Spaceflight Insider

NASA, Aerojet Rocketdyne conduct 2nd RS-25 controller test

A full duration RS-25 engine test at NASA's Stennis Space Center. The test took place May 23, 2017. Photo Credit: NASA

A full duration RS-25 engine test at NASA’s Stennis Space Center. The test took place on May 23, 2017. Photo Credit: NASA

On May 23, 2017, Aerojet Rocketdyne completed the second in a series of RS-25 engine firings, testing a new controller system. Formerly known as Space Shuttle Main Engines, the RS-25s are being upgraded to serve as the main engines for NASA’s Space Launch System (SLS) super-heavy-lift launch vehicle.

Rattling windows

Engine Controller Unit-2 was installed on RS-25 development engine No. 0528 and test fired on the A-1 Test Stand at Stennis Space Center near Bay St. Louis, Mississippi. Once the test data is certified, the controller will be removed and installed on one of four flight engines.

An RS-25 being tested at NASA's John C. Stennis Space Center. Photo Credit: NASA / Aerojet Rocketdyne RS-25 rocket engine test firing at Stennis Space Center in Mississippi NASA photo posted on SpaceFlight Insider

An RS-25 being tested at NASA’s Stennis Space Center. Photo Credit: NASA / Aerojet Rocketdyne

The firing itself rattled the neighborhood around Stennis for 500 seconds – a little under 8.5 minutes – to test this second controller unit for the duration of a full flight from liftoff to orbit, producing a massive white ground cloud. Unlike other rockets, in this case, “cloud” is an accurate description as the RS-25’s liquid hydrogen and liquid oxygen propellants produce water vapor for exhaust.

“With today’s test, the RS-25 development and flight engines have experienced 6,848 seconds of SLS test time,” said Aerojet Rocketdyne CEO and President Eileen Drake. “Between the shuttle program and the SLS program, these engines have experienced more than 1.1 million seconds of testing.”

Something old, something new

NASA is attempting to save money on engine development for SLS by re-purposing Shuttle-era RS-25s. However, unlike the Space Shuttle, which fired three engines from the back of a side-mounted orbiter, the SLS will employ four engines mounted at the base of the SLS core stage (which is also Shuttle-derived).

As such, upgrades to RS-25 are required and include modern computer systems for controlling the vectoring and thrust as the Shuttle-era controller was not fast enough to communicate with the other 21st century systems on the rocket. It is critical for commanding and controlling the engines and for tracking operating conditions, including turbopump speeds, combustion temperatures, thrust, and propellant mixture ratio.

Moreover, the engines have been upgraded to operate under different conditions. While the Shuttle kept the RS-25s at a distance from the solid rocket boosters’ exhaust, the SLS will have its engines right next to them, increasing thermal loads on the computer.

The flight controller is one of the critical new parts of the engine to enable the higher performance under SLS’ configuration. According to a NASA blog, the controller for the RS-25 also incorporated lessons learned from the J-2X engine, which was developed for the Ares upper-stage under the Constellation Program.

The SLS will sport four RS-25 engines to help power it toward orbit. Image Credit: Nathan Koga / SpaceFlight Insider

The SLS will sport four RS-25 engines to help power it toward orbit. Image Credit: Nathan Koga / SpaceFlight Insider

The long road back

The return to flight for the RS-25 has been a long time coming. The last Space Shuttle flew in 2011. The first hot-fire test for the SLS occurred in 2015. NASA and Aerojet Rocketdyne successfully tested the first flight controller for the RS-25 in late March 2017. Finally, later this year, two more engine controllers for the first SLS mission will be tested on the development engine.

The fourth controller will be tested when NASA fires the entire core stage during a “green run” on the B-2 Test Stand at Stennis. That test will require the installation of the core stage on the stand and firing its four RS-25 flight engines simultaneously. Exploration Mission 1, the first flight of the SLS, isn’t due to occur until 2019.

“This an important – and exciting – step in our return to deep space missions,” said Stennis Director Rick Gilbrech. “With every test of flight hardware, we get closer and closer to launching humans deeper into space than we ever have traveled before.”

Video courtesy of NASA Stennis Space Center



Bart Leahy is a freelance technical writer living in Orlando, Florida. Leahy's diverse career has included work for The Walt Disney Company, NASA, the Department of Defense, Nissan, a number of commercial space companies, small businesses, nonprofits, as well as the Science Cheerleaders.

Reader Comments

Impressive as always.
Does anyone know what causes the white cone eminating a short distance from the bottom of the exhaust bell? I remember seeing this during Shuttle launches as well, and it appeared pale blue in colour.

Those are mach/shock diamonds. Various camera apertures and lighting conditions can make them appear differently, but they are still mach/shock diamonds.

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