April 2024

In addition to the February 2024 Momentum article, which established the viability of additive manufacturing (AM) aluminum rocket nozzles with A6061-RAM2 powder, the RAMFIRE project used A6061-RAM2 to print a 36-inch diameter aerospike nozzle with complex integral coolant channels.

For nearly seven decades rocket engineers have considered an alternate nozzle design beyond the bell-nozzle rocket engine, the aerospike design breaks free from the traditional bell nozzle rocket engine design, which is efficient at only one point in the rocket’s trajectory.

What exactly makes this nozzle so enticing, especially after the bell-nozzle has more than proved its capabilities throughout the history of human spaceflight? 

The Aerospike’s inside-out rocket nozzle plume travels externally rather than exiting inside of the traditional bell-shaped nozzle. The main advantage of aerospike nozzle is that, as the rocket climbs, atmospheric and airstream pressure act on the plume to keep it at an optimum setting along the entire trajectory. This allows for a very efficient engine performance in flight, capable of delivering higher payloads while decreasing overall rocket weight and improved performance over a range of pressure altitudes.

So, if the aerospike nozzle design is considered a more efficient way to propel rockets to outer space, why has it never been seriously tested on the launchpad?

The lack of actual flight test data has precluded use of these nozzles in current as well as next generation space launch vehicles. In addition, the configuration of an aerospike nozzle presents unique challenges to the designer and fabricator.

The mindset of the past is changing with the introduction of AM. NASA recently validated data from hot fire tests on their 3D printed aerospike engine and reported that recent advancements in 3D printing can overcome some of the engine’s design challenges—specifically, how to manage its temperature. The positive results have green-lighted NASA engineers to develop a larger version.

NASA’s RAMFIRE (Reactive Additive Manufacturing for the Fourth Industrial Revolution) project commissioned Elementum 3D to work closely with their engineers and scientists and RPM Innovations to develop and print a 36” diameter aluminum aerospike rocket nozzle out of A6061-RAM2 material. The build was performed using RPM Innovations’ large format laser powder direct energy deposition (LP-DED) process.

Why has it taken almost 70 years to successfully produce a lightweight, high-strength aluminum rocket engine?

For one thing, conformal cooling channels are needed to keep the nozzle well below the material’s melting temperature. Curved internal voids are a specialty of additive manufacturing; these would be far more complex to achieve using a casting process and machining them would not be possible. Secondly, metal additive manufacturing via laser melting processes only became industrialized in the past few decades as computer, automation, and laser technology became simultaneously increasingly sophisticated and affordable. And finally, additive manufacturing of aerospace grade aluminum materials has only been possible since Elementum 3D invented its RAM technology in the past decade.

Aluminum alloys are highly prone to a type of cracking called hot tearing under the rapid heating and cooling conditions inherent to laser welding processes, and some popular wrought aluminum alloys including AA6061 are widely considered unweldable for this reason. Elementum 3D’s RAM chemistry serves to control the solidification process, resulting in crack free, fine-grained microstructures and printed material with strength equal – and in some cases, superior – to wrought aluminum.

Will the combination of optimized thermal and mechanical properties of A6061-RAM2 generated from RAM technology and the design freedom of additive manufacturing be the path to an operational aerospike rocket engine?

Only time and further research can answer that question. The research data acquired from the optimization of A6061-RAM2 aluminum alloy for large blown powder DED brings incredible confidence in enabling the production of an operational aerospike nozzle. NASA engineers plan to use the demonstration nozzle as a proof of concept to inform future component designs.