Hellmuth Walter was a German engineer who pioneered research into rocket engines and gas turbines. His most noteworthy contributions were rocket motors for the Messerschmitt Me 163 and Bachem Ba 349 interceptor aircraft, so-called Starthilfe jettisonable rocket propulsion units used for a variety of Luftwaffe aircraft during World War II, and a revolutionary new propulsion system for submarines known as air-independent propulsion (AIP).

A rocket engine uses stored rocket propellants as the reaction mass for forming a high-speed propulsive jet of fluid, usually high-temperature gas. Rocket engines are reaction engines, producing thrust by ejecting mass rearward, in accordance with Newton's third law. Most rocket engines use the combustion of reactive chemicals to supply the necessary energy, but non-combusting forms such as cold gas thrusters and nuclear thermal rockets also exist. Vehicles propelled by rocket engines are commonly used by ballistic missiles and rockets. Rocket vehicles carry their own oxidiser, unlike most combustion engines, so rocket engines can be used in a vacuum to propel spacecraft and ballistic missiles.