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Miss_Understands_ t1_it7u6hq wrote

If I may ask an out-of-context beginner question, I've always wondered why the nozzles aren't longer. Sometimes the pressure drives the exhaust up the sides of the rocket.

Compared to the Saturn V engine exhaust thrust, I'm surprised the sea-level air pressure even needs to be in the equation.

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NakoL1 t1_it91lxa wrote

Your assumption that the exhaust is high pressure is wrong

Generally, you want to design the nozzle so that the exhaust pressure matches the ambient pressure. So the exhaust is very fast, but low pressure. At least, that's the case by the time it leaves the nozzle (at the throat, it's high pressure, low speed, before undergoing controlled dilatation in the nozzle)

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Garo5 t1_it8ltbd wrote

In space the theoretic best performance would with a nozzle of an infinite size/length, so the real size comes from a tradeoff between performance and mass.

In atmosphere the size is heavily limited by the atmosphere itself as pressure differences between inside and outside of the nozzle would destroy the nozzle structure.

For example the planned Starship upper stage by SpaceX will have two kinds of engines and nozzles.

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thewerdy t1_it92qxc wrote

Ultimately, it's a trade off. Nozzles have a weight associated with them, after all, which is extremely important when launching something to orbit. For the first stage of a rocket, the nozzles are typically optimized for lower altitudes, as they will spend the most time at lower altitudes gaining speed. However, once they are higher up, their plumes become absolutely huge because of this. Once you get to high altitude or vacuum, the second+ stage nozzles can get pretty big and it becomes a tradeoff between how much weight the extra nozzle ends up adding.

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EBtwopoint3 t1_itattl2 wrote

I’m going to actually assume you’re an out of context beginner. The earlier responses are right but they aren’t actually helping you understand the concept.

The perfect nozzle has the ambient pressure exactly match the exhaust pressure. This because any excess pressure in the exhaust gases is wasted energy that could have been done. And if the pressure drops below ambient, that means the atmosphere will be pushing in on your exhaust gases, reducing velocity and thus performance.

But ambient pressure changes with altitude. So the perfect exit size changes as you ascend with it. In theory, an infinitely adaptable nozzle would be ideal, but to accomplish that objective requires weight. And the rocket equation tells us that the pounds of mass we have to take to orbit isn’t worth what you would have to spend to get it there .

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