A hybrid-propellant rocket is a rocket with a rocket motor that uses rocket propellants in two different phases: one solid and the other either gas or liquid. The hybrid rocket concept can be traced back to the early 1930s. Hybrid rockets avoid some of the disadvantages of solid rockets like the dangers of propellant handling, while also avoiding some disadvantages of liquid rockets like their mechanical complexity. Because it is difficult for the fuel and oxidizer to be mixed intimately (being different states of matter), hybrid rockets tend to fail more benignly than liquids or solids. Like liquid rocket engines, hybrid rocket motors can be shut down easily and the thrust is throttleable. The theoretical specific impulse ( I s p {\displaystyle I_{sp}} ) performance of hybrids is generally higher than solid motors and lower than liquid engines. I s p {\displaystyle I_{sp}} as high as 400 s has been measured in a hybrid rocket using metalized fuels. Hybrid systems are more complex than solid ones, but they avoid significant hazards of manufacturing, shipping and handling solid rocket motors by storing the oxidizer and the fuel separately.
History The first work on hybrid rockets was performed in the early 1930s at the Soviet Group for the Study of Reactive Motion. Mikhail Klavdievich Tikhonravov was responsible for the first hybrid propelled rocket launch, the GIRD-9, on 17 August 1933, which reached an altitude of 400 metres (1,300 ft). He would later supervise the design of Sputnik I and the Luna programme. In the late 1930s, there was concurrent work done in Germany at IG Farben and in the United States at the California Rocket Society. Leonid Andrussow, working in Germany, theorized hybrid propellant rockets. O. Lutz, W. Noeggerath, and Andrussow tested a 10-kilonewton (2,200 lbf) hybrid rocket motor using coal and gaseous N2O as the propellants. Oberth also worked on a hybrid rocket motor using LOX as the oxidizer and graphite as the fuel. The high heat of sublimation of carbon prevented these rocket motors from operating efficiently, as it resulted in a negligible burning rate. In 1938, the California Rocket Society experimented with a coal and gaseous oxygen hybrid rocket.
In the 1940s, the California Pacific Rocket Society used LOX in combination with several different fuel types, including wood, wax, and rubber. The most successful of these tests was with the rubber fuel, which is still the dominant fuel in use today. In June 1951, a LOX / rubber rocket was flown to an altitude of 9 kilometres (5.6 mi). Two major efforts occurred in the 1950s. One of these efforts was by G. Moore and K. Berman at General Electric. The duo used 90% high test peroxide (HTP, or H2O2) and polyethylene (PE) in a rod and tube grain design. They drew several significant conclusions from their work. The fuel grain had uniform burning. Grain cracks did not affect combustion, like it does with solid rocket motors. No hard starts were observed (a hard start is a pressure spike seen close to the time of ignition, typical of liquid rocket engines). The fuel surface acted as a flame holder, which encouraged stable combustion. The oxidizer could be throttled with one valve, and a high oxidizer to fuel ratio helped simplify combustion. The negative observations were low burning rates and that the thermal instability of peroxide was problematic for safety reasons. Another effort that occurred in the 1950s was the development of a reverse hybrid. In a standard hybrid rocket motor, the solid material is the fuel. In a reverse hybrid rocket motor, the oxidizer is solid. William Avery of the Applied Physics Laboratory used jet fuel and ammonium nitrate, selected for their low cost. His O/F ratio was 0.035, which was 200 times smaller than the ratio used by Moore and Berman. In 1953 Pacific Rocket Society (est. 1943) was developing the XDF-23, a 10-by-183-centimetre (4 in × 72 in) hybrid rocket, designed by Jim Nuding, using LOX and rubber polymer called "Thiokol". They had already tried other fuels in prior iterations including cotton, paraffin wax and wood. The XDF name itself comes from "experimental Douglas fir" from one of the first units.
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