In the first flight of a soviet rocket in August of 1933 the researchers used a hybrid design where vaporizing liquid oxygen was burned with a solid form of gasoline. The hybrid idea is to store the oxidizer as a liquid and the fuel as a solid, producing a design that can be throttled, but is much less susceptible to chemical explosion than conventional solid and liquid rockets. The fuel is contained within the combustion chamber in the form of a cylinder with a circular channel called a port hollowed out along its axis. Oxidizer is injected at the fore end of the port and, upon ignition, a diffusion flame forms over the fuel surface producing the hot gas needed for thrust. In the 86 years since the early soviet experiments, the hybrid idea has been tried several times but with only limited success. After the Space Shuttle Challenger disaster in 1986 there was a US national effort to develop hybrid rocket technology using rubberized fuels similar to the binders used in the shuttle boosters. But little progress was made, largely because the rate at which hybrids generate the hot gas needed for thrust is limited by the relatively slow breakdown and evaporation of polymeric solid fuels. In the late 1990s research in our lab at Stanford led to the identification of a class of paraffin-based fuels that burn at surface regression rates that are several times higher than that of polymeric fuels. These new fuels form a thin, hydro-dynamically unstable liquid layer on the melting surface of the fuel grain. Entrainment of droplets from the liquid-gas interface can more than triple the rate of fuel mass transfer. Using a recently developed combustion visualization tunnel, we have been able to directly see the droplet entrainment process, and imaging of OH* chemiluminescence has been used to measure the position of the flame above the fuel surface.
Since the start of our research in this area, we have carried out more than 1000 tests using a wide variety of oxidizers at thrust levels from 50N to 50,000N. The most exciting application of this technology may occur in the near future when NASA begins the decade long Mars Sample Return mission. The mission will begin with the launch of Mars 2020. Upon arrival at Mars, an unmanned rover will place 30 or so soil samples in small tubes. Later in the 2020s, a second spacecraft will land with a rover to collect those samples and place them in the payload bay of a small launch vehicle called the Mars Ascent Vehicle. At the right moment, the MAV will lift off the Martian surface and rendezvous with a third spacecraft waiting in orbit to carry the samples back to Earth. At the present time, a single-stage to orbit hybrid rocket with a paraffin-based fuel is planned for the MAV. The main reason for NASA's interest in this design stems from a somewhat unexpected advantage of paraffin fuel; it can be designed to remain structurally sound at the extreme temperatures encountered on Mars that may dip to -100C at night and reach above 20C during the day.