Fuel Information

Research at Stanford University beginning in 1997 led to a class of very high regression rate fuels for use in hybrid rockets. This class of fuels produces a very thin, low viscosity, low surface tension liquid layer on the surface when it burns. The instability of this layer is driven by the oxidizer gas flow in the port and leads to the lift-off and entrainment of droplets into the gas stream, greatly increasing the overall fuel mass transfer rate. In effect, this mechanism acts like a continuous spray injection system distributed along the port with most of the fuel vaporization occurring around droplets convecting between the melt layer and the flame front. Since droplet entrainment is not limited by diffusive heat transfer to the fuel from the combustion zone, this mechanism enables significantly higher surface regression rates than can be achieved with conventional polymeric fuels that rely on decomposition and diffusion for material removal. The liquid layer hybrid combustion theory was developed based on this observation and allows the prediction of regression rates for liquefying fuels.

It was found that members of the normal-alkane class of hydrocarbons which are solid at room temperature for carbon numbers greater than 14 have low surface tension and viscosity at the melt layer conditions typical of hybrid rockets. These fuels, which include the paraffin waxes and polyethylene waxes, are predicted to have high regression rates at oxidizer mass fluxes covering a wide range of hybrid rocket applications. SPG engineers have formulated fast burning paraffin-based fuels using the liquid layer hybrid combustion theory.  The advantages of these fuels compared with other technologies are summarized below:

  • Regression rate is 3-5 times as high as the classic polymeric fuels (including HTPB) enabling efficient, single-port designs.

  • The fuel is non-toxic, non-carcinogenic, non-hazardous and environmentally friendly.

  • The by-products of combustion of this class of fuels are carbon dioxide and water.

  • Paraffin-based fuels are inexpensive, typically one to two orders of magnitude less expensive per pound than solid propellants.

  • Regression rate can be precisely and continuously adjusted without substantially increasing the cost of the fuel or changing the processing technique.

  • Processing of the fuel grains is simple. Note that no polymerization reactions are involved. No curing agents are required. The scrap pieces of fuel can be re-melted and reused.

  • Paraffin waxes are hydrophobic, making them an ideal binder for metal, metal hydride or dense organic additives.

  • Being inert, paraffin-based fuels effectively have an infinite storage life.

  • The glass transition temperature of paraffin-based fuel is significantly lower than that of rubbery fuels (i.e., HTPB Tg ~ -70°C)

  • Paraffin based fuels are approximately 17% denser than liquid kerosene.


The high regression rates predicted with theory have been demonstrated in motors ranging from laboratory scale to 12,000 lbf. using gaseous oxygen, liquid oxygen, liquid nitrous oxide, and nitrogen tetroxide-based oxidizers.