NASA-funded scientists are designing Stirling engines, first invented in 1816, to power long-range spacecraft that travel too far from the Sun to use solar power. Decaying plutonium heats up helium until it starts a chain reaction of contraction and expansion, producing sound waves that fire a piston.
“Inside the engine, the acoustic pressure is high enough to pop your eardrums,” (Northrop Grumman researcher Mike) Petach told New Scientist. “It’s louder than a thunderclap.”
He adds that the sound does not escape the engine, so the device could be used to produce electricity for submarines, which must glide undetected beneath the ocean’s surface.