The Centaur is a family of rocket propelled

upper stage A multistage rocket or step rocket is a launch vehicle that uses two or more rocket ''stages'', each of which contains its own engines and propellant. A ''tandem'' or ''serial'' stage is mounted on top of another stage; a ''parallel'' stage is ...

s produced by U.S.

launch service provider A launch service provider (LSP) is a type of company which specialises in launching spacecraft. In 2018, the launch services sector accounted for $5.5 billion out of a total $344.5 billion "global space economy". It is responsible for the ordering ...

United Launch Alliance, with one main active version and one version under development. The diameter Common Centaur/Centaur III flies as the upper stage of the Atlas V launch vehicle, and the diameter Centaur V is being developed as the upper stage of ULA's new

Vulcan Vulcan may refer to: Mythology * Vulcan (mythology), the god of fire, volcanoes, metalworking, and the forge in Roman mythology Arts, entertainment and media Film and television * Vulcan (''Star Trek''), name of a fictional race and their home p ...

rocket. Centaur was the first rocket stage to use liquid hydrogen (LH₂) and liquid oxygen (LOX)

propellants A propellant (or propellent) is a mass that is expelled or expanded in such a way as to create a thrust or other motive force in accordance with Newton's third law of motion, and "propel" a vehicle, projectile, or fluid payload. In vehicles, the e ...

, a high-energy combination that is ideal for upper stages but has significant handling difficulties.

Characteristics

Common Centaur is built around stainless steel pressure stabilized balloon propellant tanksNASA.gov
/ref> with thick walls. It can lift payloads of up to . The thin walls minimize the mass of the tanks, maximizing the stage's overall performance. A common bulkhead separates the

LOX Liquid oxygen—abbreviated LOx, LOX or Lox in the aerospace, submarine and gas industries—is the liquid form of molecular oxygen. It was used as the oxidizer in the first liquid-fueled rocket invented in 1926 by Robert H. Goddard, an appli ...

and LH₂ tanks, further reducing the tank mass. It is made of two stainless steel skins separated by a fiberglass honeycomb. The fiberglass honeycomb minimizes heat transfer between the extremely cold LH₂ and relatively warm LOX. The main propulsion system consists of one or two

Aerojet Rocketdyne Aerojet Rocketdyne is an American manufacturer of rocket, hypersonic, and electric propulsive systems for space, defense, civil and commercial applications. Headquartered in Sacramento, California, the company is owned by Aerojet Rocketdyne Hold ...

RL10 engines. The stage is capable of up to twelve restarts, limited by propellant, orbital lifetime, and mission requirements. Combined with the insulation of the propellant tanks, this allows Centaur to perform the multi-hour coasts and multiple engine burns required on complex orbital insertions. The reaction control system (RCS) also provides

ullage Ullage or headspace is the unfilled space in a container, particularly with a liquid. Etymology The word ''ullage'' comes from the Latin word ''oculus'', used by the Romans to refer to a barrel cork hole. This word was, in turn, taken in medieval ...

and consists of twenty hydrazine

monopropellant Monopropellants are propellants consisting of chemicals that release energy through exothermic chemical decomposition. The molecular bond energy of the monopropellant is released usually through use of a catalyst. This can be contrasted with bipro ...

engines located around the stage in two 2-thruster pods and four 4-thruster pods. For propellant, of Hydrazine is stored in a pair of bladder tanks and fed to the RCS engines with pressurized helium gas, which is also used to accomplish some main engine functions.

Current versions

As of 2019, all but two of the many Centaur variants had been retired: Common Centaur/Centaur III (active) and Centaur V (in development).

Current engines

Centaur III/Common Centaur

Common Centaur is the upper stage of the Atlas V rocket. Earlier Common Centaurs were propelled by the RL10-A-4-2 version of the RL-10. Since 2014, Common Centaur has flown with the RL10-C-1 engine, which is shared with the Delta Cryogenic Second Stage, to reduce costs. The Dual Engine Centaur (DEC) configuration will continue to use the smaller RL10-A-4-2 to accommodate two engines in the available space. The Atlas V can fly in multiple configurations, but only one affects the way Centaur integrates with the booster and fairing: the diameter Atlas V payload fairing attaches to the booster and encapsulates the upper stage and payload, routing fairing-induced aerodynamic loads into the booster. If the diameter payload fairing is used, the attachment point is at the top (forward end) of Centaur, routing loads through the Centaur tank structure. The latest Common Centaurs can accommodate secondary payloads using an Aft Bulkhead Carrier attached to the engine end of the stage.

Single Engine Centaur (SEC)

Most payloads launch on Single Engine Centaur (SEC) with one RL10. This is the variant for all normal flights of the Atlas V (indicated by the last digit of the naming system, for example Atlas V 421).

Dual Engine Centaur (DEC)

A dual engine variant with two RL-10 engines is available, but only for launching the

CST-100 Starliner The Boeing CST-100 Starliner
is a class of two partially

crewed spacecraft and possibly the Dream Chaser

ISS The International Space Station (ISS) is the largest modular space station currently in low Earth orbit. It is a multinational collaborative project involving five participating space agencies: NASA (United States), Roscosmos (Russia), JAXA (J ...

logistics spaceplane. The higher thrust of two engines allows a gentler ascent with more horizontal velocity and less vertical velocity, which reduces deceleration to survivable levels in the event of a launch abort and ballistic reentry occurring at any point in the flight.

Centaur V

Centaur V will be the upper stage of the new

launch vehicle currently being developed by the United Launch Alliance to meet the needs of the

National Security Space Launch National Security Space Launch (NSSL) — formerly Evolved Expendable Launch Vehicle (EELV) from 1994 to 2019 — is a program of the United States Space Force (USSF) intended to assure access to space for United States Department of Defense and o ...

(NSSL) program. Vulcan was initially intended to enter service with an upgraded variant of the Common Centaur, with an upgrade to the

Advanced Cryogenic Evolved Stage The Advanced Cryogenic Evolved Stage (ACES) was a proposed liquid oxygen/liquid hydrogen upper-stage from 2005 to 2020 for use on a number of different launch vehicles produced by Boeing, Lockheed Martin, and after a late-2006 merger, United La ...

(ACES) planned after the first few years of flights. In late 2017, ULA decided to bring elements of the ACES upper stage forward and begin work on Centaur V. Centaur V will have ACES' diameter and advanced insulation, but does not include the

Integrated Vehicle Fluids The Advanced Cryogenic Evolved Stage (ACES) was a proposed liquid oxygen/liquid hydrogen upper-stage from 2005 to 2020 for use on a number of different launch vehicles produced by Boeing, Lockheed Martin, and after a late-2006 merger, United Lau ...

(IVF) feature expected to allow the extension of upper stage on-orbit life from hours to weeks. Centaur V will use two different versions of the RL10-C engine with nozzle extensions to improve the fuel consumption for the heaviest payloads. This increased capability over Common Centaur will permit ULA to meet NSSL requirements and retire both the Atlas V and Delta IV Heavy rocket families earlier than initially planned. The new rocket publicly became the Vulcan Centaur in March 2018. In May 2018, the Aerojet Rocketdyne RL10 was announced as Centaur V's engine following a competitive procurement process against the Blue Origin

BE-3 The BE-3 (''Blue Engine 3'') is a LH2/ LOX rocket engine developed by Blue Origin. The engine began development in the early 2010s, and completed acceptance testing in early 2015. The engine is being used on the New Shepard suborbital rocket, ...

. Each stage will mount two engines. In September 2020, ULA announced that ACES was no longer being developed, and that Centaur V would be used instead. Tory Bruno, ULA's CEO, stated that the Vulcan's Centaur 5 will have 40% more endurance and two and a half times more energy than the upper stage ULA currently flies. “But that’s just the tip of the iceberg,” Bruno elaborated. “I’m going to be pushing up to 450, 500, 600 times the endurance over just the next handful of years. That will enable a whole new set of missions that you cannot even imagine doing today.”

History

The Centaur concept originated in 1956 when

Convair Convair, previously Consolidated Vultee, was an American aircraft manufacturing company that later expanded into rockets and spacecraft. The company was formed in 1943 by the merger of Consolidated Aircraft and Vultee Aircraft. In 1953, i ...

began studying a liquid hydrogen fueled upper stage. The ensuing project began in 1958 as a joint venture among Convair, the

Advanced Research Projects Agency The Defense Advanced Research Projects Agency (DARPA) is a research and development agency of the United States Department of Defense responsible for the development of emerging technologies for use by the military. Originally known as the Adv ...

(ARPA), and the

U.S. Air Force The United States Air Force (USAF) is the air service branch of the United States Armed Forces, and is one of the eight uniformed services of the United States. Originally created on 1 August 1907, as a part of the United States Army Sign ...

. In 1959,

NASA The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the US federal government responsible for the civil List of government space agencies, space program ...

assumed ARPA's role. Centaur initially flew as the upper stage of the

Atlas-Centaur The Atlas-Centaur was a United States expendable launch vehicle derived from the SM-65 Atlas D missile. Launches were conducted from Launch Complex 36 at the Cape Canaveral Air Force Station (CCAFS) in Florida. Early development Convair, ...

launch vehicle, encountering a number of early developmental issues due to the pioneering nature of the effort and the use of liquid hydrogen. In 1994 General Dynamics sold their Space Systems division to

Lockheed-Martin The Lockheed Martin Corporation is an American aerospace, arms, defense, information security, and technology corporation with worldwide interests. It was formed by the merger of Lockheed Corporation with Martin Marietta in March 1995. It is ...

Centaur A-D (Atlas)

The Centaur was originally developed for use with the Atlas launch vehicle family. Known in early planning as the 'high-energy upper stage', the choice of the mythological Centaur as a namesake was intended to represent the combination of the brute force of the Atlas booster and finesse of the upper stage. Initial

launches used developmental versions, labeled Centaur-A through -C. The only Centaur-A launch on 8 May 1962 ended in an explosion 54 seconds after liftoff when insulation panels on the Centaur separated early, causing the LH₂ tank to overheat and rupture. After extensive redesigns, the only Centaur-B flight on 26 November 1963 was successful. Centaur-C flew three times with two failures and one launch declared successful although the Centaur failed to restart. Centaur-D was the first version to enter operational service, with fifty-six launches. On 30 May 1966, an

boosted the first Surveyor lander towards the Moon. This was followed by six more Surveyor launches over the next two years, with the Atlas-Centaur performing as expected. The Surveyor program demonstrated the feasibility of reigniting a hydrogen engine in space and provided information on the behavior of LH₂ in space. By the 1970s, Centaur was fully mature and had become the standard rocket stage for launching larger civilian payloads into high Earth orbit, also replacing the

Atlas-Agena The Atlas-Agena was an American expendable launch system derived from the SM-65 Atlas missile. It was a member of the Atlas family of rockets, and was launched 109 times between 1960 and 1978. It was used to launch the first five Mariner uncrew ...

vehicle for NASA planetary probes. By the end of 1989, Centaur-D and -G had been used as the upper stage for 63 Atlas rocket launches, 55 of which were successful.

Saturn I S-V

The Saturn I was designed to fly with a S-V third stage to enable payloads to go beyond low earth orbit (LEO). The S-V stage was intended to be powered by two RL-10A-1 engines burning liquid hydrogen as fuel and liquid oxygen as oxidizer. The S-V stage was flown four times on missions SA-1 through SA-4, all four of these missions had the S-V's tanks filled with water to be used a ballast during launch. The stage was not flown in an active configuration.

Centaur D-1T (Titan III)

The Centaur D was improved for use on the far more powerful

Titan III Titan was a family of United States expendable rockets used between 1959 and 2005. The Titan I and Titan II were part of the US Air Force's intercontinental ballistic missile (ICBM) fleet until 1987. The space launch vehicle versions contribut ...

booster in the 1970s, with the first launch of the resulting

Titan IIIE The Titan IIIE or Titan 3E, also known as the Titan III-Centaur, was an American expendable launch system. Launched seven times between 1974 and 1977, it enabled several high-profile NASA missions, including the Voyager and Viking planetary prob ...

in 1974. The Titan IIIE more than tripled the payload capacity of Atlas-Centaur, and incorporated improved thermal insulation, allowing an orbital lifespan of up to five hours, an increase over the 30 minutes of the Atlas-Centaur. The first launch of Titan IIIE in February 1974 was unsuccessful, with the loss of the Space Plasma High Voltage Experiment (SPHINX) and a mockup of the Viking probe. It was eventually determined that Centaur's engines had ingested an incorrectly installed clip from the oxygen tank. The next Titan-Centaurs launched Helios 1, '' Viking 1'', '' Viking 2'', Helios 2, '' Voyager 1'', and '' Voyager 2''. The Titan booster used to launch ''Voyager 1'' had a hardware problem that caused a premature shutdown, which the Centaur stage detected and successfully compensated for. Centaur ended its burn with less than 4 seconds of fuel remaining.

Centaur (Atlas G)

Centaur was introduced on the Atlas G and was carried over to the very similar Atlas I.

Shuttle-Centaur (Centaur G and G-Prime)

Shuttle-Centaur Shuttle-Centaur was a version of the Centaur upper stage rocket designed to be carried aloft inside the Space Shuttle and used to launch satellites into high Earth orbits or probes into deep space. Two variants were developed: Centaur G-Prime ...

was a proposed Space Shuttle upper stage. To enable its installation in shuttle payload bays, the diameter of the Centaur's hydrogen tank was increased to , with the LOX tank diameter remaining at . Two variants were proposed: Centaur G Prime, which was planned to launch the '' Galileo'' and '' Ulysses'' robotic probes, and Centaur G, a shortened version, reduced in length from approximately , planned for U.S. DoD payloads and the ''

Magellan Ferdinand Magellan ( or ; pt, Fernão de Magalhães, ; es, link=no, Fernando de Magallanes, ; 4 February 1480 – 27 April 1521) was a Portuguese explorer. He is best known for having planned and led the 1519 Spanish expedition to the East ...

'' Venus probe. After the Space Shuttle ''Challenger'' accident, and just months before the Shuttle-Centaur had been scheduled to fly, NASA concluded that it was too risky to fly the Centaur on the Shuttle. The probes were launched with the much less powerful solid-fueled IUS, with Galileo needing multiple gravitational assists from Venus and Earth to reach Jupiter.

Centaur (Titan IV)

The capability gap left by the termination of the Shuttle-Centaur program was filled by a new launch vehicle, the

Titan IV Titan IV was a family of heavy-lift space launch vehicles developed by Martin Marietta and operated by the United States Air Force from 1989 to 2005. Launches were conducted from Cape Canaveral Air Force Station, Florida and Vandenberg Air Forc ...

. The 401A/B versions used a Centaur upper stage with a diameter hydrogen tank. In the Titan 401A version, a Centaur-T was launched nine times between 1994 and 1998. The 1997 Cassini-Huygens Saturn probe was the first flight of the Titan 401B, with an additional six launches wrapping up in 2003 including one SRB failure.

Centaur II (Atlas II/III)

Centaur II was initially developed for use on the

Atlas II Atlas II was a member of the Atlas family of launch vehicles, which evolved from the successful Atlas missile program of the 1950s. The Atlas II was a direct evolution of the Atlas I, featuring longer first stage tanks, higher-performing engine ...

series of rockets. Centaur II also flew on the initial Atlas IIIA launches.

Centaur III/Common Centaur (Atlas III/V)

Atlas IIIB introduced the Common Centaur, a longer and initially dual engine Centaur II.

Atlas V cryogenic fluid management experiments

Most Common Centaurs launched on Atlas V have hundreds to thousands of kilograms of propellants remaining on payload separation. In 2006 these propellants were identified as a possible experimental resource for testing in-space cryogenic fluid management techniques. In October 2009, the Air Force and United Launch Alliance (ULA) performed an experimental demonstration on the modified Centaur upper stage of DMSP-18 launch to improve "understanding of propellant settling and

slosh In fluid dynamics, slosh refers to the movement of liquid inside another object (which is, typically, also undergoing motion). Strictly speaking, the liquid must have a free surface to constitute a slosh dynamics problem, where the dynamics of t ...

, pressure control, RL10 chilldown and RL10 two-phase shutdown operations. DMSP-18 was a low mass payload, with approximately 28% () of LH₂/LOX propellant remaining after separation. Several on-orbit demonstrations were conducted over 2.4 hours, concluding with a deorbit burn.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. The initial demonstration was intended to prepare for more-advanced cryogenic fluid management experiments planned under the Centaur-based CRYOTE technology development program in 2012–2014,Propellant Depots Made Simple
, Bernard Kutter, '' United Launch Alliance'', FISO Colloquium, 2010-11-10, accessed 2011-01-10. and will increase the TRL of the

Centaur successor.

Mishaps

Although Centaur has a long and successful flight history, it has experienced a number of mishaps: * April 7, 1966: Centaur did not restart after coast — ullage motors ran out of fuel. * May 9, 1971; Centaur guidance failed, destroying itself and the

Mariner 8 Mariner-H (Mariner Mars '71), also commonly known as Mariner 8, was (along with Mariner 9) part of the Mariner Mars '71 project. It was intended to go into Mars orbit and return images and data, but a launch vehicle failure prevented Mariner 8 ...

spacecraft bound for

Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury. In the English language, Mars is named for the Roman god of war. Mars is a terrestrial planet with a thin at ...

orbit. * April 18, 1991: Centaur failed due to particles from the scouring pads used to clean the propellant ducts getting stuck in the turbopump, preventing start-up. * August 22, 1992: Centaur failed to restart (icing problem). * April 30, 1999: Launch of the USA-143 (

Milstar Milstar (Military Strategic and Tactical Relay) is a constellation of military communications satellites in geosynchronous orbit, which are operated by the United States Space Force, and provide secure and jam-resistant worldwide communications ...

DFS-3m) communications satellite failed when a Centaur database error resulted in uncontrolled roll rate and loss of attitude control, 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 The National Reconnaissance Office (NRO) is a member of the United States Intelligence Community and an agency of the United States Department of Defense which designs, builds, launches, and operates the reconnaissance satellites of the U.S. fe ...

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. * August 30, 2018: Atlas V Centaur passivated second stage launched on September 17, 2014, broke up, creating space debris. * March 23–25, 2018: Atlas V Centaur passivated second stage launched on September 8, 2009, broke up. * April 6, 2019: Atlas V Centaur passivated second stage launched on October 17, 2018, broke up.

Centaur III specifications

Source: Atlas V551 specifications, as of 2015. * Diameter: 3.05 m (10 ft) * Length: 12.68 m (42 ft) * Inert mass: 2,247 kg (4,954 lb) * Fuel: Liquid hydrogen * Oxidizer: Liquid oxygen * Fuel and oxidizer mass: 20,830 kg (45,922 lb) * Guidance: Inertial * Thrust: 99.2 kN (22,300 lbf) * Burn time: Variable; e.g., 842 seconds on Atlas V * Engine: RL10-C-1 * Engine length: 2.32 m (7.6 ft) * Engine diameter: 1.53 m (5 ft) * Engine dry weight: 168 kg (370 lb) * Engine start: Restartable * Attitude control: 4 27-N thrusters, 8 40-N thrusters ** Propellant: Hydrazine

References

{{DEFAULTSORT:Centaur (Rocket Stage) Rocket engines using hydrogen propellant Rocket stages NASA space launch vehicles United Launch Alliance