.jpg) A single-engine Centaur III being raised for mating to anAtlas V rocket | |
| Manufacturer | United Launch Alliance |
|---|---|
| Used on |
|
| Associated stages | |
| Derivatives | Advanced Cryogenic Evolved Stage (cancelled, not flown) |
| Launch history | |
| Status | Active |
| Total launches | 273 as of October 2024[update][1] |
| Successes (stage only) | 254 |
| Failed | 15 |
| Lower stage failed | 4 |
| First flight | May 9, 1962; 63 years ago (1962-05-09) |
| Centaur III | |
| Height | 12.68 m (499 in)[2] |
| Diameter | 3.05 m (120 in) |
| Empty mass | 2,247 kg (4,954 lb), single engine 2,462 kg (5,428 lb), dual engine |
| Propellant mass | 20,830 kg (45,920 lb) |
| Powered by | 1 ×RL10A,2 × RL10A or1 ×RL10C |
| Maximum thrust | 99.2 kN (22,300 lbf), per engine |
| Specific impulse | 450.5 seconds (4.418 km/s) |
| Burn time | 904 seconds |
| Propellant | LOX /LH2 |
| Centaur V | |
| Height | |
| Diameter | 5.4 m (17.7 ft) |
| Empty mass | CV-HE: 7,100 kg (15,700 lb)[4] |
| Gross mass | CV-HE: 53,600 kg (118,200 lb)[4] |
| Powered by | |
| Maximum thrust |
|
| Specific impulse |
|
| Burn time | CV-HE: 1,077 seconds[8] |
| Propellant | LOX /LH2 |
TheCentaur is a family of rocket-propelled upper stages that has been in use since 1962. It is currently produced byUnited Launch Alliance (ULA) in two main versions. The 3.05 m (10 ft) diameterCentaur III (also known as the Common Centaur) serves as the second stage of the retiringAtlas V rocket, and the 5.4 m (17.7 ft) diameterCentaur V is used as the second stage of theVulcan Centaur rocket.[9][10] Centaur was the first rocket stage to usehydrolox propellant—liquid hydrogen (LH2) andliquid oxygen (LOX)—a high-energy combination well suited for upper stages but difficult to handle because both propellants must be stored at extremely lowcryogenic temperatures.[11]
Centaur stages are built aroundstainless steel pressure-stabilizedballoon propellant tanks[12] with 0.51 mm (0.020 in) thick walls. It can lift payloads of up to 19,000 kg (42,000 lb).[13] The thin tank walls minimize mass, maximizing overall stage performance.[14]
A commonbulkhead 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.[15]: 19
The main propulsion system consists of one or twoRL10 engines made byAerojet Rocketdyne.[12] 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.[13]
The stage is equipped with a reaction control system (RCS), which also providesullage.
On the Centaur III the RCS system consists of twentyhydrazinemonopropellant thrusters, arranged in two two-thruster pods and four four-thruster pods. Approximately 150 kg (340 lb) of hydrazine is stored in two bladder tanks and fed to the thrusters by pressurizedhelium, which also supports some main engine functions.[16]
On some Centaur V stages, the hydrazine system is replaced with hydrolox thrusters supplied by gaseous propellants from the main tanks.[17][18]
As of 2025[update], two Centaur variants are in use: Centaur III on Atlas V,[19][20] and Centaur V on Vulcan Centaur.[21] All of the many other Centaur variants have been retired.[22][23]
Common Centaur is the upper stage of theAtlas V rocket.[16] Earlier Common Centaurs were propelled by the RL10-A-4-2 version of the RL-10. Since 2014, Common Centaur has flown with theRL10-C-1 engine, which is shared with theDelta Cryogenic Second Stage, to reduce costs.[24][25] The Dual Engine Centaur (DEC) configuration will continue to use the smaller RL10-A-4-2 to accommodate two engines in the available space.[25]
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 Vpayload 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.[26]
The latest Common Centaurs can accommodate secondary payloads using an Aft Bulkhead Carrier attached to the engine end of the stage.[27]
Most payloads launch on Single Engine Centaur (SEC) with oneRL10. 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).
A dual engine variant with two RL-10 engines is available, but only for launching theCST-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 alaunch abort and ballistic reentry occurring at any point in the flight.[28]
_(cropped).jpg)
Centaur V is the upper stage of theVulcan launch vehicle developed by United Launch Alliance (ULA) beginning in 2014 to meet the requirements of theNational Security Space Launch (NSSL) program.[29]
ULA initially intended the Centaur V, an upgraded version of the Common Centaur,[30] to only be used on an interim basis until a transition to theAdvanced Cryogenic Evolved Stage (ACES) planned after the first few years of flights.[23][31]
In late 2017, the company began development of Centaur V by accelerating elements of the ACES design, including a 5.4 meters (17.7 ft) diameter and advanced insulation. TheIntegrated Vehicle Fluids (IVF) system, which had been intended to extend on-orbit lifetime from hours to weeks, was omitted.[23]
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,[32][33] and in May 2018 ULA selected Aerojet Rocketdyne’s RL10 engine overBlue Origin'sBE-3. Each Centaur V uses two RL10 engines.[34]
In September 2020, ULA confirmed that ACES would no longer be developed and that Centaur V would remain Vulcan’s upper stage.[35] The company said that the initial versions of the Centaur V offers 40% more endurance and 250% more energy than the Common Centaur.[36]
Vulcan launched successfully on January 8, 2024, with Centaur V performing as planned on its maiden flight.[37]
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.[6][38]
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.[39] CV-L is 1.94 m (6 ft 4 in) 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.[17]
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.[17] Previously, Bruno has suggested that future upper stages could offer up to 600% more endurance than the Common Centaur.[36]
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.[40]
| Engine | Upper Stage | Dry mass | Thrust | Specific impulse,vac. | Length | Diameter | Ref |
|---|---|---|---|---|---|---|---|
| RL10A-4-2 | Centaur III | 168 kg (370 lb) | 99.1 kN (22,300 lbf) | 451 s (4.42 km/s) | 2.29 m (7 ft 6 in) | 1.17 m (3 ft 10 in) | [41][42] |
| RL10C-1 | Centaur III (SEC) | 190 kg (420 lb) | 101.8 kN (22,900 lbf) | 449.7 s (4.410 km/s) | 2.12 m (6 ft 11 in) | 1.45 m (4 ft 9 in) | [43][44][45][42] |
| RL10C-1-1 | Centaur V | 188 kg (414 lb) | 106 kN (24,000 lbf) | 453.8 s (4.450 km/s) | 2.46 m (8 ft 1 in) | 1.57 m (5 ft 2 in) | [46] |
| RL10E-1 | Centaur V | 230 kg (510 lb) | 107.3 kN (24,120 lbf) | 460.9 s (4.520 km/s) | 3.312 m (10 ft 10.4 in) | 1.872 m (6 ft 1.7 in) |
The Centaur concept originated in 1956 when theConvair division ofGeneral Dynamics began studying a liquid hydrogen fueled upper stage. The ensuing project began in 1958 as a joint venture among Convair, theAdvanced Research Projects Agency (ARPA), and theU.S. Air Force. In 1959,NASA assumed ARPA's role. Centaur initially flew as the upper stage of theAtlas-Centaur launch vehicle, encountering a number of early developmental issues due to the pioneering nature of the effort and the use of liquid hydrogen.[48] In 1994 General Dynamics sold their Space Systems division toLockheed-Martin.[49]
The Centaur was originally developed for use with theAtlas launch vehicle family. Known in early planning as the 'high-energy upper stage', the choice of themythological Centaur as a namesake was intended to represent the combination of the brute force of the Atlas booster and finesse of the upper stage.[50]
InitialAtlas-Centaur launches used developmental versions, labeled Centaur-A through -C.
The onlyCentaur-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 twoRL10A-1 engines.[51]
After extensive redesigns, the onlyCentaur-B flight on 26 November 1963 was successful. This version was powered by two RL10A-3 engines.[51]
Centaur-C flew three times between 1964 and 1965,[51] with two failures and one launch declared successful although the Centaur failed to restart. This version was also powered by two RL10A-3 engines.[51]
Centaur-D was the first version to enter operational service in 1965 ,[51] with fifty-six launches.[52] It was powered by two RL10A-3-1 or RL10A-3-3 engines.[51]
On 30 May 1966, anAtlas-Centaur boosted the firstSurveyor 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.[15]: 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 theAtlas-Agena vehicle for NASA planetary probes.[15]: 103–166
An updated version, calledCentaur-D1A (powered by RL10A-3-3 engines), was used on theAtlas-SLV3D came into use during the 1970s.[53][54][51]
TheCentaur-D1AR was used for theAtlas-SLV3D andAtlas G came into use during the 1970s and 1980s.[55][51][56]
By the end of 1989, Centaur-D had been used as the upper stage for 63 Atlas rocket launches, 55 of which were successful.[1]
TheSaturn I was designed to fly with a S-V third stage to enable payloads to go beyondlow Earth orbit (LEO). The S-V stage was intended to be powered by twoRL-10A-1 engines burningliquid hydrogen as fuel andliquid oxygen as oxidizer. The S-V stage was flown four times on missionsSA-1 throughSA-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.
TheCentaur D-1T (powered by RL10A-3-3 engines) was an improved version for use on the far more powerfulTitan III booster in the 1970s,[51] with the first launch of the resultingTitan 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.[15]: 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 theViking probe. It was eventually determined that Centaur's engines had ingested an incorrectly installed clip from the oxygen tank.[15]: 145–146
The next Titan-Centaurs launchedHelios 1,Viking 1,Viking 2,Helios 2,[57]Voyager 1, andVoyager 2. The Titan booster used to launchVoyager 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.[15]: 160
TheCentaur D-1T had the following general specifications:[58]
Shuttle-Centaur was a proposedSpace 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 theGalileo andUlysses robotic probes, and Centaur G, a shortened version, reduced in length from approximately 9 to 6 m (30 to 20 ft), planned forU.S. DoD payloads and theMagellan Venus probe.[59]
After theSpace ShuttleChallenger 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.[60] The probes were launched with the much less powerful solid-fueledInertial Upper Stage, withGalileo needing multiple gravitational assists from Venus and Earth to reach Jupiter.
The capability gap left by the termination of the Shuttle-Centaur program was filled by a new launch vehicle, theTitan 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 1997Cassini-Huygens Saturn probe was the first flight of the Titan 401B, with an additional six launches wrapping up in 2003 including oneSRB failure.[61]
The upper stage of theAtlas I was theCentaur 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.[62] Centaur I was the last version of the stage to feature separating insulation panels.
Centaur II was initially developed for use on theAtlas II series of rockets.[52] Centaur II also flew on the initialAtlas IIIA launches.[16]
Atlas IIIB introduced the Common Centaur, a longer and initially dual engine Centaur II.[16]
Source: Atlas V551 specifications, as of 2015.[63]
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.[64]
In October 2009, theAir Force andUnited Launch Alliance (ULA) performed an experimental demonstration on the modified Centaur upper stage ofDMSP-18launch to improve "understanding ofpropellant settling andslosh, 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 LH2/LOX propellant remaining after separation. Severalon-orbit demonstrations were conducted over 2.4 hours, concluding with adeorbit burn.[65] The initial demonstration was intended to prepare for more-advanced cryogenic fluid management experiments planned under the Centaur-basedCRYOTE technology development program in 2012–2014,[66] and will increase theTRL of theAdvanced Cryogenic Evolved Stage Centaur successor.[22]
Although Centaur has a long and successful flight history, it has experienced a number of mishaps:
Centaur 3 (which flies on the Atlas V rocket) is 3.8 meters in diameter. The very first Centaur we fly on Vulcan will go straight to 5.4 meters in diameter.
because it proposed to make first use of the theoretically powerful but problem-making liquid hydrogen as fuel.
Mariner 8 launched in May but failed early in flight and ended its mission by splashing into the Atlantic Ocean.
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