The RL10 was USA's first liquid hydrogen fueled rocket engine. An updated version is used in several current launch vehicles. Six RL10 engines were used in the S-IV second stage of the Saturn I rocket. One or two RL10 engines are used in the Centaur upper stages of Atlas and Titan rockets. One RL10 is used in the upper stage of Delta IV rockets. There were proposals to use RL10 powered Centaur upper stages on Saturn I, Saturn IB and Saturn V rockets, and the Space Shuttle.
The first flight using production versions of the RL10 engine launched November 27, 1963. For that launch, two RL10A-3 engines powered the Centaur upper stage of an Atlas launch vehicle. The launch was used to conduct a heavily instrumented performance and structural integrity test of the vehicle.
The RL10 was first tested on the ground in 1959 and first flown in 1963.
Original RL10 specifications
Thrust (altitude): 15,000 lbf (66.7 kN)
The RL10 has been upgraded over the years. One current model, the RL10B-2, powers the Delta IV second stage, as well as the Delta III second stage. It has been significantly modified from the original RL10 to improve performance. Some of the enhancements include an extendable nozzle and electro-mechanical gimbaling for reduced weight and increased reliability. Current specific impulse is 462 s (equivalent to an exhaust velocity of 4.53 km/s).
The other current model, the RL10A-4-2, is an upper stage engine used on Atlas V.
Future use of the RL10
In 2005 NASA announced the decision to use an Apollo-like spacecraft configuration for the proposed Orion spacecraft. At that time NASA decided that the descent stage of the new Lunar Surface Access Module (LSAM) would be powered by liquid hydrogen and liquid oxygen. The original plan called for the ascent stage to use liquid methane and liquid oxygen, but that has changed and the ascent stage will now also use LH2/LOX.
Because of the choice of propellents, along with the need to land the spacecraft in the polar regions of the Moon from an equatorial orbit, NASA decided to use the RL10 as the main powerplant for the descent stage engine. Current specifications call for four RL10 engines to be used on the descent stage and a single RL10 for the ascent stage. Currently, the RL10B-2 engines used on the Delta III and Delta IV can thrust at 20% of maximum thrust. Because of the need for the LSAM to hover above the lunar surface, along with providing a smooth landing, the new RL10 engines must be able to thrust as low as 10%. The use of the RL10 will allow NASA to keep costs on the lunar program down by using existing hardware, albeit modified to enhance performance or allow for manned spaceflight.
Common Extensible Cryogenic Engine
The Common Extensible Cryogenic Engine (CECE) is a testbed to develop RL10 engines that throttle well. NASA has contracted with Pratt & Whitney Rocketdyne to develop the CECE demonstrator engine. In 2007 its operability (with some "chugging") was demonstrated at 11-to-1 throttle ratios. In 2009 NASA reported successfully throttling from 104 percent thrust to eight percent thrust, a record for an engine of this type. Chugging was eliminated by injector and propellant feed system modifications that control the pressure, temperature and flow of propellants.
Other Rockets using RL10
Four modified RL10A-5 engines, all of them with the ability to be throttled, were used in the McDonnell Douglas DC-X.
Published - July 2009
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