LR91

{{Short description|American gas-generator rocket engine used on Titan upper stages}}

{{Infobox rocket engine

| name = LR91-11

| image = Titan 4B Upper Stage Aft with a single LR91.jpg

| caption = Titan IV-B aft with a LR91-AJ-11.

| country_of_origin = {{USA}}

| date =

| first_date = {{Start date|1964|09|01|df=y}}

| last_date = {{End date|2005|10|19|df=y}}

| designer = Aerojet

| manufacturer = Aerojet

| purpose = Upper stage

| associated = Titan

| predecessor = LR91-9

| successor =

| status = Retired

| type = liquid

| oxidiser = {{chem2|N2O4|link=nitrogen tetroxide}}

| fuel = Aerozine 50

| mixture_ratio = 1.86

| cycle = Gas-generator

| pumps =

| description =

| combustion_chamber = 1

| nozzle_ratio = 49.2

| thrust =

| thrust_at_altitude =

| thrust(Vac) = {{cvt|467|kN}}

| thrust(SL) = {{cvt|232.7|kN}}

| throttle_range =

| thrust_to_weight = 80.85

| chamber_pressure = {{cvt|5.93|MPa|psi}}

| specific_impulse =

| specific_impulse_vacuum = {{cvt|316|isp}}

| specific_impulse_sea_level = {{cvt|160|isp}}

| total_impulse =

| burn_time = 247 seconds

| restarts =

| gimbal =

| capacity =

| dimensions =

| length = {{cvt|2.81|m}}

| diameter = {{cvt|1.63|m}}

| dry_weight = {{cvt|589|kg}}

| used_in = Titan III and Titan IV

| references =

| notes =

}}

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.

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.