During the 1920s, German daredevils had experimented with using solid-fuel rockets to propel cars, motorcycles, railway carriages, snow sleds, and, by 1929, aircraft such as Alexander Lippisch's Ente and Fritz von Opel's RAK.1. Solid-fuel rockets, however, have major disadvantages when used for aircraft propulsion, as their thrust cannot be throttled, and the engines cannot be shut down until fuel is exhausted.