Advanced Hybrid Rocket Fuels:

Research at Stanford University beginning in 1997 led to a class of very high regression rate fuels for use in hybrid rockets. The new fuel produces a very thin, low viscosity, low surface tension liquid layer on the fuel 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 of 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 can lead to much higher surface regression rates than can be achieved with conventional polymeric fuels that rely solely on evaporation. The liquid layer hybrid combustion theory was developed based on this observation as a practical tool for predicting regression rate of liquefying fuels.

propellantsIt 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 the new fuel are carbon dioxide and water. In contrast, the by-products of burning conventional solid rocket propellant with ammonium perchlorate (AP) oxidizer include acid forming gases such as hydrogen chloride. Moreover groundwater contamination by AP from rocket propellant manufacturing presents a significant environmental concern.
  • Paraffin-based fuels are inexpensive, typically one to two orders of magnitude less than solid propellants (per lb.).
  • The ability to precisely and continuously adjust the regression rate without substantially increasing the cost of the fuel or changing the processing technique. The adjustability of the regression rate is a critical virtue that can be quite beneficial in designing efficient hybrid systems with mission flexibility.
  • 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 estimated to be -108°C, lower than the glass transition temperature of rubbery fuels (i.e. for HTPB Tg is ~ -70°C). Moreover since paraffins are crystalline, the effect of glass transition on material properties is much less severe than for the crosslinked rubbery polymers.
  • Paraffin based fuels are approximately 17% denser than liquid kerosene.

Regression rates 3 to 5 times the predicted classical rate have been observed in test motors ranging from laboratory scale to 12,000 lbf. using gaseous oxygen, liquid oxygen and liquid nitrous oxide.