Specific Impulse Calculator

Specific Impulse

Specific impulse $I_{sp}$ measures rocket propulsion efficiency. Higher $I_{sp}$ means more thrust per unit of propellant consumed per second, which improves delta-v for a given mass budget.

From Thrust and Mass Flow

$$I_{sp} = \frac{F}{\dot{m} g_0}$$

$F$ is thrust in newtons, $\dot{m}$ is mass flow rate in kg/s, and $g_0 = 9.80665$ m/s² is standard gravity. The result is in seconds.

From Exhaust Velocity

$$I_{sp} = \frac{v_e}{g_0}$$

Effective exhaust velocity $v_e$ in m/s converts directly to specific impulse. Chemical rockets often sit near 200 to 450 s; ion thrusters can exceed 1000 s.

Frequently Asked Questions

Why is Isp measured in seconds?

Seconds represent how long one unit of weight of propellant can produce one unit of thrust at $g_0$. It is a historical but universal rocket metric.

What is a typical Isp for liquid fuel?

Liquid hydrogen/oxygen engines often reach about 400 to 450 s. Kerosene engines are lower, often near 300 to 350 s.

How does Isp relate to exhaust velocity?

$v_e = I_{sp} \times g_0$. Doubling exhaust velocity doubles specific impulse.

Does Isp include nozzle efficiency?

Reported $I_{sp}$ values are usually effective values from test stands, including real nozzle and combustion performance.

Can I use this for electric propulsion?

Yes. Ion and Hall thrusters have high $I_{sp}$ but low thrust. Enter measured thrust and mass flow, or effective exhaust velocity.

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Specific Impulse Calculator