What this calculator does
Specific impulse measures how efficiently a rocket engine uses propellant: how much thrust it produces per unit of propellant consumed. Higher is better, and it translates directly into how much velocity change a given mass of fuel can buy.
It is quoted in seconds, which seems odd for an efficiency measure. The unit comes from dividing thrust by propellant weight flow rather than mass flow, and the convenient result is that the figure is the same whether you work in metric or imperial units.
The formula
Divide thrust by the propellant mass flow rate and by standard gravity. The effective exhaust velocity is that specific impulse multiplied by standard gravity again, which recovers the physically meaningful speed.
| Term | Meaning |
|---|---|
| Specific impulse (Isp) | Thrust per unit propellant weight flow, in seconds. |
| Effective exhaust velocity | The speed the exhaust leaves at, equal to Isp times standard gravity. |
| Mass flow rate (ṁ) | Propellant consumed per second, in kilograms per second. |
The inputs explained
| Field | What to enter |
|---|---|
| Thrust (F) (N) | The engine thrust in newtons. |
| Propellant mass flow rate (ṁ) (kg/s) | The propellant mass flow rate in kilograms per second. |
When to use it
Comparing engine designs
Specific impulse is the standard efficiency figure, allowing very different engines to be compared on one number.
Feeding the rocket equation
Delta-v depends on exhaust velocity, which comes directly from specific impulse.
Trading thrust against efficiency
High-thrust chemical engines have low specific impulse; ion drives have enormous specific impulse and almost no thrust.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How does propellant flow affect efficiency?
The same thrust produced with different propellant flow rates.
| Mass flow rate | Specific impulse (Isp) | Effective exhaust velocity (ve) |
|---|---|---|
| 0.5 kg/s | 407.89 s | 4,000.0 m/s |
| 0.68 kg/s | 299.92 s | 2,941.2 m/s |
| 1 kg/s | 203.94 s | 2,000.0 m/s |
| 2 kg/s | 101.97 s | 1,000.0 m/s |
Questions
Why is it measured in seconds?
Because it divides thrust by propellant weight flow rather than mass flow, and the units of force and weight-per-second cancel to leave time. The practical benefit is that the number is identical in metric and imperial systems.
Is higher specific impulse always better?
For propellant efficiency, yes, but not for every mission. Ion drives have specific impulses ten times higher than chemical rockets yet produce so little thrust that they cannot lift off at all. Launch needs thrust; deep space cruising needs efficiency.
What are typical values?
Solid boosters are around 250 seconds, kerosene and oxygen about 300 to 350, hydrogen and oxygen roughly 450, and ion thrusters several thousand.
How does it connect to delta-v?
Through the rocket equation. Delta-v is the effective exhaust velocity times the natural logarithm of the mass ratio, and exhaust velocity is specific impulse times standard gravity.
For the velocity change it buys, see the rocket equation calculator. For the speed needed to escape a planet, see the escape velocity calculator.