This founder helped build SpaceX’s most powerful rocket engine. Now he’s building a “fighter for orbit.”

Jeff Thornburg helped turn a government research project into SpaceX’s most powerful rocket engine. Now, he’s trying to do the same thing at his startup Space Gateway Systemswhich takes an idea discarded by NASA and turns it into a high-powered engine for the next generation of spacecraft.

Founded in 2021, Portal announced a $50 million Series A funding round on Thursday that values ​​the company at $250 million. The round was led by Geodesic Capital and Mach33, along with Booz Allen Ventures, ARK Invest, AlleyCorp and FUSE.

The company is developing a technology called solar thermal propulsion. Today’s typical satellite engines either burn chemical fuels or convert solar energy into electricity, using it to power efficient but low-power thrusters. Portal’s engines will instead harvest the sun’s heat, using it to heat propellant and propel the spacecraft at high speed.

The technology has been the subject of research in government research laboratories since the 1960s, more recently as a concept to send a probe into interstellar space, but it has not yet reached orbit. Thornburg, along with co-founders Ian Vorbach and Prashaanth Ravindran, plans to change that over the next two years.

Thornburg began his career in the US Air Force, where he worked on a program to develop an efficient, powerful type of next-generation rocket engine that engineers call full-flow staged combustion. A decade later, Elon Musk lured him to SpaceX to turn those ideas into the Raptor engine that now powers the company’s massive Starship.

After stints at Stratolaunch and Amazon’s Project Kuiper, he once again turned to propulsion.

A new kind of rocket engine

Solar thermal power is, in Thornburg’s view, the next logical step in rocket technology. NASA had studied the technology extensively in the late 90s and concluded that it provides better performance in many cases. It was not developed further because there was not enough demand for space mobility, according to a 2003 report commissioned by NASA.

With satellites and space probes flying much less frequently then, it was simpler to use a more powerful rocket than invest in propulsion in space. Now, with thousands of new satellites flying every year and the US military demanding spacecraft that can fly quickly between orbits to monitor or threaten adversaries, that calculus has been upended.

“It’s no longer acceptable to move slowly into orbit,” Thornburg told TechCrunch. “You know, China is circling our spaceship. We need an equivalent capability.”

Portal has already received $45 million in strategic funding from the U.S. military, on top of $67.5 million in private capital, thanks to the potential use of its technology for orbital warfare, according to Booz Allen Ventures CEO Travis Bales.

And in a future where we may see millions of Earth-orbiting satellites providing communications and computing services, satellite operators will need cheaper means to distance each other, notes Aaron Burnett, CEO of venture fund Mach33. Burnett sees Portal becoming a “primary space mobility space,” providing propulsion to a variety of users.

The path to orbit

To get there, the company will need to get its technology up and running in orbit. Its flight electronics were launched on a shakedown cruise around the planet last week, and another prototype spacecraft is expected to launch in October. The company will demonstrate a working prototype of its engine with the launch of its first SuperNova spacecraft — a “fighter jet for orbit,” according to Thornburg — expected in 2027.

The portal benefits from recent advances in additive manufacturing and materials science, which led to the development of the company’s combined solar concentrator and nozzle, the Hex thruster.

Rocket nerds believe that nuclear rockets are the next step to unlocking transportation throughout the solar system, but the regulatory and legal challenges of building such a system make it beyond the pay grade of a startup.

But Portal’s engine also gives the company a head start on an atomic rocket version of a nuclear thermal propulsion system — essentially replacing the sun’s heat with that of a reactor. By the time the US government is ready to build it, Thornburg’s team will have already demonstrated many of the moving parts in orbit.

“I’m going to be able to help mature this technology in orbit much faster than we ever will, trying to build a $2 billion ground test facility that’s nuclear safe,” Thornburg said.