 The hypergolic rocket motor had the advantage of fast climb and quick-hitting tactics at the cost of being very volatile and capable of exploding with any degree of inattention.Other proposed combat rocket fighters like the Heinkel "Julia" and reconnaissance aircraft like the DFS 228 were meant to use the Walter 509 series of rocket motors, but besides the Me 163, only the Bachem Ba 349 "Natter" vertical launch expendable fighter was ever flight-tested with the Walter rocket propulsion system as its primary sustaining thrust system for military-purpose aircraft.The earliest ballistic missiles, such as the Soviet R-7 that launched Sputnik 1 and the U.S. Atlas and Titan-1, used kerosene and liquid oxygen.