On some missions, the payload was built directly into the Agena, which provided it with electric power, communications and three-axis stabilization.

[5] 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 Aircraft XLR81 engine produced 16,000 lbs (71 kN) of thrust using unsymmetrical dimethylhydrazine (UDMH) as the fuel, and inhibited red fuming nitric acid (IRFNA) as the oxidizer.

The parabolic shape of the chamber throat made for a difficult gun-drilling problem, which Bell Aerosystems engineers solved by arranging the cooling channels in a "One-Sheeted Circular Hyperboloid" shape, allowing machinists to gun-drill straight cooling channels through the curved surfaces of the combustion chamber.

[citation needed] 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.

[3] Attitude control of the horizontal flying Agena was provided by an inertial reference package with three gyroscopes, two horizon sensors, and cold-gas thrusters using a nitrogen-freon mixture.

This enabled the Agena to accommodate the higher pointing stability required for better ground resolution imaging with the improved Corona cameras.

[1] As the Agena was designed to hold a fixed orientation in space while orbiting Earth, a passive thermal control system was devised.

[1] The main source of the Agena's electrical power was silver peroxide-zinc batteries, which from the early 1960s on were supplemented by solar arrays.

An S-band transponder enabled the Agena to receive ground command sequences (image motion compensation, altered attitude, etc.

The Agena-A was propelled by a Bell 8048 (XLR-81-BA-5) engine, which could produce 69 kN (about 15,500 lbs) of thrust with a burn time of 120 seconds.

[8] During 1960, Lockheed introduced the improved Agena-B, which could be restarted in orbit and had longer propellant tanks for increased burn time.

This proposal originated in late 1962 when mounting frustration over the high failure rate of Thor and Atlas-Agena prompted the suggestion that greater standardization of launch vehicles would improve reliability.

David N. Spires summarizes the standardization 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.

Edwards remained responsible for the engineering for several years, until the Air Force declared the Agena-D as operational and froze its design.