The LR91 was an American liquid-propellant rocket engine, which was used on the second stages of Titan intercontinental ballistic missiles and launch vehicles.

The thrust chamber used fuel for regenerative cooling, with separate ablative skirt.

Early LR91 engines used on the Titan I burned RP-1 and liquid oxygen.

For the Titan II, the engine was converted to use Aerozine-50 and nitrogen tetroxide, which are hypergolic and storable at room temperature.

This allowed Titan II missiles to be kept fully fueled and ready to launch on short notice.