Centaur top stages

by Jean-Jacques Serra

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Centaur on Atlas

The Centaur top-stage was the first cryogenic rocket stage. The Pratt & Whitney RL10 engine consuming liquid oxygen and hydrogen had a long and difficult development. Its design started in 1958. It first flew in 1962 but didn't launch a real payload before 1966. The Centaur stage was designed to be used by Atlas. It was 3.05 m in diameter and 9.1 m in length. In its operational version it was equipped with two RL10 A-3-3 engines with a thrust of 66.7 kN each. It carried 13.8 tons propellant and could burn during 450 seconds with several ignitions.

In the late-70ies the Atlas Centaur program was to be abandonned since NASA had decided to transfer all launches to the Shuttle. In fact STS was less available than expected and the Atlas launch program was resumed (with the help of Intelsat finances) with a simplified motor. On the RL10 A-3-3A (73.4 kN thrust) the turbopump feeding was replaced by a pressurized tank system. This new Centaur was first launched in 1984 with an Atlas-G (streched by 2 meters). The GTO capacity then reached 2.3 tons.

In 1987 the augmented Atlas Centaur was selected by the USAF as the MLV2 standard launcher. This new launcher is refered to as Atlas-2. The first stage was streched by 2.7 m and the second stage by 0.9 m. This new Centaur takes 16.8 tons of propellant and can burn up to 473 seconds. Atlas-2A is different by its second stage RL10 A-4 engines with deployable nozzle. Atlas-2AS is still more powerful. It is made from Atlas-2A to which two pairs of Castor-4A boosters are added. Each pair ignites one after the other. Thus GTO capacity raises to 2.8, 3.1 and 3.7 tons respectively for Atlas 2, 2A and 2AS.

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Evolution of Centaur's propulsion system

Designation	First launch	Main engine Thrust (kN)	Main engineSI (sec.)	Applications
RL10 A-1	1962	66.7	425	Tests
RL10 A-3-1	1963	66.7	431	Atlas Centaur
RL10 A-3-3	1966	66.7	444	Atlas Centaur
RL10 A-3-3A	1984	73.4	446	Atlas-1 & -2
RL10 A-4	1991	92.6	449	Atlas-2A & -2AS

Statistics: the RL-10 motor made its 100th flight on the top stage Centaur of AC-114

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Centaur on Titan

Between 1974 and 1977, seven Centaur were used as top stages for Titan 3 rockets, designated as Titan 3E with no major transformation. In this configuration the Centaur was placed into the cap (4.1 m diameter) which provided the thermic protection. Those launchers were devoted to US and German probles and could send 3.6 tons to Mars or Venus.

In 1983 an agreement between NASA and USAF was reached to finance the development of a stage derived from Centaur to launch military satellites and probes from the Shuttle. This new cryogenic stage was a wide body of the Centaur to optimize the volume into the Shuttle's bay. The same motors remained so as the 3.05 m diamater liquid oxygen tank while the hydrogen tank was enhanced to 4.3 m diameter. Two versions of this new stage were developped: Centaur G of 6.1 m long for the USAF and a longer version named Centaur G-prime of 9.1 m long for NASA. After the Challenger accident usage of a cryogenic stage was prohibited for security reasons.

The Centaur G-prime was then adapted to the Titan 4. This version is 4.5 m diameter and about 9 m long. It weighs 26 tons including 23 tons propellant. The motors are RL10-3-3A of 73.4 kN which can operate for up to 617 seconds. The Titan 4 with Centaur top stage is designated Titan 401. It can orbit 4.5 tons in GEO. An enhanced version with Hercules SRMU boosters will enable to place 5.8 tons on such orbits.

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