*I*

_{sp}). Basically, it's how efficient your rocket engine is. It's defined by how far each unit of fuel moves your rocket.

Here's the formula:

Where:

- is the specific impulse in meters per second
- the thrust in newtons
- the fuel consumption in kg/s

If your Isp is higher, then you can get more Delta-V with the same rocket. In the rocket equation, is used instead of Isp, but the concepts are the same.