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 United Launch Alliance in two main versions. The diameter Centaur III 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 and liquid oxygen —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 LH2 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 LH2 and the comparatively warmer LOX.
The main propulsion system consists of one or two RL10 engines made by Aerojet Rocketdyne. The stage is capable of multiple restarts, constrained by propellant supply, orbital lifetime, and mission requirements. In combination with insulation on the propellant tanks, this enables Centaur to perform multi-hour coast phases and multiple engine burns for complex orbital insertions.
The stage is equipped with a reaction control system, which also provides ullage.
On the Centaur III the RCS system consists of twenty hydrazine monopropellant thrusters, arranged in two two-thruster pods and four four-thruster pods. Approximately of hydrazine is stored in two bladder tanks and fed to the thrusters by pressurized helium, which also supports some main engine functions.
On some Centaur V stages, the hydrazine system is replaced with hydrolox thrusters supplied by gaseous propellants from the main tanks.
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
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 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 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 with one RL10. This is the variant for all normal flights of the Atlas V.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 Vulcan launch vehicle developed by United Launch Alliance beginning in 2014 to meet the requirements of the National Security Space Launch 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 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 system, which had been intended to extend on-orbit lifetime from hours to weeks, was omitted.
Centaur V was designed to provide higher performance than the Common Centaur, fulfilling NSSL requirements and supporting the planned retirement of the Atlas V and Delta IV Heavy families. The stage was officially named Vulcan Centaur in March 2018, 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.
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. Unlike CV-HE, which uses a hydrolox RCS, CV-L returns to using a simpler hydrazine monopropellant RCS.ACES revival
On August 28, 2025, in an infographic by ULA posted by Tory Bruno, a variant of Centaur V was referred as "ACES", this time standing for "Advanced Centaur Endurance Stage". Few details were provided about this updated ACES concept, other than a mention of "Smart Propulsion", which was not further explained. Previously, Bruno has suggested that future upper stages could offer up to 600% more endurance than the Common Centaur.Current engines
Centaur engines have evolved over time, and three versions are in use as of 2024. All versions utilize liquid hydrogen and liquid oxygen.| Engine | Upper Stage | Dry mass | Thrust | Specific impulse, | Length | Diameter | Ref |
| Centaur III | |||||||
| Centaur III | |||||||
| Centaur V | |||||||
| Centaur V |
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, 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 LH2 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 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 vehicle for NASA planetary probes.
An updated version, called Centaur-D1A, 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.