File:Agena rocket engine (1959).jpg

English: Sitting in the corner of our office is the Agena engine that launched the Corona program, itself a spin-off of Pied Piper, the nation’s first — albeit secret — satellite program. You can’t make this up.

This engine part number is Bell Model 8048 (also known as the XLR81-BA-5) the engine that flew the first Agena A in 1959 putting the first satellite into a polar orbit (passing' over the North and South poles of the Earth, the ideal orbit for Earth observation coverage). Agena A was also the first American experience with vacuum-starting an engine; some believed at the time that engines would need atmospheric pressure for start-up.

The Agena was the first general-purpose satellite and formed the core for several different satellites and experimental space vehicles. 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. 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.

In February 1959, an Agena-A launched Discoverer 1 into a polar orbit, the first test vehicle in the Corona reconnaissance satellite program (publicly named Discoverer, the KH-1 system).

The Agena name was suggested by DARPA for the star Beta Centauri, also known as Agena, because this upper stage would "ignite in the sky". This followed Lockheed's tradition of naming products for stellar phenomena.

Back in the 50's and 60's, the Discoverer satellites' secret identity as Corona intelligence imaging platforms was closely guarded. As boosters like Thor Agena became operational, satellites gave the United States a new capability to see from space what the USSR and other communist nations were doing. Unlike reconnaissance aircraft, an orbiting satellite placed no crew members in harm's way and was immune to enemy air defenses.

Developing dependable satellite imaging systems, though, was difficult. Not only did the boosters have to place satellites in precise orbits, the satellites' cameras had to function perfectly and the film had to be recovered after the satellites successfully re-entered the atmosphere. On Aug. 19, 1960, following several earlier attempts, Discoverer XIV was the first U.S. reconnaissance satellite to finally return intelligence imagery after successful recovery from orbit.

The Agena A was the first type of Agena to be built. Eighteen Agena As were launched in 1959–61, all of them for the Discoverer, MIDAS, and Samos programs. This is one of two spares.

The Discoverer 3 flight in 1959 was the first attempt to bring back animals (4 medical research mice) from an orbital flight. Here is a 1959 newsreel on it. But it failed and was followed by a series of 6 more launch failures in 1959l

During 1960, Lockheed introduced the improved Agena B, which could be restarted in orbit and had longer propellant tanks for a 2x burn time.

Up to this point, each Agena was custom-built for both the payload and the launch vehicle it was used with. The Agena D was the result of a proposal to standardize the basic Agena configuration In late 1962, mounting frustration over the high failure rate of Thor and Atlas-Agena prompted the suggestion that greater standardization of launch vehicles would improve reliability. The Agena D was used in the Gemini program as an docking target, with first success on Gemini VIII with Neil Armstrong.

On later missions, the Agena's engine was fired while 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 kilometers (854 mi) was reached, which set a new altitude record for crewed spaceflight.

The Bell XLR81 series is a liquid-propellant rocket engine, which burned hypergolic UDMH and RFNA fed by a turbopump in a fuel-rich gas generator cycle. Since the mixture self-ignites on contact, the engine could be greatly simplified. For example, the combustion chamber ignition system was eliminated. The thrust chamber is all-aluminum, and regeneratively cooled by oxidizer flowing through gun-drilled passages in the combustion chamber and throat walls (this keeps the aluminum from melting from the heat of the engine combustion).

The engine is mounted on a hydraulic actuated gimbal which enables thrust vectoring to control pitch and yaw (±2.5° range). Engine thrust and mixture ratio were controlled by cavitating flow venturis on the gas generator flow circuit.