A military occupational specialty (MOS) for the U.S.'s version of suicide bombers. A 99 Tango is the occupation of repairing missiles in-flight. The soldier jumps from an airplane and attempts to land on the missile, complete any needed repairs, and get away before it reaches its destination.
This, of course, is a suicide mission as any damage to the missile will result in either its immediate explosion, or the missile will reach its destination before the 99 Tango can get away.
As expected, this MOS requires a Top Secret security clearance, as no U.S. Citizen can know that their government has its own suicide bombers.
PVT. Bailey was a 21 Bravo, but his drill sergeants found out he was crazy and switched him to a 99 Tango.