In aerospace engineering, mass ratio is a measure of the efficiency of a rocket. It describes how much more massive the vehicle is with propellant than without; that is, the ratio of the rocket's wet mass (vehicle plus contents plus propellant) to its dry mass (vehicle plus contents). A more efficient rocket design requires less propellant to achieve a given goal, and would therefore have a lower mass ratio; however, for any given efficiency a higher mass ratio typically permits the vehicle to achieve higher delta-v.
The mass ratio is a useful quantity for back-of-the-envelope rocketry calculations: it is an easy number to derive from either \Delta{v}
Typical multistage rockets have mass ratios in the range from 8 to 20. The Space Shuttle, for example, has a mass ratio around 16.
The definition arises naturally from Tsiolkovsky's rocket equation:where
This equation can be rewritten in the following equivalent form:
The fraction on the left-hand side of this equation is the rocket's mass ratio by definition.
This equation indicates that a Δv of
n
en
\Deltav
e2.5
n
en
Sutton defines the mass ratio inversely as:[1]
In this case, the values for mass fraction are always less than 1.
Book: Zubrin, Robert . Entering Space: Creating a Spacefaring Civilization . registration . Tarcher/Putnam . 1999 . 0-87477-975-8.