Specific Impulse (Isp)

Specific impulse (Isp) is the key measure of propulsion efficiency: the time (in seconds) that a unit weight of propellant can produce a unit force of thrust. Higher Isp means the engine extracts more velocity change from each kilogram of fuel. It directly determines the achievable delta-v via the Tsiolkovsky equation: ΔV = Isp × g₀ × ln(m₀/mf). Chemical rockets achieve 300–450 s (hydrazine 230 s, RP-1/LOX 310 s, LH2/LOX 450 s). Electric propulsion (Hall-effect, ion) achieves 1,500–10,000 s but at much lower thrust levels.