John Whiteside Parsons (born Marvel Whiteside Parsons; October 2, 1914 – June 17, 1952), better known as Jack Parsons, was an American rocket engineer and rocket propulsion researcher, chemist, inventor, businessman, writer, socialite, and Thelemite occultist. Parsons was associated with the California Institute of Technology (Caltech), and was one of the principal founders of both the Jet Propulsion Laboratory (JPL) and the Aerojet Engineering Corporation.