How this calculator works
Specific impulse measures thrust per unit propellant weight-flow rate. Its conventional unit is seconds because the mass-flow denominator is multiplied by standard gravity. It is related to effective exhaust velocity and reciprocal thrust-specific fuel consumption. Keep the default standard gravity for conventional comparisons; the inverse gravity mode is an algebra check, not a change in engine performance on another planet. Entered dry mass is used only for an optional thrust-to-weight ratio.
Formula
Isp = F/(mdot g0); veffective = Isp g0 = F/mdot; TSFC = mdot/F = 1/veffective. Thrust/dry weight = F/(mdry times 9.80665).
Worked example
Thrust 9806.65 N with flow 10 kg/s and g0 = 9.80665 m/s2 gives Isp = 100 s and effective exhaust velocity 980.665 m/s. At 200 s and 2 kg/s, thrust is 3922.66 N.
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Frequently asked questions
Does a longer specific impulse mean a longer burn?
No. It measures propellant efficiency; actual burn duration also depends on propellant mass and flow.
Is local gravity used for conventional Isp?
No. Standard gravity 9.80665 m/s2 is used for the conventional seconds value.
What mass belongs in thrust-to-weight?
Use the mass of the object whose performance you want to compare. Engine-only and full-vehicle ratios answer different questions.