"green" propellants 3-D printing 3D printing HTPB History of engineering & technology Marxman aluminum agglomeration aluminum aggregation ballistic reconstruction technique blowing factor carbon fiber composite coated aluminum combustion computational fluid dynamics c∗ efficiency diffusion-limited electric feed system energy of activation engine test error propagation analysis feedback loop control fluoropolymer fuel grain fuel regression fuel regression rate fuel-rich composite powder gas torch hybrid hybrid combustion hybrid propulsion