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The initial problem is energy and the air.

How would a mechanism like this work? Pressurised air inside a cylinder that is released when a projectile is detected?

Instead of pressurised air inside a cylinder, could it be an explosion? An explosion inside a tube? (This kind of defence is already used in tanks, but they shoot projectiles to fragment or detonate the projectile once the reactive armor is struck).

A turbine (a plane turbine or a rocket turbine) that would increase the turbulence in the air to increase the chances that the projectiles deviate from their trajectory?

Or would just be better to use electromagnetism to repel the metallic projectiles?

Projectiles could be anything from a missile to a bullet.

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    $\begingroup$ This would wok great if you could cause the airflow(wind) at great distances from the target. Like, closer to the shooter than the target. Deflecting a projectile by 1/10th of a degree when it is 10 feet from you, gains you a deviation in target of a hairs-width. The same 1/10th of a degree deviation at 1 mile gives you a miss of 7 feet. $\endgroup$ – PcMan Apr 17 at 6:54
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No, unless your incoming projectiles are bubbles or marshmallows.

Bullets and missiles have a relatively high velocity. That means that the amount of time you have to deflect with an air-stream them is very small.

Consider the frame of reference of a moving bullet. The entire time they are in flight, the air they pass through is effectively a VERY fast wind trying to slow them down. And yet they can travel for miles.

In order to massively alter their trajectory, you need to apply a force perpendicular to the direction they are moving. But by the time the bullet is anywhere near you, you can only really push on the bullet straight on with an air stream.

You simply will not have enough time to apply enough force to meaningfully change a bullets trajectory by the time it is anywhere near you.

This problem doesn't go away by trying to use magnetic fields or lasers either. The amount of power required to change the bullets momentum in a short amount of time is simply untenable.

You are much better off trying to hit it with another bullet. To make sure you don't miss, shoot a bunch of them.

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Wind affects the motion of projectile, deviating it from its ideal parabolic trajectory.

Drag, which is nothing else that air resistance, also affects the projectile trajectory, shortening it.

So, yes, an air flow can disturb the trajectory of a projectile. However it will need long distances to have a noticeable effect, unless the flow velocity is very high. But at this point, if you have a very fast air flow, you better use it to move yourself away from the path of the projectile rather than trying to deviate the projectile.

Just to give you a reference, pressurized air jets are used to clean seed from polluters: an optical detector checks the falling flow of streams and triggers a nozzle which blows away the undesired object. But a seed is way less massive than what you have in mind.

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You're describing a variant of a TROPHY-like active defense system.

If you have sci-fi tech, you could build a tank with panels containing extremely pressurized air sufficient to cause a blast wave when the panel is opened. When an onboard sensor suite detects an incoming missile, it estimates which panel will be hit and vent the panel just before impact, causing a shockwave that slows down the incoming missile.

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