LR91

LR91-11

Titan 4B aft with a LR91-AJ-11.
Country of origin	United States
First flight	1964-09-01
Last flight	2005-10-19
Designer	Aerojet
Manufacturer	Aerojet
Application	upper stage engine
Associated L/V	Titan
Predecessor	LR91-9
Status	Retired
Liquid-fuel engine
Propellant	N2O4 / Aerozine 50
Mixture ratio	1.86
Cycle	Gas generator
Configuration
Chamber	1
Nozzle ratio	49.2
Performance
Thrust (vac.)	467 kilonewtons (105,000 lbf)
Thrust (SL)	232.7 kilonewtons (52,300 lbf)
Thrust-to-weight ratio	80.85
Chamber pressure	5.93 megapascals (860 psi)
Isp (vac.)	316s
Isp (SL)	160s
Burn time	247s
Dimensions
Length	2.81 metres (111 in)
Diameter	1.63 metres (64 in)
Dry weight	589 kilograms (1,299 lb)
Used in
Titan III and Titan IV

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 - run 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.

This engine was vacuum optimized and ran the gas generator cycle. The thrust chamber used fuel for regenerative cooling, with separate ablative skirt. The LR87, which was used for the Titan first stage, was used as a template for the LR91.

Early LR91 engines used on the Titan I burned RP-1 and liquid oxygen. Because liquid oxygen is cryogenic, it could not be stored in the missile for long periods of time, and had to be loaded before the missile could be launched. 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.

Improvement and modifications required to comply with the man rating of the RD-0105 led it to have two versions: