Centaur is a rocket stage designed for use as the upper stage of space launch vehicles. Centaur boosts its satellite payload to geosynchronous orbit or, in the case of an interplanetary space probe, to or near to escape velocity. Centaur was the world's first high-energy upper stage, burning liquid hydrogen (LH2) and liquid oxygen (LOX).
Centaur, named after the centaurs of Greek mythology, was the brainchild of Karel J. "Charlie" Bossart (the man behind the Atlas ICBM) and Dr. Krafft A. Ehricke, both Convair employees. Their design was essentially a smaller version of the Atlas, with its concept of using lightweight "stainless steel balloon" tanks whose structural rigidity was provided solely by the pressure of the propellants within. To keep the tanks from collapsing prior to propellant loading, they were either kept in "stretch" or pressurized with nitrogen gas.
Centaur uses a common double-bulkhead to separate the LOX and LH2 tanks. The two stainless steel skins are separated by a 0.25 inch (6.4 mm) layer of fiberglass honeycomb. The extreme cold of the LH2 on one side creates a vacuum within the fiberglass layer, giving the bulkhead a low thermal conductivity, and thus preventing heat transfer from the relatively warm LOX to the super cold LH2. It is powered by one or two RL10 rocket engines (SEC and DEC variants respectively).
Development started in 1956 at NASA's Lewis Research Center, now the Glenn Research Center, but proceeded slowly, with the first (unsuccessful) test flight in May 1962. In the late 1950s and early 1960s Centaur was proposed as a high energy upper stage for the Saturn I, Saturn IB and Saturn V rockets, under the designation S-V ("Saturn V") in accordance with the numbering of other stages of Saturn rockets. However, the first successful Centaur flight did not take place until 1965. By then, NASA had replaced the Centaur with much larger upper stages for the Saturn.
From 1966 to 1989, the Centaur-D was used as the upper stage for 63 Atlas rocket launches. Fifty-five of these launches were successful.
From 1974 to 1977, the Centaur-D-1T was used as the third stage on seven Titan IIIE launches, six of which were successful.(Only the first launch, used to test the overall launch vehicle design rather than to launch a space vehicle, was classed as a failure; it did however, prove out the design.) Spacecraft launched by these vehicles included Viking 1, Viking 2, Voyager 1, Voyager 2, Helios 1 and Helios 2.
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