# Lockheed Martin F-35 Lightning II engine power?

1. Apr 28, 2014

### koolinoor

How can I calculate the power of the engine?

Specification:
Spec in Wikipedia

Dry thrust: 125 kN

2. Apr 28, 2014

### Staff: Mentor

There is no single answer; it depends on configuration and speed. You should read the wiki on the engine as a starting point to get some simple answers and refine your query.

One note: an engine without an output shaft doesn't make sense to rate by power. But this engine is a hybrid.

3. Apr 28, 2014

### koolinoor

How come internal combustion engines have power specified? And this one also, has Force specified, but no power. It doesn't make sense. I mean, I get the fact, that 99% of the chemical energy is probably just wasted, compared to more effective dynamics, but still.. Ford Mustang's engine's power is 157 kW either its going up or down the hill..

Even though it takes less Energy and uses less force to drive downhill.
So maybe that's the case, that the automotive industry have devalued the concept of physical Power?

Where do they get these numbers? And why is it so difficult to come up with one for this case?

4. Apr 28, 2014

### Staff: Mentor

A jet is internal combustion too, but I think you are referring to engines like car engines: they have an output shaft. They output mechanical work.
What about when the Mustang is sitting still on the hill and revving the engine against the clutch to hold still?
Force and shaft output power are easily measured.
It isn't that it is difficult, it is that it is not applicable.

5. Apr 28, 2014

### koolinoor

say F-35 weights 15 tons * 9,8 its ~ 147 kN..
more or less similar amount.
and the oxygen molecules potential energy is what forces the emission of gas from the turbofan to lift the plain.

then if the plane moves upward 1m/s, can You say, that the power is 147 kW?

its absurd, isn't it?
but say it wouldn't be an open system, but the gas is output to a barrel. what would be the power then?
is there any way of figuring that out?

If the shaft output power can be measured, why cant the turbofan output power be measured?

Last edited: Apr 28, 2014
6. Apr 28, 2014

### Staff: Mentor

No, it isn't absurd. But it may not be terribly useful either. Again, consider the Mustang hovering on a hill with its clutch. What is the power output of the car? The engine? Does it matter?
That would be another way, yes. If you knew the airflow volume, velocity and pressure you could figure out its power. But that answer will be different from one based on the speed of the plane. So which one do you think is more useful/relevant?

It can be, it's just not necessarily all that useful.

7. Apr 28, 2014

### koolinoor

You are right, that info is not all that useful. But it would still be interesting to know, just to compare the sources of power. Even so that the dynamics of the applications are not comparable.

8. Apr 28, 2014