Development in early 2015|alt=A video showing RS-25 testing. The video opens with a night view of a large scaffold structure (the test stand), lit with internal lights. The view then switches to show the nozzle of a rocket engine, mounted within the structure, lighting and beginning to fire. The view then cuts back to the view of the scaffold, from which large amounts of steam are now billowing out, towards the right of the frame. Wide and close-up views of this plume follow, before the view switches back to the engine nozzle, which shuts down. The history of the RS-25 traces back to the 1960s when
NASA's
Marshall Space Flight Center and
Rocketdyne were conducting a series of studies on high-pressure engines, developed from the successful
J-2 engine used on the
S-II and
S-IVB upper stages of the
Saturn V rocket during the
Apollo program. The studies were conducted under a program to upgrade the Saturn V engines, which produced a design for a upper-stage engine known as the
HG-3. As funding levels for Apollo wound down the HG-3 was cancelled as well as the upgraded
F-1 engines already being tested. It was the design for the HG-3 that would form the basis for the RS-25. Meanwhile, in 1967, the
US Air Force funded a study into advanced rocket propulsion systems for use during
Project Isinglass, with Rocketdyne asked to investigate
aerospike engines and
Pratt & Whitney (P&W) to research more efficient conventional
de Laval nozzle-type engines. At the conclusion of the study, P&W put forward a proposal for a 250,000 lbf engine called the
XLR-129, which used a two-position
expanding nozzle to provide increased efficiency over a wide range of altitudes. In January 1969 NASA awarded contracts to General Dynamics, Lockheed, McDonnell Douglas, and North American Rockwell to initiate the early development of the Space Shuttle. NASA specified that, prior to the Shuttle's first flight, the engines must have undergone at least 65,000 seconds of testing, a milestone that was reached on March 23, 1980, with the engine having undergone 110,253 seconds of testing by the time of
STS-1 both on test stands at
Stennis Space Center and installed on the
Main Propulsion Test Article (MPTA). The first set of engines (2005, 2006 and 2007) was delivered to
Kennedy Space Center in 1979 and installed on , before being removed in 1980 for further testing and reinstalled on the orbiter. The engines, which were of the first manned orbital flight (FMOF) configuration and certified for operation at 100% rated power level (RPL), were operated in a twenty-second flight readiness firing on February 20, 1981, and, after inspection, declared ready for flight.), which allowed their performance to be checked prior to ignition of the
Space Shuttle Solid Rocket Boosters (SRBs), which committed the shuttle to the launch. At launch, the engines would be operating at 100% RPL, throttling up to 104.5% immediately following liftoff. The engines would maintain this power level until around T+40 seconds, where they would be throttled back to around 70% to reduce aerodynamic loads on the shuttle stack as it passed through the region of maximum dynamic pressure, or
max. q. A total of 46 reusable RS-25 engines, each costing around US$40 million, were flown during the Space Shuttle program, with each new or overhauled engine entering the flight inventory requiring
flight qualification on one of the test stands at the
Stennis Space Center prior to flight.
Upgrades Over the course of the Space Shuttle program, the RS-25 went through a series of upgrades, including combustion chamber changes, improved welds and turbopump changes in an effort to improve the engine's performance and reliability and so reduce the amount of maintenance required after use. As a result, several versions of the RS-25 were used during the program: • FMOF (first manned orbital flight): Certified for 100% rated power level (RPL). Used for the orbital flight test missions
STS-1 –
STS-5 (engines 2005, 2006 and 2007). • Phase I: Used for missions
STS-6 –
STS-51-L, the Phase I engine offered increased service life and was certified for 104% RPL. Replaced by Phase II after the
Challenger Disaster. • Phase II (RS-25A): First flown on
STS-26, the Phase II engine offered a number of safety upgrades and was certified for 104% & 109% RPL (full power level, FPL) in the event of a contingency. • Block I (RS-25B): First flown on
STS-70, the Block I engines offered improved turbopumps featuring ceramic bearings, half as many rotating parts, and a new casting process reducing the number of welds. Block I improvements also included a new, two-duct powerhead (rather than the original design, which featured three ducts connected to the HPFTP and two to the HPOTP), which helped improve hot gas flow, and an improved engine heat exchanger. • Block IA (RS-25B): First flown on
STS-73, the Block IA engine offered main injector improvements. • Block IIA (RS-25C): First flown on
STS-89, the Block IIA engine was an interim model used whilst certain components of the Block II engine completed development. Changes included a new large throat main combustion chamber (which had originally been recommended by Rocketdyne in 1980), improved low-pressure turbopumps, and certification for 104.5% RPL to compensate for a reduction in
specific impulse (original plans called for the engine to be certified to 106% for heavy
International Space Station payloads, but this was not required and would have reduced engine service life). A slightly modified version first flew on
STS-96. • Block II (RS-25D): First flown on
STS-104, the Block II upgrade included all of the Block IIA improvements plus a new high-pressure fuel turbopump. This model was ground-tested to 111% RPL in the event of a
contingency abort, and certified for 109% RPL for use during an
intact abort. • RS-25E: A variant in development. It is planned to be used on the
Space Launch System for future
Artemis program missions beginning with
Artemis V, as the RS-25D stock is gradually being expended on SLS flights (the core stage is disposed in the atmosphere, along with the engines). Unlike previous versions, this engine is designed to be expendable. The powerhead is almost completely redesigned ( the specific design changes from the RS-25D have not been announced), and intended to incorporate various cost-saving measures and innovations in manufacturing. The first testing engine, E10001, passed all its qualifications and tests at NASA's Stennis Space Center, and demonstrated operation at 113% RPL. with 113% throttle being tested. These increases in throttle level made a corresponding difference to the thrust produced by the engine: •
STS-51-F – No. 2 engine caused an RSLS shutdown at T−3 seconds due to a coolant valve malfunction. •
STS-55 – No. 3 engine caused an RSLS shutdown at T−3 seconds due to a leak in its liquid-oxygen preburner check valve. •
STS-51 – No. 2 engine caused an RSLS shut down at T−3 seconds due to a faulty hydrogen fuel sensor. •
STS-68 – No. 3 engine (2032) caused an RSLS shutdown at T−1.9 seconds when a temperature sensor in its HPOTP exceeded its
redline. •
STS-93 – An Orbiter Project AC1 Phase A electrical wiring short occurred at T+5 seconds causing an under voltage which disqualified SSME1A and SSME3B controllers but required no engine shut down. In addition, a 0.1-inch diameter, 1-inch long gold-plated pin, used to plug an oxidizer post orifice (an inappropriate SSME corrective action eliminated from the fleet by redesign) came loose inside an engine's main injector and impacted the engine nozzle inner surface, rupturing three hydrogen cooling lines. The resulting three breaches caused a leak resulting in a premature engine shutdown, when four external tank LO sensors flashed dry resulting in low-level cutoff of the main engines and a slightly early main engine cut-off with a underspeed, and an 8 nautical mile lower altitude.
Constellation program and
STS-135 in storage at
Kennedy Space Center|alt=Six rocket engines, consisting of a large bell-shaped nozzle with working parts mounted to the top, stored in a large warehouse with white walls decorated with flags. Each engine has several pieces of red protective equipment attached to it and is mounted on a yellow wheeled pallet-like structure. During the period preceding final
Space Shuttle retirement, various plans for the remaining engines were proposed, ranging from them all being kept by NASA, to them all being given away (or sold for US$400,000–800,000 each) to various institutions such as museums and universities. This policy followed changes to the planned configurations of the
Constellation program's
Ares V cargo-launch vehicle and
Ares I crew-launch vehicle rockets, which had been planned to use the RS-25 in their first and second stages respectively. • The engines would not be reusable, as they would be permanently attached to the discarded stages and disposed of in the atmosphere. • Each engine would have to undergo a test firing prior to installation and launch, with refurbishment required following the test. • It would be expensive, time-consuming, and weight-intensive to convert the ground-started RS-25D to an air-started version for the Ares I second stage. Following several design changes to the Ares I and Ares V rockets, the RS-25 was replaced with a single
J-2X engine for the Ares I second stage and six modified
RS-68 engines (which was based on both the SSME and Apollo-era J-2 engine) on the Ares V core stage; these changes meant that the RS-25 would be retired along with the Shuttle fleet.
XS-1 On May 24, 2017,
DARPA announced that they had selected
The Boeing Company to complete design work on the XS-1 program. The technology demonstrator was planned to use an
Aerojet Rocketdyne AR-22 engine. The AR-22 was a version of the RS-25, with parts sourced from Aerojet Rocketdyne and NASA inventories from early versions of the engine. In July 2018 Aerojet Rocketdyne successfully completed ten 100-second firings of the AR-22 in ten days. On January 22, 2020, Boeing announced its departure from the XS-1 program, leaving no role for the AR-22. == Present use==