In the race to build the future of spaceflight, first milestones have big meaning. For Portal Space Systems, a young spacecraft company based in Washington, that milestone arrived this September when it became the first commercial player to successfully test a solar thermal propulsion system in vacuum conditions.
The achievement comes from Portal’s proprietary heat exchanger (HEX) thruster, a device called Flare. This thruster is not just another engine; it’s a novel, 3D printed technology that uses the Sun’s energy to move spacecraft across orbits faster and more efficiently than many of today’s propulsion systems.
If this concept doesn’t feel straight out of science fiction or a comic book, it’s hard to imagine what would. Portal’s solar thermal propulsion isn’t just an engineering milestone; it’s more like a glimpse at what sustainable orbital travel could look like in the next decade.
For Portal, the test wasn’t just about firing up a thruster inside a vacuum chamber. It was about proving that an idea long studied by government agencies, such as NASA and the U.S. Air Force, could finally be made practical for real-world missions.
Portal successfully tests Heat Exchanger (HEX) Thruster named Flare in vacuum.
From Research to Reality
Solar thermal propulsion (STP) has been studied since the 1960s. The concept is elegant, says Portal: collect solar energy, heat a propellant, and expel it to generate thrust. But for decades, it remained mostly an academic plan. Building a heat exchanger tough enough to handle extreme temperatures, light enough for space, and affordable enough for missions was simply too difficult.
Portal’s breakthrough is that it took this old idea and turned it into something usable. Even more interesting, by using additive manufacturing and advanced materials, the company designed and built Flare, a patented HEX thruster that passed high-temperature, full-power tests under space-like conditions.
This demonstration makes Portal the first commercial company to validate solar thermal propulsion at operational levels.
Most satellites and spacecraft today rely on either chemical propulsion, which is powerful but fuel-hungry, or electric propulsion, which is efficient but slow. Clearly, each has limits. Chemical systems can deliver quick bursts but don’t last long. Electric systems are great for steady travel, but can take months to reposition a satellite, explains Portal.
Instead, the company’s Flare thruster promises something different: both speed and endurance. Its Supernova spacecraft, powered by this system, promises to move from low Earth orbit (LEO) to medium Earth orbit (MEO) in just hours, from MEO to geostationary orbit (GEO) in under a day, and from LEO to cislunar space in only a few days.
And it can supposedly do this using ammonia, a storable, non-cryogenic propellant that doesn’t require bulky cooling systems. This makes the spacecraft simpler, safer, and more adaptable.
Jeff Thornburg.
“With maneuverability becoming a defining advantage in space operations, Supernova is built to give national security and commercial operators the ability to reposition, respond, and persist across orbits,” said Jeff Thornburg, CEO of Portal Space Systems.
Thornburg isn’t new to space technology. Before founding Portal, he held senior engineering roles at NASA, SpaceX, and Amazon’s Project Kuiper, among others. At SpaceX, he worked directly on the Raptor engine, one of the most advanced rocket engines ever built. His career has been defined by moving bold propulsion concepts from theory to reality.
With Portal, Thornburg’s vision is to build spacecraft that don’t just sit in orbit but actively maneuver — changing positions quickly, supporting real-time missions, and staying useful for years. It’s a vision shaped by his years in both government and industry, where he saw firsthand the limits of today’s satellites.
The Test Campaign
Portal’s milestone was the result of a carefully run campaign at its Bothell, Washington, facility. The company built a test setup that could simulate solar energy using high-powered electric heaters. In this environment, Flare underwent cold flow and calibration runs, high-temperature firing sequences, and full-power thrust tests inside vacuum conditions.
The company announced that the results matched its predictions, confirming that its propulsion architecture can deliver as promised. This clears the way for integration with future flight hardware, including the upcoming Supernova spacecraft.
Portal only emerged from stealth mode in 2024, but it has already made a name for itself. The company earned STRATFI (Strategic Funding Increase) support from the U.S. Space Force, raised one of the sector’s largest seed rounds, and was listed by Via Satellite as one of the “Top 10 Startups to Watch.”
Portal’s spacecraft are built to be flexible. Most satellites are made to stay in one place, but Supernova is built to move. That kind of agility is now essential, as both defense and commercial customers need satellites that can respond quickly. With Flare’s successful test, Portal proved its technology is more than just talk; it actually works.
Portal’s solar thermal testing images.
The next step is to bring this technology into orbit. Portal’s propulsion system supports refueling and offers a five-year operational life, features that could reshape how fleets of satellites are deployed and maintained. If all goes as planned, the Supernova spacecraft may soon prove in space what was just validated on Earth: that solar thermal propulsion is not just possible, but practical.
For Thornburg and his team, this is a shift from startup promises to showing what they can actually do. For the wider space industry, it’s proof that commercial innovation can sometimes achieve what decades of government research left unfinished. More than anything, it’s a reminder that the next big step in space maneuverability might not come from massive rockets, but from a 3D printed thruster called Flare.
Stay tuned for our in-depth, two-part interview with CEO Jeff Thornburg, where he shares the lessons he carried from SpaceX and Amazon to founding Portal, and what’s next for solar thermal propulsion.
Images courtesy of Portal Space Systems
