The RM-81 Agena was an American rocket upper stage and satellite support bus which was developed by Lockheed initially for the canceled WS-117L reconnaissance satellite program. Following the split-up of WS-117L into SAMOS and Corona for image intelligence, and MIDAS for early warning, the Agena was later used as an upper stage, and an integrated component, for several programs, including Corona reconnaissance satellites and the Agena target vehicle used to demonstrate rendezvous and docking during Project Gemini. It was used as an upper stage on Atlas, Thor, Thorad and Titan IIIB rockets, and considered for others including the Space Shuttle and Atlas V. A total of 365 Agena rockets were launched between February 28, 1959 and February 1987, when the last Agena D was launched.
On some missions, the payload was built directly into the Agena, which provided it with electric power, communications and three-axis stabilization. Payload components were usually located ahead of the Agena's standard bulkhead. On missions where the payload was not built into the Agena, and instead separated after launch, the Agena was known as an Ascent Agena. The Agena was upgraded twice from the original Agena-A in order to support heavier and more sophisticated satellites, such as Corona spacecraft with multiple and more powerful cameras.
The Agena was 5.0 feet (1.5 m) in diameter, three-axis stabilized (for the benefit of the reconnaissance system cameras) and its Bell 8096 engine produced 16,000 lbs. (71 kN) of thrust using unsymmetrical dimethyl hydrazine (UDMH) as the fuel, and inhibited red fuming nitric acid (IRFNA) as the oxidizer. This is a hypergolic fuel/oxidizer combination, and as such, it does not need an ignition system. This rocket engine could be restarted multiple times in orbit, by radio command, and it frequently was. The engine was notable for its unusual aluminum construction. The regeneratively-cooled channels that cooled the throat and nozzle were formed from straight gun drill formed channels. The engine was derived from the XLR-81 propulsion unit for the canceled rocket-propelled nuclear warhead pod of the Convair B-58 Hustler bomber. Until 1959, the Agena was also known as Discoverer Vehicle or Bell Hustler. The manned Project Apollo Lunar Module Ascent Propulsion System was modeled closely on the Agena engine.
Attitude control of the horizontal flying Agena was provided by an inertial reference package with three gyroscopes, two horizon sensors, and micro-jets using a nitrogen-freon mixture of cold gas. Pitch and roll were sensed by two hermetic integrating gyro units. A rate gyro unit determined yaw error by sensing orbital rate. Pitch and roll gyro errors were corrected from the horizon sensors, which were later supplemented by Sun and star trackers. This enabled Agena to accommodate the higher pointing stability required for better ground resolution imaging with the improved Corona cameras.
As Agena was designed to hold a fixed orientation in space while orbiting Earth, a passive thermal control system was devised.
The main source of Agena's electrical power were Silver peroxide-zinc batteries, which from the early 1960s on were supplemented by solar arrays. An S-band beacon enabled Agena to receive ground command sequences (image motion compensation, altered attitude, ...), which could be stored for later execution
Three versions of the Agena were flown:
|Variant||Engine||Thrust||Burn time||Rockets||number of launches|
|Agena-A||Bell 8048||69 kN||120 seconds||Atlas, Thor||20|
|Agena-B||Bell 8081||71 kN||240 seconds||Atlas, Thor||76|
|Agena-D||Bell 8096 (Bell 8247 on Agena Target Vehicle)||71 kN||265 seconds||Atlas, Thor, Thorad, Titan IIIB||269|
The Agena A was the first type of Agena to be built. It was launched atop Thor and Atlas rockets, mostly into polar orbits from Vandenberg Air Force Base Launch Complex 75 and Point Arguello Launch Complex 1 respectively. Two Atlas launches occurred from Launch Complex 14 at Cape Canaveral. The Agena-A was propelled by a Bell 8048 (XLR81) engine, which could produce 69 kilonewtons of thrust for two minutes. Agena-A launches occurred between 1959 and 1961.
The Agena B was the second type of Agena to be built. It was equipped with a Bell 8081 engine, which could generate 71 kilonewtons of thrust, and be restarted in orbit. The stage carried enough propellant for the engine to burn for a total of four minutes. These launched the SAMOS-E, SAMOS-F (ELINT Ferret), and MIDAS (Missile Defense Alarm System) military early-warning satellites and the Ranger and Lunar Orbiter lunar probes.
The Agena D was the result of a proposal by Lockheed engineering executive Lawrence Edwards, who suggested standardising the basic Agena configuration, and adding additional features depending on payload requirements, and a requirement from the Pentagon that the Agena be made compatible with the Titan rocket. David N. Spires summarizes the standardisation as follows:
- The Agena D's common configuration included four usable modules containing the major guidance, beacon, power, and telemetry equipment, a standard payload console, and a rear rack above the engine for plug-in installation of optional gear-like solar panels, "piggyback" subsatellites, and an optional Bell Aerosystems engine that could be restarted in space as many as sixteen times.
Its orbital configuration had a diameter of 60 in (1.5 m) and a length of 248 in (6.3 m), and provided 19,500 Wh of electrical power from batteries. As of 2010, the Agena-D is the most-launched American upper stage. A special production line was set up to turn out 40 Agena-D spacecraft per year. Edwards remained responsible for the engineering for several years until the Air Force declared the Agena-D operational froze its design. By the time of its retirement, the reliability of the Agena-D exceeded 95 percent.
Other variants 
In the early 1970s Lockheed studied the use of Agena as a payload in the Space Shuttle bay. An Agena-C with an increased diameter was proposed, but never built.
The Agena-2000 was intended as a modernised Agena, and would have been used on the Atlas V Light Evolved Expendable Launch Vehicle. The Atlas V Light was cancelled in favour of standardising on the Medium configuration, and as a result the Agena-2000 was never built.
Agena Target Vehicle 
The Agena Target Vehicle was based around the Agena-D, with equipment fitted to support use as a rendezvous and docking target for missions conducted as part of Project Gemini. It was equipped with a Bell Aerospace Model 8247 engine, which was qualified for up to 15 restarts. On later missions, the Agena's engine was fired whilst the Gemini spacecraft was docked, in order to boost the spacecraft to a higher orbit, and to bring it back again. During the Gemini 11 mission, an elliptical orbit with an apogee of 1,375 kilometres (854 mi) was reached, which set an altitude record for manned spaceflight that held until Apollo 8, the first manned mission to the Moon, was launched.
See also 
- Related lists
- Jacob Neufeld, George M. Watson, Jr., and David Chenoweth (1997). "Technology and the Air Force A Retrospective Assessment". Air Force History and Museums Program.
- Shuttle/Agena study. 1: Executive summary. Lockheed Aircraft Corporation. 25 February 1972. Retrieved 20 July 2010.
- "HISTORY - AGENA AS OF 31 DEC67, VOLUME I". SPACE AND MISSILE SYSTEMS ORGANIZATION AIR FORCE SYSTEMS COMMAND. 1966-06.
- Lockheed Missiles & Space Company (1972-02-25). "Shuttle/Agena study. Volume 1: Executive summary". NASA.
- "Factsheets: Bell Model 8048". National Museum of the USAF. Retrieved 2011-01-29.
- David N .Spires (1998). "Beyond Horizons: A Half Century of Air Force Space Leadership". Air Force Space Comman.
- "Feasibilty Study, Final Report, Geodetic Orbital Photographic Satellite System, Volume 2". NRO. 1966-06.
- Genesis of Agena D: America's Most-Used Space Vehicle, lead article in Cold War Space History: Programmes, Space Chronicle, May 2006. Edited by Dwayne A. Day.
- Krebs, Gunter. "Atlas-5". Gunter's Space Page. Retrieved 20 July 2010.
- Agena A and discussion of the RM-81 series
- Gemini 8 Docks with Agena Video
- Shuttle/Agena study. Annex A: Ascent agena configuration
- Reusable Agena study. Volume 2: Technical