Specific impulse facts for kids
Specific impulse, often called Isp, tells us how good a rocket or jet engine is at using its fuel. It helps us compare different engines, even if they are different sizes. Think of it like "miles per gallon" for a car. A higher specific impulse means the engine uses less fuel to do the same job. This means the rocket can go faster or farther with the same amount of fuel.
A rocket engine with a high specific impulse isn't always the most powerful. For example, ion engines have very high specific impulse but produce very little thrust (pushing force). They accelerate a rocket slowly but steadily over a long time, using tiny amounts of fuel. If two rockets had the same amount of fuel, the one with a more powerful engine would start faster. But the rocket with higher specific impulse would use its fuel more efficiently and keep accelerating for longer. Over a long distance, it would eventually go faster and farther.
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How Specific Impulse is Measured
People figure out specific impulse in two main ways. You find it by dividing the impulse by the amount of fuel used. Impulse measures how much force a rocket motor makes and for how long. A motor that makes a small force for a long time can sometimes have a higher impulse than one that makes a big force for a short time. Impulse is measured in Newton-seconds (N·s).
The amount of fuel can be measured in different ways. Sometimes it's measured by its mass (how much "stuff" is in it). When measured by mass, specific impulse is given as a speed, usually in meters per second (m/s). This is also called the effective exhaust velocity. The other way to measure fuel is by its weight (how hard gravity pulls on it). If weight is used, specific impulse is given in units of time, usually in seconds. Both ways are common and help compare how well engines perform.
When the specific impulse is higher, the engine needs less fuel to make the rocket perform well. So, a higher specific impulse means the fuel is used more efficiently.
It's important not to mix up specific impulse with thrust. Thrust is just the force a rocket makes at one moment. Specific impulse measures that force based on how much fuel is used.
When specific impulse is calculated, only the fuel inside the rocket before it fires is counted. This includes both the fuel itself and the oxidizer. The oxidizer is a substance that helps the fuel burn. It can be oxygen or other chemicals.
Examples of Specific Impulse
| Engine | Effective exhaust velocity (m/s, kg·m/s/kg) |
Specific impulse (s) |
Energy per kg of exhaust (MJ/kg) |
|---|---|---|---|
| Turbofan jet engine (actual V is ~300) |
29,000 | 3,000 | ~0.05 |
| Solid fuel rocket | 2,500 | 250 | 3 |
| Liquid fuel rocket | 4,400 | 450 | 9.7 |
| Ion thruster | 29,000 | 3,000 | 430 |
| Dual Stage Four Grid Electrostatic Ion Thruster | 210,000 | 21,400 | 22,500 |
| VASIMR | 30,000-120,000 | 3,000-12,000 | 1,400 |
Jet engines, like those on airplanes, use fuel better than rocket engines. This is because the gases don't escape as fast. Since they don't escape as fast, the exhaust carries away less energy. This means the jet engine uses much less energy to push the plane. Also, the air that goes through the engine as the jet flies helps the fuel burn faster.
Model Rocket Engines
Specific impulse is also used for model rocket motors. The table below shows some specific impulse values for motors made by Estes. Estes Industries is a well-known American company that sells model rockets. Model rocket motors often have lower specific impulse values than larger rocket engines. This is because they use black powder as fuel, which costs less.
| Engine | Total Impulse (Ns) | Fuel Weight (N) | Specific Impulse (s) |
|---|---|---|---|
| Estes A10-3T | 2.5 | .0370 | 67.49 |
| Estes A8-3 | 2.5 | .0306 | 81.76 |
| Estes B4-2 | 5.0 | .0816 | 61.25 |
| Estes B6-4 | 5.0 | .0612 | 81.76 |
| Estes C6-3 | 10 | .1223 | 81.76 |
| Estes C11-5 | 10 | .1078 | 92.76 |
| Estes D12-3 | 20 | .2443 | 81.86 |
| Estes E9-6 | 30 | .3508 | 85.51 |
Large Rocket Engines
Here are some example numbers for larger rocket engines:
| Engine type | Example Use | Specific impulse (s) | Effective exhaust velocity (m/s) |
|---|---|---|---|
| NK-33 rocket engine | Vacuum | 331 | 3,240 |
| SSME rocket engine | Space shuttle vacuum | 453 | 4,423 |
| Ramjet | Mach 1 | 800 | 7,877 |
| J-58 turbojet | SR-71 at Mach 3.2 (Wet) | 1,900 | 18,587 |
| Rolls-Royce/Snecma Olympus 593 | Concorde Mach 2 cruise (Dry) | 3,012 | 29,553 |
| CF6-80C2B1F turbofan | Boeing 747-400 cruise | 5,950 | 58,400 |
| General Electric CF6 turbofan | Sea level | 11,700 | 115,000 |
Units of Measurement
| Specific Impulse (by weight) |
Specific Impulse (by mass) |
Effective exhaust velocity |
Specific fuel consumption |
|
|---|---|---|---|---|
| SI | =X seconds | =9.8066 X N•s/kg | =9.8066 X m/s | =(101,972/X) g/kN•s |
| English units | =X seconds | =X lbf•s/lb | =32.16 X ft/s | =(3,600/X) lb/lbf•h |
Today, the most common way to measure specific impulse is in seconds. This unit is used everywhere, whether people use the metric system (SI) or English units. This means you can easily compare engine performance no matter where you are. Most companies that make rocket or jet engines use seconds to show how well their products perform.
Another common way to measure specific impulse is in meters per second (m/s). This is also called effective exhaust velocity. For many engines, the effective exhaust velocity is different from the actual speed at which the gases leave the nozzle.
Related Pages
- Jet engine
- Impulse (physics) - the change in momentum
Images for kids
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The specific impulse of various jet engines (SSME is the Space Shuttle Main Engine)
See also
In Spanish: Impulso específico para niños
