¹The back pressure of the gas outside the system on the gas inside the system is negligible once the gas exits the nozzle of the rocket engine. If we took the inside of the combustion chamber to be part of the system boundary, the results would be different, as the gas pressure there is non-negligible. At this point the gas is indeed exerting a significant force on the rocket. However, though this viewpoint is conceptually simpler, it is computationally more difficult, which is why we define the system as we do.