Phase Four

Phase Four was an aerospace company that designed and manufactured electric propulsion systems for satellites. Founded in 2015 in Hawthorne, California, the company developed radio-frequency (RF) thruster technology that eliminated electrodes, high-voltage electronics, and complex fabrication processes found in conventional Hall Effect and ion engines. Its flagship Maxwell engine used an electrodeless RF plasma design that was propellant-agnostic, enabling operation on traditional and alternative propellants including ambient atmospheric gases.

Phase Four delivered 13 flight units to orbit and held eight government contracts, including a $15 million DARPA contract under the Otter program to develop air-breathing electric propulsion for very low Earth orbit operations. The company also developed multi-mode propulsion systems capable of operating in both chemical and electric modes.

In September 2025, Phase Four's assets were divided between two acquirers: Apex acquired the Hall Effect thruster technology and personnel, while Quantum Space acquired the multi-mode propulsion assets and the Hawthorne integration and test facility.