A study of solid ramjet fuel containing boron-magnesium mixtures

Solid fuel ramjets (SFRJ) are known for their operational simplicity and high specific impulse. The performance of the SFRJ propulsion system is directly tied to the energy density and combustion behavior of the fuel. A typical solid fuel used in a ramjet application is a collection of metal particles suspended in a polymeric binder. Boron is the ubiquitous candidate when considering metal additives for fuels due to an impressive 122.5 kJ/cm3 energy density. However, boron requires long residence times in combustors due to its high melting and boiling points. Magnesium appears to be a natural complement to boron; while possessing a lower energy density (42.1 kJ/cm3), it burns with a high flame temperature and readily reacts in combustion with a low melting point. In this study, several HTPB-boron-magnesium fuels are studied on a small scale to evaluate performance for ramjet application. Holography experiments are conducted, as well as laser ignition tests, to study particle behavior just above the fuel surface. Small, center-perforated fuel grains are examined in a direct-connect SFRJ test stand configuration to measure ignition temperatures and performance parameters. Combustion efficiency of the HTPB-boron-magnesium fuel is found to significantly increase for one of the fuels studied.