Jeff Thornburg, founder of SpaceX's most powerful rocket engine, is building a high-powered propulsion system at his startup, Portal Space Systems. The company is developing a technology called solar thermal propulsion, which NASA has considered the next logical step in rocket tech. Unlike current satellite engines, Portal's engines would use the sun's heat to heat propellant and move the spacecraft at high speed. The technology has been investigated in government research labs since the 1960s but has yet to be used in orbit. Thornburg and co-founders Ian Vorbach and Prashaanth Ravindran plan to change this in the next two years. Portal has received $45 million in strategic funding from the U.S. military and $67.5 million in private capital due to its potential for orbital warfare.