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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, a higher mass fraction corresponds to a more efficient design, since there is less non-propellant mass. 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 example, the complete Space Shuttle System has:
  • 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.

The mass fraction plays an important role in the Rocket Equation :

:\Delta v = -v_e \ln (m_f / m_0)

Where m_f/m_0 is the ratio of final mass to initial mass (i.e., one minus the mass fraction), \Delta v 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:

:v_e = g_n I_{sp}

where ''I''sp 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.


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