Engineers at the Johns Hopkins University Applied Physics Laboratory are prototyping a previously theoretical rocket design that could someday take spacecraft to interstellar space. Their plan? Use heat from the sun, rather than combustion, to power a rocket engine.

Unlike a traditional engine that's mounted on the rear end of a rocket, the experimental solar-powered engine takes the shape of a flat shield made from black carbon foam. The engine would double as a heat shield, protecting the probe from the sun’s powerful rays, while coils of tubing filled with hydrogen lying beneath the surface absorb heat from the sun.

The hydrogen expands, becomes pressurized, and then explodes out from a nozzle, generating thrust. The scientists call it solar thermal propulsion.

“From a physics standpoint, it’s hard for me to imagine anything that’s going to beat solar thermal propulsion in terms of efficiency,” Jason Benkoski, a materials scientist at the Applied Physics Laboratory (APL), told WIRED. “But can you keep it from exploding?”

Benkoski and his colleagues from APL and NASA recently shared their design online at the 3rd Annual Interstellar Probe Exploration Workshop. According to Benkoski’s calculations, a real-life version of the engine could be three times more efficient than the most advanced chemical combustion engines used in today’s rockets.