Specific Impulse Calculator
Enter thrust and propellant mass-flow rate to compute specific impulse Isp=F/(ṁ·g₀), exhaust velocity v_e=Isp·g₀ and engine class. Merlin 7600 kN, ṁ=2600 kg/s → Isp≈282 s, v_e≈2767 m/s.
Input Data
Results
At a glance:Specific impulse Isp is the standard measure of rocket-engine efficiency: the impulse produced per unit weight of propellant, Isp=F/(ṁ·g₀), with F thrust (N), ṁ mass flow rate (kg/s) and g₀=9.80665 m/s². Physically Isp equals the effective exhaust velocity divided by g₀: v_e=Isp·g₀=F/ṁ, so a higher Isp engine has a higher exhaust speed. The Tsiolkovsky equation Δv=v_e·ln(m₀/m_f) shows Isp (via v_e) and the mass ratio together set the achievable Δv. History: the concept matured with Goddard's 1926 liquid rocket and the space race; it is now the headline engine spec. Classic examples: Falcon9 Merlin 1D — sea-level thrust 7600 kN, ṁ≈2600 kg/s → Isp≈7600000/(2600×9.80665)≈282 s, v_e≈2767 m/s; Shuttle RS-25 (LH₂/LOX) vacuum Isp≈452 s, the most efficient chemical engine; Long March 5 YF-77 vacuum Isp≈428 s; ion thruster (Dawn) Isp≈3100 s but thrust only mN. Applications: (1) launch-vehicle and stage design; (2) engine selection (chemical vs electric); (3) mission Δv budgeting; (4) comparing propellant types (LH₂ highest chemical Isp, solid lower).
Formula
Specific impulse: Isp = F / (ṁ·g₀)
Exhaust velocity: v_e = Isp·g₀ = F/ṁ
Rocket equation: Δv = v_e·ln(m₀/m_f)
g₀ = 9.80665 m/s²
$$I_{sp} = \frac{F}{\dot{m}\,g_0}, \quad v_e = I_{sp}\,g_0 = \frac{F}{\dot{m}}$$How to Use
- Enter engine thrust F (N).
- Enter propellant mass-flow rate ṁ (kg/s).
- The tool gives Isp (s), exhaust velocity v_e (m/s) and engine class.
Case Studies
Chemical vs ion engine
Merlin 1D: F=7600 kN, ṁ=2600 kg/s → Isp≈282 s, v_e≈2767 m/s (high thrust, low Isp).
RS-25: vacuum Isp≈452 s, LH₂/LOX, the top chemical engine.
Ion (Dawn): Isp≈3100 s but mN thrust — slow but extremely propellant-efficient for deep space.
Content review: Calculatorism Science Team. Results are for reference only; please refer to the relevant authorities for the official figures.