Aluminum built to withstand rocket power

Exploring the potential of near-infinite geometries, engineers from NASA's Marshall Space Flight Center has partnered with Elemuntum 3D to print rocket engines from a metal that, for all intents and purposes, is not ideal for millions of pounds of thrust – aluminum. The capabilities of additive manufacturing, however, may just unlock a different future into the stars.

Aluminum has many of the hallmarks of an ideal metal for building rockets – lower density, high-strength, and very lightweight.

However, due to its low tolerance to extreme heat and its tendency to crack during welding, aluminum has typically been passed over for more resilient metals, especially for use in additive manufacturing of rocket engines-until now.

Introducing NASA's latest development under the Reactive Additive Manufacturing for the Fourth Industrial Revolution, or RAMFIRE, project.

Funded under the space agency's Space Technology Mission Directorate (STMD), RAMFIRE has paid particular attention to advancing lightweight, additively manufactured aluminum rocket nozzles and using clever design enabled by the strong point of 3D printing – building nearly any shape imaginable – the nozzles have been constructed with small internal channels that are able to keep it cool enough to prevent melting during blastoff.

"Mass is critical for NASA's future deep space missions," said John Vickers, principal technologist for STMD advanced manufacturing. "Projects like this mature additive manufacturing along with advanced materials, and will help evolve new propulsion systems, in-space manufacturing, and infrastructure needed for NASA's ambitious missions to the Moon, Mars, and beyond."

With conventional manufacturing methods, a rocket nozzle may require as many as a thousand individually joined parts. Hence, meticulous care is spent to ensure no faults exist to put the entirety of the mission at risk or, worse, future astronaut's lives at risk.

As can be surmised, this requires a lot of time and even more money. With RAMFIRE nozzles, however, they can be built as a single piece, requiring far fewer bonds with significantly reduced manufacturing times.

Origins of RAMFIRE

Setting out with the goal to use the well-known lightweight metal in rocket nozzles, NASA did not have to look far, as a company had already been exploring the possibilities of metal 3D printing with aluminum.

Founded in 2014, Elementum 3D is one of the earliest trailblazers for additive manufacturing. Based out of Erie, Colorado, about a year after its inception, founder Jacob Nuechterlein filed a provisional patent for its Reactive Additive Manufacturing technology.

Shortly after that, NASA's Jet Propulsion Laboratory became its first customer with the purchase of A2024-RAM10 aluminum powder.

In the ensuing years, the company rocketed toward success, winning awards and grants such as the National Science Foundation Small Business Innovation Research grant, the Colorado State Office of Economic Development and International Trade grant, sub-awarded NASA's Tipping Point grant, or winning the Formnext 2017: "Start-up Challenge," all while maintaining development of its now widespread portfolio of specialized aluminum powders.

But what truly caught NASA's eye was the potential of RAM.

Explained by Elementum 3D as a "technology that utilizes exothermic reactions to synthesize product materials during the additive process to improve printability and properties," it could be understood as a secondary forging process that occurs due to the chemical makeup of its powders that reacts from the excess heat during printing.

What this allows then is the formation of advanced metal matrix composites, which is just a fancy way of saying that the metals become reinforced internally with fibers or particles that improve specified properties such as thermal conductivity, hardness, or yield strength.

Aiming for an aluminum alloy with special metal matrices that can withstand the force of a rocket, NASA and Elementum 3D first developed the novel aluminum variant known as A6061-RAM2 to build a nozzle and modify the powder used with a type of metal 3D printing called laser powder directed energy deposition.

However, not one to put all their eggs in one basket, NASA reached out to another commercial partner, RPM Innovations, out of Rapid City, South Dakota, to take this newly invented aluminum and specialized powder to build the RAMFIRE nozzles using their LP-DED process.

"Industry partnerships with specialty manufacturing vendors aid in advancing the supply base and help make additive manufacturing more accessible for NASA missions and the broader commercial and aerospace industry," said Paul Gradl, RAMFIRE principal investigator at NASA Marshall. "We've reduced the steps involved in the manufacturing process, allowing us to make large-scale engine components as a single build in a matter of days."

Put it to the test

With its sights on further horizons, NASA's Moon to Mars objectives will require the capability to send more cargo to deep space destinations. The functions of this new novel alloy could play an instrumental role by enabling the manufacturing of lightweight rocket components capable of withstanding high structural loads.

Typically costing upwards of US$1,500 per kilogram (2.2 pounds) to send anything outside Earth's atmosphere, if the overall weight of the vehicle can be reduced, extraplanetary travel could begin seeing more reasonable costs.

Not settling on a maybe, earlier this summer at Marshall's East Test Area, two RAMFIRE nozzles completed multiple hot-fire tests using liquid oxygen and liquid hydrogen, as well as liquid oxygen and liquid methane fuel configurations.

With pressure chambers in excess of 825 pounds per square inch – much more than anticipated testing pressures – the nozzles successfully accumulated 22 starts and 579 seconds, or nearly 10 minutes of run time.

Thus, testing demonstrated that aluminum-based 3D-printed rocket nozzles can operate in the most demanding deep-space environments.

"This test series marks a significant milestone for the nozzle," Gradl said. "After putting the nozzle through the paces of a demanding hot-fire test series, we've demonstrated the nozzle can survive the thermal, structural, and pressure loads for a lunar lander scale engine."

In addition to successfully building and testing the rocket engine nozzles, the RAMFIRE project has used the RAMFIRE aluminum material and additive manufacturing process to construct other advanced large components for demonstration purposes. These include a 36-inch diameter aerospike nozzle with complex integral coolant channels and a vacuum-jacketed tank for cryogenic fluid applications.

NASA and industry partners are working to share the data and process with commercial stakeholders and academia. Various aerospace companies are evaluating the novel alloy and the LP-DED additive manufacturing process and looking for ways it can be used to make components for satellites and other applications.

But as it has been proved successful, it is safe to say that the long-held thought of the weakness of aluminum can just be cleverly engineered into desirable strengths.