Why do missiles have fins, while modern space rockets don’t?
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I think the question has been adequately answered. I just wanted to add that plenty of missiles steer with fins/canards as well as a [jet vane control system](https://b-domke.de/AviationImages/Superbug/Images/4030.jpg). Fins/canards aren’t the only way a missile steers.
Some of the answers here are wrong. We know that fins are only effective with airflow, but some answers ignore that big rockets have stages and they even have different engines for atmospheric flight and another for vacuum.
Another problematic assumption is that “modern rockets don’t have fins”. Some of them still have, and some of the older designs didn’t have it. SpaceX Starship and the Chinese Changzheng 5 still have fins.
But if I had to guess, I would say that the trend of using fewer fins for rockets would be to how gimbal engines become more reliable and stable.
(And I don’t know about missiles, lol)
Fins are useful in atmosphere, but a space rocket quickly leaves it so fins become useless and it has to steer without anyway. If it can steer without anyway, then why bother with fins to begin with? It’s dead weight.
Still, some space launchers do have fins, Chinese launchers for example, aerodynamic stability is necessary and small fins can ease the engineering constraints in other parts of the design.
Rocket engines gimbal and are going vertices. Missiles are high thrust to weight and going parallel with lots of forces requiring fins to do the steering due to fine adjustments putting large loads in the missile.
missles have to change direction in air rapidly. Fins push on air to change direction. Space rockets gradually roll over in the air needing only minor corrections. Note the spaceX starship has fins. It has to do it funky flippy belly flop.
After many years of evolution the missiles evolved fins as they lived in the lower atmosphere. Rockets on the other hand evolved gimballed engines to allow them to navigate space.
Missiles operate either partially or entirely in atmosphere (depends on the missile) and in the atmosphere fins are very cheap and effective ways to maintain stabilization. You don’t need any fancy technology to dramatically improve the missiles, just slap on some fins and boom, a much more accurate missile.
Space craft, however, operate almost entirely out of atmosphere in a vacuum. Since vacuums have no air in them fins are literally useless. So if you add fins to a spacecraft you are just wasting money (even more so than you might think, because it’s not the fins themselves that would cost much, it’s the extra weight they would add that would mean having to use more fuel which is expensive.)
That said, there have been spacecraft that have had fins, most notably the space shuttle, but the only time that ever happens is if the spacecraft is planned to spend a decent amount of time in the atmosphere, like the space shuttle did when it would land for reuse.
Answer- modules don’t generally have pilots and anything will fly off you get it moving fast enough. The fins or vanes just reduce oscillations that would otherwise increase dynamic instability until the missile turns to an angle of attack that makes the relevant wind more like hitting a wall.
Fins require an atmosphere to work. Rockets need to be able to stabilize themselves with thrust alone, because they operate at altitudes where there isn’t enough air for the fins to provide any stabilization benefit.
Space does not have air.
Missiles have to travel through air a lot.
Space rockets not as much.
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