Sculpting Metal Using 3D Printing

metal_sculpting_redux
HUNTSVILLE, Ala. (NASA PR) — In order for humans to make that next leap beyond Earth’s orbit into the reaches of deep space, to Mars and beyond, NASA is currently constructing what will be the most powerful rocket ever built, the Space Launch System, or SLS. Its initial 70-metric-ton configuration will stand taller than the Empire State Building, provide 10 percent more thrust than the moon-trekking Saturn V and carry three times the payload of the space shuttle.

But not only is NASA aiming to take us yet again where no person has gone before, it’s also aiming to do that through the most efficient, cost-effective means possible. Toward that end, the agency is looking to break the mold of conventional rocket engine fabrication by introducing additive manufacturing — popularly known as 3D printing — to the process. The way rocket engine manufacturing works now, says NASA engineer Samuel Stephens, who’s stationed in the SLS Advanced Development Office at Marshall Space Flight Center in Huntsville, Ala., is by assembling a great majority of the parts piecemeal.

Aerojet Rocketdyne 3D printed thrust chamber made of a copper alloy is tested in October 2014. (Credit: NASA)
Aerojet Rocketdyne 3D printed thrust chamber made of a copper alloy is tested in October 2014. (Credit: NASA)

“There are specialized components such as turbo pumps, combustion devices and valves, and each of those has multiple parts such as inlet and outlet flanges, center bodies, actuators and so on that are specially made with traditional manufacturing techniques,” he says. “Then we have to assemble all those thousands of parts. There’s a lot of tough labor involved, a lot of man-hours dedicated.”
But additive manufacturing, particularly through a relatively new method called powder bed fusion, stands to rewrite the way a lot of those parts are made and assembled.

Marshall has maintained an additive manufacturing lab since 1991, when 3D printing for plastic parts first arrived on the scene. Powder bed fusion, on the other hand, prints metals. The laser technology that makes it possible to “sculpt” metal from powder matured about 4 years ago, says Ken Cooper, who leads the advanced manufacturing team at Marshall. “When the industry went away from pump lasers to using solid-state lasers, it gave us the wattage and precision needed to print metal.”

In describing how powder bed fusion works, it helps to make a comparison to skyscrapers, which are built layer by layer from the ground up. The powder bed fusion process begins with an empty steel plate that’s lowered into the machine. Next, a large windshield wiper–like device sweeps a layer of powdered metal a thousandth of an inch thick across the plate. The solid-state laser, following computerized directions, etches a shape into the bed, melting the powder together in precise patterns and creating a solid object.

NASA technicians inspect a rocket injector assembly as it's installed in the Rocket Combustion Laboratory at Glenn Research Center in July 2013. Built by Aerojet Rocketdyne using additive manufacturing, the part was successfully put trhough a series of firings by the agency. (Credit: NASA)
NASA technicians inspect a rocket injector assembly as it’s installed in the Rocket Combustion Laboratory at Glenn Research Center in July 2013. Built by Aerojet Rocketdyne using additive manufacturing, the part was successfully put trhough a series of firings by the agency. (Credit: NASA)

The plate then indexes down another thousandth of an inch, and the next layer of powder is applied and melted on top of the previous one. “The part gets continuously buried,” Cooper points out. “All you ever see is the very top layer as it gets built from the bottom up.”

The technique offers several advantages. Larger parts that are usually made by combining smaller ones can instead be printed as one piece, reducing the time and cost involved in making components. And a technology that can make virtually any component obviates the need for purchasing custom tools that might only be used to make a few different parts. There’s also less waste, because you print only what you need.

Before the technology can be used for SLS and other rockets, NASA is conducting full-scale material analyses of a range of printed metals. The results have been promising. Last year Cooper and his team tested their most complex pieces yet: rocket engine injectors responsible for sending propellant into the engine through 40 individual spray elements. Their design was similar to ones that would power the RS-25 engine that will be used to propel SLS.

Using the old method, 163 pieces would first have to be made individually and then assembled, whereas 3D printing meant that only two parts needed putting together. “This method allows us to save time and money while enhancing performance and reliability,” Cooper says, adding that the pair of injectors performed very well, producing 20,000 pounds of thrust at temperatures of 6,000 degrees Fahrenheit. Solid Concepts in Valencia, Calif., and Directed Manufacturing in Austin, Texas, fabricated the two prototypes.

Collaboration with industry is critical for making these advancements happen. In another joint effort through a Space Act Agreement, Pratt & Whitney Rocketdyne — now Aerojet Rocketdyne — partnered with NASA’s Space Technology Mission Directorate Game Changing Development Program and Glenn Research Center in Cleveland, Ohio, to complete a series of hot-fire tests on a copper alloy–based thrust chamber assembly the company had created through the Selective Laser Melting method. It’s an important milestone because while copper plays an important role in dissipating heat and maintaining integrity in rocket engines, it’s also more difficult to melt and meld with lasers due to its high reflectivity compared to other metals such as steel and nickel.

“NASA has been instrumental in helping us to understand the material and the design as well as the whole process up to getting that part successfully tested,” says Jay Littles, the company’s director of advanced launch programs.

Aerojet Rocketdyne has long contributed to American space exploration. The company provided the liquid rocket boosters for NASA’s Titan vehicle, which delivered the first crewed Project Gemini flight into orbit in 1965. More recently, in December, the firm provided propulsion technology in all stages of NASA’s successful test flight of the Orion spacecraft, which is slated to carry humans on the SLS. The goal for Aerojet eventually is to implement the technology into legacy products such as the RL10 upper stage engine, while also applying it to next-generation propulsion systems, including the Bantam engine family, as well as its new large, high-performance booster engine, the AR1.

Kristin Morgan, a strategic analyst at Marshall who is involved in the SLS development, says working with industry is critical to the agency’s success. “Ultimately, the more people we have who are working in these areas, the more knowledge will be generated and the faster we can adopt these techniques,” she explains. “NASA wins when the whole industry succeeds, because the better the quality, the better the part.”