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Space Shuttle and SLS main engine

The Aerojet Rocketdyne RS 25, also known as the Space Shuttle Main Engine (SSME), The main injector and dome assembly are welded to the hot gas manifold, and the MCC is also bolted to the hot gas manifold. These materials possess significantly lower thermal conductivities than metallic alloys, thus allowing more efficient combustion and reducing the cooling requirements. TBCs are thin ceramic oxide layers deposited on metallic components, acting as a thermal barrier between hot gaseous combustion products and the metallic shell. A TBC applied to the Inconel 718 shell during production could extend engine life and reduce cooling costs. Further, CMCs have been studied as a replacement for Ni based superalloys and are composed of high strength fibers (BN, C) continuously dispersed in a SiC matrix. An MCC composed of a CMC, though less studied and farther from fruition than the application of a TBC, could offer unprecedented levels of engine efficiency.

The engine's nozzle is long with a diameter of at its throat and at its exit. The nozzle is a bell shaped extension bolted to the main combustion chamber, referred to as a de Laval nozzle. The RS 25 nozzle has an unusually large expansion ratio (about 69:1) for the chamber pressure. At sea level, a nozzle of this ratio would normally undergo flow separation of the jet from the nozzle, which would cause control difficulties and could even mechanically damage the vehicle. However, to aid the engine's operation Rocketdyne engineers varied the angle of the nozzle walls from the theoretical optimum for thrust, reducing it near the exit. This raises the pressure just around the rim to an absolute pressure between , and prevents flow separation. The inner part of the flow is at much lower pressure, around or less. The inner surface of each nozzle is cooled by liquid hydrogen flowing through brazed stainless steel tube wall coolant passages. On the Space Shuttle, a support ring welded to the forward end of the nozzle is the engine attach point to the orbiter supplied heat shield. Thermal protection is necessary because of the exposure portions of the nozzles experience during the launch, ascent, on orbit and entry phases of a mission. The insulation consists of four layers of metallic batting covered with a metallic foil and screening. later upgraded to a system composed of two doubly redundant Motorola 68000 (M68000) processors (for a total of four M68000s per controller). Having the controller installed on the engine itself greatly simplifies the wiring between the engine and the launch vehicle, because all the sensors and actuators are connected directly to only the controller, each MEC then being connected to the orbiter's general purpose computers (GPCs) or the SLS's avionics suite via its own engine interface unit (EIU). Using a dedicated system also simplifies the software and thus improves its reliability. Two independent dual CPU computers, A and B, form the controller; giving redundancy to the system. The failure of controller system A automatically leads to a switch over to controller system B without impeding operational capabilities; the subsequent failure of controller system B would provide a graceful shutdown of the engine. Within each system (A and B), the two M68000s operate in lock step, thereby enabling each system to detect failures by comparing the signal levels on the buses of the two M68000 processors within that system. If differences are encountered between the two buses, then an interrupt is generated and control turned over to the other system. Because of subtle differences between M68000s from Motorola and the second source manufacturer TRW, each system uses M68000s from the same manufacturer (for instance system A would have two Motorola CPUs while system B would have two CPUs manufactured by TRW). Memory for block I controllers was of the plated wire type, which functions in a manner similar to magnetic core memory and retains data even after power is turned off. Block II controllers used conventional CMOS static RAM.

To control the engine's output, the MEC operates five hydraulically actuated propellant valves on each engine; the oxidizer pre burner oxidizer, fuel pre burner oxidizer, main oxidizer, main fuel, and chamber coolant valves. In an emergency, the valves can be fully closed by using the engine's helium supply system as a backup actuation system. This motion allows the engine's thrust vector to be altered, thus steering the vehicle into the correct orientation. The comparatively large gimbal range is necessary to correct for the pitch momentum that occurs due to the constantly shifting center of mass as the vehicle burns fuel in flight and after booster separation. The bearing assembly is approximately , has a mass of , and is made of titanium alloy. The low pressure oxygen and low pressure fuel turbopumps were mounted 180° apart on the orbiter's aft fuselage thrust structure. The lines from the low pressure turbopumps to the high pressure turbopumps contain flexible bellows that enable the low pressure turbopumps to remain stationary while the rest of the engine is gimbaled for thrust vector control, and also to prevent damage to the pumps when loads were applied to them. The liquid hydrogen line from the LPFTP to the HPFTP is insulated to prevent the formation of liquid air. 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. ), 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 Stennis Space Center prior to flight.

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% RPL & 109% 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% FPL in the event of a contingency abort, and certified for 109% FPL for use during an intact abort. RS 25E: It will be used on the Space Launch System for future Artemis program missions beginning with Artemis 5, as the RS 25D stock is intentionally being used up. Unlike previous versions, this engine is designed to be expendable. The powerhead is almost completely redesigned (as of 2023 the specific design changes from the 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 both a 113% FPL and a 30% increase in thrust. with 113% power being tested. These increases in throttle level made a significant 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.

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. 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 to be 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; this meant that the RS 25 would be retired along with the Shuttle fleet.

On 14 September 2011, following the retirement of the Space Shuttle, NASA announced that it would be developing a new launch vehicle, known as the Space Launch System (SLS), to replace the shuttle fleet. The design for the SLS features the RS 25 as part of its core stage, with different versions of the rocket being equipped with between three and five engines. The initial flights of the new launch vehicle are making use of previously flown Block II RS 25D engines, with NASA keeping such engines in a "purged safe" environment at Stennis Space Center, "along with all of the ground systems required to maintain them." For Artemis 1, the RS 25D units with serial numbers E2045, E2056, E2058, and E2060 from all three orbiters were used. They were installed on the core stage by November 6, 2019. For Artemis 2, the units with serial numbers E2047, E2059, E2062, and E2063 will be used. They were installed on the core stage by September 25, 2023. In addition to the RS 25Ds, the SLS program makes use of the Main Propulsion Systems (MPS, the "plumbing" feeding the engines) from the three remaining shuttle orbiters for testing purposes (having been removed as part of the orbiters' decommissioning), with the first two launches (Artemis 1 and Artemis 2) originally predicted to make use of the MPS hardware from Space Shuttles and in their core stages. The SLS's propellants are supplied to the engines from the rocket's core stage, which consists of a modified Space Shuttle external tank with the MPS plumbing and engines at its aft, and an interstage structure at the top. they will be installed in the Space Station Processing Facility at Kennedy beginning with Artemis 3. Once the remaining RS 25Ds are exhausted, they are to be replaced with a cheaper, expendable version designated the RS 25E. On 1 May 2020, NASA awarded a contract extension to manufacture 18 additional RS 25 engines, with associated services, for $1.79 billion, bringing the total SLS contract value to almost $3.5 billion. On 29 August 2022, Artemis 1 was delayed by a problem with engineering sensors on RS 25D #3 (serial number E2058) erroneously reporting that it hadn't chilled down to its ideal operating temperature. On 16 November 2022, Artemis 1 launched from Kennedy Space Center Launch Complex 39B, the first time the RS 25 engine had flown since the Space Shuttle's final flight, STS 135, on 21 July 2011.

In 2015, a test campaign was conducted to determine RS 25 engine performance with a new engine controller unit, under lower liquid oxygen temperatures, with greater inlet pressure due to the taller SLS core stage liquid oxygen tank and higher vehicle acceleration; and with more nozzle heating due to the four engine configuration and its position in plane with the SLS booster exhaust nozzles. New ablative heat shield insulation was to be tested as well. May 28 (450 seconds), June 11 (500 seconds), June 25 (650 seconds), July 17 (535 seconds), August 13 (535 seconds) and August 27 (535 seconds). Following these tests, four more engines were scheduled to enter a new test cycle. On February 28, 2019, NASA conducted a 510 second test burn of a developmental RS 25 at 113 percent of its originally designed thrust for more than 430 seconds, about four times longer than any prior test at this thrust level. On January 16, 2021, the RS 25 engines were fired again, during a hot fire test as part of the Artemis program. The test was originally scheduled as an 8 minute test but was terminated at the 67th second due to intentionally conservative test parameters being breached in the hydraulic system of Engine 2's (serial number E2056) Core Stage Auxiliary Power Unit (CAPU) during the thrust vector control (TVC) system test. Engine 2's CAPU was shut down automatically, although if this issue had occurred during flight, it would not have caused an abort, as the remaining CAPUs are capable of powering the TVC systems of all four engines. The engine also suffered a different "Major Component Failure", in the engine control system, that was caused by instrumentation failure. This would have triggered an abort of the launch countdown during an actual launch attempt. On March 18, 2021, the four RS 25 core stage engines were once again fired as part of the second SLS core stage hot fire test, which lasted the full duration of 500 seconds, successfully certifying the Artemis 1 core stage for flight. On December 14, 2022, a single development RS 25E, serial number E10001, attempted a 500 second hot fire test. The test aborted at T+209.5 due to test systems subsequently interpreting signals from a group of improperly configured accelerometers during the hot fire as exceeding acceptable vibration limits. Tests of the engine continued in 2023; on February 8, 2023, it was fired for 500 seconds at 111% power, fitted with a new production nozzle. Subsequent tests included a 600 second test at 111% power on February 22, a 520 second test at 113% power on March 8, a 600 second test at 113% power on March 21, a 500 second, 113% power level test on April 5, a 720 second fire that tested the engine's thrust vectoring gimbal system on April 26, a 630 second test on May 10, and five more 500 second, 113% power level tests without gimbaling on May 23, June 8, June 15, and June 22. The RS 25E developmental unit E0525, with significant inclusion of new components including a redesigned nozzle, hydraulic actuators, flex ducts and turbopumps, was hot fire tested to 111% power levels for 550 seconds in the first in a series of certification tests beginning October 17, 2023. It was tested to 113% power levels for 500 seconds on November 15, and to 113% for 650 seconds with gimbaling on November 29, 2023, to 113% for 500 seconds on January 17, 2024, January 23, and January 29, to 113% for 550 seconds on February 23, to 111% for 615 seconds on February 29, and to 113% for 600 seconds on March 6 and 500 seconds on March 22 and 27, and April 3.

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.