Specific impulse General considerations
There are two ways people find the number for specific impulse. To find the specific impulse, one divides the impulse by the amount of fuel. Impulse is a measurement of how much force a rocket motor makes and for how long. A motor that makes a low force for a long time can sometimes have a higher impulse than a motor that makes a high force for a short time. The amount of fuel used to find specific impulse can be measured in different ways. It is sometimes measured in mass and sometimes in weight. When the amount of fuel is measured in mass, the specific impulse is expressed as a speed. This is usually in meters per second.