The Centaur is a high-energy upper stage rocket developed by General Dynamics (now part of Lockheed Martin) that has been used on Atlas, Titan, and Delta launch vehicles since the 1960s. It was the first rocket stage to use liquid hydrogen and liquid oxygen propellants, allowing it to deliver payloads to higher orbits than other upper stages of its era.
The Centaur upper stage has been an integral part of many historic space missions over the past six decades, including the Surveyor moon landers, the Viking and Voyager missions to Mars and the outer planets, the Cassini mission to Saturn, and the New Horizons mission to Pluto. Its high performance and reliability have made it the upper stage of choice for launching many commercial and government satellites as well.
Key Characteristics
Some of the key features and specifications of the Centaur upper stage include:
- Propellants: Liquid hydrogen (LH2) fuel and liquid oxygen (LOX) oxidizer
- Engines: 1 or 2 RL10 engines built by Aerojet Rocketdyne
- Thrust: 99 kN (22,300 lbf) per RL10 engine
- Specific impulse: 449 seconds (4.40 km/s)
- Diameter: 3.05 m (10.0 ft)
- Length: Up to 12.68 m (41.6 ft) depending on variant
- Dry mass: Approximately 2,247 kg (4,954 lb)
- Propellant capacity: Up to 20,830 kg (45,922 lb) of LH2 and LOX
The exact dimensions and performance vary between the different Centaur variants used on Atlas V, Titan IV, and Delta IV rockets. The Centaur’s thin stainless steel tank walls and lack of internal insulation help maximize its propellant capacity and mass ratio.
Development History
Development of the Centaur began in the late 1950s as part of the US Air Force’s Advanced Research Projects Agency (ARPA) efforts to develop high-energy upper stages. General Dynamics was awarded the contract to develop a liquid hydrogen upper stage in 1958.
The first successful test flight of a Centaur occurred in November 1963 on an Atlas launch vehicle. However, early Centaur vehicles suffered a string of failures in 1964-1965 due to problems with propellant sloshing and resulting instability. To address this, propellant anti-slosh baffles were added to later Centaur tanks.
In the late 1960s, the Centaur was selected as the upper stage for the ambitious Surveyor lunar lander missions. Seven Surveyor spacecraft launched on Atlas-Centaur rockets between 1966-1968, with five successfully soft-landing on the Moon and paving the way for the Apollo landings.
The Centaur went on to launch the two Viking missions to Mars in 1975 and the two Voyager missions on grand tours of the outer solar system in 1977. An upgraded Centaur-G version with a widened 4.3 meter diameter tank debuted in the 1980s to launch heavier payloads on Shuttle and Titan rockets.
In the 1990s and 2000s, the Centaur continued to evolve, with the Centaur III flying on the Atlas III rocket and the Centaur IV variant used on the Atlas V. Reliability improved significantly, with the Centaur achieving a 97% success rate since 2000.
Advantages of Hydrogen Propulsion
The Centaur’s liquid hydrogen and liquid oxygen propulsion system offers significant advantages compared to other propellant combinations:
- High specific impulse: Hydrogen/oxygen combustion produces an exhaust velocity about 30-40% higher than traditional hypergolic or RP-1/LOX upper stages. This allows the Centaur to deliver payloads to higher energy orbits.
- Clean exhaust: LH2/LOX engines produce only water vapor exhaust, avoiding issues with contaminating sensitive satellite payloads that can occur with solid rocket or hypergolic upper stages.
- Efficient packaging: The low density of liquid hydrogen allows larger propellant tanks to be used while still fitting within payload fairings. The Centaur’s balloon-like tanks maximize propellant capacity.
However, the extremely low temperature of liquid hydrogen presents challenges in thermal insulation and propellant management. The Centaur’s stainless steel skin acts as insulation, with any heat entering the tank causing a small amount of hydrogen to gasify and pressurize the tank. Propellant anti-slosh baffles are also required to prevent destabilizing propellant motion.
Notable Missions
Over its long history, the Centaur has launched many groundbreaking space exploration missions and valuable commercial and government satellites. Some highlights include:
- Surveyor lunar landers (1966-1968): Demonstrated soft landing capability needed for Apollo, launched on Atlas-Centaur
- Pioneer 10 & 11 to Jupiter and Saturn (1972, 1973): First missions to the outer planets
- Viking 1 & 2 Mars landers (1975): Performed first successful landings on Mars
- Voyager 1 & 2 ‘Grand Tour’ missions (1977): Visited all four giant planets and continue exploring interstellar space
- Cassini mission to Saturn (1997): Orbited and studied the Saturnian system for over 13 years
- New Horizons mission to Pluto (2006): Performed first flyby of Pluto and is continuing to explore the Kuiper Belt
- Juno mission to Jupiter (2011): Studying Jupiter’s composition, gravity field, magnetic field, and polar magnetosphere
- Hundreds of commercial communications satellites, NASA science missions, and classified military payloads
The high performance of the Centaur upper stage was crucial to enabling these missions to reach their distant destinations.
Centaur Variants and Launch Vehicles
Several different variants of the Centaur have been developed over the years to fly on different launch vehicles:
- Atlas-Centaur: The original Centaur configuration, used on Atlas rockets from the 1960s-1980s. Launched Surveyor, Pioneer, Viking, Voyager.
- Titan IIIE-Centaur: Used on Titan rockets in the 1970s, with a larger 4.3 m diameter tank. Launched Helios, Viking, Voyager.
- Shuttle-Centaur: Designed to launch from the Space Shuttle cargo bay, but cancelled after the Challenger accident. Would have launched Galileo and Ulysses missions.
- Titan IV-Centaur: Used on Titan IV rockets in the 1990s/2000s, with the widened Centaur-G and Centaur-T variants. Launched Cassini.
- Atlas IIAS-Centaur: Improved Atlas-Centaur used in the 1990s, with a stretched Centaur stage. Launched SOHO.
- Atlas III-Centaur: Transitional Atlas version in the early 2000s, used the new Centaur III upper stage. Launched Mars Odyssey.
- Atlas V-Centaur: Current Atlas version, with the Centaur IV or Centaur V upper stage. Has launched New Horizons, Juno, OSIRIS-REx, and many others.
- Delta IV-Centaur: A proposed Centaur variant for Delta IV rockets in the early 2000s, but never flown. Delta IV continues to use the Delta Cryogenic Second Stage (DCSS) instead.
The Centaur’s flexibility in integrating with different launch vehicles has been key to its longevity. As of 2024, the Centaur V continues to fly regularly as the upper stage for Atlas V rockets.
Future of the Centaur
As the Atlas V rocket approaches retirement in the mid-2020s, the long-term future of the Centaur upper stage is uncertain. ULA’s next-generation Vulcan rocket will use the new Advanced Cryogenic Evolved Stage (ACES) hydrogen upper stage instead of Centaur.
However, Centaur hardware and design concepts will likely live on as a point of reference for future upper stages. Its 60-year service record is a testament to the advantages of high-energy hydrogen propulsion and the robust engineering behind the Centaur stage.
Some key lessons and legacies from the Centaur’s development include:
- Using balloon tanks and thin stainless steel walls to maximize propellant capacity
- Managing cryogenic propellants and propellant slosh
- Integrating high-performance liquid hydrogen engines like the RL10
- Demonstrating the value of high-energy upper stages for ambitious planetary exploration missions
As the Centaur eventually retires, its contributions to spaceflight will be remembered as future vehicles build upon its long legacy of enabling groundbreaking missions across the solar system.
