John Whiteside (Jack) Parsons (1914-1952) was a rocket propulsion researcher at the California Institute of Technology and co-founder of the Jet Propulsion Laboratory (JPL). His research was central to the United States rocket program in the 1930s and '40s, notably in the development of space-capable rocket fuels and the invention of JATO units for aircraft.
He is less well known than Wernher von Braun or Theodore von Karman in the rocket program and the origins of JPL. Nonetheless, von Braun remarked that Parsons, and not he, should be regarded as the father of the American space program.
Parsons was also an avid practitioner of the occult arts, and a follower of Thelema. He was chosen by Aleister Crowley to lead the Thelemic movement in California in 1941. He was also the sometime magickal partner of L. Ron Hubbard. The two participated in a ritual known as the "Babalon Working" which is famous in occult circles -- loosely, an attempt to summon a goddess and change the course of history. This relationship lasted until 1947, when Hubbard defrauded Parsons of a sum of money (and ran off with his wife).
Jack Parsons died in 1952 in a laboratory explosion which is generally regarded as accidental. An extensive online library of his seminal writings regarding the Babalon Working has been maintained since 1997 at http://www.babalon.net. A crater on the dark side of the Moon has been named after him.