Jeff Thornburg helped design the rocket engine powering SpaceX's Starship, but his next mission is even more ambitious: bringing a forgotten NASA technology to life that could revolutionize how spacecraft move through orbit. Portal Space Systems, the startup he founded in 2021, just announced a $50 million Series A funding round that values the company at $250 million. The goal is to launch the first operational solar thermal propulsion engine into space by 2027, a technology that has been studied by government researchers since the 1960s but never actually flown .

What Makes Solar Thermal Propulsion Different From Today's Rocket Engines?

Most satellites and spacecraft today rely on one of two propulsion methods: burning chemical fuel or converting sunlight into electricity to power ion thrusters. Portal's approach is fundamentally different. Instead of those conventional methods, the company's engines concentrate the heat of the sun to warm propellant, using that thermal energy to accelerate the spacecraft. It sounds simple, but the engineering is complex, and the potential is enormous .

NASA studied this technology extensively in the late 1990s and concluded it could deliver better performance than conventional engines in many scenarios. The reason it never went further was straightforward: there simply wasn't enough demand for in-space mobility at the time. With only a handful of satellites launching each year, it made more sense to use a more powerful rocket than to invest in advanced propulsion systems. That calculus has completely flipped .

Why Is Orbital Propulsion Suddenly Critical?

The space industry has transformed dramatically. Thousands of new satellites now launch every year, and the U.S. military is demanding spacecraft that can move quickly between orbits to surveil or respond to rivals. Thornburg explained the urgency directly: "It's no longer acceptable to move slowly on orbit. You know, China's running circles around our spacecraft. We need equivalent capability," he stated .

Thornburg

Portal has already secured $45 million in strategic funding from the U.S. military on top of its $67.5 million in private capital, reflecting the national security implications of faster orbital maneuvering. The company's technology also addresses a growing commercial need: as millions of satellites potentially fill Earth's orbit in the coming decades, operators will need cheaper ways to move spacecraft out of each other's way .

How Portal Plans to Prove Solar Thermal Propulsion Works in Space

• Flight Electronics Test: Portal launched its flight electronics on a shakedown cruise around the planet last week to validate basic systems in the space environment.
• Prototype Spacecraft Launch: Another prototype spacecraft is scheduled to launch in October 2026 to conduct additional in-orbit testing and validation.
• SuperNova Demonstration: The company plans to launch its first full SuperNova spacecraft, which Thornburg describes as a "fighter jet for orbit," in 2027 to demonstrate the engine's operational capabilities.

The technology benefits from recent breakthroughs in additive manufacturing and advanced materials science. Portal has developed a combined solar concentrator and nozzle called the Hex thruster, which represents a significant engineering achievement that would have been impossible just a few years ago .

Could This Lead to Nuclear-Powered Rockets?

Thornburg's work at Portal also positions the company to eventually support nuclear thermal propulsion, which many space experts believe is the next frontier for deep space exploration. Nuclear rockets would replace the sun's heat with energy from a reactor, unlocking transportation throughout the solar system. However, the regulatory and legal challenges of building such a system are beyond what a startup can tackle alone .

By proving out solar thermal technology in orbit first, Portal will have already validated many of the critical components needed for nuclear propulsion. "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 .

Thornburg's career trajectory reveals how innovation in aerospace often follows a pattern: government research identifies promising concepts, private companies commercialize them, and eventually those technologies enable entirely new capabilities. His journey from the U.S. Air Force to SpaceX to founding Portal demonstrates that the most transformative space technologies often come from engineers willing to take risks on ideas that have been waiting decades for their moment .