I What happens when gas is combusted while moving?

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When combustible gas mixtures like O2 and H2 combust while already in motion, their initial velocity contributes to the overall velocity of the combustion products. The principle of momentum conservation dictates that the center of mass of the gas cloud retains its motion, allowing for continued thrust even at speeds exceeding the exhaust velocity. This means that the combustion process does not negate the pre-existing velocity of the gas. However, the effective acceleration depends on the amount of propellant that has already been accelerated by prior combustion. Understanding these dynamics is crucial for applications like rocket propulsion.
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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