Additive manufacturing of ammonium perchlorate composite propellant with high solids loadings

The effective solid propellant burning rate in a rocket depends on surface area and propellant composition. Currently, the surface area geometry in a rocket is limited to what can be practically cast using molds, etc. Additive manufacturing (AM) could allow the production of unique propellant grain geometries, however printing propellants with high solids loadings and viscosities is not readily possible using currently available printers. A new AM direct write system developed recently in our laboratory, is capable of printing visibly low-void propellants with high end mix viscosities into highly resolved geometries. The system was used to print ammonium perchlorate (AP) composite propellants at 85% solids loading using hydroxyl-terminated polybutadiene (HTPB) and a UV-curable polyurethane binder. The change in HTPB propellant viscosity with time after mixing was measured and the microstructure of the strands was evaluated with X-ray tomography scans. The burning rate of printed and cast strands was measured to compare the quality of the strands at high pressure, since propellants with significant voids should catastrophically fail due to flame spreading. The printed samples burned in a planar fashion up to pressures of 10.34 MPa with consistent rates that were comparable to the cast propellants. The HTPB propellant used was not optimized and showed some porosity due to gas generation, but strands printed with the UV binder exhibited extremely low porosity. A strand printed with no gaps in one half and gaps in the other failed catastrophically where intended at high pressure, demonstrating the ability to spatially grade propellants. This new system can produce adequate strands of composite propellant with high solids loadings without the addition of solvents, special binders (low viscosity, thermal softening, etc), or restricting use to formulations with lower viscosities, and enables the fabrication of complex propellant grain geometries. (C) 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.