The W39 is a revision of the Mark 15. According to Wikipedia, the Mark 15 had a real thermonuclear second stage that generated significant yield. However, its

The W39 is a revision of the Mark 15. According to Wikipedia, the Mark 15 had a real thermonuclear second stage that generated significant yield. However, its second stage was also packed with enriched uranium, and it obtained most of its energy from fission. Essentially, it was a two-stage thermonuclear weapon that had a lower fusion fraction than most two-stage weaopns. Summary of atomic weapons: 1. Pure fission. Fusion contributes 0% of yield. 2. Boosted fission: Fusion is used solely to improve the explosive yield of the fission. Fusion contributes 1% of yield. 3. Dirty two-stage thermonuclear: Fusion contributes between 1 and 50% of yield. Most of the yield comes from fissioning of the U-238 tamper by fast neutrons. 4. Clean two-stage thermonuclear: Fusion contributes 90-99% of yield. The U-238 tamper is replaced with beryllium or lead, which does not undergo fission when hit with fast neutrons. 5. Three-stage thermonuclear: Same as two-stage. Thermonuclear bombs are considered to be an "unlimited" design, because you can keep stacking stages on top of each other, until you've got enough to blow up the planet.