W88 (nuclear weapon)
The W88 (nuclear weapon) intermediate yield strategic warhead used on the UGM-133 Trident D5 submarine-launched ballistic missile. Packaged in a conical reentry vehicle (RV) less than six feet tall with a base diameter of only 22 inches, its shape lends itself to high accuracy. It is hardened against defensive nuclear weapons, but omits certain safety mechanisms, used in the W87 warhead of the LGM-30 Minuteman III land-based missile, which are pointless in a submarine-launched missile. These safety tradeoffs allow the W88 to be more weight-efficient than the W87, although safety was increased as part of an overall redesign to meet the requirements of the START arms control treaty. It does, however, have strong safeguards against unauthorized firing.
Richard Garwin speculated that there was a considerable weight penalty to putting it into this shape,  but it was later revealed that the fission Primary section is non-spherical, allowing a much more efficient arrangement inside the reentry vehicle. Earlier warheads needed to put spherical Primaries at the bottom of the RV cone, but the W88 primary, called "oblate" or resembling the shape of an American football, can go into the bottom part of the cone without requiring additional ballast. In another change, the Secondary is spherical, not the usual cylinder, and fits into the forward part of the cone.
Yield is almost certainly variable; maximums have been described as between 350 and 500 kilotons, with 475 KT a frequently cited value. This is the highest yield of any operational U.S. missile warhead.
Some details of the design were obtained by China, although Garwin argues that it was insufficient to build a weapon, and that a weapon with the W88 special characteristics did not suit Chinese missile warhead needs.
The W88 does not use insensitive high explosives, as it was decided that a fire, in a submerged submarine, was a far greater risk to the Trident rocket engine rather than the warhead. In either case, the only plausible response would be to eject the entire missile into the sea. It does use PBX-9501 plastic bonded explosive for improved stability.
- "The W88 Warhead: Intermediate yield strategic SLBM MIRV warhead", Nuclear Weapons Archive, 1 October 1997
- Richard Garwin (April/May 1999), "Why China Won't Build U.S. Warheads", Arms Control Today
- Table of US Strategic Nuclear Forces, 2002, National Resources Defense Council