In aerospace engineering, the propellant mass fraction is the portion of a vehicle's mass which does not reach the destination, usually used as a measure of the vehicle's performance.
In a spacecraft, the destination is usually an orbit, while for aircraft it is their landing location.
A higher mass fraction represents less weight in a design.
A rocket employing staging, which are the only designs to have reached orbit, has a mass fraction higher than the propellant mass fraction because parts of the rocket itself are dropped off en route.
Propellant mass fractions are typically around 0.8 to 0.9.
In aircraft, mass fraction is related to range, an aircraft with a higher mass fraction can go farther.
When applied to a rocket as a whole, a low mass fraction is desirable, since it indicates a greater capability for the rocket to deliver payload to orbit for a given amount of fuel.
Conversely, when applied to a single stage, where the propellant mass fraction calculation doesn't include the payload, a higher propellant mass fraction corresponds to a more efficient design, since there is less non-propellant mass.
Staging increases the payload fraction, which is one of the reasons SSTOs appear difficult to build.
For example, the complete Space Shuttle system has:[1] Given these numbers, the propellant mass fraction is
The mass fraction plays an important role in the rocket equation: Where
is the change in the vehicle's velocity as a result of the fuel burn and
The term effective exhaust velocity is defined as: where Isp is the fuel's specific impulse in seconds and gn is the standard acceleration of gravity (note that this is not the local acceleration of gravity).
To make a powered landing from orbit on a celestial body without an atmosphere requires the same mass reduction as reaching orbit from its surface, if the speed at which the surface is reached is zero.