Last year, engineers at NASA’s Marshall Space Flight Center in Huntsville, AL, tested an additive manufacturing process that is being used to make some of the parts for NASA’s new rocket, the Space Launch System (SLS), more efficiently and affordably without compromising performance and safety. Selective laser melting (SLM) is a 3D printing process used to create complex parts for the engine and other components of the rocket. Four RS-25 engines and two solid rocket boosters will power the core stage of the SLS.
“The opportunity for affordability extends well beyond manufacturing because these technologies give us the ability to reduce the entire development lifecycle from design to flight,” said Chris Singer, director of the Engineering Directorate at Marshall, where the SLS Program is managed for the agency.
The selective laser melting process begins with a digital 3D design transmitted to the SLM machine in which a highenergy laser melts metal powder and fuses the powder layer-by-layer to build the desired component. The process drastically reduces or eliminates the need for multiple welds common in traditional manufacturing methods. Marshall has three SLM machines; the largest can create parts up to 24 inches long, 16 inches wide, and 19 inches high.
“Selective laser melting is changing the way we think and do things in the manufacturing world,” said Andy Hardin, a subsystems manager in Marshall's Liquid Engines Office. “We have conducted a lot of studies to see which RS-25 parts would benefit most from the SLM process. Our initial estimates show in some instances, this technology can reduce the RS-25 part cost by as much as 45 percent, and significantly shorten development and manufacturing time.”
Selective laser melting is an example of how NASA is exploring the potential of additive manufacturing in high-technology production. NASA is on the cutting edge across this emerging field, from basic research in materials, to building entire rocket engines, to 3D printing parts for use on the International Space Station. The goal of these investments is not only to reduce costs and manufacturing time, but also to explore the potential of 3D printing to radically change the way space systems are designed, built, and supported throughout their lifetime.
“A lot of foundational work is being done right now,” said Ken Cooper, advanced manufacturing team lead in Marshall’s Engineering Directorate. “We are making samples and testing them. We have to demonstrate first that we can build these parts, which isn’t a simple process. A lot of work goes into how a part is printed. You can’t just push a button and have an intricate rocket engine part come out a minute later. We have to do a lot of development work, including analyzing and testing the strength of a material being used, how it reacts to environmental factors, and other conditions that may affect how the material performs.”
Starting with Copper
Selective laser melting had been used earlier to build the first full-scale copper rocket engine part, representing a milestone for the agency. Said Steve Jurczyk, associate administrator for the Space Technology Mission Directorate at NASA Headquarters in Washington, “Additive manufacturing is one of many technologies we are embracing to help us continue our journey to Mars, and even sustain explorers living on the Red Planet.”
Additive manufacturing has the potential to reduce the time and cost of making rocket parts like the copper liner found in rocket combustion chambers where super-cold propellants are mixed and heated to the extreme temperatures needed to send rockets to space.
“On the inside of the paper-edge-thin copper liner wall, temperatures soar to over 5,000 degrees Fahrenheit, and we have to keep it from melting by recirculating gases cooled to less than 100 degrees above absolute zero on the other side of the wall,” said Singer. “To circulate the gas, the combustion chamber liner has more than 200 intricate channels built between the inner and outer liner wall. Making these tiny passages with complex internal geometries challenged our additive manufacturing team.”
Marshall’s SLM machine fused 8,255 layers of copper powder to make the chamber in 10 days and 18 hours. Before making the liner, materials engineers built several other test parts, characterized the material, and created a process for additive manufacturing with copper.
“Copper is extremely good at conducting heat,” explained Zach Jones, the materials engineer who led the manufacturing at Marshall. “That’s why copper is an ideal material for lining an engine combustion chamber and for other parts as well, but this property makes the additive manufacturing of copper challenging because the laser has difficulty continuously melting the copper powder.”
The goal is to build rocket engine parts up to 10 times faster and reduce cost by more than 50 percent, according to Chris Protz, the Marshall propulsion engineer leading the project. “We are not trying to just make and test one part. We are developing a repeatable process that industry can adopt to manufacture engine parts with advanced designs. The ultimate goal is to make building rocket engines more affordable for everyone.”
Moving to Complex Parts
One of the most complex 3D printed rocket engine parts ever made, a turbopump was tested with liquid hydrogen propellant at Marshall last summer. The turbopump is a critical rocket engine component with a turbine that spins and generates more than 2,000 horsepower — twice the horsepower of a NASCAR engine. Over the course of 15 tests, the turbopump reached full power, delivering 1,200 gallons of cryogenic liquid hydrogen per minute, or enough to power an upper stage rocket engine capable of generating 35,000 pounds of thrust.
“Designing, building, and testing a 3D printed rocket part as complex as the fuel pump was crucial to Marshall’s upcoming tests of an additively manufactured demonstrator engine made almost entirely with 3D printed parts,” said Mary Beth Koelbl, deputy manager of Marshall’s Propulsion Systems Department. “By testing this fuel pump and other rocket parts made with additive manufacturing, NASA aims to drive down the risks and costs associated with using an entirely new process to build rocket engines.”
The 3D printed turbopump has 45 percent fewer parts than similar pumps made with traditional welding and assembly techniques. Marshall engineers designed the fuel pump and its components, and leveraged the exper - tise of four suppliers to build the parts using SLM.
During the tests, the 3D printed turbopump was exposed to extreme environments experienced inside a rocket engine where fuel is burned at greater than 6,000 °F to produce thrust. The turbopump delivers the fuel in the form of liquid hydrogen cooled below 400 °F. Testing helps ensure 3D printed parts operate successfully under these harsh conditions.
“Our team designed and tested the fuel pump and other parts, such as injectors and valves, for the additive manufactured demonstrator engine in just two years,” said Nick Case, a propulsion engineer and systems lead for the turbo - pump work. “If we used traditional manufacturing processes, it would have taken us double that time. Using a completely new manufacturing technique allowed NASA to design components for an additively manufactured demonstration engine in a whole new way.”
A Complete 3D Printed Engine
In December, the Marshall team moved a step closer to building a completely 3D printed, high-performance rocket engine by manufacturing complex engine parts and test firing them together with cryogenic liquid hydrogen and oxygen to produce 20,000 pounds of thrust.
“We manufactured and then tested about 75 percent of the parts needed to build a 3D printed rocket engine,” said Elizabeth Robertson, the project manager for the additively manufactured demonstrator engine at Marshall. “By testing the turbopumps, injectors, and valves together, we’ve shown that it would be possible to build a 3D printed engine for multiple purposes such as landers, in-space propulsion, or rocket engine upper stages.”
To test the parts together, they connected the parts so that they work the same as they do in a real engine, only they are not packaged together in a configuration that looks like the typical engine you see on a test stand.
“In engineering lingo, this is called a breadboard engine,” explained Nick Case, the testing lead for the effort. “What matters is that the parts work the same way as they do in a conventional engine, and perform under the extreme temperatures and pressures found inside a rocket engine. The turbopump got its ‘heartbeat’ racing at more than 90,000 revolutions per minute (rpm), and the end result is the flame you see coming out of the thrust chamber to produce over 20,000 pounds of thrust, and an engine like this could produce enough power for an upper stage of a rocket or a Mars lander.”
Seven tests were performed, with the longest tests lasting 10 seconds. Even if methane and oxygen prove to be the Mars propellant of choice, the propellant combination of cryogenic liquid hydrogen and oxygen tests the limits of 3D printed hardware because it produces the most extreme temperatures and exposes parts to cryogenic hydrogen, which can cause embrittlement. In addition to testing with methane, the team plans to add other key components to the demonstrator engine including a cooled combustion chamber and nozzle, and a turbopump for liquid oxygen.