Centaur (rocket stage)

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 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 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.

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 took place on November 26, 1963 and 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.

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 Department of Defense meanwhile preferred to use the Titan booster family for its heavy lift needs.

Through 1989, the Centaur-D was used as the upper stage for 63 Atlas rocket launches, 55 of which were successful.

Titan III-Centaur

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. 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 operating 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.

During the developmental phase of the Shuttle in the 1970s, NASA debated the use of the solid-fueled IUS or the Centaur. The IUS was much lighter-weight and safer than Centaur, which carried many serious safety risks. On the downside, there was concern that the rough starting and extremely fast acceleration of a solid rocket motor could damage the payload, and it could not be turned off once ignited, plus it would not have as much thrust as Centaur, which meant that less complex payloads could be carried. The idea of carrying several tons of volatile liquid hydrogen and oxygen onboard a Shuttle was not an appealing idea either, especially because Centaur had never been designed as a man-rated vehicle and lacked the extra safety features of the Shuttle or the Saturn upper stages. Especially concerning was what to do in the event of an emergency or an aborted Shuttle launch. If a Shuttle had to make an emergency landing, the weight of the Centaur would be more than the orbiter's landing gear was designed to handle, nor was there a reliable, safe way to dump its propellants overboard during an emergency landing. On the other hand, Centaur's reliability in recent years had been excellent. When the first Shuttle flight was made in April 1981, there had been just two Centaur failures in 35 launches over the past decade (discounting two Atlas-Centaurs had been destroyed early in flight before the Centaur got a chance to operate). Still, several astronauts were wary about flying with a Centaur in the Shuttle's payload bay, and a few flat-out refused to do it, never mind the fact that the Shuttle itself was powered by a far larger amount of liquid hydrogen.

In the end, NASA approved Shuttle-Centaur with some hesitation, as the greater performance and smoother engine start over the IUS was too tempting to resist and in addition, the Air Force had plans for classified military Shuttle missions launching satellites that would need Centaur's extra power. Also, the only other option for planetary missions was Titan-Centaur, which ran into the difficulty of that launch vehicle being controlled by the Air Force, and two decades of history had proven the bad blood that resulted when NASA and the Air Force had to share a launch vehicle.

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

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.

Atlas V

The Atlas V rocket currently uses the Common Centaur variant. 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. 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. 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 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 use a Centaur upper stage, before later upgrading to a new 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.

Design

Centaur uses so-called "balloon tanks", made of stainless steel so thin they cannot support their own weight without pressurization. This tank design, with walls as thin as 0.03 inches, allowed for an extremely high ratio of fuel to dry mass, maximizing the stage's performance. It 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.

Attitude control and ullage are provided by means of hydrazine monopropellant thrusters located around the stage. There are two 2-thruster pods and four 4-thruster pods, sixteen in total, fed from a pair of bladder tanks carrying 340 pounds of hydrazine. Tank pressurization, as well as some engine functions, use helium gas. The main propulsion system consists of one or two RL10 engines. These engines can be restarted multiple times, given sufficient power, helium, and ullage propellant, allowing Centaur to perform complex orbital insertions and deorbit burns.

Common Centaur, on Atlas V, can accommodate secondary payloads using its Aft Bulkhead Carrier, a mounting fixture on the rear end of the stage, near the engine.

Centaur-D

Centaur-D was the first Centaur version to enter operational service.

Centaur II

Centaur II was initially developed for use on the Atlas II series of rockets. It was also used on the Atlas IIIA.

Current status

As of 2009, derivatives of the 10-foot-diameter (3.0 m) 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 the CST-100 Starliner and 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:

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 — 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 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. 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.