Feasibility Assessments of a Unique Additive Manufacturing Method for Functionally-Graded Solid Fuel Ramjet Propellants

Solid fuel ramjets are often proposed for supersonic extended-range missiles because they have a higher volumetric energy density, which enables higher thrust over liquid fuel ramjets and longer powered range than liquid ramjets and solid rocket motors for an equivalently sized vehicle. Solid fuel ramjets have been used in two primary configurations: core-burning (the typical configuration) and air-ducted rockets. HTPB is the most widely utilized fuel for both configurations due to its relatively high volumetric energy density, thermal stability, and burns efficiently. Because HTPB requires casting and curing in molds to create the fuel grain, it has limitations. Complex geometries cannot be made without destroying the molds to release the cast, and it is practically not possible to make functionally graded fuels or propellants. This paper presents a unique manufacturing approach to 3D print fuels that can be functionally graded with metals, oxidizers, and other additives with greater ease and repeatability than can be achieved with HTPB. Initial tests to validate the feasibility and effectiveness of this approach are presented. It is found that ABS thermoplastic can achieve similar performance to HTPB while providing the 3D printable advantages desired.