A
liquid-propellant rocket or
liquid rocket is a
rocket engine that uses
liquid propellants. Liquids are desirable because their reasonably high density allows the volume of the propellant tanks to be relatively low, and it is possible to use lightweight centrifugal
turbopumps to pump the propellant from the tanks into the combustion chamber, which means that the propellants can be kept under low pressure. This permits the use of low-mass propellant tanks, resulting in a high
mass ratio for the rocket.