Specific impulse

The mass-specific impulse (abbreviated ) of a drive system is the change in momentum ( mass times velocity or force times time) per mass or per unit weight of fuel. It is an essential characteristic of rocket motors and is the effective speed of the driving gases leaving the nozzle dar. It is thus a measure independent of the size of the engine for the efficiency of a drive or fuel.

With the same abbreviation and the weight specific impulse is referred to, which is additionally normalized to the former formulation of the gravitational acceleration. This has the advantage that it can be so on Anglo- American units, and back transfer.


Mass Specific Impulse

The specific impulse is formulated as a specific impulse as follows:


  • The burning time
  • The fuel mass
  • The average thrust ( force)
  • The thrust curve

The result for the mass specific impulse, the dimension length per time, for example in the SI -compliant units of meters per second ().

Weight Specific Impulse

Due to different systems of units, it is now common practice to use the weight in place of the specific impulse mass specific pulse. This is scaled by the standard acceleration due to gravity and is usually given in units of seconds:

With the standard acceleration of gravity.

Since the second unit is used internationally as a unit of time equal to the weight specific impulse is more comparable and thus has replaced the old formulation. However, it must be noted that the conversion factor is used as a constant and if changes the gravitational acceleration for the rocket that the weight specific impulse nevertheless does not change.


A specific impulse of 1000 m / s = 1,000 Ns / kg s ≙ 102 means that 1 kg of fuel, can cause a change of momentum of 1.000 ns. This corresponds to, for example, an engine including a second thrust developing a force of 1,000 N and at this time consumes 1 kg of fuel, or a small engine, the 10 s 10 N shear developed in 0.1 kg fuel consumption.

The specific impulse is dependent on the chemical propellant is released during the reaction energy and the average molecular weight. The highest specific impulses in the combinations used today have hydrogen / oxygen and hydrogen / fluorine. The highest experimentally achieved value is about 470 s ( RL- 10B2 and Vinci engine ).

Ion engines accelerate a working gas, which is ionized by electrical fields. Depending on the voltage applied, the ions can be accelerated to very high speeds, up to nearly the velocity of light theoretically. Available ion engines have specific impulses of 3,000 to to 4,000 s, the dual-stage 4 -grid ion engine ( DS4G ) of the ESA reached a specific impulse of 210,000 m / s ≙ 21,400 s


The specific impulse of a rocket engine with bell nozzle is dependent on the ambient pressure. The nozzle is optimized in design to a specific discharge pressure. In upper level, for example, yields the highest specific impulse in a vacuum. At the start of the earth is due to the atmospheric pressure, the maximum achievable specific impulse by 10 to 15 percent lower because you can not expand under about 40 percent of the external pressure at lower levels. Otherwise, it comes at the bell nozzles normally used for an outline of the flow ( Summerfield criterion). A both in air and in vacuum equally efficient working alternative is the aerospike engine.

Since the mass-specific momentum in SI units representing the effective exhaust velocity of the gases can be calculated together with the full and empty mass of the Tsiolkovsky Rocket Equation the final velocity of the rocket.

For missiles, which are used within the atmosphere, as well as military rockets that must be accommodated in a silo or as small a shipping container, and the volume specific impulse is important because then you want as compact as possible so build, low air resistance. It is formed by the specific impulse is multiplied by the density of the fuel:

For example, the specific impulse of the combination of dinitrogen tetroxide with hydrazine variants is slightly smaller than that of liquid oxygen and kerosene. The density is higher and the result is a smaller rocket.