In aerospace engineering, payload fraction is a common term used to characterize the effeciency of a particular design. Payload fraction is calculated by dividing the weight of the payload to the weight of the otherwise empty aircraft when fully fueled. Fuel represents a considerable amount of the overall takeoff weight, and for shorter trips it is quite common to load less fuel in order to carry a heavier load. For this reason the useful load fraction calculates a similar number, but based on the combined weight of the payload and fuel together.
Propeller-driven airliners had useful load fractions on the order of 25-35%. Modern jet-powered airliners have considerably higher useful load fractions, on the order of 45-55%.
For spacecraft the payload fraction is often less than 1%, while the useful load fraction is perhaps 90%. In this case the useful load fraction is not a useful term, because spacecraft typically can't reach orbit without a full fuel load. For this reason the similar term, mass fraction, is used instead.
