Jeff Thornburg played a pivotal role in transforming a government research project into SpaceX’s most powerful rocket engine, the Raptor. Today, he’s applying that same expertise at his startup, Portal Space Systems, aiming to commercialize a propulsion concept once shelved by NASA. Founded in 2021, Portal recently announced a $50 million Series A funding round, valuing the company at $250 million. The investment was led by Geodesic Capital and Mach33, with participation from Booz Allen Ventures, ARK Invest, AlleyCorp, and FUSE.
Portal is developing solar thermal propulsion, a technology that diverges from current satellite engines. Standard systems either burn chemical fuel or convert solar energy into electricity for low-powered thrusters. In contrast, Portal’s engines concentrate the sun’s heat to warm propellant, enabling high-speed spacecraft movement. This approach has been studied in government labs since the 1960s, most recently for interstellar probes, but has never reached orbit. Thornburg, alongside co-founders Ian Vorbach and Prashaanth Ravindran, intends to change that within two years.
Thornburg’s career began in the U.S. Air Force, where he worked on full-flow staged combustion engines, a next-generation rocket design. A decade later, Elon Musk recruited him to SpaceX to realize these concepts as the Raptor engine, which now powers the massive Starship. After roles at Stratolaunch and Amazon’s Project Kuiper, Thornburg returned to propulsion innovation. He views solar thermal power as the logical next step in rocket technology, noting that NASA extensively studied it in the late 1990s and found it offered superior performance in many scenarios.
A 2003 NASA-commissioned report indicated that the technology wasn’t pursued further due to insufficient demand for in-space mobility. At the time, satellites and probes launched infrequently, making it easier to rely on more powerful rockets rather than investing in advanced propulsion. However, the landscape has shifted dramatically. With thousands of new satellites launching annually and the U.S. military requiring rapid orbital maneuvers for surveillance and deterrence, the need for efficient in-space propulsion has surged.
“It’s no longer acceptable to move slowly on orbit,” Thornburg stated. “China’s running circles around our spacecraft. We need equivalent capability.” This urgency has attracted significant funding. Portal has secured $45 million in strategic military funding, in addition to $67.5 million in private capital, driven by its potential for orbital warfare applications, according to Travis Bales, managing director at Booz Allen Ventures.
Looking ahead, as millions of satellites may orbit Earth providing communications and computing services, cheaper maneuvering solutions will become critical. Aaron Burnett, CEO of aerospace-focused venture fund Mach33, notes that Portal could evolve into a “space mobility prime,” supplying propulsion to diverse users. To achieve this, Portal must first demonstrate its technology in orbit. The company recently launched flight electronics on a test mission, with another prototype spacecraft scheduled for October.
A working engine prototype is set to debut with the launch of the first SuperNova spacecraft in 2027, which Thornburg describes as a “fighter jet for orbit.” Portal leverages advancements in additive manufacturing and materials science, particularly in developing its Hex thruster, a combined solar concentrator and nozzle. While some enthusiasts advocate for nuclear-powered rockets as the future for solar system travel, regulatory hurdles make such systems impractical for startups. Portal’s engine, however, provides a foundation for nuclear thermal propulsion by substituting reactor heat for solar energy, positioning the company to support future government initiatives.
“I’ll be able to help mature this technology much faster on orbit than we ever will by trying to build a $2 billion ground test facility that’s nuclear safe,” Thornburg explained. With its funding and technical roadmap, Portal Space Systems is poised to redefine orbital mobility, bridging decades of research with modern commercial and defense needs.
