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Author Topic: Which is more powerful expansion or reaction, and why ?  (Read 4208 times)

Offline McQueen

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The problem is as follows, which force do you think is the more powerful, that of “expansion”, such as used in Guns, cannons and the IC piston engine, or “reaction forces” as used in rockets, missiles and jet engines.  The conservative view seems to be that expansion is a far more potent force because in expansion we can actually see that a force is pushing against 'something' tangible, whereas when dealing with reaction, the force seems to be pushing against thin air.  Which in fact is the more powerful force and why ?


 

Offline Atomic-S

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Which is more powerful expansion or reaction, and why ?
« Reply #1 on: 26/09/2007 06:46:01 »
It depends upon how much fuel is behind it, among other things.
 

Offline Soul Surfer

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Which is more powerful expansion or reaction, and why ?
« Reply #2 on: 26/09/2007 09:24:43 »
Expansion forces may be very large initially but are much weaker than reaction forces in the long run because expansion must be contasined and there is always a limit for the length of time such forces can be maintained ie the length of the gun barrel or crankshaft.  Also because they are contained they can be the most efficient thermodynamicaly at the relatively low speeds found in internal combustion and steam engines.

Reaction forces are "stronger" in that as long as you supply fuel they can continue indefinitely and therfore can produce a greater acceleration in the long run.
 

Offline McQueen

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Which is more powerful expansion or reaction, and why ?
« Reply #3 on: 26/09/2007 12:48:30 »
Quote
It depends upon how much fuel is behind it, among other things.
Assume that both have exactly the same amount of fuel, (take gun powder for example) and an identical load to carry.
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Reaction forces are "stronger" in that as long as you supply fuel they can continue indefinitely and therfore can produce a greater acceleration in the long run.
Absolutely, this seems to be at least half of the answer. Look at the facts relating to the supergun. It was 510 feet long (that is not an exaggeration) the breech was more than a metre thick steel, it was loaded with half a ton of propellant and this was supplemented by charges being placed at stages down the length of the barrel that could add added impetus to the 500 lb projectile and the most that it could do with all this was to lob the thing a distance of a 1000 kms. A tiny missile, only 25ft. long and weighing about a ton and a half and using the same amount of propellant could propel the same load and a little bigger one could put it into orbit. Moral of the story don't knock reaction forces. OK! there maybe some comment on this but there is really no, comparison, think of how much the supergun weighed, look at the length of the barrel, the amount of propellant, the thickness of the breech, jeez. Then there is the question of the efficiency with which the fuel is used, right! McQueen
« Last Edit: 26/09/2007 12:55:21 by McQueen »
 

Offline lightarrow

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Which is more powerful expansion or reaction, and why ?
« Reply #4 on: 26/09/2007 15:38:20 »
Quote
It depends upon how much fuel is behind it, among other things.
Assume that both have exactly the same amount of fuel, (take gun powder for example) and an identical load to carry.
Quote
Reaction forces are "stronger" in that as long as you supply fuel they can continue indefinitely and therfore can produce a greater acceleration in the long run.
Absolutely, this seems to be at least half of the answer. Look at the facts relating to the supergun. It was 510 feet long (that is not an exaggeration) the breech was more than a metre thick steel, it was loaded with half a ton of propellant and this was supplemented by charges being placed at stages down the length of the barrel that could add added impetus to the 500 lb projectile and the most that it could do with all this was to lob the thing a distance of a 1000 kms. A tiny missile, only 25ft. long and weighing about a ton and a half and using the same amount of propellant could propel the same load and a little bigger one could put it into orbit. Moral of the story don't knock reaction forces. OK! there maybe some comment on this but there is really no, comparison, think of how much the supergun weighed, look at the length of the barrel, the amount of propellant, the thickness of the breech, jeez. Then there is the question of the efficiency with which the fuel is used, right! McQueen
So, you are not actually talking about "force" but about "energy".
Anyway, I don't think that, using the same amount of propellant you could send the same load to a greater height with a rocket instead of a gun; it's not an easy thing to compute however.
The formula to compute a rocket's push is:

F = u*dm/dt

where u is the speed of the gas relative to the rocket and m their mass.
Edit. I intended: m = gas mass.
« Last Edit: 01/10/2007 12:59:47 by lightarrow »
 

lyner

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Which is more powerful expansion or reaction, and why ?
« Reply #5 on: 26/09/2007 15:53:59 »
'Powerful' is a vague term - "power' is rate of doing work and I don't think that is what you mean. If you just mean 'force, then I could push over a house with a simple hydraulic piston much more easily than using a jet engine.
 

Offline McQueen

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Which is more powerful expansion or reaction, and why ?
« Reply #6 on: 27/09/2007 01:46:27 »
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'Powerful' is a vague term - "power' is rate of doing work and I don't think that is what you mean. If you just mean 'force, then I could push over a house with a simple hydraulic piston much more easily than using a jet engine.
I came to exactly the same conclusion,'expansion' forces seem to work much better in closely contained spaces, as soon as the force has to work over any distance they seem to reduce in effectiveness exponentially. So does that mean, for instance, that an IC piston engine, is more effective than a rocket using the same amount of fuel. I don't think so because here again the volume is undergoing a change, although a little one. I would put it like this, a rocket is basically an impusle engine, i.e., it's efficiency depends upon the presence of a nozzle,each time the gases of expansion are forced through the nozzle they 'choke'  so what in effective you are getting is an almost continuous repetition of the original force.  So continuous power for as long as the fuel lasts.
Quote
Anyway, I don't think that, using the same amount of propellant you could send the same load to a greater height with a rocket instead of a gun; it's not an easy thing to compute however.
The formula to compute a rocket's push is:
Look at the size and weight  of the breech in the supergun, a small part of that would weigh more than the whole missile itself ! Take the 'scud' missile (not the most efficient of missiles) but still we have all seen pictures of them and can estimate the size. The scud was capable of carrying a 400 kg load 500 kms. McQueen
P.S I am not too sure of these facts I am mostly doing it of the top of my head but I think it is fairly accurate.
« Last Edit: 27/09/2007 01:51:27 by McQueen »
 

lyner

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Which is more powerful expansion or reaction, and why ?
« Reply #7 on: 01/10/2007 12:30:58 »
The problem here is that you are not comparing like with like..
Are you talking about the maximum force, the fastest speed you can achieve, the best  efficiency? It all depends on what you want to do with your engine.
It has got to be true to say that you can lift an object to the top of a block of flats with less fuel in a well balanced lift  system than by using a rocket. That doesn't mean that a lift is the best way to get to the Moon.
 

Offline McQueen

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Which is more powerful expansion or reaction, and why ?
« Reply #8 on: 01/10/2007 15:05:51 »
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The problem here is that you are not comparing like with like..
Pardon me sophie centaur, but I don't see the problem. you have a cannon, you have a rocket, both are fitted with equal loads and equal amounts of fuel, and from my example of the supergun it is obvious that the rocket carries its load for further. Again look at the example of the 50% efficient diesel engine, it doesn't achieve that efficieny until it has a stroke at least three times that of its bore, right. I am talking about power for powerm fuel for fuel, calories for calories, and still 'expansion' does'nt make it as far as distance is concerned!
« Last Edit: 01/10/2007 15:09:47 by McQueen »
 

lyner

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Which is more powerful expansion or reaction, and why ?
« Reply #9 on: 01/10/2007 16:09:54 »
I ask again. Do you mean power, do you mean force, do you mean energy or do you mean efficiency in your comparison? The assessment, in each case may be different. I can't deal in specifics until I know what you mean by your original statement / question.
Are you just trying to say that jet engines are better than IC engines?
Look outside - do you see any jet cars? Why would that be?
Do you see many IC aeroplanes? Why would that be?
It depends on the application and on what factor is most important.
But please define your terms first.
 

Offline McQueen

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Which is more powerful expansion or reaction, and why ?
« Reply #10 on: 06/10/2007 15:30:57 »
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I ask again. Do you mean power, do you mean force, do you mean energy or do you mean efficiency in your comparison? The assessment, in each case may be different. I can't deal in specifics until I know what you mean by your original statement / question.

Instead of physics look at the problem in a different way. You have a load of 500Kg, you have a half ton of propellant. On the one hand you have a canon that is 510 ft. long, weighs 2100 tons and on the other you have a rocket that is say 25 ft. long and weighs maybe 2 tons including the propellant and the load. The canon propels the 500 kg. projectile a distance of 1000kms, the rocket, which you will agree is much smaller and lighter can put the same load into orbit! What conclusions can be drawn from this, in terms of efficiency (i.e., use of propellant), force , power or anything else?
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Do you see many IC aeroplanes? Why would that be?
I thought World War II was fought with IC planes.
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Look outside - do you see any jet cars? Why would that be?
Turbine jet cars have been manufactured and have even won races, and there are even a few rocket powered land vehicles. But the reason that they are not common on the road is that there has not, at least so far, been a viable design.
I can understand where you are coming from, you seem to mean that different power plants work more 'efficiently' if that is the word under different circumstances. The point I am trying to make is that even with all these differences, one would not have expected such a poor perfomance from the expansion of gases, in propelling a projectile, as compared to reactive forces doing the same job, could this information be exploited so as to define a more efficient car engine? The answer to your question seems to be that it all depends on how technology is adapted, who would have thought that helicopters could be powered by turbines, even thirty years ago ?  Could it be possible that 'if' a way was found to tame rocket power, so that it yields better fuel consumption,  it could actually be used to propel a car and do it more efficiently than an IC piston engine, considering the above example between 'expansion' forces and 'reactive' forces ?
« Last Edit: 06/10/2007 17:20:04 by McQueen »
 

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Which is more powerful expansion or reaction, and why ?
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