NASA Orders 18 More RS-25 Engines for SLS Moon Rocket, at $1.79 Billion

The first SLS rocket for NASA’s Artemis moon missions rolls out of its assembly facility. Photo: NASA

With four years to go before human bootprints again dot the dusty surface of the Moon, NASA has awarded a contract worth $1.79 billion to Sacramento, Calif.-headquartered Aerojet Rocketdyne to fabricate 18 additional RS-25 engines for the mammoth Space Launch System (SLS). This powerful engine propelled 135 Space Shuttle flights off the launch pad between April 1981 and July 2011, and its next mission will provide the muscle for the SLS, which is currently targeted for its maiden voyage next year.

Friday’s agreement brings the RS-25 contract between NASA and Aerojet Rocketdyne to almost $3.5 billion, and could support as many as six SLS flights by 2029.

Video Credit: NASA

“This contract allows NASA to work with Aerojet Rocketdyne to build the rocket engines needed for future missions,” said John Honeycutt, the SLS program manager at NASA’s Marshall Space Flight Center (MSFC) in Huntsville, Ala. “The same reliable engines that launched more than 100 Space Shuttle missions have been modified to be even more powerful to launch the next astronauts who will set foot on the lunar surface during the Artemis missions.”

In its original guise as the Space Shuttle Main Engine (SSME), the cryogenic liquid oxygen/hydrogen-fueled RS-25 came into existence a half-century ago when NASA selected what was then the Rocketdyne Division of North American Rockwell Corp. to design, build, test and deliver engines for the shuttle program. In April 1972, NASA contracts estimated at $450 million authorized the start of work on the engine, with Critical Design Review (CDR) passed in September 1976 and the first test of a complete engine in March 1977. Beginning with STS-1 in April 1981, three reusable RS-25s were affixed to the end of each shuttle, producing almost 1.2 million pounds (535,000 kg) of thrust at liftoff.

Each shuttle was powered by a trio of RS-25 Space Shuttle Main Engines (SSMEs). These were test-fired before each orbiter’s maiden voyage, during the Flight Readiness Firing (FRF). Image Credit: Aerojet Rocketdyne

One shuttle flight in July 1985 suffered a dramatic in-flight abort and the program experienced five on-the-pad shutdowns during the engine-start sequence, whilst another narrowly averted a hairy Return to Launch Site (RTLS) abort, but all told a total of 46 RS-25s powered 135 shuttle missions until July 2011 with an estimated 99.95-percent success rate. Those engines were significantly modified over the years, benefiting from certification to operate at higher power levels, improved welds and main injector functionality, better turbopumps and an extended service lifetime.

The final shuttle-era iteration of the engine incorporated a robust high-pressure fuel turbopump and enabled a full power setting of up to 109 percent. It was also stronger and more reliable than earlier engines, thanks to the elimination of several welds, the inclusion of more durable bearings and a sophisticated casting process for its turbopump housing.  

The four RS-25 engines which will power the maiden SLS launch of NASA’s Artemis moon missions. Photo: NASA

Following the retirement of the shuttle fleet, the RS-25 was expected to play a role in the Constellation program, powering the core and second stages of the Ares V heavylifter, although that role was eventually taken by other engines. Constellation itself was canceled in 2010 and consideration was given to selling or donating NASA’s RS-25 inventory to U.S. museums or universities. However, with the birth of the SLS program in September 2011 the venerable old engine was revived to lift the new rocket’s core stage. Four RS-25s would power each SLS, but unlike the shuttle they would not be reused and as such the remaining inventory of engines would quickly be depleted.

As such, in November 2015 NASA awarded Aerojet Rocketdyne a $1.16 billion contract over nine years to restart RS-25 production to extensively modernize the engine to make it more affordable for SLS. It would benefit from fewer parts and welds and would be certified for higher thrust settings of up to 111 percent rated performance, generating 521,000 pounds (236,300 kg) at liftoff. Sixteen surviving shuttle-era RS-25s have been extensively upgraded and tested and will power the first four missions by the new booster.

When fully operational, the Space Launch System (SLS) promises to be the world’s most powerful rocket and the only booster capable of sending a human-rated craft to the Moon by 2024. Image Credit: NASA

NASA also exercised a contract option for Aerojet Rocketdyne to build six all-new engines. “We’ve already begun production on the first six new RS-25 engines,” said Johnny Heflin, the SLS engines manager. “Aerojet Rocketdyne has restarted the production lines, established a supplier base and its building engines using advanced techniques that reduce both the cost and time for manufacturing each engine.” Those six engines, the delivery of which is expected to begin in 2023, target more than a 30-percent cost reduction on future production when compared to their shuttle-era predecessors.

And with Friday’s contract award for a further 18 engines, NASA will have the capability to support six more SLS missions; four engines per booster. According to Aerojet Rocketdyne, the 18 engines continue to leverage supply-chain optimization and the incorporation of modern methodologies, including additive manufacturing. “The nation is returning to the Moon and moving forward with plans to explore other deep-space destinations, including Mars,” said Aerojet Rocketdyne CEO and President Eileen Drake. “This contract ensures our flight-proven RS-25 engines will be there every step of the way as NASA successfully and safely achieves these objectives.”

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