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Is a 203mm ovoid implosion bomb possible? Is there a continuum between conventional spherical implosion and linear implosion?

chimeric oncogene

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Most small-diameter nuclear artillery shells (e.g. W82, W79) use linear implosion to force a long cylinder/ovoid of plutonium into a spherical critical shape, allowing them to generate nuclear yields with very narrow weapons.

Linear implosion is generally understood to be less efficient (requiring >1 critical mass, ~13 kg, of plutonium) than spherical implosion, which compresses a sphere of plutonium to a critical configuration. The smallest spherical implosion bomb, the W54 Davy Crockett (and its higher-yielding derivatives), obtained a yield in the vicinity of 1-2kt in a package ~250-260mm across, presumably with less plutonium than a similar-yield W82 (155mm) or W79 (203mm) linear implosion bomb.

Modern MIRVs are believed to use ovoid (or even egg-shaped) spherical implosion weapons as primaries, in a bid to save space in the MIRV nosecone.

My question is thus: Is it possible to make a spherical implosion bomb (perhaps an ovoid or egg-shaped device) <203mm in diameter? Would this lower the cost of my 203mm artillery round? Would this increase the efficiency of use of expensive plutonium in my 203mm round?

I have a conceptual question: Do linear implosion weapons, ovoid spherical implosion weapons, and spherical implosion weapons exist on a continuum? Or are they completely different devices?
Commonsensically, it seems they should, so if you thicken the HE around a linear implosion weapon you can make more efficient use of your plutonium, and if you add even more HE you end up with an ovoid, and then eventually a sphere.
 

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Difficult to say, what you're really trading off is the efficiency of the actual warhead vs the packing efficiency of the warhead within the device. The best assessment probably comes from Little Boy vs Fat Man. These bombs both used 64kg of fissile material, U235 and Pu239 respectively. They weighed approximately the same overall weight (9,700lb vs 10,300lb respectively), Little Boy was linear and Fat Man was spherical. Fat Man's yield was 40% higher but the diameter of the bomb was 2.15 larger. For artillery this has very bad implications. If we assume yield is proportional to volume, them a bomb with 2.15x the diameter has about 9x the volume, if we divide that by 1.4 to bring it down to the same yield, that's 9/1.4 = 6.4, so 1.86x diameter. This implies that a spherical critical shape shell will need to be 1.86x the calibre, so a 203mm round becomes a 378mm shell to attain the same yield. Or your yield goes down to <16% of that of a linear shape with the same calibre.

The W54 (spherical, implossion) was 273mm diameter and packed 1kT, the W33 (gun-type) was 203mm and packed 40kT, W79 (linear) was 203mm and 1.1kT.

You may also notice that the trend with the 203mm warhead was tending to lower yields for application suitability.

I think it boils down to this, the Davy Crockett was round and lent itself to the spherical implosion, regular artillery rounds do not. Interestingly the W79 article actually has a paragraph suggesting spherical implosion warheads are cheaper, but it is uncited and makes bad assumptions about the cost of the material by assuming the added cost is all fissile material rather than warhead design and manufacture.

Modern MIRVs are fusion weapons and therefore vastly different in shape.

615112
 

Ravinoff

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The W54 (spherical, implossion) was 273mm diameter and packed 1kT, the W33 (gun-type) was 203mm and packed 40kT, W79 (linear) was 203mm and 1.1kT.

You may also notice that the trend with the 203mm warhead was tending to lower yields for application suitability.
On the differences between implosion and linear artillery shells, things get a little complicated because much of the details remain classified. Most specifically I'm referring to how the W33 achieved such a highly variable yield while still using the "Fleegle" oralloy linear-implosion (possibly weird linear implosion with a double gun or hollow projectile mass) physics package. It's strongly suspected that the 40kt variant of the W33 had to be a boosted fission device, because the math for getting 40 kilotons out of a complete round weight of ~250lbs just doesn't work out.
 
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chimeric oncogene

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W33 had to be a boosted fission device
I never knew they figured out how to boost a gun-type device. I thought they told Teller he was crazy for even encouraging it... thanks!
And a 40 kt 203mm shell? Would that have even been safe to use near friendly forces?
 

chimeric oncogene

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with nuclear device ?
Davy Crockett was 20 tons for killing 1-3 tanks (reportedly), and Atomic Annie was 15kt. Mid-Late Cold War nuclear artillery was 1-2kt, reportedly for safe use near friendly forces; I presume they figured this out after the late 50s. :D

Do you know how they were intended to be used on the nuclear battlefield? I've heard that Davy Crockett was geared to hit forces in contact, nuclear arty was supposed to hit Soviet tanks as they advanced to contact (like do all the things normal arty does), and all the big ones (Pershing II, Pershing I, etc) were basically battlefield air interdiction weapons for wide-area destruction.
 

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W33 had to be a boosted fission device
I never knew they figured out how to boost a gun-type device. I thought they told Teller he was crazy for even encouraging it... thanks!
And a 40 kt 203mm shell? Would that have even been safe to use near friendly forces?
Presumably they just used 'deal with it' shades.
 

Michel Van

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Davy Crockett was 20 tons for killing 1-3 tanks (reportedly)
Actually this thing was design to kill massively Soviet armor and troops
absolut deadly 150 meter radius from ground zero and a probably fatal dose at 400 meter radius.
and further out to make troop too sick to fight...
 

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The W54 warhead in it could go up to 1kT. That would have made a terrible mess.
 

chimeric oncogene

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The W54 warhead in it could go up to 1kT. That would have made a terrible mess.
I was under the impression that the recoilless-gun fired Davy Crockett could only go up to 20t, and that a modified variant was needed for the 1kt yield of the SADM.
 

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I was under the impression that the recoilless-gun fired Davy Crockett could only go up to 20t, and that a modified variant was needed for the 1kt yield of the SADM.
I'm not sure but all 4 variants were the same size, so at least theoretically it could have carried a 1kT warhead.

All four variants share the same basic core: a nuclear system which is 10.75 inches (273 mm) diameter, about 15.7 inches (400 mm) long, and weighs around or slightly over 50 pounds (23 kg).
 

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Davy Crockett was neutron bomb. Primary oval shape is related to flying plate design with 2 detonators only. It is easy for detonation mechanism - you need to synchronize and deliver energy only for 2 initiators - no 60 or 90. That why it is cheaper.
 
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