Spaceflight Insider

Lucky 113: NASA tests RS-25 engine at highest-ever power level

An RS-25 engine undergoes a test at NASA’s Stennis Space Center. Photo credit: NASA

Engineers at NASA’s Stennis Space Center conducted a test of the Space Launch System’s (SLS) RS-25 engine, pushing the design to the highest level ever recorded for the powerhouse previously used to send Space Shuttles into orbit. The Aerojet Rocketdyne-manufactured engine reached a peak output of 113 percent of rated power during the Feb. 21, 2018, firing at the coastal Mississippi site.

Aerojet Rocketdyne's Owen Brayson highlight's the 3-D-printed pogo accumulator assembly on the RS-25. Photo credit: Aerojet Rocketdyne

Aerojet Rocketdyne’s Owen Brayson highlight’s the 3-D-printed pogo accumulator assembly on the RS-25. Photo credit: Aerojet Rocketdyne

Testing the limits

The firing took place at the A-1 test stand where Engine 0528, a development article being used by NASA and Aerojet Rocketdyne to evaluate new hardware and software, was pushed to 113 percent of rated power for 50 seconds of the 260-second test to explore the limits of the design.

“Increased thrust requirements for the RS-25 are just one of the many changes in the SLS rocket’s performance that will facilitate our nation’s deep space exploration goals and objectives,” said Aerojet Rocketdyne’s RS-25 program director, Dan Adamski, in a press release issued by the company. “While we can analytically calculate engine performance and structural capabilities at these higher power levels, actually demonstrating that performance with an engine hot fire provides the added confidence that these engines will meet all specification requirements demanded of SLS.”

During their previous life as Space Shuttle Main Engines, RS-25 engines regularly ran at 104.5 percent of rated power while pushing Space Shuttles into orbit. Once the storied spacecraft were retired, 16 of the reusable engines were left over. They will be used on the first four flights of SLS (four per launch) and run at 109 percent.

Because the SLS is expendable, the current stock of engines will be depleted by flight four. According to Aerojet Rocketdyne, new RS-25s are being developed under its restart program to be used for SLS flight five and beyond. Those will fly at 111 percent. 

Testing to 113 percent will allow engineers to discover how the RS-25 and its components react at higher levels.

The RS-25 engines were designed more than 40 years ago for a specific power level during development, which engineers considered 100 percent. Over the decades, the design was refined and upgraded. Rather than revising documentation, the initial 100 percent mark was kept, with ratings over that marking higher power levels.

SLS Block-1 on launchpad at night

An artist’s rendering of an SLS Block 1 rocket on the launch pad at night. Four RS-25 engines and two five-segment Solid Rocket Boosters will power it toward orbit. Image Credit: NASA

Beyond evaluating the engine at higher throttle levels, this test also saw the inclusion of both an RS-25 flight controller and a 3-D-printed pogo accumulator assembly. The RS-25’s flight controller, or “brain,” will communicate with the SLS’s flight computers to relay the health and performance of the engine during the powered phase of its flight.

The pogo accumulator assembly, however, marks the largest component of the engine to be manufactured via a 3-D printing, or additive manufacturing, process. The use of this modern technique eliminates more than 100 welds on the vibration-reducing component, shortening production time by more than 80 percent, according to NASA.

“With modern fabrication processes, including additive manufacturing, the ‘next generation’ of the RS-25 will have fewer parts and welds, reducing production time as well as costs,” said Carol Jacobs, RS-25 engine lead at Marshall Space Flight Center, in a news release issued by the space agency.

These solo engine evaluations are precursors to the SLS core stage’s full-up hot fire test—also called the “Green Run”—during which all four RS-25 engines will undergo a flight-duration firing. This test will mimic the output levels expected on a nominal flight and will certify the hardware for use on Exploration Mission 1 (EM-1) in 2020. The uncrewed flight will validate the rocket and the Orion crew vehicle before carrying astronauts on EM-2 no earlier than 2023.

This test is the latest in a line of evaluations of the super heavy-lift vehicle’s development.

“One of the key features of SLS is its versatility to support human and robotic missions, launching spacecraft, habitats and astronauts to a variety of deep space destinations,” said Eileen Drake, CEO and president of Aerojet Rocketdyne, in the company’s release. “The lifting power of the SLS will permit NASA to get bigger payloads to distant planets more quickly than any other launcher operating today.”

Video courtesy of NASA


Curt Godwin has been a fan of space exploration for as long as he can remember, keeping his eyes to the skies from an early age. Initially majoring in Nuclear Engineering, Curt later decided that computers would be a more interesting - and safer - career field. He's worked in education technology for more than 20 years, and has been published in industry and peer journals, and is a respected authority on wireless network engineering. Throughout this period of his life, he maintained his love for all things space and has written about his experiences at a variety of NASA events, both on his personal blog and as a freelance media representative.

Reader Comments

Enjoy that sweet welfare money Owen Brayson! How high won;t that welfare baby fliy? What a joke.

What’s the improvement in specific impulse at the higher power? I know the chamber pressure & exhaust velocity have come up, but how proportional is the increase in SI? Just curious. Not the best for every situationn but always liked theS SSME/RS-25.

… and anyway to delete Weel-Fare’s post. The sentiment he or she is trying to convey is fine, but he’s trashing a technician, not anyone who’s actually a higher up at Aerjet R or NASA.

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