Characteristics
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 stage engine 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
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