What happens when gas is combusted while moving?

[Limebat]

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.

 

[A.T.]

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.

 

[hutchphd]

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"