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13

The hard part is that $P_e$ isn't a completely independent variable. As the gas expands past the throat, thermal energy is being converted into kinetic energy. The gas cools down and speeds up. So if you shorten the nozzle (creating an underexpanded flow), there is greater pressure at the exit (good). But the exhaust speed $v_e$ is lower (bad). The \$\...

2

Yes, but... Ignoring plume interactions the propulsive effect of an engines thrust does depend on the spacecraft's center of mass being along the line of thrust. If for example a single-engine spacecraft vectors its thrust slightly to turn there is a tiny loss of linear acceleration as there is now angular acceleration. For a single-engine craft is small ...

7

Yes. Thrust is additive. If you want more thrust, adding more engines or scaling up the size of your engines are both valid ways of doing it. There is a balancing act you have to make, however. At one extreme, single large engines can have unstable exhausts where the combustion products 'stick' to one side of the nozzle, to a first degree of ...

2

I thought of this question, also, but decided that since everything in the universe is moving, "standing still" would be a moot point. The question would always have to include "standing still" in relation to what?

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