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serves as a catalyst to bring about a greater explosion. Calibration of this
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device is essential. Too small of a detonating head will only cause a
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colossal dud that will be doubly dangerous since someone's got to disarm and
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re-fit the bomb with another detonating head. (an added measure of discomfort
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comes from the knowledge that the conventional explosive may have detonated
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with insufficient force to weld the radioactive metals. This will cause a
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supercritical mass that could go off at any time.) The detonating head will
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receive an electric charge from the either the air pressure detonator or the
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radar altimeter's coordinating detonator, depending on what type of system is
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used.
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[8] Conventional Explosive Charge(s)
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--------------------------------
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This explosive is used to introduce (and weld) the lesser amount of
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Uranium to the greater amount within the bomb's housing.
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Plastic explosives work best in this situation since they can be
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manipulated to enable both a Uranium bomb and a Plutonium bomb to detonate.
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[11] Neutron Deflector
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-----------------
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The neutron deflector is comprised solely of Uranium-238. Not only is
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U-238 non-fissionable, it also has the unique ability to reflect neutrons back
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to their source.
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The U-238 neutron deflector can serve 2 purposes. In a Uranium bomb, the
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neutron deflector serves as a safeguard to keep an accidental supercritical
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mass from occurring by bouncing the stray neutrons from the `bullet'
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counterpart of the Uranium mass away from the greater mass below it (and vice-
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versa). The neutron deflector in a Plutonium bomb actually helps the wedges
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of Plutonium retain their neutrons by `reflecting' the stray particles back
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into the center of the assembly.
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[6] Lead Shield
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The lead shield's only purpose is to prevent the inherent radioactivity
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of the bomb's payload from interfering with the other mechanisms of the bomb.
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The neutron flux of the bomb's payload is strong enough to short circuit the
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internal circuitry and cause an accidental or premature detonation.
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[14] Fuses
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-----
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The fuses are implemented as another safeguard to prevent an accidental
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detonation of both the conventional explosives and the nuclear payload. These
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fuses are set near the surface of the `nose' of the bomb so that they can be
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installed easily when the bomb is ready to be launched. The fuses should be
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installed only shortly before the bomb is launched. To affix them before it
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is time could result in an accident of catastrophic proportions.
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============================================================================
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1994
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