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Comment by jleahy | Hacker News Reader

Energy. If you fire a missile to your rear then it needs to first accelerate to your original speed just to be staying still, only then can it accelerate towards the target.

An AIM-9X can do this, but the kill probability drops rapidly as you move off boresight.

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This was addressed by the Pye Wacket missile (http://astronautix.com/p/pyewacket.html) which was to be developed for the B-70 bomber. It was to be a 500lb circular (lenticular) missile which could be launched with its thrust vector pointing in any direction relative to the direction of flight.

> only then can it accelerate towards the target

but in a dogfight (where this move is presumably useful) the target is not stationary, it's moving towards the missile. if all the missile does is accelerate to stay still, it will still hit the target at 700mph

In a vacuum. The aerodynamics aren't possibly the same.

Surely the aerodynamics are better for firing a missile backwards.

If you fire it forwards, the missile has to accelerate through whatever air resistance you're both already experiencing through even more air resistance to get to the target. That's hard.

If you fire backwards, the missile uses air resistance to accelerate towards (i.e. slow down) towards the target. Even when it goes through 0mph relative to the air and continues to accelerate, the resistance will be much less as it approaches an even higher closing velocity in a shorter period of time.

I would guess that the speed at which air moves over the missile's control surfaces has a large effect on its ability to steer to meet the target.

Modern fighters are all about sensing and targeting the enemy first since missiles are so deadly. The missiles are much faster than the planes so which way the missile is pointed at matters less and most of the time it is better to be pointed forward.

So the optimizations and tradeoffs are in sensing and targeting further away or being able to rotate a sensor to target an enemy off bore.

But, missiles can't maneuver at 0mph airspeed even if traveling at 600mph in the frame of reference of a plane.

If you fire it backwards, when it first launches it will be traveling backwards relative to the airflow. So you would have to make a missile that can fly forwards or backwards. Probably not impossible but it must add some difficulty.

hmm, yeah that sounds challenging, but spacex does it when landing the first stage :)

What about guns? Relative speed against the enemy is the same no matter if you shoot forward or backward. Or am I missing something?

Bullets don't move at the speed of light.[1] Shoot a bullet backwards off a bullet train, that bullet's ground speed is less than if shot from the ground while stationary. Similarly, shoot a bullet forward off a bullet train, that bullet's ground speed is more than if shot from the ground from a stationary position. Shoot bullets backwards off a jet moving Mach 3 (just sayin') those 1700mph bullets will still be moving in the forward direction of the jet at 600mph relative to the ground.

[1] They should use rear-facing lasers!

But the bullet will still hit the plane following you at roughly 1700mph (you could say, the plane will crash into the bullet at that speed, but it‘s the same result), because it‘s also moving at Mach 3.

Good luck trying a Cobra with a Blavkbird, a plane that needs a third of the US to make a 180 turn at full speed.

That's true in the ground's frame of reference.

But in the plane's frame, where both planes are roughly stationary and the wind is moving 100s of mph backwards, bullets fired backwards move faster.

And for aerial combat I would argue the plane's frame is the important one.

But the guy that is pursuing you will still be going after you in around Mach 3 so he will go through your bullets like they would be traveling his direction at 600mph.

Yeah but your enemy is also moving forward because they’re trailing you.

This really only applies if shooting at a target behind you that is either stationary or moving much slower than you.

If the target is moving at (or nearly at) the same speed as you then those velocities effectively cancel out

You need a backward facing radar for modern fire control targeting calculating a solution. These are heavy and large and will interfere with engine placement.

R-73 and AGM-88 can also launch 'over the shoulder'

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