Why do missiles have fins, while modern space rockets don’t?
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Fins are only effective at low altitude where the air is thick. A guided missile will usually stay down in those low altitudes, while a space rocket only spends the first 3 or 4 minutes of flight in thick air, after that the air is getting so thin that fins wouldn’t help much.
Space Rockets usually steer by “engine gimbal” which means the exhaust nozzle can deflect. This is steering similar to a boat with an outboard motor where you steer by angling the motor rather than by rudder. While fins *would* work for the first stage of a rocket for those few minutes that it’s still in thick air, the engine gimbal also works both in and out of thick air, so you may as well just engineer one system and use it for both.
Space rockets also tend to do gentler steering. The entire point is to get it aimed the right way at the start so you don’t have to make large turns later and you’re mostly just going straight the way you’re already pointed with steering deflections of only a few degrees. In fact, having to steer too much in a space rocket is a source of inefficiency. The goal is to get the software to predict as early as possible how the rest of the flight will get shaped so you can do small corrections now so you don’t have to perform large corrections later.
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