You Might Want to Know: How to “miniaturize” an atomic bomb
The first atomic bombs weren’t miniatures. In size, I mean. Fat Man, the bomb the United States dropped on Nagasaki, was more than ten feet long and five feet in diameter. It weighed over ten-thousand pounds. If our B-29 Superfortress hadn’t come into service in 1944, just a year before the Trinity test, we wouldn’t have had a bomber big enough to carry it on over from Tinian Island to Nagasaki.
Those ten-thousand pounds yielded the equivalent of over forty-million pounds of TNT.
After the war, our weapons designers started trying to make nuclear weapons that would weigh less and yield more. They knew the weapons could be made to yield more. Even before the end of the war J. Robert Oppenheimer, the Scientific Director of the Manhattan Project, and other scientists had told our leaders that if we couldn’t find a way to keep a nuclear arms race from starting, these bombs would get bigger, much bigger. Bigger in yield, he meant. More powerful.
The designers realized, first of all, that bombs like the Nagasaki bomb, which imploded plutonium, were more efficient than bombs like the Hiroshima bomb, which had shot one slug of highly enriched uranium into another slug. In the implosion design, you’d need less fissile fuel and more of it would “fish” than in the gun-type design. They had demonstrated this even before the war ended. The gun-type Hiroshima bomb had yielded fifteen kilotons. The Nagasaki bomb, which had used much less fissile fuel, had yielded over twenty kilotons, 30% more than the Hiroshima bomb.
You could make the bombs more powerful and smaller by enriching the uranium or purifying the plutonium even more so you’d need less of it. You could also experiment with the shape of the explosives that did the compressing to try to make them compress the fissile fuel faster and more evenly. We did both of these things.
Bombs like the Nagasaki bomb go off when a ball of fissile fuel--like plutonium or highly enriched uranium--is very quickly squeezed into a smaller size by imploding conventional explosives that compress the non-critical ball of fission fuel--which is the size of a grapefruit, say--into a super-critical ball-- the size, say, of an eyeball. The faster you can compress the fissile fuel and the longer you can keep it compressed before the atomic energy being generated in the uncontrolled nuclear chain reaction pushes the fuel too far apart, the more explosive energy you’ll get. After a couple of millionths of a second, you’re out of time.
One way to make the fissile fuel compress faster, the weapons designers found, was to leave a cavity at the center of the ball of fissile fuel. Another was to leave some space between the surface of the ball of fuel and the tamper that was placed around it. When the tamper was compressed by the implosion, it would be going faster when it slammed into the ball of fuel. You could also get more fission by making the tamper out of materials like beryllium that would reflect neutrons back into the fissile fuel.
Then somebody realized that we might be able to get more yield by shooting some fusion fuel gas—isotopes of hydrogen called deuterium and tritium--into the hollow core of the ball just before it was imploded. The fission that followed the compression would produce enough heat and pressure to cause fusion in this fuel. The fusion would release energy and neutrons that would cause more fission in the fission fuel. This was called “boosting.” In the Spring of 1951, in Operation Greenhouse at our Pacific Proving Ground, we successfully tested our first “boosted” bombs. Boosting, it turned out, worked really well. Before long, we were boosting all our fission bombs.
In the 1950’s, we’d gotten serious about developing missiles to deliver our nuclear weapons. If we were to going to put nuclear warheads on missiles, we would need to make the warheads weigh less.
By 1952, we had developed the Mark 7 bomb. It was small enough in size--2.5 feet in diameter, 15.2 feet in length, 1680 pounds—to be carried by a fighter airplane. If you went by how it was delivered, by a fighter not a bomber, that made it our first “tactical” nuclear weapon. In a streamlined case, it could be carried by one of our newer jet fighter planes, like the F-100 Super Sabre. The yield of the Mark 7’s could be set at eight- or at sixty-one kilotons. Sixty-one kilotons was four Hiroshimas. Still tactical? I guess so.
In 1953, we put the Mark 7 on the first nuclear-capable missile we ever deployed, the Honest John, which we issued to our Army. In 1954, we deployed the Honest John over in England.
In 1953, the biggest news was that in the Mike test in Operation Ivy at our Pacific Proving Ground, we had succeeded in detonating the first staged hydrogen device. The device in Mike used one of our fission bombs to trigger fusion in a second stage that then caused more fission. Now you could begin to think that the yield of a nuclear weapon might have no upper limit. Just add more stages.
By 1959, six years later, we had a thermonuclear (fission-fusion) warhead--the W-49--in service in England on our new Thor Intermediate Range Missiles. The W-49 weighed two-thousand two-hundred pounds, five hundred twenty pounds more than the Mark 7, but it yielded 1.44 megatons, more than twenty-three times what the Mark 7 yielded. The Thor could deliver those two-thousand two-hundred pounds and 1.44 megatons to targets fifteen-hundred miles away, which was enough to get it from England to Moscow. That had to have gotten their attention.
Two years later, in 1961, we introduced the B41, a bomb that had the highest design yield yet for our bombs. The B41 was a three-staged hydrogen bomb that weighed a little more than Fat Man and was a little bigger in size, so too heavy and big to go on a missile but not too heavy for one of our newer B-52 bombers.
We’d never did test the B-41 but in Operation Redwing in 1956, in the Zuni test, we had successfully tested a three-stage device. The B-41 was designed to yield twenty-five megatons, one-thousand two-hundred fifty times more than the Nagasaki bomb. This would give it the best yield-to-weight ratio ever for our nuclear bombs, like 50,000,000 to 1. For every pound it weighed, it would produce, we thought, the equivalent of 50,000,000 pounds of TNT. Try doing some “precision” bombing with that boy.
In 1963, we put into service the thermonuclear W53, the highest yielding warhead we’d ever put on a missile. The W53 was a little longer than Fat Man, just over twelve feet, but not as fat, only a little over four feet in diameter. It weighed less than Fat Man, just under nine-thousand pounds, and had a design yield of nine megatons, four-hundred fifty times Fat Man. The Titan II missiles we installed in underground silos in Arizona, Arkansas, and Kansas and mounted the W53s on were Intercontinental Ballistic Missiles, powerful enough to carry the nine-thousand pound W53s all the way from Tucson, Little Rock, or Wichita, where we’d installed them in underground silos, to Moscow.
On the other end of the scale, back in 1961, we had put into service the W-54, one of the smallest warheads we would ever make. It was less than a foot-and-a-half long and weighed around fifty pounds. That meant it could be carried around in the field by soldiers. By one soldier even. It could be set to yield the equivalent of ten tons of TNT, which would flatten two city blocks, or to one-thousand tons, one hundred times more.
The diameter of the implosion system of the W-54 was 10 ¾ inches. The designers thought this might be the smallest diameter possible for an implosion device.
Since developing the Mark 7 in the early 50’s, we’d developed other “tactical” weapons. The new ones weren’t as small and light as the Davy Crockett, but small enough to be driven around by squads of soldiers in the field, with yields that some thought might be small enough to allow our soldiers to use them in actual battles they were having without killing our own soldiers too.
Some of these smaller “tactical” nuclear devices were put into artillery shells, some into short-range missiles, some into depth-charges and torpedoes, some into mines, some into demolition munitions, and some into bombs that could be dropped by fighter airplanes to support troops in a battle, provided the troops weren’t too close to the troops that were your target. They would have to be several miles away, not in battling hand-to-hand.
We could set some of these weapons to different yields and do it just before use. Our military leaders had taken twenty kilotons as the limit for a tactical nuclear weapon, which is what the Nagasaki bomb had yielded. But the W31 we now used on our Honest John surface-to-surface tactical missile, could yield two-, twenty-, and forty kilotons. Forty kilotons is two times Nagasaki. Still tactical. Do I have that right?
In the 70’s, we had thousands of tactical weapons in several of the countries in Europe that were in NATO. In some of the countries, especially Germany, there were protests about this. Big angry violent protests sometimes. Safeguarding the weapons from our own allies might have been more of a concern for the troops we’d given them to than were the Soviets.
In the 70’s, we were still working to make smaller the thermonuclear nuclear warheads we would be putting on our missiles. If we could keep making them smaller, we might at some point be able to put several warheads on a single missile. Multiplying our warheads without having to multiply our missiles. Wouldn’t that be great?
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Some of our best scientists continue to devise" better", faster and more powerful nuclear weapons and delivery systems and probably will until a launch occurs, probably by error, but when the computers are through, everything else will be through too. How do they explain that to their children in the final hours of existence?
That's a chronicle of insane behavior. Herman Kahn (aka Dr. Strangelove) was completely insane, some of the leading military people were batshit crazy, and some of the so-called "scientists" at this time (e.g. von Braun) were as amoral as can be. Miraculous that humanity has -- so far! -- survived such madness.