The Heinkel name was also behind pioneering work in jet engine and rocket development, and also the German aviation firm that attempted to popularize the use of retractable tricycle landing gear, a relative rarity in early WW II German airframe design. In 1939, flown by Erich Warsitz,[3] the Heinkel He 176 and Heinkel He 178 became the first aircraft designs to fly under liquid-fuel rocket and turbojet power respectively.