Most thrusters burn chemicals or accelerate ions electrically. A third path heats propellant with raw sunlight. US12467421B2, granted to Trans Astronautica Corporation on November 11, 2025, pursues it, claiming an "omnivorous solar thermal thruster, cooling systems, and thermal energy transfer in rockets."

The CPC spans propulsion and solar collection: F02K 9/64 (thrust-chamber and nozzle arrangements), B64G 1/401 / 1/402 / 1/446 (propellant tanks, propulsion, solar arrays), F02K 9/42 / 9/44 / 9/60, and a notable run of F24S solar-concentrator codes (F24S 20/20, 23/30, 23/70, 70/16). The F24S solar-thermal-collector codes alongside rocket codes are the unmistakable signature of solar-thermal propulsion.

"Omnivorous solar thermal thrusters and adjustable cooling structures are disclosed."— U.S. Patent No. 12,467,421 source

The mechanism is heat from light. Concentrators focus sunlight onto a heat exchanger or absorber, raising propellant to high temperature; the hot gas then expands through a nozzle for thrust. Solar-thermal propulsion sits between chemical (high thrust, low efficiency) and electric (high efficiency, tiny thrust) — offering moderate thrust at efficiency better than chemical, without the heavy power-processing electronics electric thrusters need.

What the granted claims actually protect, though, is narrower and more interesting than "a sunlight thruster": they protect how you start one without melting it. Claim 1 is a method of "starting up a solar thermal rocket engine," and it reads as a careful thermal ramp. It provides "a gaseous propellant... via one or more propellant channels," then introduces "a liquid propellant... via the one or more propellant channels," and then "gradually reduc[es] a gas-to-liquid propellant ratio... to ensure that the solar thermal rocket engine is maintained in an instantaneous steady state." The reason this is claim 1 and not an afterthought is physical: a solar-thermal absorber holds a lot of heat, and dumping cold liquid propellant into it too fast would shock the structure or quench the chamber. So the engine is brought up on gas first and weaned onto liquid as it stabilizes.

The dependent claims fill in the startup choreography. Claim 2 reduces the gas fraction "until the gaseous propellant is no longer provided." Claim 3 raises engine temperature "using an electric heater before providing the gaseous propellant," and claim 4 then "heat[s] absorbing materials... using solar energy in response to the temperature... reaching an operating temperature" — an electric pre-heat that hands off to sunlight. Claim 5 puts that under a "control loop and a temperature sensor"; claim 6 keeps "a rate of change of the gas-to-liquid propellant ratio... below a threshold level to prevent components... from rising above a threshold temperature." Claims 7 and 8 name the handoff explicitly: a transition "from an electro-thermal thruster mode to a solar-thermal thruster mode" that works by "reducing power to an electric heating element and increasing concentrated light input." A parallel apparatus claim (claim 9) recasts the whole sequence as a "solar thermal rocket" configured to do it. The thruster, in short, runs as an electric heater at ignition and only becomes a true sunlight engine once it is hot enough to take the energy.

"Omnivorous" propellant tolerance is the strategic twist behind all this thermal caution: the abstract states the thruster is "configured to use a plurality of different propellant types, either singly or in combination simultaneously," and "in both liquid and gaseous states." A thruster that can use whatever volatile is available — and that can manage the thermal transition between gas and liquid feeds — is well suited to space-resource and in-situ-propellant ambitions, which is squarely Trans Astronautica's stated focus.

The named inventor, Joel Sercel, recurs across an unusually visionary body of solar-thermal and space-mining filings — the company's portfolio (including US12297792B2 earlier in 2025) is one of the more distinctive in the propulsion landscape for its commitment to sunlight-as-energy architectures.

It is telling that the granted independent claims are about startup and steady-state management rather than the optics of sunlight collection. A solar-thermal absorber is a large thermal mass; the risk is not lighting it but feeding it propellant without thermally shocking the structure or quenching the chamber. The claimed answer — gas first, then a metered introduction of liquid, then a gradual reduction of the gas-to-liquid ratio while holding "an instantaneous steady state" — is a control law for crossing that transition safely, with the rate of change explicitly capped (claim 6) to keep components below a threshold temperature. That is the unglamorous, genuinely hard part of making sunlight-as-energy propulsion work, and it is what the patent chose to protect.

The electric-to-solar handoff is the other distinctive move. Claims 3 through 8 describe pre-heating the engine with an electric heater, transitioning "from an electro-thermal thruster mode to a solar-thermal thruster mode" once an operating temperature is reached, and effecting that transition by "reducing power to an electric heating element and increasing concentrated light input." Functionally the device is two thrusters in one body — an electrothermal heater for cold starts and a solar-thermal engine for cruise — governed by a control loop reading a temperature sensor. Read against Trans Astronautica's stated space-resource ambitions, the "omnivorous" propellant tolerance and the disciplined thermal startup make sense together: an engine meant to run on whatever volatile a mission can mine has to handle propellants of varying state and quality without flaming out, and the claimed startup sequence is the mechanism that lets it.

The honest discipline: solar-thermal propulsion has been studied for decades and flown rarely; this grant protects a specific omnivorous-thruster startup-and-thermal-management method — the gas-to-liquid ramp, the electric-to-solar handoff, the adjustable cooling structures — not the concept, and a granted claim is not a demonstrated engine. Read it as a clear marker of a contrarian propulsion bet — using abundant sunlight and flexible propellant — and as a reminder that the gap between an elegant thermal architecture and a working in-space thruster is exactly where such bets are won or lost.