Solid Fuel Ramjets

Leveraging from its deep expertise in the field of hybrid rocket propulsion and the fast burning solid fuels, SPG has started a research and development program in the area of Solid Fuel Ramjets (SFRJ). Due to their inherent simplicity SFRJ's present a cost effective option for a wide range of applications that demand a sustained thrust force during a substantial portion of their mission profile. UAV's and target drones are believed to be the primary candidates for SFRJ propulsion. Testing program with a 2,000 lb thrust class paraffin-based SFRJ is ongoing.

Background:

solid fuel ramjets

The SFRJ cycle is the same as the ramjet cycle except that the fuel exists in solid form within the chamber and the stoichometry of combustion is controlled by the regression rate of the fuel. The fuel is not a propellant in the solid rocket motor sense but a pure fuel, inert without external oxidizer much like in a hybrid rocket motor. A wide range of fuels can be used from polymers such as PMMA or PE to long-chain alkanes such as paraffin or cross-linked rubbers such as HTPB. Because the fuel exists in the solid form, inclusion of solid metals is significantly easier than in a liquid fueled ramjet. SFRJ's offer some very significant advantages over liquid fuel ramjets such as:

  • Extremely simple compared with liquid fueled rockets or ramjets? In its simplest form, a SFRJ is basically a tube with a fuel grain cast in it.
  • Higher fuel density in the solid phase for pure hydrocarbons and even higher if metal additives are used
  • Easy inclusion of metal fuels such as boron, magnesium or beryllium which raise the heat of combustion and/or the density and therefore the density impulse capability compared with liquid ramjets
  • Solid fuel acts as an ablative insulator, allowing higher sustained combustion chamber exit temperature levels (and hence specific thrust) with less complexity
  • Fuel is stored within the combustion chamber allowing for more efficient packaging and higher mass fractions than liquid ramjets
  • No need for pumps, external tankage, injectors or plumbing for fuel delivery