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Is reactive pressure as efficient as expansive pressure?

  1. Sep 1, 2007 #1
    Right, I am django, and the question , as stated "Is a reactive force as effective as an expansive force."(a) Take a cylinder put a bullet in it, put a propellant behind the bullet, ignite the propellant, the propellant expands and the bullet is expelled out of the cylinder with a certain amount of force! (b) Take a cylinder put a bullet in it, put a propellant behind the bullet, ignite the propellant and vent the gases of the combustion chamber through a CDN (Convergent / divergent nozzle) in the rear of the cylinder. In which case would the bullet travel further and why? In one case we have a cannon and in the other we have a rocket. What is the difference? django
     
    Last edited: Sep 1, 2007
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  3. Sep 1, 2007 #2

    russ_watters

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    No, in both cases you have a cannon. Putting the nozzle in there just acts as an obstruction. By the law of conservation of energy, there is only a certain amount of energy in the compressed gas and even an efficient nozzle will lose some of it.
     
  4. Sep 1, 2007 #3
    Should I take that to mean that in your opinion the cannon with the nozzle would be less effective. Let me put it anotherway.
    For example, take an open ended cylinder of a certain diameter, equipped with a piston and a cork pushed tightly into one end, and call this cylinder A. When the piston is pushed up the cylinder it reaches a compression ratio of 3 : 1, or 44 psi approx. before the cork is pushed out of the cylinder. OK< take another cylinder, of exactly the same dimensions as cylinder A and call it cylinder B, it also has a cork stuck into one end. However, instead of a piston, it is equipped with a venturi at one end, the venturi is equipped with a valve. Cylinder B is then filled with compressed air throughj a compressor until its internal pressure reaches 43 psi (i.e., a pressure just a little less than that needed to blow the cork out of the hole, in cylinder.} The valve to the venturi is opened? What happens ? will the cork fly out of the end of cylinder B. If so how far will it go? My guess is that they will travel the same distance or that B will travel further because it has more compressed air behind it. In order to be exactly equal, when cylinder B is filled with compressed air, its size should also change correspondingly, so that it is the size that remains of cylinder A after the piston has compressed the air to 44 psi.
    OK, take another scenario, now cylinder A is fully sealed at both ends, this time it is fitted with piston rings that enable it to be placed in an air-tight manner down a long barrel. The barrel is equipped with an air-tight piston that allows the air behind the cylinder to be compressed in a ratio of 3 : 1 or to 44psi. When the piston is depressed cylinder A flies out of the barrel. Cylinder B has exactly the same dimensions as Cylinder A, however instead of piston rings it has a venturi equipped with a valve. Cylinder B is filled with compressed air to a pressure of 44 psi and the venturi is opened. Which cylinder would go further, my guess is cylinder B, because it retains the pressure for longer.
     
  5. Sep 1, 2007 #4

    Astronuc

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    A properly designed nozzle would likely provide greater range for a given mass flow rate, and in fact some shells use rocket assist in addition to the detonation charge.

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

    http://exploration.grc.nasa.gov/education/rocket/nozzle.html

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

    Cannons (or artillery rifles) are designed with a large bore for the shells, whereas nozzles in a shell would be smaller.

    The cannon charges are explosive or detonate (supersonic combustion), whereas as propellants deflagrate (subsonic combustion). So the two propulsion mechanisms are somewhat different.

    In a cannon, there is a limited distance over which the force can be applied. In contrast, a rocket can burn for many seconds or minutes depending on size and mass flow rate. The downside of the rocket is that the propellant mass must be accelerated with the rocket, so it starts slowly before picking up speed. One could ideally optimize a combination such as rocket assisted artillery to maximize range.

    Cruise missles have much longer ranges than artillery shells.
     
  6. Sep 1, 2007 #5

    russ_watters

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    Astronuc, the OP was talking about a nozzle inside the combustion chamber, not attached to the shell.
     
  7. Sep 1, 2007 #6
    It comes to the same thing, the shell takes the places of the combustion chamber. Am I right Astronuc? I think that it is a pretty fair post, always remembering that rate of flow can be speeded up to optimise power. Further, strictly speaking because of the fact that the propellant is expoanded before leaving the combustion chamber in the RPJ, it can be seen as equivalent with an IC piston engine in terms of power produced. django
     
    Last edited: Sep 1, 2007
  8. Sep 2, 2007 #7

    Astronuc

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    I agree with Russ's initial response.

    Putting a CDN in the breech doesn't buy anything, unless it reduces the recoil of the cannon - which is the basic principle behind a recoilless rifle. By reducing recoil, more energy/momentum is transfered to the shell, but it costs from the standpoint of energy lost in the backblast.

    The only practically effective use of a nozzle to enhance the range would be to attach it to the shell.

    In an RPG, the objective of the nozzle is to eliminate the recoil because it is a shoulder fired rocket. If the system was closed, the person firing the relatively heavy shell would be knocked down from the recoil.
     
  9. Sep 5, 2007 #8
    Isn't this what the Rotary Pulse Jet Engine
    is all about. See the thread on the Rotary Pulse Jet engine in this forum. django
     
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