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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| RS-25 | 6/9 | https://en.wikipedia.org/wiki/RS-25 | reference | science, encyclopedia | 2026-05-05T13:21:33.138292+00:00 | kb-cron |
Each Space Shuttle had three RS-25 engines, installed in the aft structure of the Space Shuttle orbiter in the Orbiter Processing Facility prior to the orbiter being transferred to the Vehicle Assembly Building. If necessary, the engines could be changed on the pad. The engines, drawing propellant from the Space Shuttle external tank (ET) via the orbiter's main propulsion system (MPS), were ignited at T−6.6 seconds prior to liftoff (with each ignition staggered by 120 ms), 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. The engines would then be throttled back up until around T+8 minutes, at which point they would be gradually throttled back down to 67% to prevent the stack exceeding 3 g of acceleration as it became progressively lighter due to propellant consumption. The engines were then shut down, a procedure known as main engine cutoff (MECO), at around T+8.5 minutes. After each flight the engines would be removed from the orbiter and transferred to the Space Shuttle Main Engine Processing Facility (SSMEPF), where they would be inspected and refurbished in preparation for reuse on a subsequent shuttle flight. 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 2 seconds (0.020 km/s) 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 (as of September 2023 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.
==== Engine throttle/output ==== The most obvious effects of the upgrades the RS-25 received through the Space Shuttle program were the improvements in engine throttle. Whilst the FMOF engine had a maximum output of 100% (RPL), Block II engines could throttle as high as 109% or 111% in an emergency, with usual flight performance being 104.5%. Existing engines used on the Space Launch System are throttled to 109% RPL during normal flight, while new RS-25E engines produced for the Space Launch System can be run at 111% RPL, with 113% throttle being tested. These increases in throttle level made a corresponding difference to the thrust produced by the engine: