Centaur (rocket stage)
Centaur-2A upper stage of an Atlas IIA
United Launch Alliance
|Country of origin||United States|
Saturn I (unflown)
Space Shuttle (unflown)
|Maiden flight||9 May 1962|
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.[not verified in body] 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 history 
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.
A major change to the Centaur was made in the early 1980s. A hydrazine monopropellant attitude control system replaced the previous hydrogen peroxide system. The hydrogen peroxide powered boost pumps were deleted and the RL-10 engines fed directly via tank pressure — a significant reduction in system complexity.
A new version, the Centaur-G, was developed for use with the Space Shuttle. It was optimized for installation in the Orbiter payload bay by increasing the hydrogen tank diameter to 14 feet while retaining the 10-foot-diameter (3.0 m) oxygen tank. Its initial mission was to boost the Galileo scientific probe to Jupiter.
The Centaur, as carried in the Shuttle payload bay, required an extremely complex airborne support system, which was integrated into the Centaur Integrated Support System, the CISS. In addition to controlling Centaur pressurization in flight, the CISS had to be able to dump propellants overboard quickly in the event of a Return To Launch Site Abort, a capability needed to permit the Orbiter to land safely. After the Challenger accident, NASA realized that it was far too risky to fly the Centaur on the Shuttle.
The decision to terminate the Shuttle/Centaur program spurred the United States Air Force to create the Titan IV, which, in its 401A/B versions, used the Centaur-T, also with a 14-foot-diameter (4.3 m) hydrogen tank, as its final stage. This vehicle was capable of launching payloads which had originally been designed for the Shuttle-Centaur combination. In the Titan 401A version, a Centaur-T was launched nine times between 1994 and 1998. In the Titan 401B version, a Centaur-T was launched seven times, with one failure, between 1997 and 2003. The last Titan-Centaur launch was in 2003.
Another major Centaur 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 a single RL-10 is optimal (or adequate), hence a substantial reliability and cost benefit was realized.p. 251.
A "Common Centaur" was unveiled by Lockheed Martin Space Systems on November 30, 1999. The stretched Centaur stage was 11.68 m in length, 1.7 m longer than the Centaur used on Atlas IIA and IIAS rockets at that time.
Although Centaur has a long and successful history in planetary exploration, it has had its share of problems, especially early on:
- May 8, 1962: Centaur weather shield separated early; stage exploded. Footage of this was used in the film Koyaanisqatsi.
- June 30, 1964: RL-10 hydraulic actuator pump shaft broke, preventing one of the two RL-10 engines from vectoring. This led to no roll control, and uncovering of LOX inlet.
- December 11, 1964: Restart attempt failed, due to problem with ullage rockets.
- April 7, 1966: Centaur did not restart after coast — ullage motors ran out of fuel.
- August 10, 1968: Centaur restart failed.
- May 9, 1971; Centaur guidance failed, destroying itself and the Mariner 8 spacecraft bound for Mars orbit.
- Feb 11, 1974: Titan-Centaur; boost pump failed.
- June 9, 1984: Centaur LOX tank failed; no restart.
- April 18, 1991: Centaur failed due to icing of hydrogen pump impeller blades (not understood at the time)
- August 22, 1992: Centaur failed to restart (icing problem again)
- April 30, 1999: Launch of the USA-143 (Milstar-3) communications satellite failed when a Centaur programming error resulted in incorrect burn times, placing the satellite in a useless orbit.
- June 15, 2007: the engine in the Centaur upper stage of an Atlas V shut down early, leaving its payload — a pair of National Reconnaissance Office ocean surveillance satellites — in a lower than intended orbit. The failure was called "A major disappointment", though later statements claim the spacecraft will still be able to complete their mission. The cause was traced to a stuck-open valve that depleted some of the hydrogen fuel, resulting in the second burn terminating four seconds early. The problem was fixed and the next flight was nominal.
Current status 
As of 2009[update], derivatives of the 10-foot-diameter (3.0 m) Centaur-3, with either one or two RL-10A4-2 engines, continue to be used as the upper stage of the Atlas V EELV rocket, the successor of the Titan-Centaur configuration.
Although ULA has an extensive launch manifest for future Centaur flights, ULA has been working on an upper stage design concept that would bring the Delta and Centaur stages together into a single new cryogenic second stage design. The Advanced Common Evolved Stage is intended as a lower-cost, more-capable and more-flexible upper stage that would supplement, and perhaps replace, the existing ULA (Lockheed Martin legacy) Centaur and the ULA (Boeing legacy) Delta Cryogenic Second Stage (DCSS) upper stage vehicles.
Future uses 
There was 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 have even been used as a high-energy "kick motor" for planetary probes launched on board the 125-ton Ares V, which would have seen its first flight around 2018.
The Centaur has a planned evolutionary upgrade which changes the tank diameter to 5.4 m 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 5.4 m 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 5.4 m 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 V-Heavy (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.
Test bed for cryogenic fluid management experiments 
By 2006, Lockheed Martin Space Systems had described the ability to use existing Centaur hardware, with little modification, as a test bed for in-space cryogenic fluid management techniques. Most Centaurs launched on Atlas have excess propellants, ranging from hundreds to thousands of pounds, which could be used for "rideshare" experiments flown as secondary payloads conducted after separation of the primary spacecraft.
In October 2009, the Air Force and United Launch Alliance (ULA) performed an experimental on-orbit demonstration on a modified Centaur upper stage on the DMSP-18 launch to improve "understanding of propellant settling and slosh, pressure control, RL10 chilldown and RL10 two-phase shutdown operations. "The light weight of DMSP-18 allowed 12,000 pounds (5,400 kg) of remaining LO2 and LH2 propellant, 28% of Centaur’s capacity," for the on-orbit demonstrations. The post-spacecraft mission extension ran 2.4 hours before executing the deorbit burn. The initial mission demonstration in 2009 was preparatory to the more-advanced cryogenic fluid management experiments planned for the Centaur-based CRYOTE technology development program in 2012-2014 and to a higher-TRL design for the Advanced Common Evolved Stage Centaur successor.
Source: Atlas V551 Specifications.
- Diameter: 3.05 m (10 ft)
- Length: 12.68 m (42 ft)
- Inert mass: 2,247 kg (4,954 lb)
- Propellant: Liquid hydrogen
- Oxidizer: Liquid oxygen
- Fuel & oxidizer mass: 20,830 kg (45,922 lb)
- Guidance: Inertial
- Propulsion: 1 RL 10A-4-2
- Thrust: 99.2 kN (22,300 lbf)
- Engine length: 2.32 m (7.6 ft)
- Engine diameter: 1.53 m (5 ft)
- Engine dry weight: 168 kg (370 lb)
- Burn time: Variable
- Engine start: Restartable
- Attitude control: 4 27-N thrusters, 8 40-N thrusters
- Propellant: Hydrazine
- Krebs, Gunter. "Centaur". Gunter's Space Page.
- Wade, Mark. "Titan 3E". Encyclopedia Astronautica.
- Virginia Dawson and Mark Bowles (2004). Taming Liquid Hydrogen: The Centaur Upper Stage Rocket 1958-2002 (Technical report SP-2004-4230). http://history.nasa.gov/SP-4230.pdf.. See footnote 40 on page 232.
- Mangels, John (December 11, 2011). "Long-forgotten Shuttle/Centaur boosted Cleveland's NASA center into manned space program and controversy". The Plain Dealer (Cleveland, OH). Retrieved December 11, 2011.
- "Lockheed Martin Builds First "Stretched" Centaur for Next-Generation Atlas". LM.
- MILSTAR 3 — Description.
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- Craig Covault (2007-07-03). "AF Holds To EELV Schedule". Aerospace Daily & Defense Report.
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- Justin Ray. "AV-011: Mission Status Center". Spaceflight Now.
- Zegler, Frank; Bernard Kutter (2010-09-02). "Evolving to a Depot-Based Space Transportation Architecture". AIAA SPACE 2010 Conference & Exposition. AIAA. Retrieved 2011-01-25. "ACES design conceptualization has been underway at ULA for many years. It leverages design features of both the Centaur and Delta Cryogenic Second Stage (DCSS) upper stages and intends to supplement and perhaps replace these stages in the future. The baseline ACES will contain twice the Centaur or 4m DCSS propellant load, providing a significant performance boost compared to our existing upper stages. The baseline 41-mT propellant load is contained in a 5m diameter, common bulkhead stage that is about the same length as ULA's existing upper stages. ACES will become the foundation for a modular system of stages to meet the launch requirements of a wide variety of users. A common variant is a stretched version containing 73t of propellant."
- "Atlas Rocket Overview: Ares V Cargo Launch Vehicle". NASA.
- Sakla, Steven; Kutter, Bernard; Wall, John (2006). "Centaur Test Bed (CTB) for Cryogenic Fluid Management". NASA.
- Successful Flight Demonstration Conducted by the Air Force and United Launch Alliance Will Enhance Space Transportation: DMSP-18, United Launch Alliance, October 2009, accessed 2011-01-23.
- Propellant Depots Made Simple, Bernard Kutter, United Launch Alliance, FISO Colloquium, 2010-11-10, accessed 2011-01-10.
- "Atlas V 551". Spaceflight101.
|Wikimedia Commons has media related to:|
- Gunter's Space Page — information on Centaur versions and launch list
- Taming Liquid Hydrogen: The Centaur Upper Stage Rocket, 1958-2002 NASA History Publication.