An RS-68 engine undergoing hot-fire testing at NASA's Stennis Space Center during its developmental phase.) |
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| Country of origin | United States |
|---|---|
| Manufacturer | Pratt & Whitney Rocketdyne |
| Application | first stage |
| Liquid-fuel engine | |
| Propellant | LOX / LH2 |
| Cycle | Gas-generator cycle |
| Configuration | |
| Nozzle ratio | 21.5 |
| Performance | |
| Thrust (Vac.) | 758,000 lbf (3,370 kN) |
| Chamber pressure | 1,488 psia (9.7 MPa) |
| Isp (Vac.) | 410 s |
| Dimensions | |
| Length | 5.20 m |
| Diameter | 2.43 m |
| Dry weight | 14,560 lb (6,600 kg) |
| Used in | |
| Delta IV | |
The Pratt & Whitney Rocketdyne RS-68 (Rocket System 68) is a liquid-fuel rocket engine that burns liquid hydrogen (LH2) with liquid oxygen (LOX). It is the largest hydrogen-fueled engine in the world.[1] Development of the engine started in the 1990s with the goal of producing a simpler, less-costly, heavy-lift engine for the Delta IV launch system. The engine has three versions: the original RS-68, the improved RS-68A, and the RS-68B for NASA.
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The RS-68 was developed at Rocketdyne Propulsion and Power, located in Canoga Park, Los Angeles, California, to power the Delta IV Evolved Expendable Launch Vehicle (EELV). The combustion chamber burns liquid hydrogen and liquid oxygen at 1,486 lbf/in² (10.25 MPa) at 102% with a 1:6 engine mixture ratio.
At a maximum 102% thrust, the engine produces 758,000 pounds-force (3,370 kN) in a vacuum and 663,000 pounds-force (2,950 kN) at sea level. The engine's mass is 14,560 pounds (6,600 kg) at 96 inches (2.4 m). With this thrust, the engine has a thrust-to-weight ratio of 51.2, and a specific impulse of 410 s (4 kN·s/kg) in a vacuum and 365 s (3.58 kN·s/kg) at sea level.[2] The RS-68 is gimbaled hydraulically and is capable of throttling between 58% and 101% thrust.[3]
A leading goal of the RS-68 program was to produce a simple engine that would be cost-effective when jettisoned after a single launch. To achieve this, the RS-68 has 80% fewer parts than the multi-launch Space Shuttle Main Engine (SSME). Simplicity came at the cost of lower thrust-efficiency versus the SSME: the RS-68's thrust-to-weight ratio is significantly lower and the RS-68's specific impulse is 10% lower. The benefit of the RS-68 is its reduced construction cost: To build an RS-68 for the Boeing Delta IV program costs about $14 million, compared to $50 million for the SSME. While the SSME's higher costs were designed to be spread across multiple launches, the larger, less-costly, more powerful (50% more thrust) RS-68 was a more cost-effective engine for an expendable launch vehicle.
The engine itself is a gas generator cycle engine with two independent turbopumps. The combustion chamber uses a channel-wall design to reduce cost. This design, pioneered in the former Soviet Union, features inner and outer skins brazed to middle separators, forming cooling channels. This method is heavier, but much simpler and cheaper than the tube-wall design (using hundreds of tubes, bent into the shape of the combustion chamber and brazed together) used in other engines. The lower nozzle has an expansion ratio of 21.5 and is made from an ablative material. The nozzle's lining is designed to burn away as the engine runs, dissipating heat. This is also heavier than the tube-wall nozzles used in other engines, but is also much easier and cheaper to manufacture. The presence of a Carbonaceous fuel, or in this case an ablative coating utilizing a carbon containing material, can be inferred by the yellow color of the engine exhaust, this is in contrast to the engine exhaust of another LOX/LH2 engine, the RL-10.
The engine design was done at the Canoga Park, California facility, where the SSME is manufactured. The initial development engines were assembled at the nearby Santa Susana Field Laboratory where the Saturn V F-1 engines were developed and tested for the Apollo missions to the Moon. The RS-68 had initial testing done at Air Force Research Lab, Edwards AFB and later at NASA's John C. Stennis Space Center. The first successful test firing at AFRL was completed on September 11, 1998, and the first successful test firing at Stennis on September 22, 1999. The RS-68 was certified for use on Delta IV in December 2001.[4] The first successful launch using the new engine and launch vehicle occurred on November 20, 2002.
The RS-68 is part of the Common Booster Core (CBC) used to create the five variants of the Delta IV family of launch vehicles. The largest of the launch vehicles includes three CBCs mounted together for the Heavy.
On May 18, 2006, NASA announced that five RS-68 engines would be used instead of SSMEs on the planned Ares V (CaLV). NASA chose the RS-68 because of its lower cost, about $20 million per engine after NASA upgrades. The modifications to the RS-68 for the Ares V include a different ablative nozzle to accommodate a longer burn, a shorter start sequence, hardware changes to limit free hydrogen at ignition, and changes to reduce helium use during countdown and flight. Thrust and specific impulse increases will occur under a separate upgrade program for Delta IV.[5] Later the Ares V was changed to use six RS-68 engines. The engine version for NASA's Ares V is designated RS-68B.[6] The DIRECT alternative launch project included two or three RS-68 engines in "version 2.0" of the team's proposal, but switched to the Space Shuttle Main Engine (SSME) for "version 3.0".
On April 4, 2008, the Air Force awarded Boeing Launch Services of Huntington Beach, Calif., a modified contract for $20 million. This contract modification will authorize Boeing to perform demonstration testing on a rebuilt RS-68 engine. The government has authorized work under the Assured Access to Space initiative to develop hardware that will reduce or eliminate these risks and increase the reliability of the RS-68 engine.[7]
On September 25, 2008, the upgraded RS-68A successfully completed its first test firing. The RS-68A is an updated version of the RS-68, with changes to provide increased specific impulse and thrust (to over 700,000 pounds-force (3,100,000 N) at sea level). The RS-68A is planned to be certified in 2010, with initial launch capability in early 2011.[8] The first flight that will use RS-68A engines will use three of them. On March 8, 2011 Pratt & Whitney Rocketdyne announced that the Hardware Acceptance Review for the first of those three engines had been successfully completed. The remaining two engines for that flight are scheduled for Hardware Acceptance Review in March and April 2011, respectively.[9]
It would reportedly require over 200 changes to the RS-68 to meet human-rating standards.[10] NASA states several changes are needed to human-rate the RS-68, including health monitoring, removal of fuel-rich environment at liftoff, and improved subsystems robustness.[11][12]
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