# Centaur (rocket stage)

The Centaur is a family of rocket propelled upper stages that has been in use since 1962. It is currently produced by U.S. launch service provider United Launch Alliance, with one main active version and one version under development. The 3.05 m (10 ft) diameter Common Centaur/Centaur III flies as the upper stage of the Atlas V launch vehicle, and the 5.4 m (18 ft) diameter Centaur V has been developed as the upper stage of ULA's new Vulcan rocket. Centaur was the first rocket stage to use liquid hydrogen (LH$_2$) and liquid oxygen (LOX) propellants, 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 tanks with 0.51 mm (0.020 in) thick walls. It can lift payloads of up to 19,000 kg (42,000 lb). The thin walls minimize the mass of the tanks, maximizing the stage's overall performance.
A common bulkhead separates the LOX and LH$_2$ 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 LH$_2$ and less cold LOX.$^{:19}$
The main propulsion system consists of one or two Aerojet Rocketdyne 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 and consists of twenty hydrazine monopropellant thrusters located around the stage in two 2-thruster pods and four 4-thruster pods. For propellant, 150 kg (340 lb) of Hydrazine is stored in a pair of bladder tanks and fed to the RCS thrusters with pressurized helium gas, which is also used to accomplish some main engine functions.

## Current versions

As of 2024, 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.

### 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.
Centaur engines in use as of 2024 :
| Engine | Upper Stage | Dry mass | Thrust | I$_{sp}$, vac. | Length | Diameter |
| RL10A-4-2 | Centaur III | 168 kg (370 lb) | 99.1 kN (22,300 lbf) | 451 s | | 1.17 m (3.8 ft) |
| RL10C-1 | Centaur III (SEC) | 190 kg (420 lb) | 101.8 kN (22,900 lbf) | 449.7 s | 2.12 m (7.0 ft) | 1.45 m (4.8 ft) |
| RL10C-1-1 | Centaur V | 188 kg (414 lb) | 106 kN (24,000 lbf) | 453.8 s | 2.46 m (8.1 ft) | 1.57 m (5.2 ft) |

### 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 5.4 m (18 ft) 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 4 m (13 ft) 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 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

Centaur V is the upper stage of the new Vulcan launch vehicle developed by the United Launch Alliance to meet the needs of the National Security Space Launch (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 (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' 5.4 m (18 ft) diameter and advanced insulation, but does not include the Integrated Vehicle Fluids (IVF) feature expected to allow the extension of upper stage on-orbit life from hours to weeks. Centaur V uses 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 was intended to 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. 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.”
Vulcan finally launched on 8 January 2024 and the stage performed flawlessly on its maiden flight.
On 4 October 2024, in a pre-recorded message during the broadcast of the Vulcan Cert-2 mission, Upgrades Development Director Amanda Bacchetti had stated that ULA would be developing a "LEO Optimized Centaur" set to launch aboard a Vulcan launch vehicle sometime in 2025. She had stated that this variant of Centaur V would be shorter (and therefore more mass efficient for LEO orbits), however specifications for this variant were not given.

## History

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)

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$_2$ 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.
Centaur-C flew three times between 1964 and 1965, with two failures and one launch declared successful although the Centaur failed to restart. This version was also powered by two RL10A-3 engines.
Centaur-D was the first version to enter operational service in 1965 , with fifty-six launches. It was powered by two RL10A-3-1 or RL10A-3-3 engines.
On 30 May 1966, an Atlas-Centaur 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 LH$_2$ in space.$^{:96}$
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 vehicle for NASA planetary probes.$^{:103–166}$
An updated version, called Centaur-D1A (powered by RL10A-3-3 engines), was used on the Atlas-SLV3D came into use during the 1970s.
The Centaur-D1AR was used for the Atlas-SLV3D and Atlas G came into use during the 1970s and 1980s.
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)

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, with the first launch of the resulting Titan IIIE 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.$^{:143}$
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.$^{:145–146}$
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.$^{:160}$

#### Centaur D-1T specifications

The Centaur D-1T had the following general specifications:
* Diameter: 3.2 m (126 in)
* Length: 9.6 m (31.5 ft)
* Inert mass: 1,827 kg (4,028 lb)
* Fuel: Liquid hydrogen
* Oxidizer: Liquid oxygen
* Fuel and oxidizer mass: 13,490 kg (29,750 lb)
* Guidance:
* Thrust:
* Burn Capability: 3 to 4 burns
* Engine: 2 x RL10A-3-3
* Engine start: Restartable
* Attitude control: 4 x 27 N (6 lbf) thrusters

### Shuttle-Centaur (Centaur G and G-Prime)

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 4.3 m (14 ft), with the LOX tank diameter remaining at 3.0 m (10 ft). 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 9 to 6 m (30 to 20 ft), planned for U.S. DoD payloads and the Magellan 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 T (Titan IV)

The capability gap left by the termination of the Shuttle-Centaur program was filled by a new launch vehicle, the Titan IV. The 401A/B versions used a Centaur upper stage with a 4.3-meter (14 ft) 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 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)

Centaur II was initially developed for use on the Atlas II 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.

#### 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 x 27 N (6.1 lbf) thrusters, 8 x 40 N (9.0 lbf) 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% (5,400 kg (11,900 lb)) of LH$_2$/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.

## 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.
* August 10, 1968: AC-17. Centaur did not restart after coast — 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 — 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.
* 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.