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- Aug 13, 2014
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I'm going to take a stab at this. First they need to recognize the pushing or accelerating anything, particle, baseball, car, pushes back against the object doing the accelerating. This should not be debated reading the post on the site. Really simply if you push an object, say a punching bag you will have to brace yourself or be pushed back a bit. This is true, through to a lesser, extent for lower mass objects, and more so for objects you push faster. This is conservation of momentum, and Newton's 3rd law. So accelerating a punching bag to a low velocity can produce the same reaction force as accelerating a baseball to a hundred miles per hour, or exhaust gasses to a number of times the speed of sound (rocket or jet), or ions to near the speed of light (ion engine). On a side a more powerful engine will either accelerate a larger mass of exhaust or accelerate the same mass to a higher speed, this is why jets have bypass air which increases the mass of accelerated exhaust without requiring more fuel making the engine more efficient. I think you might get them to accept this point in air without much arguing.
The harder part of this is to explain why you don't need a medium to push against, though it can be done simply. If you accept the above argument you can now equate a rocket engine throwing exhaust particles out of the nozzle to a baseball pitcher or the like. If the pitcher (rocket) throws a ball (exhaust) at a pop can (air), it is not the ball hitting the pop can that that pushes against the pitcher, he is no longer coupled to it, it is the act of the pitcher pushing on the ball that pushes back against him. I fear the deniers will say the push the pitcher feels is because of the air resistance of the ball as he accelerates it through the air, not the reaction of newtons laws, but I'm not certain of a strong counter argument.
As to the counter arguement that rocket engines are less efficient at lower pressures and therefore must be completely ineffective in a vacuum. First the efficiency does drop in a vacuum but no where near to zero. More important is the physics behind it. The rocket engine works by accelerating the exhaust out the back of the engine and since the total momentum in the system needs is conserved the backward gain in momentum of the exhaust is matched by the forward gain of momentum of the rocket, the more momentum imparted to the exhaust the more reaction of the rocket. Since the exhaust mass is fixed as the mass of the products of combustion the only way to increase the backward momentum of the exhaust it by maximizing it's velocity opposite of the desired direction of travel. The exhaust is largely accelerated by the pressure in the engine forcing the exhaust out of the of the throat of the rocket, this can be seen in an estes rocket engine which has no significant nozzle. This component of thrust would be present regardless of whether or not there is a nozzle or air. The efficiency gains from a nozzle come in two forms. First, the nozzle directs exhaust particles that would have spilled in directions other than opposite of flight carrying momentum in other directions reducing the net backwards momentum of the exhaust, the nozzle redirects these improving thrust. Again this component doesn't rely on the surrounding air pressure. Second, the nozzle contains the exhaust gasses as they expand from the pressurized state inside the engine to the ambient pressure around them. This expansion accelerates the exhaust further effectively pushing forward against the nozzle increasing thrust. This expansion is dependent on nozzle design and ambient pressure so that at other pressures this component of thrust is compromised reducing overall efficiency, but only by the relatively small amount it contributed to start with.
The harder part of this is to explain why you don't need a medium to push against, though it can be done simply. If you accept the above argument you can now equate a rocket engine throwing exhaust particles out of the nozzle to a baseball pitcher or the like. If the pitcher (rocket) throws a ball (exhaust) at a pop can (air), it is not the ball hitting the pop can that that pushes against the pitcher, he is no longer coupled to it, it is the act of the pitcher pushing on the ball that pushes back against him. I fear the deniers will say the push the pitcher feels is because of the air resistance of the ball as he accelerates it through the air, not the reaction of newtons laws, but I'm not certain of a strong counter argument.
As to the counter arguement that rocket engines are less efficient at lower pressures and therefore must be completely ineffective in a vacuum. First the efficiency does drop in a vacuum but no where near to zero. More important is the physics behind it. The rocket engine works by accelerating the exhaust out the back of the engine and since the total momentum in the system needs is conserved the backward gain in momentum of the exhaust is matched by the forward gain of momentum of the rocket, the more momentum imparted to the exhaust the more reaction of the rocket. Since the exhaust mass is fixed as the mass of the products of combustion the only way to increase the backward momentum of the exhaust it by maximizing it's velocity opposite of the desired direction of travel. The exhaust is largely accelerated by the pressure in the engine forcing the exhaust out of the of the throat of the rocket, this can be seen in an estes rocket engine which has no significant nozzle. This component of thrust would be present regardless of whether or not there is a nozzle or air. The efficiency gains from a nozzle come in two forms. First, the nozzle directs exhaust particles that would have spilled in directions other than opposite of flight carrying momentum in other directions reducing the net backwards momentum of the exhaust, the nozzle redirects these improving thrust. Again this component doesn't rely on the surrounding air pressure. Second, the nozzle contains the exhaust gasses as they expand from the pressurized state inside the engine to the ambient pressure around them. This expansion accelerates the exhaust further effectively pushing forward against the nozzle increasing thrust. This expansion is dependent on nozzle design and ambient pressure so that at other pressures this component of thrust is compromised reducing overall efficiency, but only by the relatively small amount it contributed to start with.
