Principles of rocket thrust

[Obidiah]

A question if I may.

I am in an ongoing debate with a university professor from a non aviation/aerospace background but versed in physics. I on the other hand am of a non university background but with 27 years as pilot with what I would like to believe a good understanding of the applied physics.

Regarding rocket thrust in space.

My understanding is that a rocket in space exerts a pushing force (thrust) that can be said to act as an opposing pressure upon the inner surface of the rocket nozzle/s and the engine as a whole. This I feel is a physical “pushing” force.

We both understand Newton’s 2nd and 3rd laws. My interpretation of the application specific to this is it is the accelerated mass which in turn places an opposing force. But I could be wrong.

The professor’s line on this is that it is simply the application of the 3rd law and there is no force, push or thrust as such. Simply it is the movement of mass in the opposing direction that causes accelerated forward movement.

Can there be thrust without push? At first glance it would appear absurd,.....but..?

My manuals don’t get this deep and everything I have pulled from the net appears to support my view. Perhaps my understanding has reached a limit and I am missing something or maybe the professor is thinking pure science and the real world application does exert a measurable force.

One method I use in my mind is to imagine a sealed pressure chamber a gas bottle if you will. Pressure is exerted equally in all directions. Create an opening at one point and pressure can no longer act at that point. The pressure force on the opposing side is now out of equilbrium and pushes the vessel forward.

Can someone throw some light on this?

[Intruder]

Look up the word "semantics."

[Slatye]

I'm not sure how the professor can claim that it's an application of the third law (ie f = ma) but there's no force involved; this would imply that there's no acceleration, which is clearly wrong.

There is acceleration. Therefore there is also a force, assuming that the rocket has mass.

Exactly where the force is applied is just a matter of design. The term "rocket" covers a lot of different designs and the force may not always be applied at the same point on each of them. As you say, in a conventional rocket the force is largely applied to the engine bell. In a simple compressed air rocket, force is applied pretty much everywhere except the exhaust opening.

[StainesFS]

Where rockets are concerned, Newton's laws are not the first bit of science to turn to. Rather, as the basis of the Rocket Equation, conservation of linear momentum is better.

Starting with momentum = mass x velocity, for a rocket at rest, the momentum is obviously zero. Start the rocket and propellant/oxidiser/combustion products shoot out of the back. If we take the forward direction as the positive one for vectors like velocity then these substances have a negative momentum. To preserve the zero momentum of our rocket/fuel "system", the rocket must have a positive momentum of equal magnitude, achieved by its mass moving forward with a certain positive velocity.

Over the course of a journey, we have to use calculus since the mass of the rocket is gradually changing due to loss of fuel/propellant.

A rocket, or indeed a turbine engine on a static test can, indeed, be said to have a certain amount of thrust. Stand behind the engine and you will, of course, only be too aware of this. However, in explaining the movement/velocity of the rocket, we do not need to invoke the term "thrust".

SFS

[barit1]

Pretty simple if you ask me.

The rocket engine is physically attached to the rest of the vehicle by some pretty stout mount structure, which transmits the F (= Ma) to the fuel & oxidizer tank(s), payload etc.

So yes, in the big picture, there is a summation of (pressure x area) over the rocket engine combustion chamber and expansion nozzle, and this force summation is the de facto thrust applied to the vehicle.

You might ask the prof about the shuttle external solid-fuel rockets - if there is no "force" involved, then what good are the solids? :confused:

[Obidiah]

To play the devils advocate, is it not the case the external thrusters are depleted during the atmospheric stage?

To my way of thinking if you had rocket gently orbiting and the entire combuster nozzle array were suspended by tensional springs attached to the body of the rocket itself and you fired the engine the whole array would tension forward on its support springs. Thus in turn dragging the body forward with it.

If this is in fact the case it would be proof postive that the thrust is a pushing force and not as the good professor claims simply thrust without push based on the 3rd law.

[3holelover]

I think it's an interesting question.... The professor has my vote. "Simply it is the movement of mass in the opposing direction that causes accelerated forward movement."
Your pressure can't act upon anything in the opposite direction, so there's no resultant push, of the sort there would be in the atmosphere..... but the mass being accelerated aft.

[italia458]

StainesFS and barit1 have essentially answered your question.

maybe the professor is thinking pure science and the real world application does exert a measurable force.

You seem to have the mindset that science isn't "real". That couldn't be further from the truth. If scientific theory doesn't match experiment, it's wrong! If someone is using 'pure science' to explain something and it doesn't match what you know to be true from 'real life', it's usually because he/she isn't considering all factors that would affect the result or is misapplying a scientific theory to the 'real life' problem.

Feynman Chaser - The Key to Science - YouTube (http://youtu.be/b240PGCMwV0?t=38s)

[Brian Abraham]

Thrust is generated in accordance with the third law, and that thrust acts upon the walls of the combustion chamber. On the latter point your Professor errs.

A primer

Basics of Space Flight: Rocket Propulsion (http://www.braeunig.us/space/propuls.htm)

[Obidiah]

Yes that would be my understanding, 3rd law is the principal player and the force is measurable in convential terms ie. lbs. newtons, etc.

To me it would appear clear, a drifting space vehicle fires its rockets, its combuster/nozzle array is spring attached within a plenum housing.
The feed in fuel pipes are flexible so the combuster is free to wobble on its springs.

As the rocket powers, the nozzle/combuster assembly strains against the mounting springs and drags/pushes/pulls, how ever you have assembled this in your mind, but it transfers the thrust to the main body via the springs and hi ho Cisco we're off.

But I could be wrong, one does not disagree with a Professor lightly.

"Thrust' is not a pushing force, it merely denotes movement." to quote the Professor. So the search is for an example of such, the rocket in space has been put forward.

As I read through the great weight of text on this subject i become aware of drawings which I already knew of. These show magnitude arrows on the inner and outer surface of the combustor/nozzles, the inner being greater, proof positive perhaps from those that explain the theory. But what is the magnitude of this force? Calculus will take me there I guess but not my forte. I am also drawn to comments regading optimised pressure on the bell and it does not appear high, I wonder is this pressure a simple correlation such as wing loading or is there more at play here.

[Brian Abraham]

The nozzle converts the thermal energy of the hot chamber gases into kinetic energy, and directs that energy along nozzle axis. The nozzle gradually increases in cross-sectional area allowing gases to expand and push against walls creating thrust. The ultimate purpose of the nozzle is to expand gases as efficiently as possible so as to maximize exit velocity.

Thrust of a rocket can be calculated by

MVe+(Pe-Pa)Ae

Where

M = engine mass flow rate
Ve = gas velocity at nozzle exit
Pe = exhaust gas pressure at nozzle exit
Ae = cross-sectional area of nozzle exhaust exit
Pa = external ambient pressure, (free stream pressure)

A nozzle is designed to operate at one particular static pressure, and any deviation will see losses in the form of over or under expansion of the exhaust plume. Where the exhaust plume is perfectly expanded (ie at the nozzle design point) thrust = MVequivalent (or effective) exhaust gas velocity at nozzle exit

Basics of Space Flight: Rocket Propulsion (http://www.braeunig.us/space/propuls.htm)

[henra]

To my way of thinking if you had rocket gently orbiting and the entire combuster nozzle array were suspended by tensional springs attached to the body of the rocket itself and you fired the engine the whole array would tension forward on its support springs. Thus in turn dragging the body forward with it.


That is correct. But it does not invalidate Newton's 3rd law.
You have to differentiate the external effect of the mass flow leading to the accelleration of the rocket and the mechanical effect inside the rocket. In order to achieve the mass flow you have to accelerate the exhaust flow. this happens in the Nozzle. And acceleration causes an opposing force.
It is simply the result of the static pressure created in the combustion chamber by the expansion of the hot gases that is subsequently converted to dynamic pressure in the Nozzle.

[Turbavykas]

This is philosophical discussion all I can say that rocket nozzle shape is very important on the performance of the rocket and it's different for rockets that fly in vacuum or atmosphere.

[Obidiah]

Thanks,

Not for a moment would I suggest the force described is not in total accordance with Newton's laws.

The question was/is does this force push, the good Professor suggests that Newtons 3rd law does not require a push force to exist. It is simply a mass transfer equation and that a rocket in space has nothing to push on, ie. absence of an atmosphere. Explaining the push force acts throught the combustor/nozzle does not sit with him.

I can think of a limitless list of examples to visualise the effects.

Shouldering a shotgun and firing it in space, I have no doubt the bruise on my shoulder would be just the same on earth as in space, and I don't bruise easy.

Explaining holding a fire hose while pointed at a wall (on earth) as opposed to pointing the stream into the open. In the first case the stream has a lot more to push on, ie. the wall as opposed to just air. The fireman of course will not notice any difference in hanging onto the hose in either case. For good reason there is none, it is not what it is pushing on but the mass ejection of the fluid. This example does not work for him either.

Frustration led me to question myself, so I sort other explanations here, just to be sure to be sure.