Phase Four tested its Maxwell engine with water propellant. The U.S. Air Force Research Laboratory's green propellant, Advanced Spacecraft Energetic Non-Toxic, contains a significant percentage of water. Credit: Phase Four

COLORADO SPRINGS – Phase Four plans to fuel its Maxwell engine with a green propellant developed by the U.S. Air Force Research Laboratory, under a contract the Southern California startup won Aug. 18 at the U.S. Space Force’s first SpaceWERX Pitch Day.

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Debra Werner is a correspondent for SpaceNews based in San Francisco. Debra earned a bachelor’s degree in communications from the University of California, Berkeley, and a master’s degree in Journalism from Northwestern University. She is a recipient...