A hybrid-propellant rocket is a rocket with an engine that uses propellants in two different phases, one solid and the other gaseous or liquid. The concept can be traced back to at least the 1930s.
This picture shows a transparent 3D-printed hybrid-rocket fuel grain with dual helical fuel ports, a post-combustion chamber, and a de Laval nozzle, photographed prior to the hot-fire test. The motor used nitrous oxide as the oxidizer and Pyrodex pellets for the igniter.Photograph credit: Matt Steiner and Kees08
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