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PropulsionHow does propulsion work? |
Rocket engine performance can be roughly calculated by the thrust equation:
T = mVe + (pe - pa) Aewhere
T = thrust force
m = flow rate = feVeAe
fe = fluid density at nozzle exit
Ve = exhaust velocity at nozzle exit
pe = exhaust pressure at nozzle exit
pa = ambient pressure
Ae = nozzle exit area
This means that the produced by a rocket is sum of two forces:
However the dominant term (the one that is numerically much bigger than the other) is mVe, and therefore the thrust can sometimes be approximated as:
T = mVe
You can see that the major predictor of the performance of a rocket is how much stuff can you throw out the back multiplied by how fast you can throw it out. The larger the thrust force, the more mass the rocket can lift off the ground. However there are other factors that can affect the rocket engine performance.
| Rocket type | Limiting factor | Effects |
| chemical rocket | mass limited | run out of stuff to throw out the back |
| ion/electrical rocket | energy limited | not enough energy to throw out large amounts of propellant |
The fundamental measure of rocket engine performance is how much mass can you propel out the back and at what speeds the propellant is driven out the back. Chemical rockets can spew a large quantity of propellant out at any one time, but because of this, they are less efficient then ion/electrical rockets which spew propellant out at higher speeds.
What is thrust?
