Jeff Thornburg played a pivotal role in transforming a government research concept into SpaceX‘s most powerful rocket engine, the Raptor. Today, he is applying that same engineering rigor at his startup, Portal Space Systems, to revive a propulsion technology that NASA abandoned in the late 1990s. Portal, established in 2021, 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.
The core innovation at Portal is solar thermal propulsion, a method that concentrates solar heat to energize propellant, enabling high-speed spacecraft movement. This approach diverges sharply from conventional satellite engines, which either rely on chemical combustion or convert sunlight into electricity for low-thrust operations. Government laboratories have explored solar thermal propulsion since the 1960s, most recently considering it for interstellar probes, but no implementation has reached orbit. Thornburg, alongside co-founders Ian Vorbach and Prashaanth Ravindran, intends to change that within the next two years.
Thornburg’s career trajectory underscores his expertise in advanced propulsion. He began in the U.S. Air Force, working on full-flow staged combustion engines—a next-generation design known for efficiency and power. A decade later, Elon Musk recruited him to SpaceX, where he helped develop the Raptor engine that now propels the massive Starship rocket. After roles at Stratolaunch and Amazon’s Project Kuiper, Thornburg returned to propulsion with a focus on solar thermal technology, which he views as the logical evolution in rocket science.
NASA extensively studied solar thermal propulsion in the late 1990s, concluding it offered superior performance in many scenarios. However, a 2003 NASA-commissioned report noted that limited demand for in-space mobility stalled further development. At the time, satellites and probes launched infrequently, making it easier to rely on more powerful rockets rather than investing in orbital propulsion systems. That calculus has inverted dramatically. Today, thousands of new satellites deploy annually, and the U.S. military requires spacecraft capable of rapid orbital shifts for surveillance or 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 urgency has translated into significant financial backing. Portal has already received $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. The technology’s potential for orbital warfare drives much of this interest.
Beyond military applications, the commercial satellite sector faces a growing need for affordable maneuvering solutions. Aaron Burnett, CEO of the aerospace-focused venture fund Mach33, predicts that as millions of satellites populate Earth’s orbit for communications and computing, operators will require cost-effective ways to avoid collisions. Burnett envisions Portal evolving into a “space mobility prime,” supplying propulsion systems to diverse users.
To achieve these goals, Portal must first demonstrate its technology in space. The company recently launched flight electronics on a test mission, with another prototype spacecraft scheduled for October. The critical milestone will be the 2027 launch of the first SuperNova spacecraft, which Thornburg describes as a “fighter jet for orbit.” This vehicle will showcase a working prototype of Portal’s engine, leveraging advancements in additive manufacturing and materials science that enabled the development of the Hex thruster—a combined solar concentrator and nozzle.
While some enthusiasts advocate for nuclear-powered rockets as the ultimate solution for solar system travel, the regulatory and legal hurdles make such projects impractical for startups. Portal’s solar thermal engine, however, provides a foundational step toward nuclear thermal propulsion, where a reactor would replace solar heat. Thornburg argues that proving these components in orbit accelerates development far more efficiently than constructing ground-based nuclear test facilities.
“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. By commercializing a dormant NASA concept, Portal aims not only to enhance orbital mobility but also to lay the groundwork for future deep-space exploration, positioning Thornburg once again at the forefront of propulsion innovation.
