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.

"A multi-mode thruster system for use in a spacecraft includes a microwave source; a cavity coupled to the microwave source and including a first inlet to receive a first fluid and a second inlet to receive a second fluid; and a nozzle provided at one end of the cavity."— U.S. Patent No. 11,527,387 source

The word doing the most work in claim 1 is "multi-mode." The same hardware — microwave source, dual-inlet cavity, nozzle — is claimed as operating two ways. In "microwave electrothermal thruster (MET) mode" the system "generate[s] a standing wave in the cavity using the microwave source" and uses it to "raise a temperature of the first fluid to generate a first hot gas that exits the cavity via the nozzle to generate thrust." In "chemical propulsion mode" the same cavity instead "produce[s] a reduction-oxidation reaction between the first fluid and the second fluid" to make "a second hot gas" out the same nozzle. One thruster, an efficient low-thrust electric mode and a higher-thrust chemical mode, sharing the cavity and nozzle — that dual personality, not the microwave heating alone, is what claim 1 stakes out.

The dependent claims pin down the chemistry and control that make the dual mode practical. Claim 2 lists the MET-mode working fluids — water, hydrazine, hydrogen peroxide, or ammonia — and claim 3 has the cavity operate "as a resonant cavity" in that mode, which is how the standing wave couples energy in efficiently. Claim 4 routes "an oxidation agent via the first inlet and a reducing agent via the second inlet" for chemical mode, and claim 5 captures the cleverness of the plumbing: "the first inlet receives a same fluid in the MET mode and the chemical propulsion mode," where that fluid "operates as propellant in the MET mode and as an oxidation agent in the chemical propulsion mode." Claim 8 gives the canonical pairing — water in MET mode; oxygen and hydrogen in chemical mode — and claim 9 stores them with "a propellant tank configured to store water" plus "a fuel cell configured [to] separately store oxygen and hydrogen," i.e., the water can be electrolyzed into the chemical-mode propellants. Claim 7 adds a controller that picks MET mode for "a first amount of thrust" and chemical mode for "a second amount of thrust larger than the first" — slow, efficient station-keeping versus a bigger burn on demand. Claims 10 and 11 even cover an additive in the propellant "for depositing a coating material onto an inner surface of the nozzle" to protect it, and method claims 12 through 19 restate the whole scheme as operation.

The mechanism in MET mode 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 a resonant cavity, how the same cavity tolerates a combustion reaction, how a shared nozzle survives both duty cycles. The dependent claim is the moat, and in a microwave thruster the coupling efficiency — and the ability to switch a single cavity between plasma-heating and combustion without destroying it — is the difference between a working tug and a science project.

The shared-fluid claims are the clearest window into why "multi-mode" is more than a marketing label. Claim 5 has the first inlet receive "a same fluid" that "operates as propellant in the MET mode and as an oxidation agent in the chemical propulsion mode," and claims 8 and 9 make that concrete: store water, run it as plasma propellant for efficient low-thrust maneuvers, and — via "a fuel cell configured [to] separately store oxygen and hydrogen" — electrolyze it into the oxidizer and fuel for a higher-thrust chemical burn. A single onboard commodity, water, becomes both an electric propellant and the feedstock for a chemical one. For a transfer vehicle that wants efficiency for long cruises and punch for time-critical maneuvers, that dual use of one tank is the architectural payoff the patent is built around.

The remaining dependent claims show the engineering risks the inventors anticipated. Claim 3 requires the cavity to act "as a resonant cavity" in MET mode, the condition under which microwave energy actually couples into the plasma efficiently rather than reflecting away. Claims 10 and 11 add a propellant additive "for depositing a coating material onto an inner surface of the nozzle" — an oxide or non-oxide ceramic, refractory metal, or diamond or sapphire film — an acknowledgment that running both a hot plasma and a combustion reaction through the same throat is brutal on nozzle materials. The controller of claim 7, choosing MET mode for small thrust and chemical mode for larger, is the logic that ties the two personalities into a single mission profile. The claim set, in short, is not just "heat propellant with microwaves" but a worked-through dual-mode system with the resonance, materials, and control problems each addressed in their own dependent claim.

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 multi-mode microwave-excitation propulsion approach — MET plus chemical mode in a shared cavity — 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.