The LR91 was an American liquid-propellant rocket engine, which was used on the second stages of Titan intercontinental ballistic missiles and launch vehicles. While the original version - the LR91-3 - ran on RP-1/LOX (as did the companion LR87-3) on the Titan I, the models that propelled the Titan II and later were switched to Aerozine 50/N2O4.

Titan IV was a family of heavy-lift space launch vehicles developed by Martin Marietta and operated by the United States Air Force from 1989 to 2005. Launches were conducted from Cape Canaveral Air Force Station, Florida and Vandenberg Air Force Base, California.