In aerospace engineering, the mass fraction is an important measure of a rocket's efficiency. For a given target orbit, a rocket's mass fraction is the portion of the rocket's pre-launch mass (fully fueled) that does not reach orbit. In the cases of a single stage to orbit vehicle the mass fraction is simply the fuel mass divided by the mass of the full spaceship, but with a rocket employing staging, which is the vast majority of them, the mass fraction is higher because parts of the rocket itself are dropped off en route. Mass fractions are typically around 0.8 to 0.9, with lower numbers being better. Aerospace engineering is the branch of engineering that concerns spacecraft and related topics. ... A Redstone rocket, part of the Mercury program A rocket is a vehicle, missile or aircraft which obtains thrust by the reaction to the ejection of fast moving exhaust gas from within a rocket engine. ... Efficiency is the capability of acting or producing effectively with a minimum amount or quantity of waste, expense, or unnecessary effort. ... In physics, an orbit is the path that an object makes, around another object, whilst under the influence of a source of centripetal force, such as gravity. ... A single-stage to orbit (or SSTO) launcher describes an as-yet theoretical class of spacecraft designed to place a load into orbit as a self-contained vehicle without the use of multiple stages. ... The word staging can mean more than one thing: Staging (rocketry) Staging (pathology) Staging (theatre) Staging (stagecoaches) Staging area (military) This is a disambiguation page — a navigational aid which lists other pages that might otherwise share the same title. ...

For example, the complete Space Shuttle system has: The Space Shuttle Columbia seconds after engine ignition, 1981 (NASA). ...

• weight at liftoff: 4,500,000 lb (2,040,000 kg)
• weight at end of mission: 230,000 lb (104,000 kg), and
• maximum cargo to orbit: 63,500 lb (28,800 kg)

Given these numbers, the mass fraction is 1 − (293,500 / 4,500,000) = 0.935 or perhaps a little less because of the fuel brought to orbit for use when returning: this may not have been counted as cargo, in which case the figure 293,500 should be a little higher.

A lower mass fraction for the rocket means that it uses fuel efficiently, reserving a larger portion of its mass as payload. Without the benefit of staging, SSTO designs are typically designed for mass fractions around 0.9. Staging increases the mass fraction, which is one of the reasons SSTO's appear difficult to build.

For individual stages, however, a higher mass fraction is better, meaning that there is less non-propellent mass.

The mass fraction plays an important role in the rocket equation: Tsiolkovskys rocket equation, named after Konstantin Tsiolkovsky who first derived it, considers the principle of a rocket: a device that can apply an acceleration to itself (a thrust) by expelling part of its mass with high speed in the opposite direction, due to the conservation of momentum. ...

Δv = − v_eln(m_f / m₀)

Where m_f / m₀ is the ratio of final mass to initial mass (i.e., one minus the mass fraction), Δv is the change in the vehicle's velocity as a result of the fuel burn and v_e is the effective exhaust velocity.

The term specific impulse is defined as: The specific impulse (commonly abbreviated Isp) of a propulsion system is the impulse (change in momentum) per unit of propellant. ...

v_e = g_nI_sp

where I_sp is the fuel's specific impulse in seconds and g_n 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.

See also

• Payload fraction
• Mass ratio