Last year Addaero started an in-house project to design, fabricate and fire an EBM additively manufactured rocket. In December, we shared with you the first finished component, the rocket nozzle. We are now happy to announce the completion of the second major step in the process with the fabrication of the injector assembly! Utilizing our new Arcam Q20+ EBM system, the injector took 30 hours to build. The part went through standard powder recovery and support removal processes and then, like the nozzle, steel wire was used to remove sintered powder from the internal channels.
The injector is responsible for delivering and mixing the propellants (liquid oxygen and kerosene) to achieve our design thrust of 1500 lb. The propellant mixture ratio is roughly 2.2 parts LOX to 1 part kerosene by mass. The injector utilizes a like on like (L-O-L) injection style with the oxidizer pairs forming a concentric ring around the fuel ports. Concentrating the fuel near the center of the combustion chamber while having oxidizer rich pockets near the walls helps prevent flameouts and promotes combustion stability. There are 9 LOX injection pairs and 12 kerosene injection pairs. The LOX ports have a diameter of 2.4 mm and the kerosene ports have a diameter of 2 mm. The injection velocities are predicted to be roughly 13.22 m/s and 16.74 m/s with a pressure drop of 51 psi and 59 psi for LOX and kerosene respectively. Although a larger difference in injection velocities is desirable for better mixing (usually one propellant should be at least twice the speed of the other), the upstream pressurization system cannot provide the excess pressure to facilitate a greater difference in injection velocities. There are coolant channels along the edge of the injector plate that will feed to the injector holes on the nozzle. We plan to machine small notches to ensure proper mating of the nozzle and injector assembly. There is one input for each propellant, and two inputs for the coolant (water), which will be machined with NPT fittings for hose connections. The protruding flange will be used to mount the injector to the test stand and was engineered with COMSOL MultiPhysics with a factor of safety of 10+.
As with the nozzle, this injector assembly was designed specifically for Arcam EBM using Ti6Al4V Grade 5 powder. As the system must transport liquid, supports in internal channels were avoided at all costs. Through several in-house experiments with the Arcam Q20+, we determined the maximum unsupported hole sizes the machine could handle with a build failure. The largest internal channels are roughly 7.5 mm in diameter, and the upper edges of these channels were contoured to account for droop during the melting process. The actual pattern and style of the injection ports was determined through iteration. The two primary injection styles that were considered were unlike triplet impinging jets (O-F-O) and like on like doublet impinging jets (L-O-L). While studies have shown that higher theoretical specific impulse (ISP) values can be obtains with O-F-O injector systems, L-O-L systems are traditionally more reliable and exhibit greater combustion stability. The assembly was drafted in SolidWorks.
The injector assembly went through the HIP process (Check out our blog post on the Effect of HIPing on EBM Produced Ti6Al4V) and is now going through a multi-step process to improve the surface finish across the injector and remove the remaining powder stuck inside the internal channels. In the coming months, we will machine the injector and fabricate an igniter, all moving towards the first pressure test this fall!