A Technological Appendix to Postblogging Technology, July 1950: Tactical Nuclear Weapons

 

The M-29 "Davy Crockett Weapon System" is, understandably, the most famous American recoilless rifle of the Cold War era. 

Because, you see, it fired a nuclear bomb. A tiny atom bomb, to be sure, but the only way you can make an atom bomb that gives a 10--1000 ton yield is by letting all the neutrons get out. This was a generation before the whole "neutron bomb" fuss, but the concept seems to have been the same. Other uses of the same warhead include atomic demolition --crater making, but that wasn't the Davy Crockett's job. The Davy Crockett was intended to stop tank armies. For that, a ten ton detonation is not enough, but the estimated instantly lethal radiation flux radius of 500m is. 

The Wikipedia article goes on to point out that Davy Crockett was a particular favourite of Franz Josef Strauss, a name I haven't heard in a long time, perhaps because that particular strain of postwar German thinking is so embarrassing. Strauss thought of it as a force multiplier, says Wikipedia, allowing a single 8" howitzer to control the same amount of terrain as a much larger formation. I'm not sure that thta accurately captures what was going on in Strauss's head, and Wikipedia has evidently conflated the Mk 54-powered M-388 round of the Davy Crockett with the conceptually similar W33/W48 warheads intended to arm first the 203mm howitzer and later even the 155mm weapon. Still, the concept remains the same. 

. . . And that concept would appear to be that Gamma World is far too much fun to not be real. 

Though unfortunately radiation doesn't actually work that way and NATO took all the toys away. Spoilsports!

Politicians have a well-established tradition of being party poopers on the nuclear front, a tradition established in the Korean War. Well, at least comparatively, considering that we've encountered politicians who want to hand America's nuclear bombs over to General MacArthur and letters to the editor calling for settling the Korean problem with a generous distribution of atomic explosives around the Communist world. MacArthur asked the Pentagon for nuclear capability on 10 July, arguing that atomic bombs would make it much easier to interdict the communications routes from China or the Soviet Union in the event that either power intervened in the Korean conflict.

(We've already watched a landing party from USS Juneau trying to blow up a tunnel on the coastal rail route from Vladivostok-Sonbon route.) 

This actually wasn't unreasonable, although the AEC wasn't about to allow SCAP to have warheads, and neither would LeMay let go of any nuclear-capable B-29s. A squadron from SAC's 509 Group was detached to Guam, loaded with Mark 4s but not the nuclear pits. The aircraft remained under the JCS, or, operationally LeMay. 

David Halberstam says that President Truman deliberately leaked word of the transfer. However, the propeller on the number 2 engine of B-29 44-87651 ran away on takeoff from Fairfield-Suisun. Unable to retract the undercarriage and with trouble coming in from the number 3 engine, the pilot found a crash landing vector that didn't go through a trailer park and set the plane down at 120mph. Richard Rhodes quotes LeMay downplaying the accident (and getting key details wrong in his recollection) as "not . . . [that] hard." The nose did break off. The plane did catch fire. Telve of twenty aboard were killed, including all ten of the passengers in the rear compartment, who were unable to escape the fire. But eight survived, and that's not at all bad by the standards of the time! Among the passengers killed was Brigadier General Robert F. Travis, commander of 5th and 9th Strategic Reconnaissance Wing under SAC, which might have allowed the suspicious to suss out the true purpose of the movement. That is, if the Mark 4 hadn't blown up twenty minutes into the crash, killing eight people and spreading debris, including the natural uranium of the tamper, over a two square kilometer area. 

Oops! It may just be me, but the accident report also seems pretty damning:

The crash investigators found that the number two propeller was indeed feathered. The cause of the problem was improper adjustment during maintenance on 22 July 1950, when all four propellers were changed. A problem was detected with number two during a test flight, and the ground crew was instructed to install new contactors. There was no paperwork indicating that this was done, and the entire maintenance crew was killed in the crash. The number three engine and propeller were not found. The generators were found to be working, so there was sufficient electrical power to retract the landing gear. No crew members recalled hearing the landing gear motors. The switches could not be checked due to the degree of destruction of the aircraft, but the fuse was intact. It was noted that there were only six seat belts for the ten men in the forward compartment.^[6]

Their report made four recommendations:

1. That procedures regarding propellers be reviewed and improved so as to identify malfunctions on the ground;
2. That more emphasis be placed on training pilots and flight engineers in procedures for handling propeller problems;
3. That there should be better follow-up of maintenance problems; and
4. That escape exits and tunnels should be kept clear of baggage.^[6]

USAF B-29 operating procedures were changed as a result of the investigation; aircraft with the same type of propellers as 44-87651 were required to be test-flown after corrective maintenance, and the number of persons permitted aboard an operational flight was reduced to 16, as it was felt that overloading and an inadequate number of safety belts in the accident aircraft contributed to the high loss of life.^[10]

 It also seems to me that some housecleaning at SAC wouldn't have been out of line. The SILVERPLATE bombers were recalled in September after the JCS agreed that there was no real point in using Nagasaki-sized weapons against bridges and rail tunnels, and a great deal of possible downside. Time is clearly quite sensitive about the charge that the conflict is a race war as it is. 

On the other hand, clearly the lesson of all of this is that America needs different flavours of atom bombs so it can use them without bringing on WWIII. Indeed, a great deal of modern commentary on nuclear weapons and Korea seems premised on the idea that America had available either hydrogen bombs or tactical nuclear weapons already. The former comes up in a 1954 interview with General MacArthur, posthumously published in 1964, in which he talks about blocking the North Korean borders with a belt of radioactive cobalt. Although we've also seen that thought in Time, albeit on the somewhat more ambitious scale of cutting an atomic Cordon sanitaire right across Eurasia, it isn't actually practical without hydrogen bombs to generate the necessary neutrons. Nice hustle, though, general.

Also, it's my understanding that the original Cordon sanitaire  was the Austrian Black Plague stop line intended to prevent the dreaded Asiatic epidemic from spreading into Europe. So there you go on that.

The idea that America had a "small" atom bomb in its arsenal in 1950 makes intuitive sense, but is, in fact quite wrong. A lump of fissile material will only achieve "prompt criticality" if the mean free path of a neutron through the lump is less than the dimensions of the said lump. In other words, the neutron has to pass through a volume occupied by plutonium/highly enriched uranium/other exotic material for sufficiently long as to encounter the neutron cross section of the relevant isotope. This will also depend on the density of the lump. This is kept low in the W 33/48, and, we assume, W 54, by separating the fission mass into one or more lumps and then combining them with an explosive. The W33/48 was a gun-type warhead designed for some degree of earth penetration in order to make craters in front of advancing Red tank armies. The W48, on the other hand, was a "hybrid gun-assembly/implosion design" based on a cylindrical "primary" originally developed for two-stage hydrogen bombs.  Cylindrical designs are popular because it is easy to inject tritium axially, boosting

the yield. "Dial-a-yield" capability with tritium gas injectors seems like an unnecessary extravagance in a nuclear artillery shell, although that ship may have sailed at the point where you are using great lumps of fissionable material as a conventional artillery force multiplier. On the other hand, it may not have been possible to deliver a bomb capable of fitting into a 155mm howitzer shell without tritium boosting. Remember, tritium has a ten year half life and has to be made in nuclear reactors, soaking up precious neutrons that would otherwise be used to make plutonium.

If it's not clear by this point, I'm shaking my head a bit. 

The W54 is another matter, since it was designed specifically to have a recoilless-rifle compatible blast radius. Persuading a critical mass of fissionable material to explode with the effect of only 10 tons of high explosive (but all of those lethal neutrons!) is hard. The W54 was the most-tested warhead in the American arsenal at the time it entered service, previous iterations having frequently run away, yielding many times the specified charge. 

The basic task in designing such a weapon is to create a promptly critical mass that then "disassembles" itself so quickly that it only has time to produce the specified 10 tons of high explosive-equivalent energy. The assumption that this requires skipping the reflector that mirrors back escaping neutrons, which is why the neutron flux is relatively high, although official information is evasive on this point, and understandably so given the neutron bomb's reputation as being an unusually barbaric weapon even by the standards of atom bombs. 

This does, however, leave us a bit puzzled about how the hot/dense mass of fissionable material required to capture all those escaping neutrons is created in the first place. It's not enough to just coat a hollow sphere of fissionable material with explosives and let them go. Not only will this just create random jets of molten metal, the energy required to pack solid metal into a denser form is enormous, and you don't want to be wasting it. There's too much going on here to hope for converting the metal into a hot plasma, although plasma does begin to form. We're actually looking for "crystalline collapse," which is to say that the crystalline matrix is smushed and the atoms are forced together. The physical chemistry sounds complicated, especially considering that plutonium has 6 phases, although only one exists above about 600 degrees C, so maybe it's not as complicated as I just suggested. 

Our attention then turns to the geometry of the explosive shell, which is described as a "lens." The imploding shockwave of the explosive material passes through a material with a different propagation velocity, producing a refraction of the wave per the familiar principles of Snell's Law, although in practice using the much more challenging mathematics of Fourier optics. (Just in case someone was reading this and wondering what use Fourier transforms are in the real world. Start studying or give up on your dream of being a nuclear weaponeer!) Casey Sublette points out that there was a lot of experimental work between the original Fat Man design and the W54 era, and proposes that the refractory material was an "advanced inert material--" perhaps, some have suggested, a polystyrene of some kind. Finding room for that may have something to do with the development of levitated pit designs apart from its advantages for shock wave formation. 

Finally, and of some interest here, is the development of a driving plate, which sounds to me like a tamper plate but isn't. An arrangement of metal plates ensures an instantaneous linear detonation and the best-shaped convergent high explosive jets. The W54, because it was so very small, was by far the greatest challenge for the weaponeers yet, since the time between reaching prompt criticality and disassembly had to be so small.  So, at least on the technical side, congratulations on that!

The mathematics of various kinds of lens symmetries is obviously very complicated compared with the issues that present themselves to the antitank weapon designer, who is only concerned with a single jet. But the rest of the design problem, basically bringing the maximum amount of energy to bear across a specified target area --if only in the sense of "the minimum possible"-- is the same. The standard American infantry antitank weapon of the Cold War era, designed to replace the bazooka while retaining the desired single operator functionality of the German Panzerfaust, was the M72 LAW, developed at the Redstone Arsenal by the Rohm and Haas research laboratory.* No-one treats the M72 as a high technology device, but we really can stand to dwell for a moment on a man-portable chemical munition capable of melting its way through 200mm of steel, or 300mm in the M72A2. It's more-or-less the same problem and a nice bit of calculation and experimentation. It's also going to lead to composite armours, as the amount of metal a tank has to carry around in order to defeat improved HEAT weapons reaches ridiculous levels.

After all, if you can't stop Red tank armies with your Chieftain tanks, you may end up shooting atom bombs at them. 

_____

*I can't help reminding everyone at this point, as I always do, that Rohm and Haas maded its bones by developing a synthetic feces substitute.