Centaur (rocket stage)

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Model of Centaur with Surveyor as payload.

The Centaur is an upper stage rocket designed for use on satellite launch vehicles, boosting the satellite into its final orbit or, in the case of interplanetary probes, to escape velocity. Centaur was the world's first high-energy upper stage, burning liquid hydrogen (LH2) and liquid oxygen (LOX). Centaur has had a long and successful history in planetary exploration.

Centaur, named after the centaurs of Greek mythology, was the brain child of Karel J. "Charlie" Bossart (the man behind the Atlas ICBM) and Dr. Krafft A. Ehriche, both Convair employees. Their design was essentially a smaller version of the Atlas missile concept, using the Atlas's lightweight stainless steel "balloon tanks" whose structural strength was provided by the pressure of the fuel within.

Centaur uses an ingenious 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. 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 was to be powered by one or two RL-10 rocket engines.

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 1950's and early 1960's 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 which point NASA had replaced the Centaur with much larger upper stages on their designs.

A major change to the Centaur occurred in the early 1980's with the removal of the hydrogen peroxide powered boost pumps and attitude control system from the vehicle. Instead the RL-10 engines were fed directly via tank pressure- resulting in significant reduction in system complexity. A hydrazine monopropellant attitude control system replaced the previous hydrogen peroxide system.

A version of the Centaur was developed for use with the Space Shuttle but was never used due to tougher safety rules imposed after the Challenger accident. This Shuttle/Centaur configuration changed the hydrogen tank diameter to 14 feet while retaining the 10 foot diameter oxygen tank. The geometry was optimized for installation into the Space Shuttle Orbiter payload bay. Its initial mission was to be the Galileo scientific probe to Jupiter. The Centaur systems were dwarfed in their complexity by the supporting fluids, avionic and structural systems which were integrated into the Centaur Integrated Support System or CISS. In addition to more mundane tasks these systems were required to rapidly dump propellants overboard in the event of a Return to Launch Site (RTLS) abort. This was required to permit the orbiter to land safely. These contingency, emergency and abort provisions effectively amplified system complexity to an extreme level and drove the majority of the systems design.

The decision to terminate the Shuttle/Centaur program spurred the US Air Force to create the Titan IV, which used a similar Centaur with a 14 foot hydrogen tank diameter as its final stage. This vehicle was capable of launching payloads which had originally been designed for the Shuttle-Centaur combination. The last Titan-Centaur launch was in 2005. The 14 ft diameter Centaur design has now been effectively retired.

Another major reconfiguration was done for the Atlas III vehicle with a change from dual RL-10 engines as standard to a single RL-10. This change was accomplished while retaining the ability to revert to dual engines should mission requirements dictate. For most missions however a single RL-10 is optimal or adequate and hence a substantial reliability and cost benefit was realized.

As of 2006, the 10 foot diameter Centaur continues to be used as the upper stage of the Atlas V EELV rocket, the successor of the Titan-Centaur configuration. There is a possible use of the Centaur as an upper stage on the new Delta IV (Heavy) rocket, which started test flights in 2004, and may even be used as a high-energy "kick motor" for planetary probes launched onboard the 125-ton Ares V, which will see its first flight after 2015.

The Centaur has a planned evolutionary upgrade which changes the tank diameter to 5.4m and increases propellant load from 1.5 to 6.0 times that of the present Atlas V configuration. This diameter matches the existing Contraves-built 5.4m payload fairing, thus eliminating many structural elements and permitting the vehicle to fly from existing launch complexes with minimal modifications. This design reverses the internal common bulkhead and streamlines many systems while permitting many existing systems and components to fly unchanged. Modular design enables multiple engine configurations from one to six RL-10s or up to three advanced next-generation high performance engines. This evolved Centaur can be flown either on existing Atlas boosters ( designated a Phase 1 configuration) or on next generation 5.4m diameter boosters (designated Phase 2).

Performance levels for the Evolved Centaur based Phase 1 vehicles envelope all Atlas V capabilities. In certain circumstances a single Atlas booster vehicle with five solids and with an evolved Centaur upper-stage can replace a three-booster core Atlas HLV. This has obvious reliability and cost benefits. Phase 2 vehicles open the door to a vastly higher performance capability. Up to 80 metric tons can be lifted to low earth orbit on a Phase 2 HLV vehicle — a substantial fraction of a Saturn V or Ares V vehicle. This performance level, mandated only by NASA crewed exploration missions, can be achieved using hardware identical to that used for traditional commercial and USG missions thus allowing development and support costs to be diluted by rate.

Studies have been conducted showing the extensibility of the basic Centaur and Evolved Centaur designs to long duration space flight for exploration purposes and even for use as a Lunar Lander. Complementing these basic performance capabilities is the ability to rate the vehicle for crewed operation. Extensive work has been conducted showing that achieving this "man-rating" is straightforward and does not mandate wholesale design changes to the Centaur vehicle.