Thor-Agena A & B:  Photospy Launcher
Part Three of a Series Reviewing Thor Family History
by Ed Kyle, Updated 5/31/2009

Thor-Agena

At the dawn of the Space Age, the highest priority U.S. space program, in terms of money, manpower, facilities, hardware flown, and launch pads used, was not run by NASA.  It was U.S. Air Force Weapons System 117L (WS-117L), also known as the Advanced Reconnaissance System.    

RAND, the U.S. Air Force think tank, studied the idea of space-based reconnaissance during the early 1950s.  One study considered development of "television" based orbital reconnaissance.  Other work evaluated the possibility of space-based infrared observation to detect ballistic missile launches.  A third option, space-based photographic reconnaissance using film capsules that would reenter from orbit to be recovered, was proposed by RAND physicist Richard Raymond in 1956.  WS-117L would encompass all three ideas.  They would eventually evolve into the "Samos", "Midas", and "Corona" programs, respectively.  

In June 1956, Lockheed's new Missiles and Space Company of Palo Alto (later Sunnyvale), California, won a contract from the U.S. Air Force Air Research and Development Command to develop the WS-117L systems, including a versatile new upper stage named "Agena".  Thor would serve as the initial Agena booster, for the Corona program.

Agena was more than a rocket stage, it was an orbiting platform for the reconnaissance systems - a spacecraft in its own right.  It was powered by a Bell XLR81 hypergolic turbopump-fed liquid propellant engine originally developed to power a weapons pod for the B-58 Hustler bomber, leading many to call the stage "Hustler" during its early flights.  Initial versions of the engine, used on Agena A stages, produced 7.03 tonnes of thrust for up to 120 seconds.  Later, restartable versions that flew on Agena B and D stages, produced 7.26 tonnes of thrust for up to 240 seconds.
 

Agena A weighed 3.79 tonnes when loaded with nitric acid and UDMH propellants and 0.885 tonnes empty.  It was 4.73 meters long and 1.52 meters in diameter.  Its forward section contained a three-axis gyroscopic inertial guidance and flight control system that used horizon sensors to provide updates.  Cold-gas thrusters located in its aft section provided flight control.  The smaller-diameter aft section, which could mount solar arrays or subsatellites or both, fit within the Thor-Agena interstage at launch.       

A DM-18 Thor, modified with guidance removed and warhead replaced by an interstage, served as the first stage.  It was powered by a 68 tonne sea-level thrust MB-3 Block 1 engine.  At liftoff, Thor-Agena A weighed more than 53 tonnes.  It was capable of lifting more than 0.85 tonnes to near polar low earth orbit.

Thor Agena contractors included Douglas, Rocketdyne, Lockheed, Bell, General Electric (recovery capsule), Itek (Keyhole camera), and Eastman Kodak (film).

Thor-Agena A

Fifteen Thor-Agena A launches occurred between February 28, 1959 and September 13, 1960.  All launched toward near-polar orbits from converted Thor IRBM pads 75-3-4 and 75-3-5 at Vandenberg AFB.   The missions were given the "Discoverer" cover name.  Discoverer was said to be a scientific research effort, but it was actually a Corona development program.    

The effort was more than difficult.  One Thor-Agena A launch vehicle was damaged even before the first flight when, on January 21, 1959, a sneak circuit triggered by a pre-launch test initiated the partially-fueled Agena's internal timer.  The Agena behaved as if the Thor had completed its burn, firing staging ordenance that included two small solid propellant ullage rockets.  The accident, dubbed "Discoverer Zero", fortunately spared several workers who were on the pad at the time.   Agena 1019 was subsequently scrapped, but the Thor 160 booster was refurbished and used on the Discoverer 12 flight in 1960.    

Six of the 15 Thor-Agena A launches failed to reach orbit, and a seventh launch failed to achieve the proper orbit due to a guidance system failure.  Not until Discoverer 13, flown in August 1960, would a capsule be orbited and successfully recovered - the first man-made object recovered from space.  Discoverer 14 returned film containing images taken by the spacecraft's Keyhole camera.  The images covered 1.5 million square miles of Warsaw Pact territory and revealed the presence of 64 previously unknown airfields and 26 surface to air missile sites.  It also revealed the presence of a previously unknown launch center at Plesetsk.
 

Thor-Agena B

More capable Thor Agena B began flying on October 26, 1960.  It launched 43 times, failing eight times, during its five-years of service.  It used an upgraded DM-21 Thor first stage powered by an MB-3 Block 2 engine that produced 77.1 tonnes of liftoff thrust.  The Agena B second stage, which was powered by the restartable 7.26 tonne thrust XLR81-BA-7 Bell engine, was 7.09 meters long and 1.52 meters in diameter.  It weighed 7.17 tonnes fueled, and 0.867 tonnes empty.  Thor Agena B weighed 56.5 tonnes at liftoff and could boost at least 0.95 tonnes into polar orbit. 

Thor Agena B launched Keyhole 2, 3, and 4 series film return spysats.  It launched several Samos electronic intelligence satellites.  It also flew several times for NASA, lifting second generation weathersat Nimbus 1, Canadian ionespheric satellites Alouette 1 and 2, and NASA's Echo 2 and Explorer 31. 

Thor Agena B flew from the two Thor Agena A pads, with Complex 75-1-1 added to the mix in 1961.  The two-stage, mass-produced rocket and its mass-produced payload flew often.  There were 17 launches in 1961 and 18 in 1962.

Even as Thor-Agena B flight rates increased, and as the Keyhole camera system began to prove itself in regular service, plans were being laid for even more powerful upgrades - and for even higher flight rates.