The Mark 5 nuclear weapon was the first lightweight bomb produced by the US at the dawn of the Cold War, and also became the first missile warhead.
Mark 5 nuclear bomb
At the end of World War II, the United States possessed just one unused nuclear weapon, a Fat Man pit that had not yet been assembled into a bomb. Plutonium production at Hanford, however, was reaching 33 lbs of plutonium per month, enough for two Fat Man bombs. Production of enriched uranium at Oak Ridge, in which the output of the S-50 thermal diffusion process produced 1% enriched uranium, which was then fed to the K-25 gaseous diffusion plant for intermediate enrichment before being purified to weapons-grade in the Y-12 calutrons, would reach 135 pounds per month by October 1945. This was enough for four implosion uranium bombs (after the prototype was dropped on Hiroshima, the inefficient gun-type design was never put into production by the US).
Since the Fat Man had been essentially hand-made and was slow to produce and assemble, it was not suited for large-scale military use, and a new version was needed that could be mass-produced from standardized parts on an assembly line. This new version was dubbed the Mark 4 (the Thin Man was Mark 1, the Little Boy was Mark 2, and Fat Man was Mark 3.)
As the war ended, however, priority for nuclear weapons production fell. The Y-12 plant at Oak Ridge proved too costly to operate, and was closed in 1946. The two plutonium-production reactors at Hanford were suffering from unanticipated radiation damage to their components; they were operated at greatly reduced levels, and were closed as soon as replacements became available. One year after Hiroshima, the US had a total of 9 Fat Man bombs (but only had initiators available for 7 of these); by July 1947, the entire US nuclear arsenal consisted of just 13 implosion weapons – none of them of the new Mark 4 type.
Meanwhile, in the late 1940’s, Soviet domination of eastern Europe and the perceived threat to Europe increased the tensions between the US and the USSR. The event that really sparked the Cold War, however, happened in August 1949, when the USSR successfully tested its first atomic bomb, which was codenamed “Joe-1” by the US. That event sparked a nuclear arms race which was to rage for almost half a century and hold the entire world under the threat of nuclear annihilation.
At the end of the Second World War, Los Alamos officials realized that there was no “atomic secret”, and that any nation with sufficient resources would be able to produce their own atomic bombs. However, they had estimated, the technical difficulties were so great that it would probably take the Soviet Union at least 15 years to solve all the problems and produce their own weapon.
Unknown to the Los Alamos scientists, however, the Russians had help. Several sources in the United States, England and Canada were passing atom bomb information to the Soviets. The most important of these was nuclear physicist Klaus Fuchs, a communist German exile living in England who was assigned to Los Alamos to work on the implosion bomb. The information provided by Fuchs was corroborated by David Greenglass, an Army soldier who was assigned to work in the Los Alamos machine shop that made the explosive lenses. Greenglass’s information was funneled to the Soviets through a network run by Julius Rosenberg, a Communist Party member who felt that the Soviets, though allies with the Americans and British, were being unfairly denied access to this important military program. Fuchs and Greenglass provided the Russians with virtually complete descriptions of the implosion process, allowing the Soviet atomic program, lead by physicist Igor Kurchatov, to discard its early gun-type designs and focus exclusively on Los Alamos’s already-tested implosion design (and saving the Russians several years of research and testing). Joe-1 was a virtual copy of Fat Man. The Russian bomb prompted a scramble in the US to keep up, and the new Mark 4 implosion design was rushed into production before the end of 1949.
Los Alamos, meanwhile, had already prepared two major improvements to the implosion bomb design that would be incorporated into the new version. The first advance was to use a hollow composite core, consisting of an inner layer of plutonium and an outer layer of uranium-235, surrounded by a natural uranium-238 tamper. Using a hollow core rather than the Fat Man’s solid core allowed a larger amount of material to be used while still allowing enough neutrons to escape to keep the mass subcritical. The composite core allowed supplies of scarce plutonium to be stretched using the more readily available uranium.
In another improvement, this hollow core was “levitated” – a series of small support struts held it several inches away from the surrounding tamper. This air space gave the tamper some room to accelerate as it was imploded into the core, building up more momentum and increasing the efficiency of the core by holding it together for a few additional microseconds. The uranium tamper used in Fat Man was also later replaced by a beryllium tamper, which better reflected neutrons.
The Mark 4 also introduced a vitally important safety feature known as “in-flight insertion”. During the missions to Hiroshima and Nagasaki, there were fears that an accidental crash on takeoff might be enough to set off the nuclear weapon and produce an explosion that would destroy the entire airbase. So the Little Boy had been “safed” by a set of electrical plugs that had to be physically switched from “safe” to “armed” after takeoff. The Fat Man design, however, had no such capability, and the B-29 had to take off with a fully-armed weapon.
In the new Mark 4 design, two concepts were combined for greater safety. The first was a set of wires was added to the firing circuit which physically connected to the airplane, and were pulled out as the bomb fell away. The firing circuit was disabled until these wires were pulled.
In Fat Man, the nuclear pit had been buried deep inside the explosive lens assembly, and once the bomb was assembled the pit was inaccessible. Now, as a second safety feature, the Mark 4 was designed with an open pathway through the explosive lens layer that led to an empty space inside the bomb, where the pit would settle. During takeoff, the pit itself would be stored outside of the bomb, carried inside the plane in a metal storage frame called the “birdcage”. Now, if the plane should crash on takeoff or in flight, there would be no pit inside the bomb, making a nuclear explosion impossible. Just before the bomb would be dropped, a weaponeer would manually insert the pit into the bomb through a doorway in the front of the bomb casing, add the final explosive layer, and close the bomb to arm it.
Around 550 Mark 4 bombs were produced and deployed over the next four years. But work was already continuing on a better version. Although the Mark 4 was much easier to manufacture than the Fat Man, it was still big and heavy—at a length of ten feet eight inches, a diameter of five feet, and a weight of 5.5 tons, the Mark 4 was actually a little bit larger than the Fat Man. The Air Force wanted a bomb that was smaller and lighter.
Bomb designers had already discovered that using a larger number of individual explosive lens units in the implosion design allowed the lenses to produce the same implosion wave within a shorter distance, making the explosive lens layer thinner and decreasing the size and weight of the bomb. So the new Mark 5 bomb was designed to use 92 individual explosive lenses instead of the 32 used in Fat Man and the Mark 4. The effect of this was drastic: the Mark 5, while roughly the same length as the Mark 4, measured only 44 inches in diameter and weighed only 1.5 tons. In addition, a new more efficient core design gave a yield of up to 120kt—three times larger than the Mark 4.
The Mark 5 also used a new version of the in-flight insertion safety procedure. Now, a switch in the cockpit allowed the pilot to use an automated piston to push the nuclear core through a set of “barn doors” at the front of the bomb to arm it, or to withdraw the pit to “safe” the weapon.
The Mark 5 began deployment in 1952, with about 140 of them seeing service with the US. Another 72 Mark 5 bombs were delivered to the UK for use by the Royal Air Force (though the weapons remained under US command and control).
As the first lightweight nuclear weapon, the Mark 5 also made it possible to consider placing atomic warheads on smaller missiles, and in 1954 a number of Mark 5 bombs were modified slightly to fit inside a small space and mounted onto Regulus and Matador missiles. These were early versions of cruise missiles which could deliver nuclear weapons at ranges up to 700 miles. The Matador was deployed by the Army as a surface-to-surface missile, while the Regulus was carried atop a submarine, which had to surface in order to launch the missile.
The deployment of nuclear-armed missiles changed the nomenclature of American weapons. Henceforth, nuclear weapons that were intended for use as air-dropped bombs received a “B” designation (such as B-5), and weapons that were intended for missile delivery were designated with a “W” (such as W-5). Later, the “Mark” designation was replaced by a “B” or “W” classification, followed by the year the weapon was designed.
The small size and weight of the Mark 5 also made it ideal for use as the “primary” nuclear trigger for the first thermonuclear hydrogen bombs. These later versions were usually “boosted” with tritium and deuterium.
The Mark 5 remained in service until the end of 1962.
Today, a bomb casing from a Mark 5 is on display at the Museum of Nuclear History in Albuquerque NM.