Centaur (rocket stage)

Atlas-Centaur

An Atlas-Centaur rocket launches Surveyor 1

The Centaur was originally designed for use with the Atlas launch vehicle family, which shared its balloon structure. Known in early planning as the "high-energy upper stage", its eventual name was proposed by Ehricke, who was directing its development for General Dynamics. In recognition of the mythological half-man-half-horse Centaur, the horse portion represented the "workhorse" Atlas as the "brawn" of the launch vehicle, while the man represented the "brain" of the combination in the Centaur.^[5]

Centaur was considered essential for both launching the Surveyor probes and proving the viability of LH2 as a high energy fuel. Both were important to the Apollo program. The Surveyor probes were tasked with studying the lunar regolith and confirming the feasibility of crewed landings. LH2 had been selected as the ideal propellant for the Saturn I, IB, and V upper stages.^[3]:29

Initial Atlas-Centaur launches used developmental versions, labeled Centaur-A through -C. The first launch on May 8, 1962, ended in an explosion 54 seconds after launch when insulation panels on the Centaur failed and caused the LH2 tank to rupture. After extensive redesigns, the next test on November 26, 1963, was successful.

On May 30, 1966, an Atlas-Centaur boosted the first Surveyor lander towards the Moon. The soft landing of Surveyor 1 in the Ocean of Storms was NASA's first landing on any extraterrestrial body.^[3]:92 This was followed by six more Surveyor missions over the next two years, with the Atlas-Centaur always performing as expected. These missions demonstrated the feasibility of reigniting a hydrogen engine in space, a capability vital to Apollo, 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. In addition, it replaced the Atlas-Agena vehicle for NASA planetary probe missions. The U.S. Department of Defense meanwhile preferred to use the Titan booster family for its heavy lift needs.

By the end of 1989, the Centaur-D had been used as the upper stage for 63 Atlas rocket launches, 55 of which were successful.^[2]

Titan III-Centaur

A Titan IIIE-Centaur rocket launches Voyager 2

The Centaur stage was mated with the far more powerful Titan III booster in 1974, producing the Titan IIIE or Titan III-Centaur, with more than triple the payload capacity of Atlas-Centaur. This version of Centaur featured improved thermal insulation, allowing it to coast for five hours in orbit, up from the Atlas-Centaur's 30 minute maximum.^[3]:143

The first launch of Titan-Centaur in February 1974 was unsuccessful, with Centaur's engines failing to ignite after separation from the Titan booster. Without power, the Centaur was ordered to self-destruct by a range safety command. The original plan was to carry a simulated mockup of the $1 billion Viking probe to be launched the following year. The Space Plasma High Voltage Experiment (SPHINX), which would have studied the interaction between spacecraft and high energy plasma, was instead added as a secondary payload and was destroyed. It was eventually determined that Centaur's engines had ingested an incorrectly installed clip from the oxygen tank.^{[3]:145–146}

The next Titan-Centaur flew in December 1974 and carried the joint West German-American Helios 1 probe to study the sun at close range. The West Germans were concerned that NASA was using the Helios launch as a second test flight of Titan/Centaur in preparation for the upcoming Viking missions. This concern gained believability when the Helios launch used an unnecessary two-burn profile, presumably a rehearsal for the Viking launch that needed the two burns. Centaur completed a further two burns after separation, proving the stage's in-space multi-restart capability.^{[3]:149–150}

In 1975, Titan-Centaur launched the Viking 1 and Viking 2 spacecraft to Mars. The original plan was to launch them on Saturn V rockets^[3]:154 as the Vikings were the most massive interplanetary missions to date, with each including an orbiter and a lander. These missions were highly successful, with the Viking 1 lander operating until 1982, and were the only NASA missions to study Mars until the Mars Global Surveyor was launched in 1996.

These launches were followed by the 1976 launch of Helios 2, another West German-U.S. solar probe that flew closer to the Sun than Helios 1. Helios 2 still holds the record for the highest speed of any spacecraft, with a heliocentric velocity of 70 kilometres per second (43 mi/s; 250,000 km/h; 160,000 mph) at its closest approach to the Sun.^[6]

The Titan-Centaur's next two launches were the Voyager 1 and Voyager 2 spacecraft. Voyager 1 visited Jupiter and Saturn. Voyager 2 completed the "Grand Tour" of the outer planets by visiting Jupiter, Saturn, Uranus, and Neptune. A once-in-175-years alignment of those planets allowed gravitational assists to boost the probe from one planet to the next. Voyager 2 was launched on August 20, 1977, followed 16 days later by Voyager 1. Voyager 2 is still the only spacecraft to have visited Uranus and Neptune. Voyager 1 is the first spacecraft to have entered interstellar space. While the Titan-Centaur that launched Voyager 2 performed flawlessly, the Titan booster used to launch Voyager 1 had a hardware problem that caused a premature shutdown. The Centaur stage detected and successfully compensated for this problem, although Centaur ended its burn with less than 4 seconds of fuel remaining.^[3]:160 This was the final launch of the Titan IIIE-Centaur.

Shuttle-Centaur

Illustration of Shuttle-Centaur with Ulysses

With the introduction of the Space Shuttle, NASA and the Air Force needed an upper stage to boost shuttle payloads out of low Earth orbit. The Centaur-G was developed for this purpose, with both Challenger and Discovery being modified to carry the stage. To enable its installation in shuttle payload bays, the diameter of the Centaur-G's hydrogen tank was increased to 14 feet (4.3 m). The diameter of its oxygen tank, however, remained at 10 feet (3.0 m). During its first mission on May 16, 1986, a Centaur-G was to have boosted the Galileo probe towards Jupiter. Just six days later, another Centaur-G would have boosted the Ulysses probe towards Jupiter where it eventually - with an IUS - used the planet's gravity to reach a highly inclined solar orbit to observe the Sun's polar regions. A shortened version of the Centaur-G was planned to be used on shuttles carrying payloads for the U.S. Department of Defense and for boosting the Magellan probe to Venus.^[7]

During the development of the shuttle in the 1970s, NASA debated whether to use a solid-fueled Inertial Upper Stage (IUS) instead of a Centaur. An IUS was much lighter and safer than a Centaur. NASA was concerned, however, that the rough starting and extremely fast acceleration of a solid rocket motor could damage payloads. Unlike a Centaur, an IUS could not be turned off once ignited. In addition, an IUS would have a lower thrust than a Centaur, which meant that a shuttle would have to carry less complex payloads. However, the presence of several tons of volatile liquid hydrogen and oxygen on a shuttle was concerning, especially because no Centaur had been designed as a human-rated vehicle and lacked the safety features of both a shuttle and a Saturn upper stage. NASA was especially worried about a shuttle having to make an emergency landing with a fully fueled Centaur in its payload bay. The Centaur's weight would exceed the design capacity of the shuttle's landing gear, and there was no safe and reliable method for dumping the Centaur's fuel before the shuttle landed. In its favor, the Centaur had an excellent reliability record in recent years. As of April 1981, when the first shuttle flight occurred, a Centaur had failed to operate only twice in the 35 launches during the previous decade. Still, several astronauts were wary about flying a shuttle with a Centaur in its payload bay, and a few flat-out refused to do it. Ultimately, NASA decided that shuttles could carry Centaurs. Their superior performance and smoother engine starts versus an IUS were too important to disregard. Additionally, the Air Force needed a Centaur's extra power to launch its satellites during classified military shuttle missions. Another important factor was that the Titan-Centaur launch vehicle was the only alternative for planetary missions. The problem was that the Air Force controlled those vehicles, and two decades of those entities sharing a launch vehicle had proven that bad blood always resulted.

A Centaur, when carried in the shuttle payload bay, needs a complex airborne support system, known as the Centaur Integrated Support System (CISS). The CISS controls Centaur pressurization in flight and enables Centaur's cryogenic propellants to be dumped overboard quickly in the event of an abort. Shuttle-Centaur flights would have needed the shuttle's main engines to run at 109 percent instead of the typical 104 percent, and the Shuttle would have had to orbit at its lowest possible altitude.^{[3]:208–209}

After the Challenger accident, and just months before the Shuttle-Centaur was scheduled to fly, NASA concluded that it was far too risky to fly the Centaur on the Shuttle.^[8] Galileo, Ulysses, and Magellan were boosted by the much less powerful solid-fueled IUS, with Galileo needing multiple gravitational assists from Venus and Earth to reach Jupiter.

Titan IV-Centaur

A Titan-Centaur rocket launches Cassini-Huygens

Termination of the Shuttle-Centaur program spurred the Air Force to create the Titan IV. In the 401A/B versions, the Centaur-T was used as its final stage. The diameter of its hydrogen tank was 14 feet (4.3 m). This vehicle was capable of launching the same payloads as the Shuttle-Centaur combination. In the Titan 401A version, a Centaur-T was launched nine times between 1994 and 1998. Titan-Centaur launched the Cassini-Huygens probe to Saturn in 1997 on the debut flight of the Titan 401B, which launched an additional six times, with one failure. The last flight of the Titan IV/Centaur was in 2003^[9]

Atlas III

Both versions of Atlas III used Centaur variants. Atlas IIIA used the Centaur II upper stage, developed for the Atlas II series. Atlas IIIB used a new version, Common Centaur.^[10]

Atlas V

The Atlas V rocket currently uses the Common Centaur variant.^[10] In 2014, on the NROL-35 mission, Atlas V's Common Centaur first flew in a reengined configuration with an RL10-C-1 replacing its previous RL10-A-4-2. This engine is meant to be common between Centaur and the Delta Cryogenic Second Stage to reduce costs.^[11][12] RL10-A-4-2 will continue to be used on some future flights. Atlas V launches using the Dual Engine Centaur configuration must use RL10-A-4-2 because the new engine is too wide to accommodate two side-by-side.^[12] To date, all Atlas V launches have used the Single Engine Centaur variant, however CST-100 Starliner and Dream Chaser missions will require the dual engine variant, because it allows a "flatter" trajectory which is safer for aborts.

As on Titan-Centaur, Atlas V 500 launches encapsulate the upper stage inside the payload fairing, to reduce aerodynamic loads. Atlas V 400 flights carry the fairing on top of Centaur, exposing it to the air.

Vulcan-Centaur

The new Vulcan launch vehicle currently being developed by United Launch Alliance will initially fly with an upgraded variant of the Common Centaur stage flown on Atlas V known as Centaur V^[13], before later upgrading to the new "Advanced Cryogenic Evolved Stage", which will include the Integrated Vehicle Fluids technology to allow long on-orbit life of the upper stage measured in weeks rather than hours.^[14][15][16] When the new ULA successor to the Atlas V was initially announced in 2015, it was slated to initially use a Centaur upper stage, but no mention was made at that time about an upgrade to a newer version.^[14]

Centaur-D

Centaur-D was the first Centaur version to enter operational service.^[19]

Centaur II

Centaur II was initially developed for use on the Atlas II series of rockets. It was also used on the Atlas IIIA.^[19][10]

Centaur V

In late 2017, ULA decided to bring elements of the ACES upper stage forward and begin work on Centaur V. This upper stage would be expanded to the same 5.4-meter (18 ft)-diameter of Vulcan core, using a cluster of four LH2/LOX engines (BE-3U or RL-10), but not include the Integrated Vehicle Fluids (IVF) feature expected with ACES. Bringing critical items from ACES to Centaur V, at this time, is expected to increase the lift capacity of the first generation Vulcan, so it can carry planned national security payloads. Centaur V would permit ULA to retire both the Atlas V and Delta IV rocket families on a faster schedule.

By March 2018, ULA had begun to publicly refer to the new Vulcan first stage with the Centaur V second stage as the Vulcan Centaur.^[20][21]

In May 2018 United Launch Alliance (ULA) announced that Aerojet Rocketdyne RL10 upper stage engine was selected for ULA’s next-generation Vulcan Centaur rocket following a competitive procurement process.^[22]

Test bed for cryogenic fluid management experiments

By 2006, Lockheed Martin Space Systems had described the ability to use existing Centaur hardware, with little modification, as a test bed for in-space cryogenic fluid management techniques.^[30] Most Centaurs launched on Atlas have excess propellants, ranging from hundreds to thousands of pounds, which could be used for "rideshare" experiments flown as secondary payloads conducted after separation of the primary spacecraft.

In October 2009, the Air Force and United Launch Alliance (ULA) performed an experimental on-orbit demonstration on a modified Centaur upper stage on the DMSP-18 launch to improve "understanding of propellant settling and slosh, pressure control, RL10 chilldown and RL10 two-phase shutdown operations. "The light weight of DMSP-18 allowed 12,000 pounds (5,400 kg) of remaining LO₂ and LH₂ propellant, 28% of Centaur’s capacity," for the on-orbit demonstrations. The post-spacecraft mission extension ran 2.4 hours before executing the deorbit burn.^[31] The initial mission demonstration in 2009 was preparatory to the more-advanced cryogenic fluid management experiments planned for the Centaur-based CRYOTE technology development program in 2012–2014^[32] and to a higher-TRL design for the Advanced Common Evolved Stage Centaur successor.^[24]