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There is a lot riding on NASA's Space Launch System (SLS). Not only does the agency's first new heavy-lift booster since the Saturn 5 that took U.S. astronauts to the moon play a central role in the future of the American spaceflight, it also provides a critical test for technology expected to figure prominently in revamping the country's ailing manufacturing industry.
NASA's Marshall Space Flight Center in Huntsville, Ala., is testing an approach called selective laser melting (SLM) to create parts for the J-2X and RS-25 rocket engines that will power the SLS, whose maiden voyage is slated for 2017 (pdf). The space agency expects SLM to simplify the process of making certain parts and in some cases halve the cost of producing them—a huge advantage for NASA, provided the components can withstand the rigors of lifting the largest launch vehicle ever built into space.
The first version of the SLS is a 70-metric-ton rocket that will lift around 70,000 kilograms while providing 10 percent more thrust than the Saturn 5. This SLS will power the 2017 Exploration Mission 1, which will launch an unmanned Orion spacecraft on a circumlunar voyage as a precursor to Exploration Mission 2. That mission, scheduled for 2021, will use a 130-metric-ton version of the SLS to launch Orion and a crew of up to four astronauts. This second SLS will be capable of lifting more than 130,000 kilograms and provide 20 percent more thrust than the Saturn 5.
Cash-strapped NASA is counting on SLM to speed SLS's development and reduce the program's costs. SLM is a type of additive manufacturing technology, which uses computer-aided design (CAD) files to build parts layer by layer (3-D printing is perhaps the most well known example of additive manufacturing). With SLM, a finely powdered alloy is deposited in a layer as thin as 20 microns and then fused together by a focused laser beam inside a chamber containing inert gas such as argon or nitrogen. Once the laser has turned that layer into solid metal, another layer of powder is deposited and the process is repeated.
NASA is testing the viability of making engine parts from nickel-based alloys using an SLM machine (pdf) with a square cubical build chamber measuring 250 millimeters on each side and a depth of 280 millimeters. These same alloys are already used to make 90 percent of the parts in the RS-25 and J-2X engines. The key difference is that the engines' current elements are forged and then milled into their final shapes. Often several pieces must be welded together to create a part.
Marshall engineers began evaluating alternative approaches to building parts for the next-generation J-2X engine a few years ago. In late 2010 they turned to SLM to create a duct for a gas generator in the engine. "The part itself is not necessarily complex—it's a [10-centimeter] in diameter duct that's bent in a U-shape," says Andy Hardin, SLS Liquid Engines Office engine integration hardware lead. However, "because of the thickness and the radius of the bend, it's very difficult to make. We were having trouble getting vendors to do this properly."
After printing the duct, the engineers set about deconstructing it to study its metallurgy and microscopic structure. They found that although the part was not as strong as a forged and milled duct, it fell within the "minimal acceptable range," Hardin says. "If you made a part [using SLM], the material properties would be degraded somewhat but not much." One structural advantage is that the part required no welding. "When you make a part out of multiple pieces, welds are always the weakest points," he adds. This opened the door for the engineers to consider using SLM to make other engine parts as well.
SLM, and additive manufacturing in general, is not a viable option for all J-2X or RS-25 engine parts. For starters, the printed parts must be small enough to fit in the machine's build chamber. And a lot more testing is required to determine whether components such as turbines, which operate under the most intense conditions, could be made properly using SLM, Hardin says. Good candidates for SLM are those with complex geometries that are difficult to make and require multiple welds to achieve those geometries. Depending on how well printed J-2X parts fare in tests, Marshall engineers hope to at some point use SLM to likewise make parts for the older RS-25, which served as the space shuttle's main engine throughout its 30-year history.
Another incentive for NASA to transition to additive-manufactured parts: their contractors are beginning to adopt the technology in their factories. "As a big customer for many of these manufacturers, we thought it was important that we understand the technology," Hardin says. NASA does not want to hold manufacturers back by failing to create specifications for parts made using SLM or some other additive process, he adds.
As such, NASA's success with SLM could be a boon to a flagging U.S. manufacturing industry that seeks to create more domestic jobs but has been reluctant do so because of high costs.
