Of course, the weight of the rocket has to be taken out of consideration and so does the reduction in fuel weight as it’s expended the basic idea is “how long can any given amount of x hold itself up”. While the unit of seconds can seem confusing to laypeople, it is fairly simple to understand as “hover-time”: how long a rocket can “hover” before running out of fuel, given the weight of that propellant/fuel. A propulsion method and system with a higher specific impulse is more propellant-efficient. Specific impulse is a useful value to compare engines, much like miles per gallon or liters per 100 kilometers is used for cars. The higher the specific impulse, the lower the propellant flow rate required for a given thrust, and in the case of a rocket, the less propellant needed for a given delta-v, per the Tsiolkovsky rocket equation.

The conversion constant between these two versions is the standard gravitational acceleration constant (g0). If weight (newton or pound force) is used instead, then specific impulse has units of time (seconds). If mass (kilogram or slug) is used as the unit of propellant, then specific impulse has units of velocity. By definition, it is the impulse delivered per unit of propellant consumed, and is dimensionally equivalent to the thrust generated per unit propellant flow rate. Specific impulse (usually abbreviated Isp) is a measure of the efficiency of rocket and jet engines.

Propulsion time calc