The definition
Isp = F / (ṁ · g0)
F is the thrust, ṁ the propellant mass flow and g0 standard gravity. Dividing by g0 turns the result into seconds, which has the useful property of being the same in metric and imperial units. An engine with 300 s of specific impulse produces one kilogram-force of thrust for 300 seconds from one kilogram of propellant.
Why it matters
The rocket equation links specific impulse to how much speed a vehicle can gain:
Δv = Isp · g0 · ln(m0 / mf)
Because the mass ratio enters only through a logarithm, a few percent more specific impulse is worth a lot of propellant. That is why engine designers fight for every second.
Typical values
Ideal vacuum values from our NASA CEA calculation at 10 bar chamber pressure and an area ratio of 40, each at its best mixture ratio, plus typical electric thrusters:
| Propulsion | Vacuum Isp |
|---|---|
| Liquid oxygen / kerosene (RP-1) | about 353 s |
| Liquid oxygen / methane | about 365 s |
| Liquid oxygen / hydrogen | about 454 s |
| Hall thruster (electric) | about 1,500–3,000 s |
Real engines reach somewhat less than the ideal chemical values because of friction, heat loss and incomplete combustion. Sea-level values are lower again, because the surrounding air pressure pushes back on the nozzle exit.