A liquid-propellant rocket or liquid rocket utilizes a rocket engine burning liquid propellants. (Alternate approaches use gaseous or solid propellants.) Liquids are desirable propellants because they have reasonably high density and their combustion products have high specific impulse (Isp). This allows the volume of the propellant tanks to be relatively low.
Bipropellant liquid rockets are simple in concept but due to high temperatures and high speed moving parts, very complex in practice.
The NMUSAF's Me 163B Komet rocket plane
Titan II
Rocket 09 (left) and 10 (GIRD-09 and GIRD-X). Museum of Cosmonautics and Rocket Technology; St. Petersburg.
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.
RS-68 being tested at NASA's Stennis Space Center
Viking 5C rocket engine used on Ariane 1 through Ariane 4
Armadillo Aerospace's quad vehicle showing visible banding (shock diamonds) in the exhaust jet