John Drury Clark, Ph.D. was an American rocket fuel developer, chemist, and science fiction writer. He was instrumental in the revival of interest in Robert E. Howard's Conan stories and influenced the writing careers of L. Sprague de Camp, Fletcher Pratt, and other authors.

Rocket propellant is the reaction mass of a rocket. This reaction mass is ejected at the highest achievable velocity from a rocket engine to produce thrust. The energy required can either come from the propellants themselves, as with a chemical rocket, or from an external source, as with ion engines.