Stuhlinger spent much of his spare time developing designs for solar-powered spacecraft. The most popular of those designs relied on ion thrusters, which ionize either caesium or rubidium vapor and accelerate the positively charged ions through gridded electrodes. The spacecraft would be powered by one kilowatt of solar energy. He referred to the concept as a "sunship". He is considered as one of the pioneers of electric propulsion having, among many contributions, authored the classic textbook "Ion Propulsion for Space Flight" (McGraw-Hill, New York, 1964). In 2005, he was honored by the Electric Rocket Propulsion Society, and awarded its highest honor "The Medal for Outstanding Achievement in Electric Propulsion", which was renamed the Stuhlinger Medal shortly following his death.