The Agena upper stage began development in 1955, two years before Sputnik, as part of the US Air Force's WS-117L strategic photo-reconnaissance satellite. Lockheed was named prime contractor in October 1956. The concept was an integrated spacecraft with its own maneuvering system. It would be boosted to suborbital velocity by a Thor, Atlas, or Titan ICBM. The spacecraft's own propulsion system would then complete the rocket burn to orbit. It would be restarted in space to make orbital adjustments to better photograph targets of opportunity on the ground. At the end of the mission, it would burn a final time to brake the spacecraft to a reentry in the earth's atmosphere. A separate reentry capsule would bring the camera film safely to earth.
Lockheed selected the Bell XLR-81 storable liquid propellant rocket engine originally developed for a separable rocket-powered thermonuclear weapons pod for the B-58 Hustler bomber. In many early sources the stage and engine are referred to as 'Hustler' before the more correct name of Agena was selected (in line with Lockheed's naming its air- and spacecraft with 'Star' names). The first launch of an Agena A in January 1959 was a failure, but a month later it orbited Discoverer 1, the first test vehicle in the Corona reconnaissance satellite program. Early launches used the Bell XLR81-BA-3 engine, but most Agena A's used the XLR81-BA-5 (Bell Model 8048).
The stage was also used, with an Atlas booster, to launch the USAF Midas (Missile Detection And Surveillance) early-warning satellites and Samos (Satellite and Missile Observation System) ELINT satellite series. The Agena B had an improved XLR81-BA-7 engine (Bell Model 8081), allowing multiple restarts in space. The stage had double the propellant load of the A model. Later models used a further improved XLR81-BA-9 (Bell Model 8096) engine. The Agena B, which had different configurations for each payload/booster combination, was succeeded by the standardized Agena D.
Unit Cost $: 8.700 million.
Gross mass: 7,167 kg (15,800 lb).
Unfuelled mass: 867 kg (1,911 lb).
Height: 7.09 m (23.26 ft).
Diameter: 1.52 m (4.98 ft).
Span: 1.52 m (4.98 ft).
Thrust: 71.17 kN (15,999 lbf).
Specific impulse: 285 s.
Number: 94 .