Jeff Thornburg played a pivotal role in transforming a government research concept into the Raptor engine, the powerhouse behind SpaceX’s Starship. Today, he’s applying that same engineering rigor at Portal Space Systems, a startup he co-founded in 2021, to bring a long-dormant NASA idea—solar thermal propulsion—into operational reality. Portal recently secured a $50 million Series A funding round, valuing the company at $250 million. The investment was spearheaded by Geodesic Capital and Mach33, with participation from Booz Allen Ventures, ARK Invest, AlleyCorp, and FUSE.
Solar thermal propulsion represents a radical departure from conventional satellite engines. Current systems either rely on chemical combustion or convert solar energy into electricity to drive low-thrust, high-efficiency ion thrusters. Portal’s approach harnesses concentrated sunlight to directly heat propellant, generating high-speed thrust. This technology has languished in government labs since the 1960s, most recently considered for interstellar probes, but has never reached orbit. Thornburg, alongside co-founders Ian Vorbach and Prashaanth Ravindran, aims to change that within the next two years.
Thornburg’s career trajectory is steeped in advanced propulsion. He began in the U.S. Air Force, working on full-flow staged combustion engines—a complex, high-performance design. A decade later, Elon Musk recruited him to SpaceX, where he helped evolve those concepts into the Raptor. After roles at Stratolaunch and Amazon’s Project Kuiper, Thornburg returned to propulsion innovation, convinced that solar thermal power is the logical next evolution in rocket technology.
NASA extensively studied solar thermal propulsion in the late 1990s, concluding it offered superior performance in many scenarios. A 2003 NASA-commissioned report noted that development stalled due to insufficient demand for in-space mobility. At the time, satellites and probes launched infrequently, making it easier to rely on more powerful launch rockets rather than investing in sophisticated orbital propulsion. That calculus has inverted dramatically.
Today, thousands of new satellites deploy annually, and the U.S. military urgently requires spacecraft capable of rapid orbital maneuvers for surveillance and deterrence. “It’s no longer acceptable to move slowly on orbit,” Thornburg stated. “China’s running circles around our spacecraft. We need equivalent capability.” This strategic shift has fueled significant interest in Portal’s technology.
The company has already attracted $45 million in strategic funding from the U.S. military, supplementing $67.5 million in private capital, according to Travis Bales, managing director at Booz Allen Ventures. This investment underscores the potential application of Portal’s engines in orbital warfare. In a future where millions of satellites might populate Earth’s orbit, providing global communications and computing, affordable maneuvering solutions will become critical. Aaron Burnett, CEO of aerospace-focused venture fund Mach33, envisions Portal evolving into a “space mobility prime,” supplying propulsion systems to diverse users.
Portal’s path to orbit is already underway. The company recently launched its flight electronics on a shakedown mission, with another prototype spacecraft scheduled for October. The first full demonstration is slated for 2027 with the launch of the SuperNova spacecraft—described by Thornburg as a “fighter jet for orbit.” This vehicle will showcase a working prototype of the solar thermal engine.
Advances in additive manufacturing and materials science have been crucial, enabling the development of Portal’s integrated solar concentrator and nozzle, dubbed the Hex thruster. While some propulsion enthusiasts advocate for nuclear thermal rockets as the ultimate solution for deep-space travel, the regulatory and financial hurdles make such systems impractical for startups. However, Portal’s solar thermal engine provides a foundational stepping stone. It essentially mimics a nuclear thermal system by substituting concentrated solar heat for reactor heat, allowing the team to validate key components in orbit without the prohibitive costs and safety concerns of nuclear testing.
“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. This pragmatic approach positions Portal to accelerate development while aligning with future government ambitions for advanced propulsion. As orbital dynamics grow increasingly contested, Thornburg’s vision for high-speed, solar-powered mobility could redefine how we operate in space.
