The Agena was a series of upper stages used with Thor, Atlas, and even Titan boosters (although I am
unaware of any current museum display consisting of a Titan with an Agena).
The Agena came in three variants, the Agena A, Agena B, and Agena D (the Agena
C, a proposed up-rating of the Agena B, was never built). The A and B variants
were used to launch a number of "Discoverer" satellites, which were a
cover story for the Corona
reconnaissance satellite program, although Atlas/Agena B boosters also
launched several Ranger probes to the
moon and launched Mariner 2 to Venus.
An improvement in the Agena B was that its
engine, which used unsymmetrical dimethyl hydrazine (UDMH) as the fuel and
inhibited red fuming nitric acid (IRFNA) as the oxidizer, could be restarted in
In addition to being used as a satellite launcher, the Agena D was used in NASA's Gemini
program as a docking target. Gemini
8 became the first spacecraft to dock with another spacecraft.
Agena A configuration
Click image for a 6044x2284 pixel version of this image in a new window.
Taken from p. 14/15 (p. 17/18 in the PDF) of the Agena
Extraction, reconstruction, and clean-up by heroicrelics.org.
Agena B schematic
Click image for a 1354x932 pixel version of this image in a new window.
Taken from p. 237 (p. 248 in the PDF) of the Technology and the
Air Force: A Retrospective Assessment
Extraction and clean-up by heroicrelics.org.