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Astrodynamics
Orbital mechanics
Orbital elements Apsis Argument of periapsis Eccentricity Inclination Mean anomaly Orbital nodes Semi-major axis True anomaly
Types of two-body orbits by eccentricity Circular orbit Elliptic orbit Transfer orbit (Hohmann transfer orbit Bi-elliptic transfer orbit) Parabolic orbit Hyperbolic orbit Radial orbit Decaying orbit
Equations Dynamical friction Escape velocity Kepler's equation Kepler's laws of planetary motion Orbital period Orbital velocity Surface gravity Specific orbital energy Vis-viva equation
Celestial mechanics
Gravitational influences Barycenter Hill sphere Perturbations Sphere of influence
N-body orbits Lagrangian points (Halo orbits) Lissajous orbits Lyapunov orbits
Engineering and efficiency
Preflight engineering Mass ratio Payload fraction Propellant mass fraction Tsiolkovsky rocket equation
Efficiency measures Gravity assist Oberth effect
Propulsive maneuvers Orbital maneuver Orbit insertion
v t e

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 ${\displaystyle \Delta {v))$ or from rocket and propellant mass, and therefore serves as a handy bridge between the two. It is also a useful for getting an impression of the size of a rocket: while two rockets with mass fractions of, say, 92% and 95% may appear similar, the corresponding mass ratios of 12.5 and 20 clearly indicate that the latter system requires much more propellant.

Typical multistage rockets have mass ratios in the range from 8 to 20. The Space Shuttle, for example, has a mass ratio around 16.

Derivation

The definition arises naturally from Tsiolkovsky's rocket equation:

$${\displaystyle \Delta v=v_{e}\ln {\frac {m_{0)){m_{1))))$$

• Δv is the desired change in the rocket's velocity
• v_e is the effective exhaust velocity (see specific impulse)
• m₀ is the initial mass (rocket plus contents plus propellant)
• m₁ is the final mass (rocket plus contents)

This equation can be rewritten in the following equivalent form:

$${\displaystyle {\frac {m_{0)){m_{1))}=e^{\Delta v/v_{e))}$$

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 ${\displaystyle n}$ times the exhaust velocity requires a mass ratio of ${\displaystyle e^{n))$. For instance, for a vehicle to achieve a ${\displaystyle \Delta v}$ of 2.5 times its exhaust velocity would require a mass ratio of ${\displaystyle e^{2.5))$ (approximately 12.2). One could say that a "velocity ratio" of ${\displaystyle n}$ requires a mass ratio of ${\displaystyle e^{n))$.

Alternative definition

Sutton defines the mass ratio inversely as:^[1]

$${\displaystyle M_{R}={\frac {m_{1)){m_{0))))$$

In this case, the values for mass fraction are always less than 1.