Momentus pitched investors on cheap, water-fueled space tugs. The physics behind that pitch is in US11527387B2, granted to Momentus Space LLC on December 13, 2022, which claims "spacecraft propulsion devices and systems with microwave excitation."
The CPC is precise: H01J 37/32247 (microwave plasma generation), B64G 1/405 and 1/409 (electric and other propulsion arrangements), and F03H 1/0093 (plasma-based electric propulsion). That combination — microwave plasma generation feeding an electric-propulsion arrangement — is the microwave electrothermal thruster in classification form.
The mechanism is heat by radio. Microwave energy is coupled into the propellant to strike a plasma and heat it; the hot gas then expands through a nozzle to make thrust. The appeal is propellant flexibility — water can work — and a relatively simple, electrode-free heating scheme that avoids the cathode-erosion problems that limit some other electric thrusters.
The dependent claims are where the real engineering lives: how microwave power is coupled into the propellant, how the plasma is confined, how the system survives operation. The dependent claim is the moat, and in a microwave thruster the coupling efficiency is the difference between a working tug and a science project.
A sober note this desk must add: Momentus faced well-documented commercial and regulatory turbulence, and a patent is not proof of a fielded, reliable capability. US11527387B2 protects a specific microwave-excitation propulsion approach, not the concept of electrothermal thrusters generally. Read it as a clear map of the technology Momentus was betting on — and a reminder that the gap between a granted claim and a dependable in-space system is exactly where space ventures are tested.