Solar‑thermal propulsion raises $50M

Most satellite engines force a bad trade. Chemical propulsion gives a hard shove but burns through fuel fast, while electric propulsion sips propellant so slowly that big orbit changes can take weeks or months. (nasa.gov) Solar thermal propulsion tries a simpler trick. Instead of turning sunlight into electricity first, it uses mirrors to focus sunlight directly into heat, then blows a heated propellant out a nozzle like steam from a pressure cooker. (ntrs.nasa.gov) The performance yardstick here is specific impulse, which is just how long a kilogram of propellant keeps pushing. Higher specific impulse means you get more motion from the same tank, the way a fuel-efficient car goes farther on the same gallon. (nasa.gov) Portal Space Systems says its engine is aiming for about 350 to 450 seconds of specific impulse. That puts it above many chemical systems and below the most efficient electric thrusters, which is exactly the gap the company is trying to fill. (portalsystems.space) (nasa.gov) The second yardstick is thrust, which is the actual push. Portal is targeting roughly 100 to 200 newtons, which is far more shove than the tiny millinewton-level thrust common in many small electric systems. (spacenews.com) (nasa.gov) Its propellant choice is ammonia, which is cheap, storable, and easier to handle than cryogenic hydrogen. Academic work on ammonia solar thermal systems has long shown the attraction: lower performance than hydrogen, but much easier tanks and operations for small spacecraft. (sciencedirect.com) (link.springer.com) Portal’s hardware uses deployable mirror concentrators to focus sunlight onto a thermal battery and a 3D-printed heat exchanger thruster. The company says that combined part has no moving parts or internal interfaces, which is meant to cut failure points in orbit. (portalsystems.space) This idea is old enough that NASA was studying solar thermal propulsion in the 1990s, and one NASA history traces the concept back to 1956. What changed is manufacturing: Portal says its heat exchangers use additive techniques pioneered on Raptor-class engines, and its founder Jeff Thornburg previously worked on SpaceX’s Raptor program. (archive.org) (ntrs.nasa.gov) (portalsystems.space) (techcrunch.com) Now investors are betting that this old physics has finally found the right market. Portal said on April 9, 2026 that it raised a $50 million Series A led by Geodesic Capital and Mach33, with Booz Allen Ventures, ARK Invest, AlleyCorp, and FUSE also participating. (portalsystems.space) The company is not selling a loose engine by itself. It is building a spacecraft called Supernova and says the vehicle is designed for about 6 kilometers per second of delta-v, which is the total change in speed a spacecraft can buy with its propellant budget. (portalsystems.space) That number matters because modern satellite operators increasingly want to do more than sit in one assigned slot. Portal’s pitch is that a maneuverable spacecraft can reposition between orbits, extend mission life, service other satellites, or respond quickly to military tasking instead of drifting wherever it was first dropped. (portalsystems.space) (techcrunch.com) The catch is that solar thermal propulsion still has to prove itself in orbit. NASA and Air Force researchers have studied the concept for decades, but TechCrunch reported on April 9, 2026 that it has not yet made it into orbit, so Portal now has to show that the mirrors, thermal storage, plumbing, and repeated heating cycles all work together outside a vacuum chamber. (archive.org) (techcrunch.com)