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Ascent propulsion system (APS)
Apollo Lunar Module Ascent Engine.jpg
Apollo LM ascent engine
Country of origin United States
Manufacturer Bell Aircraft / Rocketdyne
ApplicationLunar Ascent Stage/ Spacecraft propulsion
PredecessorBell 8247
Successor RS-18
Liquid-fuel engine
Propellant N
/ Aerozine 50
Mixture ratio1.6
Cycle Pressure-fed
Nozzle ratio46
Thrust, vacuum3,500 pounds-force (16 kN)
Thrust-to-weight ratio16.7
Chamber pressure120 psia
Specific impulse, vacuum311 seconds (3.05 km/s)
Burn time465 seconds
RestartsDesigned for 2 restarts
Length51 inches (130 cm)
Diameter31 inches (79 cm)
Dry weight210 pounds (95 kg)
Used in
Lunar module as ascent engine
References [1]

The ascent propulsion system (APS) or lunar module ascent engine (LMAE) is a fixed- thrust hypergolic rocket engine developed by Bell Aerosystems for use in the Apollo Lunar Module ascent stage. It used Aerozine 50 fuel, and N
oxidizer. Rocketdyne provided the injector system, at the request of NASA, when Bell could not solve combustion instability problems. [2]


The LMAE traces its origin to the earlier Bell Aerosystems engines (8096, 8247) used in the RM-81 Agena, the rocket upper stage and satellite support bus developed by Lockheed initially for the canceled WS-117L reconnaissance satellite program. [3] The Agena served as an upper stage for several defense, intelligence, and exploration programs: SAMOS-E, SAMOS-F (ELINT Ferret) and MIDAS (Missile Defense Alarm System) military early-warning satellites, Corona photo intelligence program, and the Ranger and Lunar Orbiter lunar probes.

The Lockheed Agena target vehicle using the Bell 8247 engine was qualified for 15 restarts for NASA's Project Gemini. [4]

A total of 365 Agena rockets were launched by NASA and the U.S. Air Force between February 28, 1959, and the last Agena D launched on 12 February 1987, configured as the upper stage of a Titan 34B. [5] [6]

Apollo 17 LM Ascent Stage


During the spring of 1963, Grumman hired Bell to develop the lunar module ascent engine, on the assumption that Bell's experience in development of the Air Force Agena engine would be transferable to the lunar module requirements. Grumman placed heavy emphasis upon high reliability through simplicity of design, and the ascent engine emerged as the least complicated of the three main engines in the Apollo space vehicle, including the LM descent and CSM service propulsion system engines.

Embodying a pressure-fed fuel system using hypergolic (self-igniting) propellants, the ascent engine was fixed-thrust and nongimbaled, capable of lifting the ascent stage off the Moon or aborting a landing if necessary. [7]

The engine developed about 3,500 pounds-force (16 kN) of thrust, which produced a velocity of 2,000 meters per second from lunar launch, to LOR, and CM docking. [7] [2] It weighed 180 pounds (82 kg), with a length of 47 inches (120 cm) and diameter of 34 inches (86 cm). [8]

RS-18 Engine

Rocketdyne brought the lunar module ascent engine out of its 36-year retirement in 2008 for NASA's Exploration Systems Architecture Study (ESAS) engine testing, re-designated it as RS-18, and reconfigured the non-throttleable hypergolic engine to use LOX/methane. [9]


  1. ^ Bartlett, W.; Kirkland, Z. D.; Polifka, R. W.; Smithson, J. C.; Spencer, G. L. (7 February 1966). Apollo spacecraft liquid primary propulsion systems (PDF). Houston, TX: NASA, Lyndon B. Johnson Space Center. pp. 8–9. Archived from the original on 23 August 2022. Retrieved 23 August 2022.{{ cite book}}: CS1 maint: bot: original URL status unknown ( link)
  2. ^ a b "LM Ascent Propulsion". Encyclopedia Astronautica. Archived from the original on 17 November 2002. Retrieved June 7, 2012.
  3. ^ Jacob Neufeld; George M. Watson, Jr. & David Chenoweth (1997). "Technology and the Air Force A Retrospective Assessment" (PDF). Air Force History and Museums Program. Archived (PDF) from the original on October 7, 2012.
  4. ^ Lockheed Missiles & Space Company (1972-02-25). "Shuttle/Agena study. Volume 1: Executive summary". NASA.
  5. ^ "HISTORY - AGENA AS OF 31 DEC67, VOLUME I" (PDF). SPACE AND MISSILE SYSTEMS ORGANIZATION AIR FORCE SYSTEMS COMMAND. June 1966. Archived from the original (PDF) on 2012-09-16. Retrieved 2012-06-07.
  6. ^ Andreas Parsch (2010). "Lockheed RM-81 Agena". Directory of U.S. Military Rockets and Missiles. Retrieved June 6, 2012.
  7. ^ a b Courtney G. Brooks; James M. Grimwood; Loyd S. Swenson (September 20, 2007). "Engines, Large and Small; Chariots for Apollo: A History of Manned Lunar Spacecraft". Retrieved June 7, 2012.
  8. ^ "LM Ascent Engine Specifications".
  9. ^ "New RS-18 builds upon LM Ascent Engine heritage". September 3, 2008.

Public Domain This article incorporates public domain material from websites or documents of the National Aeronautics and Space Administration.

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