# A rockets force in different enviornments

1. Dec 11, 2006

### wakejosh

a rocket will not produce the same force under water, in air, or in space. is this correct? I would think that according to Newtons 3rd law the force would be different in each enviornment. am i right?

2. Dec 11, 2006

### OlderDan

Why does Newton's third law lead you to this conclusion? There is a difference, but it may be for other reasons.

3. Dec 11, 2006

### wakejosh

because there is an equal and opposite force. actually, now that I think about it, the force that the rocket expends should be the same no matter what, but the enviornment may provide different results due to friction etc. so I guess its more accurate to say that the force from the rocket is the same. thanks.

4. Dec 11, 2006

### OlderDan

Actually, it is not the same. It has to do with what happens to the expelled gases. In space the gas mollecules go on their way pretty much unimpeded. In a fluid, there would be a high pressure bubble formed by the exhaust that does provide some additional thrust. Of course this pressure probably also affects the exhaust velocity, so it is a bit complicated.

http://www.grc.nasa.gov/WWW/K-12/airplane/rockth.html

5. Dec 11, 2006

### wakejosh

now im confused. Isn't the thrust different than the force or are they the same? I understand that the thrust would change in the different enviornments, but wouldnt the force produced by the rocket be the same?

6. Dec 11, 2006

### OlderDan

Thrust is the force on the rocket produced by the engine gasses. Resistive forces that retard the motion of the rocket would not be considered part of the thrust, but any force on the rocket from the gasses it expells would be. I think the problem is talking about thrust, not all the forces that might be acting.

7. Dec 11, 2006

### wakejosh

ok.. so as far as Force is concerned will the force by the rocket be the same in water, air, or in space? im tottally confused now that thrust was brought in.

8. Dec 11, 2006

### wakejosh

im more confused than when i started.

9. Dec 11, 2006

### wakejosh

thrust is the force ON the rocket, not OF the rocket right? so isn't it accurate to say that the force would be the same, but the thrust would be different?

10. Dec 11, 2006

### OlderDan

I certainly do not mean to confuse you. The original statement above is talking about the force produced by a rocket in different envioronments. In empty space, the only rocket force you are likely to care about is the force ON the rocket. By Newton's third law, there is an equal and opposite force on the gas expelled by the rocket, but we have no reason to care about the resulting motion of the gas mollecules once they leave the rocket. Both of these forces are produced at the same time (you can't have one without the other) and both are produced by the burning of rocket fuel.

In a different environment, the ejected gas interacts with the stuff it gets ejected into. Are the forces involved in this interaction produced by the rocket? I suppose you could argue that they are not, but there would be no such interaction if the rocket were not spitting all that gas into the surrounding fluid. You wil have to decide what the intent of the problem is. My interpretation is that they want to know about the forces acting ON the rocket that result from the burning and exhaustion of fuel. Other interpretations are possible, but I don't think anybody cares about the forces on the fluid apart from the effect they have on the forces that act on the rocket.

The ejected gas encounters different stuff in the three environments, and although the primary effect of the burning of fuel is the equal and opposite forces between the expelled gas and what is left behind at any moment in time, the external environment does have some effect on the expelled gas that is transmitted to the rocket.

Consider a hypothetical example. Suppose a rocket expels one blob of mass at very high speed. By conservation of momentum, the rocket is accelerated in one direction and the blob is accelerated in the opposite direction. If the blob encounters no obstacle, it goes on its way forever and the rocket goes on forever the other way. Now suppose that behind the rocket is a massive wall. When the blob hits the wall, it bounces off and goes back and runs into the back of the rocket. This secondary collision is going to impart additional momentum to the rocket, so the total impulse deliverd to the rocket is greater with the wall there than it would be without it. Did the rocket produce this additional force? Not all by itself, but the fact is the force would not have existed if the rocket had not spit out the blob in the first place.

Ejecting gases into different environments is like spitting out billions of little blobs, some of which bounce back and hit the rocket again. That is what pressure in a fluid is all about. It is the forces from many little collisions averaged over time and distributed over an area. If there is nothing behind the rocket for the gas to bounce off (empty space) the net effect on the rocket is going to be different than it would be if there is stuff there to bounce the gas back to the rocket.

11. Dec 11, 2006

### Clausius2

Just to add that technically there is no difference. What matters is the external pressure. I wouldn't say anything about water because as far as I know there is no rocket working under water, so it doesn't make sense. So yes, for technical purposes the outer substance, air, oxygen, argon whatever you want has a second order effect on the thrust. The most important thing is the external pressure which tunrs out to modulate the whole dynamics of the engine. A rocket engine eventually works with supersonic flow if the external pressure is low enough for causing choking conditions in the nozzle throat. From that point on the flow is Hyperbolic, so that nothing what happens downstream matters to the calculation of the thrust.

As Olderdan pointed out a second order effect comes from the viscous dissipation and the drag on the body, but it is not included on the drag. Also, the friction of the fluid with the nozzle walls is dependant on the nature of the fluid, but that's a second order effect too.

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