Phase Four

Phase Four was an in-space propulsion company founded in 2015 and based in Hawthorne, California. It designed and manufactured electric propulsion systems for satellites, originally centered on a radio-frequency thruster developed in response to the proliferation of satellite constellations. Over time the company positioned itself as a full-spectrum in-space electric propulsion provider spanning hardware, software and services, working with constellation operators and satellite bus manufacturers.

Its product line included the Maxwell thruster family and a multi-mode propulsion system in which a single thruster can operate either as a chemical thruster for high thrust or as an electric thruster for higher efficiency. The company said it demonstrated the multi-mode capability in its Maxwell Block 3 thruster using hydrazine and the nontoxic alternative propellant ASCENT. Phase Four also operated an integration and test facility in Southern California, in Hawthorne, with a high bay, clean room and related labs, and capacity described as up to 100 engines per year.

In September 2025 the company's technology was split between two buyers: Apex acquired its Hall-effect thruster technology, including hardware and personnel, and Quantum Space acquired the multi-mode propulsion technology together with the Hawthorne integration and test facility, for undisclosed terms.

Business model

Phase Four designed and manufactured in-space electric propulsion hardware, and also offered software and services around propulsion systems, teaming with constellation operators and satellite bus manufacturers to optimize mission effectiveness and control cost.

Latest developments

In January 2025 Phase Four announced the first close of a Series C round led by Artemis Group Capital, securing roughly 60% of the target raise. In September 2025, Apex acquired the company's Hall-effect thruster technology, hardware and personnel as part of Apex's $200 million funding round, and roughly ten days later Quantum Space acquired Phase Four's multi-mode propulsion technology and its Hawthorne integration and test facility on undisclosed terms, intending to apply the technology to its Ranger spacecraft line. CEO Steve Kiser described the Quantum Space transaction as a partnership to scale the technology into defense, science and commercial infrastructure missions.