John Whiteside Parsons (born Marvel Whiteside Parsons; October 2, 1914 – June 17, 1952) was an American rocket engineer, chemist, and Thelemite occultist. Parsons was one of the principal founders of both the Jet Propulsion Laboratory (JPL) and the Aerojet Engineering Corporation. He invented the first rocket engine to use a castable, composite rocket propellant, and pioneered the advancement of both liquid-fuel and solid-fuel rockets.
Parsons was raised in Pasadena, California. He began amateur rocket experiments with school friend Edward Forman in 1928. Parsons was admitted to Stanford University but left before graduating due to financial hardship during the Great Depression. In 1934, Parsons, Forman, and Frank Malina formed the Caltech-affiliated Guggenheim Aeronautical Laboratory (GALCIT) Rocket Research Group, with support by GALCIT chairman Theodore von Kármán. The group worked on Jet-Assisted Take Off (JATO) for the U.S. military, and founded Aerojet in 1942 to develop and sell JATO technology during World War 2. The GALCIT Rocket Research Group became JPL in 1943.