Specific Impulse

Specific impulse (abbreviated as \(I_{sp}\) or Isp) is a measure of the efficiency of an engine. It's in many ways comparable to fuel milage for a car or aircraft.

Do note that in some cases, with very high Isp engines, the advantages of high Isp can sometimes become negligible, or even undesirable. In some electric thrusters, it's more important to care about things like thrust density and electrical efficiency.

As a very rough rule of thumb, for the smallest spacecraft, try to aim for an Isp roughly equal to the total Delta-V of the mission plan. Again, this is very rough -- and does not apply for atmospheric escapes.