Centaur (rocket stage)

Centaur is a rocket stage designed for use as the upper stage of space launch vehicles and is currently used on the Atlas V. Centaur boosts its satellite payload to geosynchronous orbit or, in the case of an interplanetary space probe, to or near to escape velocity. Centaur was the world's first high-energy upper stage, burning liquid hydrogen (LH2) and liquid oxygen (LOX), and has enabled the launch of some of NASA's most important scientific missions over its 50-year history.

Centaur was the brainchild of Karel J. "Charlie" Bossart (the man behind the Atlas ICBM) and Dr. Krafft A. Ehricke, both Convair employees. Their design was essentially a smaller version of the Atlas, with its concept of using lightweight "stainless steel balloon" tanks whose structural rigidity was provided solely by the pressure of the propellants within. To keep the tanks from collapsing prior to propellant loading, they were either kept in "stretch" or pressurized with nitrogen gas.

Centaur is powered by one or two RL10 rocket engines (SEC and DEC variants respectively).

History
In 1956 Krafft Ehricke of Convair began to study a liquid hydrogen upper stage rocket. In 1958 the project started through a joint between Convair, Advanced Research Projects Agency (ARPA) and U.S. Air Force. In 1959 NASA assumed ARPA's role. Development started at NASA's Marshall Space Flight Center and then at Lewis Research Center, now the Glenn Research Center, but proceeded slowly, with the first (unsuccessful) test flight in May 1962. In the late 1950s and early 1960s Centaur was proposed as a high energy upper stage for the Saturn I, Saturn IB and Saturn V rockets, under the designation S-V (pronounced "ess five") in accordance with the numbering of other stages of Saturn rockets. However, Centaur never flew on any Saturn vehicle, though the Saturn I used a cluster of six RL10 engines on its second stage.

Atlas-Centaur
The first launch vehicle to carry Centaur was the Atlas, which featured a pressure-stabilized "balloon" tank from which the structure of Centaur would be based. Originally known as the "high-energy upper stage", its eventual name was proposed by Krafft Ehricke of General Dynamics, who also directed its development, in recognition of the mythological half-man-half-horse: 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.

Centaur was considered essential for the launch of the Surveyor probes, as well as proving the viability of liquid hydrogen as a high energy fuel. Both were important to the Apollo program—the Surveyor probes to study the lunar regolith and confirm that crewed landings would be possible, while liquid hydrogen had been selected as the ideal propellant for the Saturn I, IB, and Saturn V upper stages.

The first Atlas-Centaur launch on May 8, 1962 ended in failure when the insulation protecting the Centaur stage sheared off in flight, allowing liquid hydrogen to vaporize. This caused the hydrogen tank to rupture 54 seconds into the flight, destroying the vehicle. Following extensive redesigns, the next launch on November 27, 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. This was followed by six more Surveyor missions over the next two years, four of which were successful, though Atlas-Centaur performed as expected for each launch. Further, these missions demonstrated the feasibility of reigniting a hydrogen engine in space, a capability vital to Apollo, and provided information on the behavior of liquid hydrogen in space.

From 1966 to 1989, the Centaur-D was used as the upper stage for 63 Atlas rocket launches. 55 of these launches were successful.

Titan III-Centaur
The Centaur stage was coupled 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. Centaur would also feature improved thermal insulation, allowing it to coast up to five hours in orbit, up from Atlas-Centaur's 30 minute maximum.

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. Originally planned to carry only a simulated mockup of the Viking probe to be launched the following year to test the vehicle's capabilities before launching the nearly $1 billion spacecraft, the Space Plasma High Voltage Experiment (SPHINX), intended to study the interaction between spacecraft and high energy plasma, was 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.

The next Titan-Centaur flew in December 1974 and carried the joint German-American Helios 1 probe to study the sun at close range. While there were concerns from the Germans that NASA was using the Helios launch as a further test flight of Titan/Centaur in preparation for the upcoming Viking missions, including using a two-burn profile (which would be required for Viking) when Helios required only one, this flight was successful. Centaur completed a further two burns after separation, proving the stage's in-space multi-restart capability.

In 1975, Titan-Centaur launched the Viking 1 and Viking 2 spacecraft to Mars. Originally planned to be launched on the Saturn V, the Vikings would be the most massive interplanetary missions to that time, with each spacecraft consisting of both an orbiter and a lander. These missions were highly successful, with the Viking 1 lander functioning until 1982, and would be the only NASA missions to study Mars for the next 20 years, until the Mars Global Surveyor was launched in 1996.

These launches were followed by the 1976 launch of Helios 2, another German solar probe, which approached the sun even more closely than Helios 1. Helios 2 still holds the record for the highest speed of any spacecraft, with a heliocentric velocity of 70 km/s at closest approach to the Sun.

The following two launches were the Voyager 1 and Voyager 2 spacecraft, bound for a "grand tour" of the outer solar system enabled by an alignment of the planets that allowed gravitational assists to boost the probes from one planet to the next. Voyager 2 was launched on August 20, 1977, followed 16 days later by Voyager 1. Voyager 2 is the only spacecraft to have visited Uranus and Neptune, while Voyager 1 was the first spacecraft to enter interstellar space. While the Titan-Centaur that launched Voyager 2 performed flawlessly, the Titan booster used to launch Voyager 1 burned out early due to a hardware problem, which the Centaur stage detected and successfully compensated for. Centaur ended its mission with less than 4 seconds of burn time remaining. This was the final launch of Titan IIIE-Centaur.

Shuttle-Centaur
With the introduction of the Space Shuttle, NASA and the Air Force needed an upper stage to boost payloads out of low Earth orbit. A new version of Centaur, the Centaur-G, was developed, with both Challenger and Discovery modified to carry the stage. Centaur-G was optimized for installation in the Orbiter payload bay by increasing the hydrogen tank diameter to 14 feet while retaining the 10-foot-diameter (3.0 m) oxygen tank. Its initial mission, scheduled for May 16, 1986, was to boost the Galileo probe to Jupiter, then, just six days later, the Ulysses probe. Ulysses would also be boosted to Jupiter in order to use the planet's gravity to reach a highly inclined solar orbit to allow observation of the Sun's polar regions. A shortened version of the Centaur-G was also planned for use on shuttle missions involving Department of Defense payloads and was to be used for launching the Magellan probe to Venus.

The Centaur, as carried in the Shuttle payload bay, required a complex airborne support system, the Centaur Integrated Support System (CISS). The CISS controlled Centaur pressurization in flight and enabled Centaur's cryogenic propellants to be dumped overboard quickly in the event of an abort. Shuttle-Centaur flights would have run the Shuttle's main engines at 109%, higher than the typical 104%, and the Shuttle would have had to orbit at its lowest possible altitude.

After the Challenger accident, just months before Shuttle-Centaur was scheduled to fly, NASA realized that it was far too risky to fly the Centaur on the Shuttle. Galileo, Ulysses, and Magellan would all eventually be boosted by the much less powerful solid-fueled Inertial Upper Stage, with Galileo requiring multiple gravitational assists from Venus and Earth to reach Jupiter.

Titan IV-Centaur
The decision to terminate the Shuttle-Centaur program spurred the United States Air Force to create the Titan IV, which, in its 401A/B versions, used the Centaur-T, also with a 14-foot-diameter (4.3 m) hydrogen tank, as its final stage. This vehicle was capable of launching payloads which had originally been designed for the Shuttle-Centaur combination. In the Titan 401A version, a Centaur-T was launched nine times between 1994 and 1998. Titan-Centaur would launch the Cassini-Huygens probe to Saturn in 1997 on the debut flight of the Titan 401B, which would launch an additional six times, with one failure. The last flight of the Titan IV/Centaur was in 2003

Vulcan-Centaur
The new launch vehicle being developed by United Launch Alliance—Vulcan—will initially use a Centaur upper stage, before later upgrading to a new ULA upper stage—the "Advanced Cryogenic Evolved Stage" which will include the Integrated Vehicle Fluids technology that could allow long on-orbit life of the upper stage measured in weeks rather than hours.

Double Bulkhead
Centaur uses a common double-bulkhead to separate the LOX and LH2 tanks. The two stainless steel skins are separated by a 0.25 inch (6.4 mm) layer of fiberglass honeycomb. The extreme cold of the LH2 on one side creates a vacuum within the fiberglass layer, decreasing the bulkhead's thermal conductivity, and thus reducing heat transfer from the relatively warm LOX to the super cold LH2.

Current status
, derivatives of the 10 ft Centaur-3, with one RL-10A4-2 engine, continue to be used as the upper stage of the Atlas V EELV rocket. There is an option to fly the Atlas V with a two engined Centaur, which is planned to be used for manned launches with Dream Chaser.

United Launch Alliance (ULA) had been working on an upper stage design concept that would bring the Delta and Centaur stages together into a single new cryogenic second stage design, called the Advanced Common Evolved Stage, was originally intended as a lower-cost, more-capable and more-flexible upper stage that would supplement, and perhaps replace, the existing ULA (Lockheed Martin legacy) Centaur and the ULA (Boeing legacy) Delta Cryogenic Second Stage (DCSS) upper stage vehicles. ACES design conceptualization has been underway at ULA for many years, and leverages design features of both the Centaur and Delta Cryogenic Second Stage (DCSS) upper stages. With the decision to discontinue both the Delta IV and Atlas V lines by the 2020s, ULA also abandoned work on replacing their upper stages. ACES will continue to be developed, and will be deployed on Vulcan.

Mishaps
Although Centaur has a long and successful history in planetary exploration, it has had its share of problems, especially early on:
 * May 8, 1962: Centaur weather shield separated early; stage exploded. Footage of this was used in the film Koyaanisqatsi.
 * June 30, 1964: RL-10 hydraulic actuator pump shaft broke, preventing one of the two RL-10 engines from vectoring.  This led to no roll control, and uncovering of LOX inlet.
 * December 11, 1964: Restart attempt failed, due to problem with ullage rockets.
 * April 7, 1966: Centaur did not restart after coast &mdash; ullage motors ran out of fuel.
 * August 10, 1968: Centaur restart failed.
 * May 9, 1971; Centaur guidance failed, destroying itself and the Mariner 8 spacecraft bound for Mars orbit.
 * Feb 11, 1974: Titan-Centaur; boost pump failed.
 * June 9, 1984: Centaur LOX tank failed; no restart.
 * April 18, 1991: Centaur failed due to icing of hydrogen pump impeller blades (not understood at the time)
 * August 22, 1992: Centaur failed to restart (icing problem again)
 * April 30, 1999: Launch of the USA-143 (Milstar-3) communications satellite failed when a Centaur database error resulted in uncontrolled roll rate and thus incorrectly directed and timed engine burns, 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 &mdash; a pair of National Reconnaissance Office ocean surveillance satellites &mdash; 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.

Future uses
Performance levels for a planned Evolved Centaur based Phase 1 vehicles envelope all Atlas V capabilities. In certain circumstances a single Atlas booster vehicle with five solids and with an evolved Centaur upper-stage can replace a three-booster core Atlas V-Heavy (HLV). This has obvious reliability and cost benefits. Phase 2 vehicles open the door to a vastly higher performance capability. Up to 80 metric tons can be lifted to low earth orbit on a Phase 2 HLV vehicle &mdash; a substantial fraction of a Saturn V or Ares V vehicle. This performance level, mandated only by NASA crewed exploration missions, can be achieved using hardware identical to that used for traditional commercial and USG missions thus allowing development and support costs to be diluted by rate.

Studies have been conducted showing the extensibility of the basic Centaur and Evolved Centaur designs to long duration space flight for exploration purposes and even for use as a Lunar Lander. Complementing these basic performance capabilities is the ability to rate the vehicle for crewed operation. Extensive work has been conducted showing that achieving this "man-rating" is straightforward and does not mandate wholesale design changes to the Centaur vehicle.

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. 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 12000 lbs 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. 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 and to a higher-TRL design for the Advanced Common Evolved Stage Centaur successor.

Specifications
Source: Atlas V551 Specifications.


 * 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 & oxidizer mass: 20,830 kg (45,922 lb)
 * Guidance: Inertial
 * Propulsion: 1 RL 10A-4-2
 * Thrust: 99.2 kN (22,300 lbf)
 * Engine length: 2.32 m (7.6 ft)
 * Engine diameter: 1.53 m (5 ft)
 * Engine dry weight: 168 kg (370 lb)
 * Burn time: Variable
 * Engine start: Restartable
 * Attitude control: 4 27-N thrusters, 8 40-N thrusters
 * Propellant: Hydrazine