I have some doubts waken up by one of the last threads in the Classical physics forum. Before anything, please open the attached file below. It is opened with word. Please assume subsonic flow in every picture. Cases a) and b): P<Pa. To where does it move?. You can see I have re-scaled figure b). This is because I want to emphasize the pòssible force exerted by the inlet air jet on the opposite wall. I mean, the jet is not completely dissipated in the inner atmosphere like in case a). Cases b) and c). They are put across with the same intention as in a) and b). Do you think the exhausting structure in the welding with the vessel would resist the case b)?. Cases d) and e). P>Pa. Both of them are subsonic rockets. If this sounds wrong imagine a torpedo inside water. They are at rest initially. At instant t=0, both of them init the boosting. But the torpedo b) has an static platform just behind him, where outflow jet is going to shock against. Which of them would you say have the most initial thrust? Thanks in advance.