Theexpander cycle that the engine uses drives theturbopump with waste heat absorbed by the engine combustion chamber, throat, and nozzle. This, combined with the hydrogen fuel, leads to very highspecific impulses (Isp) in the range of 373 to 470 s (3.66–4.61 km/s) in a vacuum. Mass ranges from 131 to 317 kg (289–699 lb) depending on the version of the engine.[4][5]
The RL10 was the first liquid hydrogen rocket engine to be built in the United States, with development of the engine byMarshall Space Flight Center andPratt & Whitney beginning in the 1950s. The RL10 was originally developed as a throttleable engine for the USAFLunex lunar lander.[6] The engine was electric spark ignited.[7]
The RL10 was first tested on the ground in 1959, atPratt & Whitney's Florida Research and Development Center inWest Palm Beach, Florida.[8][9] The first successful flight took place on November 27, 1963.[10][11] For that launch, two RL10A-3 engines powered theCentaur upper stage of anAtlas launch vehicle. The launch was used to conduct a heavily instrumented performance and structural integrity test of the vehicle.[12]
RL10A information and overview from Saturn V
Multiple versions of this engine have been flown. TheS-IV of theSaturn I used a cluster of six RL10A-3S, a version which was modified for installation on the Saturn[13] and theTitan program includedCentaur D-1T upper stages powered by two RL10A-3-3 Engines.[13][14]
A flaw in thebrazing of an RL10B-2 combustion chamber was identified as the cause of failure for the 4 May 1999Delta III launch carrying the Orion-3communications satellite.[16]
TheDIRECT version 3.0 proposal to replaceAres I andAres V with a family of rockets sharing a common core stage recommended the RL10 for the second stage of the J-246 and J-247 launch vehicles.[17] Up to seven RL10 engines would have been used in the proposed Jupiter Upper Stage, serving an equivalent role to theSpace Launch SystemExploration Upper Stage.
In the early 2000s, NASA contracted withPratt & Whitney Rocketdyne to developthe Common Extensible Cryogenic Engine (CECE) demonstrator. CECE was intended to lead to RL10 engines capable of deep throttling.[18] In 2007, its operability (with some "chugging") was demonstrated at 11:1 throttle ratios.[19] In 2009, NASA reported successfully throttling from 104 percent thrust to eight percent thrust, a record for an expander cycle engine of this type. Chugging was eliminated by injector and propellant feed system modifications that control the pressure, temperature and flow of propellants.[20] In 2010, the throttling range was expanded further to a 17.6:1 ratio, throttling from 104% to 5.9% power.[21]
Second stage of a Delta IV Medium rocket featuring an RL10B-2 engine
In 2012 NASA joined with the US Air Force (USAF) to study next-generation upper stage propulsion, formalizing the agencies' joint interests in a new upper stage engine to replace the Aerojet Rocketdyne RL10.
"We know the list price on an RL10. If you look at cost over time, a very large portion of the unit cost of the EELVs is attributable to the propulsion systems, and the RL10 is a very old engine, and there's a lot of craftwork associated with its manufacture. ... That's what this study will figure out, is it worthwhile to build an RL10 replacement?"
— Dale Thomas, Associated Director Technical, Marshall Space Flight Center[22]
From the study, NASA hoped to find a less expensive RL10-class engine for the upper stage of theSpace Launch System (SLS).[22][23]
USAF hoped to replace the Rocketdyne RL10 engines used on the upper stages of the Lockheed Martin Atlas V and the Boeing Delta IVEvolved Expendable Launch Vehicles (EELV) that were the primary methods of putting US government satellites into space.[22] A related requirements study was conducted at the same time under the Affordable Upper Stage Engine Program (AUSEP).[23]
The RL10 has undergone multiple upgrades over the decades. The RL10B-2, used on theDCSS, incorporated anextendable nozzle made fromcarbon–carbon, electro-mechanicalgimbaling to reduce weight and increase reliability, and achieved aspecific impulse of 465.5 seconds (4.565 km/s).[24][25]
Beginning in the 2000s, Aerojet Rocketdyne introduced3D printing (additive manufacturing) into RL10 production. The RL10C-1-1 was the first engine to include a 3D-printed component, featuring a nickel superalloy main injector.[26] Building on that experience, in 2015 the company began developing a more extensive upgrade that employed an additively manufactured copperthrust chamber. According to the company, the new process reduced chamber fabrication time from approximately 20 months to 4–6 months compared with earlier hand-fabricated stainless steel chambers, enabling production of up to one engine per week rather than one per month. This variant, designated RL10C-X during development, entered production as the RL10E-1 and is planned for use on United Launch Alliance’s Vulcan Centaur rocket, scheduled for its first flight in 2025.[27][28]
Centaur III: The single engine centaur (SEC) version uses the RL10C-1,[3] while the dual engine centaur (DEC) version retains the smaller RL10A-4-2.[29] An Atlas V mission (SBIRS-5) marked the first use of the RL10C-1-1 version. The mission was successful but observed unexpected vibration, and further use of the RL10C-1-1 model is on hold until the problem is better understood.[30] The engine was used again successfully on SBIRS-6.
Centaur V stage: On May 11, 2018,United Launch Alliance (ULA) announced that the RL10 upper stage engine had been selected for itsVulcan Centaur rocket following a competitive procurement process.[31] Early versions of the Centaur V will use the RL10C-1-1,[3] but later versions will transition to the RL10E in 2025.[32] Vulcan flew its successful maiden flight on January 8, 2024.[33]
Interim Cryogenic Propulsion Stage: The Interim Cryogenic Propulsion Stage or ICPS is used for the SLS and is similar to the DCSS, except that the engine is an RL10B-2 and it is adapted to fit on top of the 8.4 meter diameter core stage with fourRS-25 Space Shuttle Main Engines.
OmegA Upper Stage: In April 2018,Northrop Grumman Innovation Systems announced that two RL10C-5-1 engines would be used onOmegA in the upper stage.[35]Blue Origin'sBE-3U andAirbus Safran'sVinci were also considered before Aerojet Rocketdyne's engine was selected. OmegA development was halted after it failed to win a National Security Space Launch contract.[36]
Advanced Cryogenic Evolved Stage: As of 2009[update], an enhanced version of the RL10 was proposed to power the Advanced Cryogenic Evolved Stage (ACES), a long-duration, low-boiloff extension of existingULACentaur andDelta Cryogenic Second Stage (DCSS) technology for theVulcan launch vehicle.[37] Long-duration ACES technology is intended to supportgeosynchronous,cislunar, andinterplanetary missions. Another possible application is as in-spacepropellant depots inLEO or atL2 that could be used as way-stations for other rockets to stop and refuel on the way to beyond-LEO or interplanetary missions. Cleanup ofspace debris was also proposed.[38]
^Zegler, Frank; Bernard Kutter (September 2, 2010)."Evolving to a Depot-Based Space Transportation Architecture"(PDF).AIAA SPACE 2010 Conference & Exposition. AIAA. Archived fromthe original(PDF) on October 20, 2011. RetrievedJanuary 25, 2011.ACES design conceptualization has been underway at ULA for many years. It leverages design features of both the Centaur and Delta Cryogenic Second Stage (DCSS) upper stages and intends to supplement and perhaps replace these stages in the future. ...
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