What happens when gas is combusted while moving?

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SUMMARY

When a combustible gas mixture, such as O2 and H2, is combusted while in motion, the existing velocity of the gas adds to the velocity achieved from combustion. For instance, if the gas is moving at 150 feet per second, this velocity combines with the combustion velocity, which can reach several kilometers per second. The principle of momentum conservation dictates that the center of mass of the gas cloud retains its motion, allowing for continued thrust from a rocket motor even when exceeding the exhaust velocity of the fuel. This phenomenon illustrates that there is no "free lunch" in propulsion, as all propellant must be accelerated prior to combustion.

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  • Understanding of basic combustion chemistry, specifically the reaction of O2 and H2 to form H2O.
  • Knowledge of classical mechanics, particularly the concept of momentum conservation.
  • Familiarity with rocket propulsion principles and exhaust velocity.
  • Basic grasp of thermodynamics, especially regarding energy transfer during combustion.
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Limebat
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If a combustible gas mixture (say O2 and H2 are combusted to form H2O) are already moving before combustion, what would happen? Intuition tells me the previous velocity (say the gas was moving 150 feet per second) would just add to the velocity achieved when standing still and then combusted (say a few kilometers per second). Basically, net velocity. However, I haven't taken classic thermodynamics, so I am unsure and would like to double check with others.
 
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Limebat said:
If a combustible gas mixture (say O2 and H2 are combusted to form H2O) are already moving before combustion, what would happen? Intuition tells me the previous velocity (say the gas was moving 150 feet per second) would just add to the velocity achieved when standing still and then combusted (say a few kilometers per second). Basically, net velocity.
If the gas cloud is not interacting with anything else, its center of mass will retain its motion, due to momentum conservation.
 
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This is why a rocket motor can continue to produce thrust even when accelerating to faster than the exhaust velocity of the fuel. Of course all the remaining propellant has to have been accelerated up to speed by the fuel previously burned, so there is no "free lunch"
 
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