New commercial rocket engines require fast and low-cost AM processes giving sufficient flexibility to react to the changing demand of launches. For these reasons, in the past few years, significant attention has focused on AM processes and developments, particularly powder bed fusion AM due to its design freedom and prevalence on the market. However, powder bed fusion AM techniques face challenges limiting the utilization and scalability for combustion chamber manufacturing such as limited build envelope dimensions; limited processing of metals and alloys, including multi-material layers; and high surface roughness, particularly in the cooling channel inner walls, which can reduce cooling efficiency significantly.
The Cold Spray Additive Manufacturing (CSAM) process, developed by Impact Innovations, has the potential to overcome all the limitations and offers a potential solution to manufacture combustion chambers with superior properties and no envelope size restrictions. Impact Innovations set up a collaborative project with Airborne Engineering (AEL), a UK-based company specializing in propulsion system design and testing. AEL designed a combustion chamber demonstrator according to Impact Innovations guidelines. The regeneratively cooled liner is a high-strength Cu-alloy, and the outer jacket material is Inconel.
The demo sample proved that the CSAM process is suitable for manufacturing combustion chambers and many advantages compared to other additive manufacturing processes have been identified: no protective atmosphere required, the simple joining technique of dissimilar materials/alloys, negligible thermal stress, no cooling channel surface roughness issues, access for inspection during production steps, abilities to re-work/repair areas for prototypes and to join additional parts without welding; and powder is only required for the material to be deposited, rather than in powder bed fusion AM processes where it is required to fill the entire build volume. Buy-to-fly ratio close to 1.
To demonstrate the dimensional flexibility of the process at Impact Innovations’ spray-lab, the spray lathe allows manufacturing of components up to Φ1500 mm diameter, 2000 mm length at a max component weight of 1500 kg. A full-size combustion chamber, as shown in Picture 2 is under fabrication at Impact Innovations and soon be available for fire testing at the AEL site.
The mechanical properties of the combustion chambers are very critical and depend on the materials used. A special cooling channel demo sample was manufactured to determine the mechanical properties of the high-strength Cualloy and Inconel. The resulting mechanical properties are shown in the charts: as-deposited, after heat treatment at room temperature and at elevated temperature (427°C).
The second important aspect is the deposition rate which has a significant effect on costs. The CSAM process developed by Impact Innovations has a deposition rate of 10 kg/h for Cu-alloy and 6.7 kg/h for Inconel, which is in comparison to powder bed fusion AM processes more than 20 times faster