Changes

Impulse (or total impulse) is defined as equal to force multiplied by time - it is a measure of how powerful motors are, and can easily give you your change in velocity (Impulse / Mass, assuming no drag or gravity losses. There will always be drag and gravity losses.) Total impulse can be thought of as the area under the thrust curve.

Impulse (or total impulse) is defined as equal to force multiplied by time - it is a measure of how powerful motors are, and can easily give you your change in velocity (Impulse / Mass, assuming no drag or gravity losses. There will always be drag and gravity losses.) Total impulse can be thought of as the area under the thrust curve.

[[File:L1_Guide_Thrust_Curve.png|400px|center|frame|Thrust curves of motors with the same total impulse]]

[[File:L1_Guide_Thrust_Curve.png|400px|thumb|frame|center|Thrust curves of motors with the same total impulse]]

In the above graph, each of those three motors have the same total impulse (area under the curve), but with very different thrust profiles.

In the above graph, each of those three motors have the same total impulse (area under the curve), but with very different thrust profiles. The letter designation for each motor (e.g. in E15, H125, or M1250) are a measure of total impulse and each successive letter represents double the total impulse.

It turns out that impulse alone is not a good measure for rocket fuel performance; it is technically possible to use wood and air as rocket fuels, and to get an insanely large impulse by making the engine really big. That will never get you to space. Instead, rockets really care about a number called the specific impulse, defined as impulse divided by the mass of the propellant and the gravitational constant. This gives a far better picture of what constitutes a good rocket fuel, although there are definitely other considerations.

It turns out that impulse alone is not a good measure for rocket fuel performance; it is technically possible to use wood and air as rocket fuels, and to get an insanely large impulse by making the engine really big. That will never get you to space. Instead, rockets really care about a number called the specific impulse, defined as impulse divided by the mass of the propellant and the gravitational constant. This gives a far better picture of what constitutes a good rocket fuel, although there are definitely other considerations.