A critical leftover from the space shuttle program is scheduled to continue flying well into the 2020s, but with a key difference. NASA has 16 space shuttle main engines (SSMEs), plus two ground-test articles, and it plans to use them all—four at a time—to power the first stage of its heavy-lift Space Launch System (SLS). Designed for multiple flights, the reusable powerplants will get one more mission each before winding up in the ocean.

Work on the big new rocket is moving toward a first flight in 2018, paced by the Orion crew capsule that will ride it to orbit). Three more missions with the surplus engines are planned after that. Now NASA and Aerojet Rocketdyne (AJR), successor to the companies that helped develop the engine in the 1970s, are beginning work in earnest on a throwaway version of the shuttle engine that conceivably could power human missions to Mars.

“It’s kind of a two-pronged effort,” says Steve Wofford, who runs the SLS Liquid Engines Office at NASA’s Marshall Space Flight Center. “You’ve got to commence production restart. We’ve been down for a while on RS-25 production, so you’ve got to restart all the vendors, restart all the lines, work all the factory layouts. What we’re also doing is reengineering everything to make the engine an expendable versus a reusable engine. All of our affordability objectives are wrapped into that as well.”

Aerojet Rocketdyne is working with Wofford’s shop to prepare a sole-source proposal for six new RS-25 engines that will be ready in time to serve as backups for the fourth and final SLS flight with the original SSMEs. Those engines will be built with new techniques to save money, and designed with an eye to incorporating additive manufacturing and other advanced tools to continue wringing out cost. The industrial team that AJR is pulling together must balance heritage from the SSME with affordability gains available with modern manufacturing, says Vice President Julie Van Kleeck, who heads the company’s Advanced Space and Launch business unit.

“[Finding] the right plan for bringing the cost down and taking advantage of modern materials and processes, but not going through a full NASA development and qualification program, that’s the fine line that we’re straddling right now,” she explains.

Company and government engineers are identifying the work that will be required to restart engine production, including identifying obsolete manufacturing techniques that can be replaced with contemporary approaches, according to Jim Paulsen, a longtime SSME engineer who is Van Kleeck’s deputy.

“The main combustion chamber used to be a plated design, structural nickel that was done in [Rocketdyne’s] Canoga Park [facility in Los Angeles],” he says by way of example. “Now over the years we’ve developed a more cost-effective way of doing that called hit bonding. Basically we use high pressure and heat together, and we can build them a lot quicker with a lot less money.”

Similarly, modern “net forging” produces an initial cast for the hot-gas manifold that is much closer to the final product, reducing the machining required to produce the component. The industrial team is using value-stream mapping—a process ATK has applied to the shuttle-heritage SLS solid-rocket boosters—to improve component producibility.

While they get ready to build the new engines, NASA and AJR also are beginning to test modifications that the surplus engines will need to make the shift from a three-engine configuration at the back of a space shuttle orbiter to a four-engine array on the SLS core stage. A series of hot-fire tests that began on Jan. 9 also marked the first use of a prototype advanced controller (photo) that was derived from the controller developed for the mothballed J-2X upper-stage engine.

On the SLS, the engine inlets will receive colder liquid oxygen at higher pressures because the LOX tank is taller and the vehicle will have higher acceleration than the shuttle. Plumes from the SLS solid boosters will be much closer to the liquid-fuel engines, so the RS-25 nozzles will need shielding from the heat.

“You always like to put the component into the engine environment relative to vibration and loads and so forth,” says Paulsen, noting that the next test in April will mark the first time the new controller will be operated with the new inlet conditions.

Like many other aerospace concerns, AJR is experimenting with the use of additive manufacturing to build parts for less money—and in some cases greater design complexit—than is possible with traditional machining. The company has used 3-D printing techniques to build a pressure-fed 5,000-lb.-thrust “Baby Bantam” rocket engine, and a hydrazine propulsion system for cubesats. Both have been hot-fired, the company says, and it is now procuring the equipment for larger additive manufacturing demonstrations under a contract with the Air Force. That growing expertise may eventually find its way into future variants of the RS-25.

“You’ve got to design it correctly to make sure you can leverage additive manufacturing and build various components,” Paulsen says. “We’ve hot fired engines with additive manufacturing components, J-2X being one of them . . . . Taking the technology and putting it into a flight program where we’re going to fly humans on it, we’ve got to be very careful that we have a good understanding of the material properties.”

The first six throwaway RS-25 engines “probably won’t” contain parts built with additive manufacturing, he says, but likely will be included in the next generation of changes after that because of the “huge” cost saving.

Aside from the new controller and the ablative thermal protection on the nozzle to protect it from the heat of the solid-fuel booster plumes, the first new RS-25s will be a lot like the surplus shuttle engines in performance, with a specific impulse of 452 sec., and probably weigh a little more.

“You’re going for a simpler engine, easier to manufacture, and as a result we have a weight bogey that the program has given us that increases the weight,” says Paulsen. “We’re trying to keep the reliability the same, the performance the same—they’re high-performance engines—but the program has said ‘well, if you can get me some affordability but it costs some weight, I’ll accept it.’ It’s not a lot, I think it’s 300 lb.” 

A version of this article appears in the January 26 issue of Aviation Week & Space Technology.