May, 2021. The 212-foot-tall rocket, the core stage and final major piece of NASA’s new Space Launch System (SLS), arrives at Kennedy Space Center. Assembly begins within the towering, iconic Vehicle Assembly Building, which hasn’t seen a human-rated deep space rocket since the end of the Apollo program 50 years ago.

But the SLS isn’t an all-new ride; it’s got some familiar parts. Mounted at the bottom of the center stage are four RS-25 engines supplied by Aerojet Rocketdyne. Originally designed in the 1970s, the engines are seasoned, upgraded veterans, with 25 previous Space Shuttle flights among them. The most poignant of the four is engine number 2060, used on July 8, 2011 to launch the shuttle’s final mission, STS-135.

Doug Bradley was at Kennedy Space Center during that final launch, working as a chief engineer for Rocketdyne. “I’ve been to many flights, but it was different for 135,” Bradley tells Popular Mechanics. “It was electric. It was a very emotional flight.”

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Space Shuttle Atlantis on the shuttle’s final mission, STS-135, on July 10, 2011. The three RS-25 engines are visible on the back.
NASAGetty Images

The grounding of the Space Shuttle left Rocketdyne with a backlog of 16 unused RS-25 engines, which the company mothballed. “For us, it was a bit sad to have engines in the stable that could fly dozens of times more,” says Bradley.

Now with the title of Chief Engineer of Advanced Space and Launch at Aerojet Rocketdyne, Bradley is overseeing the rebirth of the RS-25 engine. (A 2013 merger with Aerojet led to the company’s current two-word name.) And it’s quite a comeback: SLS is the custom-built, off-planet ride for NASA’s Artemis program, a multiyear campaign to return humans to the lunar surface and later to land people on Mars. That puts the RS-25, originally created with slide rules and graph paper, at the foundation of 21st-century human exploration of the solar system.

“It’s very gratifying to see those engines resurrected,” Bradley says. “And what’s better than going to the moon and Mars?”

A Legend Is Born

Rocketdyne began production of the RS-25 on March 31, 1972, but it’d be another five years before Bradley would meet the RS-25. A fresh engineering graduate from California Polytechnic State University in San Luis Obispo, Bradley got a tip from a neighbor and applied at Rocketdyne to work on the shuttle program. But because he didn’t want to linger in his hometown of Pasadena, his plan was to stay with the company for only a couple of years.

“I was old school. Every design you were getting down on paper, on a physical drawing board with pens or pencils,” Bradley says. “They started testing hot fire testing engines in 1975, but in ’77 we were still burning up a fair amount of engines. I was in turbomachinery and we were taking the brunt of some of the damage, so it was very exciting.”

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A RS-25 undergoing a test firing at the National Space Technology Laboratories in Mississippi, 1981.
Smith Collection/GadoGetty Images

Four turbopumps are at the fiery heart of the RS-25’s design. These spinning, whirling fans create the extreme pressure that carefully shoots liquid hydrogen (LH2) and liquid oxygen (LOX) to the main combustion chamber. These machines operate in truly brutal conditions, including heat as intense as 6,000°F inside the chamber—hot enough to transform an iron bar into a molten puddle. The extreme highs are rough enough, but it’s the temperature swings that introduce especially dangerous stress to the engine’s delicately machined innards.

“Just on the turbo pumps alone, you’ve got a couple of thousand degrees gradient from one end of it to the other,” Bradley says. “The combustion chamber is even worse. They’re at thousands of degrees with (physical) tolerances of fractions of inches. So there are challenges all over.”

Creating central air conditioning for the main combustion chamber—a pressure cooker suitable for Dante’s sixth level of hell—required some novel thinking. In the early 1970s, Rocketdyne researchers created a new copper-zirconium alloy called NARloy-Z expressly for use on the engine. The new material could withstand pressure and temperatures that threatened to deform the tiny channels that deliver superchilled liquid hydrogen through the main combustion chamber’s lining to keep it from burning up.

The RS-25 was born of ceaseless tests conducted in an era before computer simulations. NASA dictated that the engines endure at least 65,000 seconds of flame and smoke on stands before making the first flight, even though the engine works for just 510 seconds during a mission. The trials included challenging the RS-25 to perform at power levels greater than what the mission required, mostly to satisfy demands during emergency situations. Decades later, when NASA was evaluating SLS engines, that extra thrust would prove invaluable.

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Thousands of spectators watch the first launch of Columbia in the early hours of April 12, 1981
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The first Space Shuttle launched on April 12, 1981, becoming the world’s first reusable crewed spacecraft, and the RS-25 the first reusable space rocket motor. When the orbiter landed, its RS-25s were removed, inspected, and refurbished before being readied for another mission.

“I call these my children because I’ve lived with all these engines from the day they were born, when they were put together, when they were tested, when they were flown,” says Bill Muddle, Aerojet’s RS-25 field integration engineer, who worked on the shuttle program in Florida since 1989. “Being here at KSC, I got to touch and be part of all of those motors through their history for 125 flights.”

It turns out that the reusable engines, built to the same precise standards, developed what engineers call “personalities” based on their performance and maintenance demands. It’s a phenomenon seen in airplane hangars and shipyards—places where hardware is reused and the maintainers can get to know it better during a service life.

A Deeper Dive Into a Historic Spacecraft

Popular Mechanics met the new Space Shuttle in the months leading up to its inaugural launch in the April 1981 issue. Here's everything we knew about the metal beast—and its 3 RS-25 engines—back then.

And Muddle sees the existing stable of RS-25s in a similar, more personal way.

“I’ve got 16 unique children,” he remarks. “I love them all to death, but they all have their idiosyncrasies.”

The RS-25 powered shuttle missions for 30 years, boasting a 99.95% reliability rate. The only official “in-flight main engine failure” occurred during the Space Shuttle Challenger’s fatal mission (STS-51) when a solid rocket booster seal failed during launch.

As the missions changed, the engine was asked to run at higher power levels, up to 104% of its rated power level, mostly to handle emergency situations.

"I’ve got 16 unique children...I love them all to death, but they all have their idiosyncrasies."

“They asked it to go run at those high pressures and temperatures, asking it to do a lot of work, and it came back every single time,” Muddle ways. “It did everything it had to go do; when you started that engine to put astronauts into space, it did.”

When the shuttle retired in 2011, so did the RS-25—or so it seemed.

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Space Shuttle Columbia, with three RS-25 engines, sits at Cap Canaveral, Florida.
Print CollectorGetty Images

The Rocket Roars Again

In 2015, NASA prepared to announce its choice of engines that would be used in its new deep space rocket, the Space Launch System. With a 30-year track record and more than 1 million seconds of total ground test and flight firing time, the venerable engine’s history became a selling point, especially in a traditionally risk-averse NASA program. SLS will be the largest launch system ever built, and efficient engines get more benefits as rockets scale up, which was good news for the RS-25’s bid.

“We measure efficiency in specific impulse (ISP), like gas mileage for a rocket. Our ISP gets 452 seconds, which is very, very hot. For other rockets that will be in the 300s,” Bradley says. “The more efficient you are, the less propellant you have to lift. Efficiency means, in reality, cargo. That’s really crucial for this particular mission.”

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A look at the RS-25 engines on the SLS.
NASAGetty Images

In November 2015, NASA awarded $1.16 billion to adapt the 16 mothballed RS-25s to the new launch system and restart the production line for six new engines. “You hear that decision, you celebrate for 10 minutes, and then you get to work,” says Bradley. “It’s a different vehicle, so we got a lot to do. The rocket’s taller, so the pressure coming into the engines are higher when it’s started, so that’s important to note. The boat tail is a little colder, so that’s important.”

The Space Shuttles used three RS-25s, but the behemoth SLS demands four, which changes the operating environment. There’s not as much room under the megarocket as it seems, especially given the fact that people need room to work on them. “There’s only 8 inches between the wall of that vehicle and the edge of the engine,” Muddle says. “I’ve got to put a technician in there that has to lean over the hardware on the engine and try to torque something.”

The nearby flames from the motors of 17-story solid rocket boosters, one affixed to each side of the core stage, also now pose a threat. “In the shuttle program, our engines were 20 or 30 feet higher than the boosters,” Bradley says. “On this rocket, we’re adjacent to them, and those are big, powerful beasts.” Extra thermal shielding now protects the four core engines from the two raging SRBs.

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Rocketdyne’s RS-25 engine No. 0525 is readied for installation on the A-1 Test Stand at Stennis Space Center on July 23, 2018.

During SLS launches, RS-25 engines will have to run beyond what it was designed for—111% of the space shuttle main engine power, to be precise. This is where the reams of testing from the Space Shuttle era came in handy, since the versatile engine was already certified for those high levels.

“Without any redesign per se, we can go ahead and throttle up a little bit higher. And we think we can go higher than that,” Bradley says. “But right now we’re committed to 111%. That gets us to the next generation, but it’s already poised to evolve with the rocket.”

The enduring use of the RS-25 speaks to the strength of its original design, says Muddle. “You ask something to throttle up to 111%, 114% and then also be able to throttle down to 65%, that’s a huge range to ask a rocket engine to go do,” he says, “And for those [original engineers] to be able to design an engine able to work for all those different power levels is amazing to me.”

NASA doubled down on the RS-25 in May 2020 when it awarded Aerojet Rocketdyne a $1.79 billion contract to produce 18 more RS-25 engines, on top of the six already on order. The combined price between this new RS-25 contract and the earlier one—resulting in a $146 million tab for each single-use SLS engine—has raised critical eyebrows.

After all, it cost $40 million per RS-25 shuttle engine, while Russian RD-180 engines go for just over $20 million each, and in 2018 United Launch Alliance paid Blue Origin around $16 million per pair of new BE-4 engines that it will use for its Vulcan Centaur rocket.

Aerojet says a per-engine calculation is unfair, since the contract’s fee includes new hardware for additional testing, as well as investments in manufacturing tech that will generate overall savings. That’s still a lot of money for engines that will only be used once before being discarded into the ocean, but that’s just the way SLS is structured.

"You hear that decision, you celebrate for 10 minutes, and then you get to work...It's a different vehicle, so we got a lot to do."

Dealing with an expendable launch system is new for the RS-25 veterans at Aerojet Rocketdyne, where the primary design and manufacturing ethos had changed from certain reuse to cost reduction. Company facilities in California and Florida are creating legacy engine components using 21st-century manufacturing techniques, like manufacturing and laser printing, to reduce the time needed to make and quality check engines. “Most of the things we’re working on right now are to retain reliability, but make it less expensive,” Bradley says.

Still, the idea that a famously reusable rocket engine will be left to sink in the ocean seems to many like a waste. The new generation of RS-25s will only be fired during one test and one launch, so the engines won’t have the chance to develop those personalities that come from contact with maintainers.

The engine’s return on a one-and-done rocket makes for an emotional, bittersweet return. “I was sad to see my children go, but they’re going off to college now,” Muddle says. “They’re going off to do whatever they need to do next.”

Memories of Smoke and Fire

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A 500-second hot firetest of the RS-25 engine at NASA’s Stennis Space Center in Mississippi, February 28, 2019.

On January 28, 2021, an RS-25 developmental engine fired for a full duration, the entire 500 seconds it takes to vault Orion into orbit. Bradley went to Stennis Spaceflight Center in Mississippi to work the test—familiar ground for the now high-ranking executive engineer.

The visit triggered memories of the first rocket engine test Bradley observed as a fresh Aerojet hire at the same location. He watched from the roof of a building just a quarter of a mile away from where the engine roared to life, assaulting his ears and spreading shock waves through his chest cavity. “My thought at that time as a young engineer was: ‘How does that thing stay together?’” Bradley says.

Watching the RS-25 flare to life in January 2021, thundering well past the parameters of its design, reignited those memories. “I’ve got to admit that during the last single engine test I had the same thought,” he says. “I've been in this business for a long time, but the amount of power that we’re harnessing—I just never take it for granted.”

SLS is scheduled to fly later this year, but widespread belief is that a delay until 2022 is inevitable. But whenever SLS makes its maiden voyage, carrying an Orion capsule with an instrumented mannequin in the seat where astronauts will one day sit, Bradley plans to see his old friend once again.

“I’ll probably have a job to do,” Bradley says.”I’m hoping secretly that I'm in a certain position of responsibility that I can sneak up three minutes before the launch and watch it with my eyeballs.”`