Specific impulse

**Specific impulse** (often shortened to *I*sp) is a way to describe how well a rocket performs. It is also used to describe how well a jet engine performs. It can be used to compare rockets of different sizes. It is a way to know how much force a given engine makes for each bit of fuel. To do this, one needs to know how much fuel is in the engine. A high specific impulse means that a rocket needs less fuel to perform as well. If the specific impulse is lower, it means that the rocket needs more fuel to perform as well. 

Specific impulse is a useful value to compare rocket or jet engines. It is used much like *miles per gallon* or *litres per 100 kilometres* is used to compare cars.  An engine with a higher specific impulse uses fuel better. This means that the same amount of fuel lets the rocket go faster after all fuel is used up.

Be careful not to confuse thrust and specific impulse. Thrust is only the force made by a rocket at one point in time. Specific impulse is a measure of the force based on how much fuel there is.

When people find specific impulse, the only fuel counted is in the rocket before it gets fired. This includes the fuel and the oxidizer (the part of the fuel that helps the fuel burn).   The oxidizer is sometimes oxygen, or often something else (see Rocket engine#Liquids, solids and hybrids).

## Examples
Jet (airplane) engines use fuel better than rocket engines. This is because the gases do not escape as fast. Because they do not escape as fast, the exhaust does not carry away as much energy. This means that the jet engine uses a lot less energy to push the jet.  It is also because the air that goes through the engine as the jet flies through the air helps the fuel to burn faster.

### Model rocketry
Specific impulse is also used to describe how well model rocket motors work. In the table below are some of Estes' claimed values for specific impulses for several of their rocket motors: Estes Industries is a large, well known American seller of model rockets and rocket parts. The specific impulse for model rocket motors is much lower than for many other rocket motors because the black powder is used as a fuel. Black powder is used in model rocket motors because it costs less. 

### Larger Rocket Engines
Here are some example numbers for larger rocket engines:

## Units
The most common way to measure specific impulse today uses seconds. This is used both in the SI (metric system) world as well as where English units are used. This way the units are identical everywhere. This means specific impulse can be used to compare engine performance in any country. Most companies that make rocket motors or jet engines use seconds to advertise their product’s performance.

The other common way to measure specific impulse is in meters per second (m/s), which is also called effective exhaust velocity. For many engines the effective exhaust velocity is different from the speed that the gases actually come out of the nozzle.

## Related pages
*Jet engine
*Impulse (physics) - the change in momentum

## References
## Other websites 
*RPA - Design Tool for Liquid Rocket Engine Analysis
*List of Specific Impulses of various rocket fuels