Mk.5 / W5
The Mark 5 was a smaller and lighter implosion weapon than previous designs. The Mark 5 was the first lightweight strategic nuclear bomb. Its weight was one-half that of the Nagasaki weapon. It was an improved version of the "Fat Man" bomb that was dropped on Nagasaki by a B-29 during World War II. This high efficiency implosion bomb could be configured to yield three times the explosive power of the Fat Man. The Mark 5's nuclear material was normally kept separate from the casing and inserted just before it was dropped. The nuclear warhead was loaded through the doors in the casing.
The experience gained in the successful development of the Mark 4 put Los Alamos Laboratory in a position to move much more rapidly and with more assurance on the development of other new systems. A smaller and lighter weapon, called the Mark 5, was tested successfully in 1951. First deployed in 1952 and remained in the inventory until 1963. The Mark 5 could be carried by the B-29, B-36, B-45, B-47, and the B-50. It weighted about 3,175 pounds with an explosive yield in the kiloton range (1 kiloton is equivalent to 1,000 tons of TNT).
In contrast with the Mk 6, which requires manual insertion of the nuclear material, the Mk 5 has a built in mechanism to perform the insertion of the capsule and the cored high explosives. Thus, by a switch on his infilght control box, the bomb commander of the delivery aircraft can automatically perform a nuclear insertion or extraction at any time. After the cartridge has been checked and installed, the tail section is attached, completing the assembly of this internally carried weapon.
The Mk5 was a large open pit weapon containing a large amount of high explosives. The front end contained the automatic in-flight insertion mechanism (AIFI), eight MC-300 impact fuses and two Horn Antennae. The fusing and firing set, collectively known as the cartridge, was attached to the HE sphere at the aft end. Two power supply assemblies were attached to the cartridge. The arm/safe switch, also a part of the cartridge, could be monitored and set from an access door in the tail assembly. The ballistic case followed aircraft construction, the aluminum skin was riveted to support rings. The case was in three sections, the front, the center (made up of three panels), and the tail assembly.
