Rocket engine AJ10

The hypergolic rocket engine AJ10 has been used to propel upper stages of several launch vehicles, such as Delta II and Titan III. Some variants were used as the service propulsion module for the Apollo Service Module, and in the Space Shuttle Orbital Maneuvering System. 

AJ10-118K; source: wiki

The AJ10-138 engine was originally developed for Vanguard and Able, and was flown from 1964 to 1980. Two of these engines were used in the Titan III GTO Transtage, with thrust uprated from 35 kN to 36 kN, and with a higher specific impulse of 311 s (3.05 km/s).

The AJ10-137 engine (91 kN of thrust) was used in the Apollo service module's service propulsion system from first flight in 1966. Trans-Earth injection, from lunar orbit, was the most critical usage of this engine during the Apollo program. This version used Aerozine 50 (a 1:1 mix of UDMH and hydrazine) as fuel and nitrogen tetroxide (N2O4) as oxidizer, rather than the previous nitric acid/UDMH.

The AJ10-118F engine produced 40 kN of thrust and was derived from the AJ10-138 engine used on the Transtage. It was used by the Delta-F upper stage of the Delta 1000 Straight Eight series rocket, starting in 1972. This version also used Aerozine 50 as fuel and nitrogen tetroxide (N2O4) as oxidizer.

The AJ10-190 engine was used on the Space Shuttle Orbital Maneuvering System (OMS) for orbital insertion, on-orbit maneuvers, and de-orbiting, first flown in 1981. They produced 26.7 kN  of thrust with a specific impulse (Isp) of 316 seconds. Following the retirement of the Shuttle, these engines were repurposed for use on the Orion spacecraft's service module. This variant used Monomethylhydrazine as fuel, with nitrogen tetroxide (N2O4) as oxidizer.

The AJ10-118K engine was used on the Delta II rocket's upper stage, Delta-K. It used Aerozine 50 as fuel and nitrogen tetroxide (N2O4) as oxidizer. The AJ10-118K engine variant was used from 1989 and retired at the conclusion of the ICESat-2 launch on 15 September 2018.

The Orbital Maneuvering System on Space Shuttle Orbiter

The Orbital Maneuvering System (OMS) is a system of hypergolic liquid-propellant rocket engines used on the Space Shuttle. The system allowed the orbiter to perform various orbital maneuvers according to requirements of each mission profile: orbital injection after main engine cutoff, orbital corrections during flight, and the final deorbit burn for reentry.

From STS-90 onwards the OMS were typically ignited part-way into the Shuttle's ascent for a few minutes to aid acceleration to orbital insertion. Notable exceptions were particularly high-altitude missions such as those supporting the Hubble Space Telescope (STS-31) or those with unusually heavy payloads such as Chandra (STS-93). An OMS dump burn also occurred on STS-51-F, as part of the Abort to Orbit procedure.

The OMS consists of two pods mounted on the orbiter's aft fuselage, on either side of the vertical stabilizer. Each pod contains a single AJ10-190 engine, based on the Apollo Service Module's Service Propulsion System engine. AJ10-190 produced 26.7 kN of thrust with a specific impulse (Isp) of 316 seconds. The oxidizer-to-fuel ratio is 1.65-to-1, The expansion ratio of the nozzle exit to the throat is 55-to-1, and the chamber pressure of the engine is 8.6 bar. The dry weight of each engine is 118kg. Each engine could be reused for 100 missions and was capable of a total of 1,000 starts and 15 hours of burn time.

The engines never failed and never required replacement during the life of the shuttle program.

An OMS pod detached from an orbiter for maintenance; source: wiki

The propellant storage and distribution system consists of one fuel tank and one oxidizer tank in each pod. It also contains propellant feed lines, interconnect lines, isolation valves and crossfeed valves.

Hypergolic propellant tanks of the Orbital Maneuvering System of Space Shuttle Endeavour; source: wiki

Spacecraft attitude control and orbital maneuvering thrusters are almost universally pressure-fed designs. Examples include the Reaction Control (RCS) and the Orbital Maneuvering (OMS) engines of the Space Shuttle orbiter; the RCS and Service Propulsion System (SPS) engines on the Apollo Command/Service Module; the SuperDraco (in-flight abort) and Draco (RCS) engines on the SpaceX Dragon 2; and the RCS, ascent and descent engines on the Apollo Lunar Module.

Pressure-fed rocket cycle. Propellant tanks are pressurized to directly supply fuel and oxidizer to the engine, eliminating the need for turbopumps. source: wiki

The pressure-fed engine is a class of rocket engine designs. A separate gas supply, usually helium, pressurizes the propellant tanks to force fuel and oxidizer to the combustion chamber. To maintain adequate flow, the tank pressures must be higher than the combustion chamber pressure.

Pressure fed engines have simple plumbing and have no need for complex and occasionally unreliable turbopumps. A typical startup procedure begins with opening a valve, often a one-shot pyrotechnic device, to allow the pressurizing gas to flow through check valves into the propellant tanks. Then the propellant valves in the engine itself are opened. 

If the fuel and oxidizer are hypergolic, they burn on contact; non-hypergolic fuels require an igniter. Multiple burns can be conducted by merely opening and closing the propellant valves as needed. If the pressurization system also has activating valves, they can be operated electrically, or by gas pressure controlled by smaller electrically operated valves.