The Centaur is a family of rocket-propelled upper stages that has been in use since 1962. It is currently produced by United Launch Alliance (ULA) in two main versions. The diameter Centaur III (also known as the Common Centaur) serves as the second stage of the retiring Atlas V rocket, and the diameter Centaur V is used as the second stage of the Vulcan Centaur rocket. Centaur was the first rocket stage to use hydrolox propellant—liquid hydrogen (LH<sub>2</sub>) and liquid oxygen (LOX)—a high-energy combination well suited for upper stages but difficult to handle because both propellants must be stored at extremely low cryogenic temperatures.

Characteristics

Centaur stages are built around stainless steel pressure-stabilized balloon propellant tanks with thick walls. It can lift payloads of up to . The thin tank walls minimize mass, maximizing overall stage performance.

A common bulkhead separates the LOX and LH<sub>2</sub> tanks, further reducing weight. The bulkhead consists of two stainless steel skins separated by a fiberglass honeycomb, which limits heat transfer between the extremely cold LH<sub>2</sub> and the comparatively warmer LOX.

The main propulsion system consists of one or two RL10 engines made by Aerojet Rocketdyne.

Current versions

, two Centaur variants are in use: Centaur III on Atlas V, and Centaur V on Vulcan Centaur. All of the many other Centaur variants have been retired.

Centaur III/Common Centaur

thumb|upright|Single Engine Centaur (SEC) stage

Common Centaur is the upper stage of the Atlas V rocket. 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 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 crewed spacecraft. 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

thumb|upright|Centaur V stage on [[Vulcan Centaur rocket carrying Peregrine lunar lander]]

Centaur V is the upper stage of the Vulcan launch vehicle developed by United Launch Alliance (ULA) beginning in 2014 to meet the requirements of the National Security Space Launch (NSSL) program.

Development

ULA initially intended the Centaur V, an upgraded version of the Common Centaur, to only be used on an interim basis until a transition to the Advanced Cryogenic Evolved Stage (ACES) planned after the first few years of flights.

In late 2017, the company began development of Centaur V by accelerating elements of the ACES design, including a diameter and advanced insulation. The Integrated Vehicle Fluids (IVF) system, which had been intended to extend on-orbit lifetime from hours to weeks, was omitted. and in May 2018 ULA selected Aerojet Rocketdyne's RL10 engine over Blue Origin's BE-3. Each Centaur V uses two RL10 engines.

In September 2020, ULA confirmed that ACES would no longer be developed and that Centaur V would remain Vulcan's upper stage. The company said that the initial versions of the Centaur V offers 40% more endurance and 250% more energy than the Common Centaur.

Vulcan launched successfully on January 8, 2024, with Centaur V performing as planned on its maiden flight.

Starting in late 2025, ULA plans to upgrade the stage with the RL10E engine, featuring a fixed nozzle extension and modest improvements in thrust and specific impulse, offering minor improvements to payload capacities.

In February 2026 NASA announced changes to the SLS program including a plans to develop a "Standardized Block 1 Configuration" alongside a rendering that appears to show an SLS variant where the Interim Cryogenic Propulsion Stage (ICPS) is replaced by a Centaur V.

CV-L

During the Vulcan Cert-2 mission broadcast on October 4, 2024, ULA announced a "LEO Optimized Centaur" variant, later designated CV-L, scheduled to debut in 2025. CV-L is shorter than the baseline Centaur V, which was redesignated CV-HE (Centaur V High Energy). Unlike CV-HE, which uses a hydrolox RCS, CV-L returns to using a simpler hydrazine monopropellant RCS.

Current engines

Centaur engines have evolved over time, and three versions (RL10A-4-2, RL10C-1 and RL10C-1-1) are in use as of 2024 (see table below). All versions utilize liquid hydrogen and liquid oxygen.

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History

thumb|Centaur stage during assembly at General Dynamics, 1962

thumb|upright|Diagram of the Centaur stage tank

The Centaur concept originated in 1956 when the Convair division of General Dynamics 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 (ARPA), and the U.S. Air Force. In 1959, NASA assumed ARPA's role. Centaur initially flew as the upper stage of the Atlas-Centaur 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.

Centaur A-D (Atlas)

thumb|upright|An [[Atlas-Centaur rocket (Centaur D stage) launches Surveyor 1]]

thumb|Centaur D1-AR diagram

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 Atlas-Centaur 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 LH<sub>2</sub> tank to overheat and rupture. This version was powered by two RL10A-1 engines.

After extensive redesigns, the only Centaur-B flight on 26 November 1963 was successful. This version was powered by two RL10A-3 engines. It was powered by two RL10A-3-1 or RL10A-3-3 engines.

By the end of 1989, Centaur-D 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)

thumb|upright|A [[Titan (rocket family)|Titan IIIE-Centaur rocket (Centaur D-1T stage) launches Voyager 2]]

The Centaur D-1T (powered by RL10A-3-3 engines) was an improved version for use on the far more powerful Titan III booster in the 1970s, 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.

  • Diameter: ; with the Centaur standard shroud (CSS)
  • Length:
  • Inert mass:
  • Fuel: Liquid hydrogen
  • Oxidizer: Liquid oxygen
  • Fuel and oxidizer mass:
  • Guidance: Inertial
  • Thrust: 2 x
  • Burn Capability: 3 to 4 burns
  • Engine: 2 x RL10A-3-3
  • Engine start: Restartable
  • Attitude control: 4 x thrusters

Shuttle-Centaur

thumb|Illustration of Shuttle-Centaur GPrime with Ulysses

Shuttle-Centaur 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 GPrime, 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 Venus probe.

After the Space Shuttle Challenger disaster, 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.

Centaur I (Atlas I)

The upper stage of the Atlas I was the Centaur I stage, derived from earlier models of Centaur that also flew atop Atlas boosters. Centaur I featured two RL-10-A-3A engines burning liquid hydrogen and liquid oxygen, making the stage extremely efficient. To help slow the boiloff of liquid hydrogen in the tanks, Centaur featured fiberglass insulation panels that were jettisoned 25 seconds after the first stage booster engines were jettisoned. Centaur I was the last version of the stage to feature separating insulation panels.

Centaur II (Atlas II/III)

thumb|Centaur-2A second stage of an Atlas IIA rocket

Centaur II was initially developed for use on the Atlas II series of rockets.

Centaur III/Common Centaur (Atlas III/V)

thumb|Common Centaur upper stage for an Atlas V

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

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

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, pressure control, RL10 chilldown and RL10 two-phase shutdown operations. DMSP-18 was a low mass payload, with approximately 28% () of LH<sub>2</sub>/LOX propellant remaining after separation. Several on-orbit demonstrations were conducted over 2.4 hours, concluding with a deorbit burn. 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, and will increase the TRL of the Advanced Cryogenic Evolved Stage Centaur successor.

  • August 10, 1968: AC-17. Centaur did not restart after coast&nbsp;— icing of the hydrogen peroxide supply lines.
  • May 9, 1971: Centaur guidance failed, destroying itself and the Mariner 8 spacecraft bound for Mars orbit.
  • April 18, 1991: AC-70. Centaur failed to restart (icing problem). Incomplete failure investigation initially stated that 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: AC-71. Centaur failed to restart (same icing problem as the prior incident).
  • April 30, 1999: Launch of the USA-143 (Milstar 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&nbsp;— a pair of National Reconnaissance Office ocean surveillance satellites&nbsp;— 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. and the next flight was nominal.
  • March 23–25, 2018: Atlas V Centaur passivated second stage launched on September 8, 2009, broke up.
  • August 30, 2018: Atlas V Centaur passivated second stage launched on September 17, 2014, broke up, creating space debris.
  • April 6, 2019: Atlas V Centaur passivated second stage launched on October 17, 2018, broke up.
  • September 6, 2024: Atlas V Centaur passivated second stage launched on March 1, 2018, broke up.

References