Aero Fact: Specific Impulse (Isp)

Isp measures thrust per unit propellant flow—higher Isp means more efficient engines.

By AircoreDynamics • October 22, 2025 • 1 min read

Definition: Thrust per unit propellant weight flow — a measure of propulsive efficiency.

Physics insight: Isp = Thrust/(ṁ g0) and relates to exhaust velocity via ideal flow relations.

Example: Cryogenic LH2/LOX engines ~450 s; RP-1/LOX ~330 s.

I_sp = V_e / g0 (idealized)

Rules of thumb: Higher Isp isn’t always best; density and volume constraints matter.

Common pitfalls: Confusing sea-level and vacuum ratings.

References: Sutton