W21

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The W21 was a planned U.S. hydrogen bomb design from the 1950s. It would have used the same basic physics package as the TX-21, which was based on the Shrimp device tested during Bravo in Operation Castle. The TX-21C was also tested as the Navajo shot in Operation Redwing. The TX-21 was a smaller version of the Runt (M-17) hydrogen bomb, and it was smaller than the Mk-17.

The Mk-21 was considered as a potential missile warhead. It would have been more powerful than the TX-13, which came from the Mk-6. The XW21 was planned for deployment in the B-58 bomber’s weapons pod and for the SM-64 Navaho missile.

At the same time, the Mk-15 was developed, with missile versions for the Navajo, Matador, and Regulus missiles (an XW29 was designed for Snark and Redstone). The XW15 design evolved into the XW39, which was deployed on Redstone and Snark missiles. The XW21 was canceled in 1957 in favor of the smaller XW-39.

Although several hundred Mk-21 bombs were briefly stockpiled, no W21 warheads were ever built. The W21 shows how the rapid development of hydrogen bombs in the mid-1950s produced many designs that were quickly replaced by smaller, lighter, more efficient weapons.