3D Systems Injector Earns the Nickname “Old Reliable”

Pictures: https://imgur.com/a/ExpjtEA

5, 4, 3, 2, 1!” Members of the Student Space Initiative huddle in silence and trepidation, peering out at a bundle of tubes, pipes, and electrical connectors set up in the distance. For about a year, the engine test stand had been collecting dust and the test site had been silent. The team had stood down from firing their first liquid rocket and over the course of a year, developed a new prototype to eliminate the problems of the old. Multiple attempts at firing up this new engine had been thwarted by faulty pyrotechnics and electrical snafus. Now they watch and wait as the engine startup sequence unfolds, hoping for ignition. The engine roars to life, spewing flames, rocking the test stand, and sending out a rolling thunder across campus.

Video of Hotfire: https://www.youtube.com/watch?v=9osoALNrB40

This was the first engine test witnessed by a couple of the observers, an awe-inspiring experience. The team was ecstatic – at last all of that hard work had been rewarded. However, the high spirits were quickly taken down a notch by a discovery of damage to the engine. The injector, the heart of the engine, was melted and scorched by the flames. The team had developed a new prototype, 3D printed from stainless steel. The very flames that power the engine, reaching about 4000°F, had consumed the outer wall of the injector itself, exposing the internal manifolds. This new design, created in partnership with Protolabs, a prototype manufacturing company, had pushed the limits of the hardware, cutting away unnecessary material and riding the edge of what was possible to print. The design would need further iteration. 

With a few weeks left in the year, it looked like any hope of certifying the engine for flight was lost. However, one shot remained for the team. Buried in a box of old hardware was the green Inconel injector from the previous year, the “3D Systems injector,” named after the company that printed it. The chief designer of this part, James Kolano, had since graduated and went to work at SpaceX. Little did he know that his old part was about to be put to use again. 

The part was a marvel of engineering for a student project. The students had worked with engineers at 3D Systems and their Design for Additive Manufacturing (DfAM) tools to craft a printable injector that, to a casual observer, resembled a circus tent. The structure resembled that of a cathedral, with sloped ceilings rising up from the injector face. A complicated pattern of colliding orifices shoots the propellant into the fires of the combustion chamber, releasing an incredible amount of power – about 1.5 MW or equivalent to the P-51 Mustang fighter plane – all within a 4” combustion chamber.  

A key DfAM capability the team took advantage of with its 3D Systems fuel injector was parts count reduction: going from a product that would traditionally require multiple assembled components to a monolithic structure that requires no assembly. The student designer, James Kolano, says removing fasteners from the design not only helped the team lower final weight, but also helped remove known points of failure. In a non-3D printed design, the team would have had to make several parts and fasten them together with O-rings and bolts. Not only is this assembly typically difficult, but it is not always reliable. “O-rings notoriously have leaks,” says Kolano. “By using 3D printing we had a single part with no failure points.” 

Beyond eliminating the need for fasteners, the team requested 3D Systems’ assistance in removing additional weight from a few internal cavities. 3D Systems’ application engineers incorporated internal lattice structures within the designated areas using 3DXpert®, an all-in-one software that covers the entire metal additive manufacturing process. In addition to this weight reduction, 3D Systems performed pre-printing operations to facilitate powder removal in post-processing, as well as a printability check using 3DXpert to ensure the part would build without complication. 
The final part was printed on a 3D Systems ProX® DMP 320 metal printer in LaserForm® Ni718 (A), an oxidation and corrosion-resistant Inconel alloy. Once printed, 3D Systems’ team removed unused material from the part’s interior, heat-treated the part for stress relief, and removed the part from the build plate using wire Electrical Discharge Machining (EDM). The final part weighed 1.16 pounds (0.53 kg), quite a small amount in comparison to the 350 pounds of thrust it produced.

The part had performed fantastically the year prior, leading to the first successful firing of a student liquid rocket engine at Stanford University. But as the team’s second prototype firing revealed shortcomings in their new prototype injector’s design, all attention was turned yet again to “Old Reliable,” the 3D Systems injector. 

On June 14th, 2019, the students of SSI pulled together one final effort to re-test their engine. Attempting a full-duration burn, all stakes were riding on this final test. Deep within the engine lie the 3D Systems injector, freshly oxygen-cleaned. When the engine fired up this second time, reports came in from across campus that SSI was “at it again” with their rocket testing. The engine eclipsed all previous power records of the team, coming in at an average sustained thrust of 310 pounds for a whopping ten seconds, enough to send SSI’s rocket past the student amateur altitude record. The 3D Systems injector, “Old Reliable,” endured the test completely unscathed. 

“3D Systems’ guidance and design reviews were very helpful to the students, and I think the benefits worked both ways,” says Greg Zilliac, NASA Research Scientist and consulting professor in Stanford’s Department of Aeronautics and Astronautics. “I believe 3D Systems added to its knowledge of the concerns involved in manufacturing high temperature parts for aerospace applications, which are different from other applications. This project was a definite win-win on both ends.”

-Nick Gloria