Fused Filament Fabrication of Micro-Turbojet Engines for Small UAVs: Energy-Efficiency Perspectives on High-Performance Polymers and Sinterable Bound-Metal Filaments

The growing operational demands placed on small uncrewed aerial vehicles (UAVs) favor propulsion systems with high onboard energy density, which has renewed interest in micro-turbojet engines in the 100–500 N thrust class. Manufacturing these miniature, geometrically complex engines by subtractive machining or laser powder bed fusion (L-PBF) entails high capital expenditure and, in the case of L-PBF, a substantial feedstock energy burden, which limits their use on attritable (expendable) platforms. This paper is a narrative review; it does not fabricate, instrument, or test a micro-turbojet engine, and none of the arguments presented here constitute experimental validation. Its purpose is to assess, against the published evidence, whether fused filament fabrication (FFF) both in its high-performance polymer form and in its sinterable bound-metal form offers a credible manufacturing route for this engine class, and to identify where the evidence is insufficient. We examine a hybrid material strategy: high-performance thermoplastics (CF-PEEK, PEI) for cold-section lightweighting, and the shaping, debinding and sintering (SDS) route for bound-metal (17-4 PH, 316L, Inconel 718) and ceramic filaments in the hot section. Three findings temper the optimism of earlier accounts. First, a simplified per-part energy model, benchmarked against published life-cycle inventories, shows that the manufacturing-energy advantage of SDS over L-PBF is conditional on furnace loading: with the assumptions adopted here, batch sintering must accommodate roughly 15 or more parts per cycle before SDS becomes the lower-energy route, and a single-specimen comparison favors L-PBF. Second, a first-order conduction analysis indicates that the 2–5% residual porosity of sintered FFF metals lowers the through-wall temperature drop of a combustor liner by only a few kelvin, two orders of magnitude less than a thermal barrier coating, so residual porosity should not be presented as a thermodynamic benefit. Third, applying a classical admissible-roughness criterion to representative micro-turbojet blade chords and Reynolds numbers shows that as-built FFF surfaces exceed the admissible equivalent sand-grain roughness by a factor of three to seven, making aerodynamic post-processing a quantified requirement rather than a qualitative recommendation. We conclude that FFF is presently credible for static cold- and hot-section hardware on attritable platforms, that its use for rotating hot-section components is not supported by the available data, and that the economic case is stronger and better evidenced than the manufacturing-energy case.