Centaur (rocket stage)
Centaur is a rocket stage designed for use as the upper stage of space launch vehicle
Launch vehicle
In spaceflight, a launch vehicle or carrier rocket is a rocket used to carry a payload from the Earth's surface into outer space. A launch system includes the launch vehicle, the launch pad and other infrastructure....

s. Centaur boosts its satellite
In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....

 payload to geosynchronous orbit
Geosynchronous orbit
A geosynchronous orbit is an orbit around the Earth with an orbital period that matches the Earth's sidereal rotation period...

 or, in the case of an interplanetary space probe
Space probe
A robotic spacecraft is a spacecraft with no humans on board, that is usually under telerobotic control. A robotic spacecraft designed to make scientific research measurements is often called a space probe. Many space missions are more suited to telerobotic rather than crewed operation, due to...

, to or near to escape velocity
Escape velocity
In physics, escape velocity is the speed at which the kinetic energy plus the gravitational potential energy of an object is zero gravitational potential energy is negative since gravity is an attractive force and the potential is defined to be zero at infinity...

. Centaur was the world's first high-energy upper stage, burning liquid hydrogen (LH2
Liquid hydrogen
Liquid hydrogen is the liquid state of the element hydrogen. Hydrogen is found naturally in the molecular H2 form.To exist as a liquid, H2 must be pressurized above and cooled below hydrogen's Critical point. However, for hydrogen to be in a full liquid state without boiling off, it needs to be...

) and liquid oxygen (LOX
Liquid oxygen
Liquid oxygen — abbreviated LOx, LOX or Lox in the aerospace, submarine and gas industries — is one of the physical forms of elemental oxygen.-Physical properties:...


Centaur, named after the centaur
In Greek mythology, a centaur or hippocentaur is a member of a composite race of creatures, part human and part horse...

s of Greek mythology
Greek mythology
Greek mythology is the body of myths and legends belonging to the ancient Greeks, concerning their gods and heroes, the nature of the world, and the origins and significance of their own cult and ritual practices. They were a part of religion in ancient Greece...

, was the brainchild of Karel J. "Charlie" Bossart
Karel Bossart
Karel Jan Bossart was a pioneering rocket designer and creator of the Atlas ICBM...

 (the man behind the Atlas ICBM) and Dr. Krafft A. Ehricke
Krafft Arnold Ehricke
-External links:* * *...

, both Convair
Convair was an American aircraft manufacturing company which later expanded into rockets and spacecraft. The company was formed in 1943 by the merger of Vultee Aircraft and Consolidated Aircraft, and went on to produce a number of pioneering aircraft, such as the Convair B-36 bomber, and the F-102...

 employees. Their design was essentially a smaller version of the Atlas, with its concept of using lightweight "stainless steel
Stainless steel
In metallurgy, stainless steel, also known as inox steel or inox from French "inoxydable", is defined as a steel alloy with a minimum of 10.5 or 11% chromium content by mass....

 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
Glenn Research Center
NASA John H. Glenn Research Center at Lewis Field is a NASA center, located within the cities of Brook Park, Cleveland and Fairview Park, Ohio between Cleveland Hopkins International Airport and the Cleveland Metroparks's Rocky River Reservation, and has other subsidiary facilities in Ohio...

, now the Glenn Research Center
Glenn Research Center
NASA John H. Glenn Research Center at Lewis Field is a NASA center, located within the cities of Brook Park, Cleveland and Fairview Park, Ohio between Cleveland Hopkins International Airport and the Cleveland Metroparks's Rocky River Reservation, and has other subsidiary facilities in Ohio...

, 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 I
The Saturn I was the United States' first heavy-lift dedicated space launcher, a rocket designed specifically to launch large payloads into low Earth orbit. Most of the rocket's power came from a clustered lower stage consisting of tanks taken from older rocket designs and strapped together to make...

, Saturn IB
Saturn IB
The Saturn IB was an American launch vehicle commissioned by the National Aeronautics and Space Administration for use in the Apollo program...

 and Saturn V
Saturn V
The Saturn V was an American human-rated expendable rocket used by NASA's Apollo and Skylab programs from 1967 until 1973. A multistage liquid-fueled launch vehicle, NASA launched 13 Saturn Vs from the Kennedy Space Center, Florida with no loss of crew or payload...

 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
Titan IIIE
The Titan IIIE or Titan 3E, also known as Titan III-Centaur was an American expendable launch system, launched seven times between 1974 and 1977...

 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 1
Viking 1 was the first of two spacecraft sent to Mars as part of NASA's Viking program. It was the first spacecraft to successfully land on Mars and perform its mission, and until May 19, 2010 held the record for the second longest Mars surface mission of 6 years and 116 days .- Mission :Following...

, Viking 2
Viking 2
The Viking 2 mission was part of the American Viking program to Mars, and consisted of an orbiter and a lander essentially identical to that of the Viking 1 mission. The Viking 2 lander operated on the surface for 1,281 Mars days and was turned off on 11 April 1980 when its batteries failed...

, Voyager 1
Voyager 1
The Voyager 1 spacecraft is a 722-kilogram space probe launched by NASA in 1977, to study the outer Solar System and eventually interstellar space. Operating for as of today , the spacecraft receives routine commands and transmits data back to the Deep Space Network. At a distance of as of...

, Voyager 2
Voyager 2
The Voyager 2 spacecraft is a 722-kilogram space probe launched by NASA on August 20, 1977 to study the outer Solar System and eventually interstellar space...

, Helios 1 and Helios 2.

A major change to the Centaur was made in the early 1980s. A hydrazine
Hydrazine is an inorganic compound with the formula N2H4. It is a colourless flammable liquid with an ammonia-like odor. Hydrazine is highly toxic and dangerously unstable unless handled in solution. Approximately 260,000 tons are manufactured annually...

 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
Space Shuttle
The Space Shuttle was a manned orbital rocket and spacecraft system operated by NASA on 135 missions from 1981 to 2011. The system combined rocket launch, orbital spacecraft, and re-entry spaceplane with modular add-ons...

. 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 be 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
Space Shuttle Challenger disaster
The Space Shuttle Challenger disaster occurred on January 28, 1986, when Space Shuttle Challenger broke apart 73 seconds into its flight, leading to the deaths of its seven crew members. The spacecraft disintegrated over the Atlantic Ocean, off the coast of central Florida at 11:38 am EST...

, 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
United States Air Force
The United States Air Force is the aerial warfare service branch of the United States Armed Forces and one of the American uniformed services. Initially part of the United States Army, the USAF was formed as a separate branch of the military on September 18, 1947 under the National Security Act of...

 to create the Titan IV
Titan IV
The Titan IV family of space boosters were used by the U.S. Air Force. They were launched from Cape Canaveral Air Force Station, Florida, and Vandenberg Air Force Base, California. At the time of its introduction, the Titan IV was the "largest unmanned space booster used by the Air Force."The...

, 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. The 14 ft diameter Centaur design has now been effectively retired.

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 however a single RL-10 is optimal or adequate and hence a substantial reliability and cost benefit was realized.

A "Common Centaur" was unveiled by Lockheed Martin Space Systems
Lockheed Martin Space Systems
Lockheed Martin Space Systems Company is one of the four major business divisions of Lockheed Martin. It is headquartered in Denver, Colorado with additional sites in Sunnyvale, California; Newtown, Pennsylvania; Huntsville, Alabama; and elsewhere in the US and UK...

 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
    Koyaanisqatsi also known as Koyaanisqatsi: Life Out of Balance, is a 1982 film directed by Godfrey Reggio with music composed by Philip Glass and cinematography by Ron Fricke....

  • 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
    Ullage motor
    Ullage motors are relatively small, independently-fueled rocket engines that may be fired to accelerate the rocket prior to main engine ignition, when the vehicle is in a zero-g situation....

  • 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
    Mariner 8
    Mariner-H, also commonly known as Mariner 8, was part of the Mariner Mars 71 project. It was intended to go into Mars orbit and return images and data.-Mission description:...

     spacecraft bound for Mars
    Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...

  • 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
    Atlas V
    Atlas V is an active expendable launch system in the Atlas rocket family. Atlas V was formerly operated by Lockheed Martin, and is now operated by the Lockheed Martin-Boeing joint venture United Launch Alliance...

     shut down early, leaving its payload — a pair of National Reconnaissance Office
    National Reconnaissance Office
    The National Reconnaissance Office , located in Chantilly, Virginia, is one of the 16 U.S. intelligence agencies. It designs, builds, and operates the spy satellites of the United States government.-Mission:...

     ocean surveillance satellite
    Spy satellite
    A spy satellite is an Earth observation satellite or communications satellite deployed for military or intelligence applications....

    s — 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

, derivatives of the 10 feet (3 m) Centaur-3, with either one or two RL-10A4-2 engines, continue to be used as the upper stage of the Atlas V
Atlas V
Atlas V is an active expendable launch system in the Atlas rocket family. Atlas V was formerly operated by Lockheed Martin, and is now operated by the Lockheed Martin-Boeing joint venture United Launch Alliance...

 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
Advanced Common Evolved Stage
The Advanced Common Evolved Stage, or ACES, is a proposed upper stage rocket for use on space launch vehicles. The design concept is from the U.S. company United Launch Alliance . ACES is intended to boost satellite payloads to geosynchronous orbit or, in the case of an interplanetary space...

 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
Ares V
The Ares V was the planned cargo launch component of the Constellation program, which was to have replaced the Space Shuttle after its retirement in 2011. Ares V was also planned to carry supplies for a human presence on Mars...

, 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
Oerlikon Contraves
Rheinmetall Air Defence AG is a division of German armament manufacturer Rheinmetall, created when the company's Oerlikon Contraves unit was renamed on 1 January 2009 and integrated with Rheinmetall's other air-defence products...

-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
United Launch Alliance
United Launch Alliance is a joint venture of Lockheed Martin and Boeing. ULA was formed in December 2006 by combining the teams at these companies which provide spacecraft launch services to the government of the United States. U.S...

 (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
Slosh dynamics
In fluid dynamics, slosh refers to the movement of liquid inside another object . Strictly speaking, the liquid must have a free surface to constitute a slosh dynamics problem, where the dynamics of the liquid can interact with the container to alter the system dynamics significantly...

, pressure control, RL10 chilldown and RL10 two-phase shutdown operations. "The light weight of DMSP-18 allowed 12000 lbs 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
Technology Readiness Level
Technology Readiness Level is a measure used by some United States government agencies and many of the world's major companies to assess the maturity of evolving technologies prior to incorporating that technology into a system or subsystem...

 design for the Advanced Common Evolved Stage
Advanced Common Evolved Stage
The Advanced Common Evolved Stage, or ACES, is a proposed upper stage rocket for use on space launch vehicles. The design concept is from the U.S. company United Launch Alliance . ACES is intended to boost satellite payloads to geosynchronous orbit or, in the case of an interplanetary space...

Centaur successor.

External links

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