Metal additive manufacturing (AM) has transformed the design and production of rocket propulsion systems, particularly combustion chambers and nozzles. The ability to fabricate complex geometries as single, integrated parts without costly tooling has shortened the path from design to testing. NASA’s work with Virgin Orbit on a copper alloy combustion chamber liner (GrCOP-84) encased in a nickel superalloy jacket demonstrated significant manufacturing time savings. “Traditionally, it takes many months to manufacture, test and deliver a conventional combustion chamber. We can reduce that time considerably,” said Paul Gradl, senior engineer at NASA.
Despite these advances, AM’s as-built surface finish remains a critical challenge, especially for components subjected to high-cycle fatigue. Complex geometries make uniform material removal difficult, and the roughness inherent in AM parts can impair performance. NASA’s Game Changing Development Program, under the Space Technology Mission Directorate, aims to raise the technology readiness level of AM processes, funding efforts that move concepts from lab prototypes to engineered systems. Within this framework, the Rapid Analysis and Manufacturing Propulsion Technology (RAMPT) program targets the combustion chamber and nozzle—high-cost, long-lead, heavy components—by scaling AM technologies and integrating special processes like regenerative cooling channels and bimetallic manifolds.
REM Surface Engineering, a family-owned firm with expertise in isotropic superfinishing, entered the AM field around 2010, tackling Ti6Al4V parts with surface roughness far higher than machined components. Conventional helicopter gear finishing removes about 5 µm of material to achieve Ra 0.025–0.10 µm, but AM parts often require 25–750 µm removal. REM developed Extreme ISF® processes—Chemical-Polishing (CP) and Chemical-Mechanical Polishing (CMP)—to meet this need. CP flattens surfaces and radiuses micro-valleys, while CMP uses alloy-matched chemistry to create a brittle surface layer that is easily removed in a tumbling environment, enabling precise, uniform refinement.
In 2017, NASA Marshall Space Flight Center sought REM’s expertise for nickel-base superalloy Inconel 625 rocket nozzles, requiring removal of 500 µm from hotwall surfaces with improved roughness. REM’s Phase 1 Small Business Innovation Research (SBIR) project evaluated CMP, electropolishing, and standard chemical milling. X-ray CT scans revealed porosity concentrated in the first 200 µm, within removal targets. The optimal finishing technique combined chemical milling (with plans to optimize into CP) followed by CMP, meeting NASA’s goals for roughness, defect elimination, scalability, and automation.
Phase 2 SBIR work expanded to alloys JBK-75, NASA HR-1, and IN718, adding fatigue and corrosion testing. For IN718, REM’s CP reduced roughness from ~8 µm Ra to 1.8–3.5 µm Ra, outperforming traditional chemical milling. CMP alone or combined with CP achieved full isotropic superfinish levels (<0.10 µm Ra), removing fatigue initiation sites. Scaling efforts included installing a CP cell in Texas capable of processing components up to 700 mm in each dimension, accommodating 35K-sized nozzles and chambers. A Phase 3 SBIR contract focuses on applying the optimal finishing technique to thrust chamber hardware, including BP-DED nozzles in JBK-75 and NASA HR-1, coupled with PBF-LB combustion chambers in NASA HR-1/GrCOP-42. Masking methods protect copper alloys from aggressive chemistries and shield coldwall surfaces with tighter material removal tolerances. Parallel collaborations with ASRC Federal Astronautics advanced CP and CMP chemistries for GrCOP alloys, applied to components like the 7K Long Life Additive Manufacturing Assembly combustion chamber. REM’s NASA-driven developments have spilled into commercial projects in rocket propulsion, turbomachinery, aircraft engines, land-based gas turbines, RF systems, and fusion reactors. Over fifteen significant customer engagements have emerged since Phase 1, many in private rocket propulsion, where traditional finishing is inadequate. REM’s AM business has grown over 500% from pre-SBIR levels. Future work may include optimizing chemistries for copper and nickel alloys, scaling to larger components, and refining internal geometries like cooling channels. Expanding material property datasets across alloys, build parameters, and heat treatments will aid industrial adoption, as surface texture’s influence on fatigue life is complex and not fully captured by Ra values alone. NASA’s public-private collaborations continue to accelerate AM’s maturation, supporting ambitions for human exploration of the Moon, Mars, and beyond.