Jack Parsons, American chemist, occultist, and engineer (d. 1952)
John Whiteside Parsons (born Marvel Whiteside Parsons; October 2, 1914 – June 17, 1952) was an American rocket engineer, chemist, and Thelemite occultist. Associated with the California Institute of Technology (Caltech), 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.

Born in Los Angeles, Parsons was raised by a wealthy family on Orange Grove Boulevard in Pasadena. Inspired by science fiction literature, he developed an interest in rocketry in his childhood and in 1928 began amateur rocket experiments with school friend Edward S. Forman. He dropped out of Pasadena Junior College and Stanford University due to financial difficulties during the Great Depression, and in 1934 he united with Forman and graduate Frank Malina to form the Caltech-affiliated Guggenheim Aeronautical Laboratory (GALCIT) Rocket Research Group, supported by GALCIT chairman Theodore von Kármán. In 1939 the GALCIT Group gained funding from the National Academy of Sciences (NAS) to work on Jet-Assisted Take Off (JATO) for the U.S. military. After the U.S. entered World War II, they founded Aerojet in 1942 to develop and sell JATO technology; the GALCIT Group became JPL in 1943.

Following some brief involvement with Marxism in 1939, Parsons converted to Thelema, the new religious movement founded by the English occultist Aleister Crowley. Together with his first wife, Helen Northrup, Parsons joined the Agape Lodge, the Californian branch of the Thelemite Ordo Templi Orientis (O.T.O.) in 1941. At Crowley's bidding, Parsons replaced Wilfred Talbot Smith as its leader in 1942 and ran the Lodge from his mansion on Orange Grove Boulevard. Parsons was expelled from JPL and Aerojet in 1944 owing to the Lodge's infamous reputation and to his hazardous workplace conduct.

In 1945, Parsons separated from Helen, after having an affair with her sister Sara; when Sara left him for L. Ron Hubbard, Parsons conducted the Babalon Working, a series of rituals intended to invoke the Thelemic goddess Babalon on Earth. He and Hubbard continued the working with Marjorie Cameron, whom Parsons married in 1946. After Hubbard and Sara defrauded him of his life savings, Parsons resigned from the O.T.O., then held various jobs while acting as a consultant for Israel's rocket program. Amid McCarthyism, Parsons was accused of espionage and left unable to work in rocketry. In 1952 Parsons died at the age of 37 in a home laboratory explosion that attracted national media attention; the police ruled it an accident, but many associates suspected suicide or murder.

Parsons's libertarian and occult writings were published posthumously. Historians of Western esoteric tradition cite him as one of the more prominent figures in propagating Thelema across North America. Although academic interest in his scientific career was negligible, historians have come to recognize Parsons's contributions to rocket engineering. For these innovations, his advocacy of space exploration and human spaceflight, and his role in founding JPL and Aerojet, Parsons is regarded as among the most important figures in the history of the U.S. space program. He has been the subject of several biographies and fictionalized portrayals.