The W21 was an hydrogen bomb design for the US military. It would have used the physics package of the TX-21 bomb. The TX-21 was a weaponized version of the "Shrimp" device tested in the Bravo shot of Operation Castle. A TX-21C was tested as the Navajo shot, Operation Redwing. The TX-21, was a scaled-down version of the Runt device (M-17 hydrogen bomb). Smaller in size and weight to the Mk-17, the Mk-21 was considered as a potential missile warhead. Far more powerful than the TX-13, which was a high-yield atomic bomb developed from the Mk-6 bomb, the XW21 was to replace the XW13 in the weapons pod of the B-58 bomber and for the SM-64 Navaho missile.