Although hybrids have been in development over the last 50 years, they have not made it into mainstream commercial applications because they did not produce as much thrust as liquid and solid systems. “Hybrid rockets tend to be sort of anaemic in their ability to produce thrust,” Cantwell said. This is because the fuel burns too slowly, relying on a process limited by the rate at which fuel evaporates and mixes with oxidizer. By contrast, the fuel and oxidizer are forced together in liquid systems and pre-mixed in solid systems.
In 1995, the U.S. Air Force began to address this problem with a new type of hybrid fuel — a simple hydrocarbon, pentane, frozen using liquid nitrogen. The pentane burned three to four times faster than conventional fuels. The Air Force engineers explained their results by saying that less heat was required to gasify the pentane than was needed for conventional solid
fuels. Cantwell and Karabeyoglu felt that this explanation was inadequate in light of the “blocking effect,” which limits the amount by which fuel evaporation can be increased simply by increasing the rate of heating.
The effect occurs because the increasing evaporation pushes the flame away from the surface and blocks heat transfer even as the heating rate increases. Karabeyoglu proposed an alternate mechanism. He suggested that as oxidizer flows over it, the surface of the pentane melts to form a low -viscosity liquid layer that becomes unstable and forms waves that are easily
pulled off the liquid surface as a spray of droplets that evaporate, mix and burn to produce thrust. Karabeyoglu then set out to find materials that have the same physical properties as frozen pentane but that are naturally solids at room temperature.
The paraffin waxes were the perfect candidates.
The Stanford team first tested paraffin in a laboratory-scale rocket motor in November 1998 and found that like solid pentane, it burned three to four times faster than conventional solid fuels. To date, they have conducted more than 250 laboratory and field tests in collaboration with engineers at NASA Ames Research Center.
They have tested rocket motors with 2,500 pounds of thrust, the amount that might be needed for a third stage rocket in a launch system. “Further scale-up tests are needed before paraffin-fuelled rockets can be utilized in lower stage rockets requiring thrust levels of 200,000 pounds or more,” Cantwell said. Cantwell projects that commercial application of paraffin
fuels could become a reality in as few as three years.
Stanford has secured a patent on the use of paraffin in rocket fuel applications, and Cantwell and Karabeyoglu have started Space Propulsion Group Inc., a company geared toward commercializing the technology. David Altman, a consulting professor of engineering who is a co-founder and co-inventor of the technology, heads the company. Cantwell has high hopes for paraffin - fuelled motors. “Solid rocket boosters remain among the most dangerous part of any space shuttle mission,” he said. “I think [the paraffin-based hybrid] would be a good candidate for replacing the shuttle solid rocket boosters.”
