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Old 17th April 2019, 07:29 AM   #7
JayUtah
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Originally Posted by Belz... View Post
Well it's not that the rockets push against something, it's that the equal reaction pushes the rocket forward as it throws stuff backward.

ETA: Or, in other words, the rocket pushes against the stuff that it shoots out its back.
The equation describes momentum thrust and pressure thrust. For something like the Apollo lunar module DPS, pressure thrust accounts for about 40 percent of the overall thrust. This "proof" conflates the concepts of propellant mass flow rate, which is needed to understand how much momentum is in the exhaust, and the exhaust flow rate. It ignores the most exciting part about rocket engines, which is the massive acceleration imparted to the working fluid by the release of stored chemical energy as heat. The de Laval nozzle converts the results of that energy to velocity, which is where Ve comes from.
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