Mark 15 nuclear bomb
The Mark 15 nuclear bomb, or Mk-15, was a 1950s American thermonuclear bomb, the first relatively lightweight (7,600 lb / 3450 kg) thermonuclear bomb created by the United States.
The Mark 15 was first produced in 1955, and a total of 1,200 units were made before production ended in 1957. The Mark 15 design was in service from 1955 to 1965.
There were three production variants of the Mark 15 bomb, the Mod 1, Mod 2, and Mod 3.
The Mark 15 is widely described as a transitional design between fission and thermonuclear weapons. The Mark 15 was a staged weapon (see Teller-Ulam design), using radiation implosion from a fission nuclear primary (Cobra) to implode a secondary stage. Unlike most modern thermonuclear bombs, the Mark 15 used a secondary which was primarily HEU (highly enriched uranium), which generated most of its energy from nuclear fission reactions once the primary imploded it. There was a thermonuclear core which underwent fusion reactions, but most of the energy came from the HEU fissioning. The HEU fission was enhanced by fusion stage neutrons, but would have generated a very significant fission yield by itself.
Some later bombs used depleted uranium fusion stage tampers, and neutrons from the fusion would fission some of the tamper, but the primary energy release (50% or more) was from the fusion reaction.
All three models were generally physically similar; weight of around 7,600 lb / 3,450 kg, diameter of 34.4 to 35 inches, length of 136 to 140 inches. 
The Mod 3 also appears to have had a 3.8 megaton yield.
A missile warhead variant of the Mark 15, the W15 Warhead, was an ongoing project in the mid 1950s. It was canceled in early 1957. Before cancellation, it had been intended for use on the SM-62 Snark missile. Instead, the Snark ended up using the W39 (see below).
The W39 nuclear warhead and B39 nuclear bomb used a common nuclear physics package which was derived from the Mark 15. The experimental W39 devices were initially tested as the TX-15-X3 (which is identical to the W39 Mod 0 design).
Dropped and Lost
|Wikimedia Commons has media related to Mark 15 nuclear bomb.|
- Allbombs.html list of all US nuclear warheads at nuclearweaponarchive.org
- tybeebomb.com—information regarding lost nuclear bombs