Hacker News Reader

Comment by kkielhofner | Hacker News Reader

Random question - anyone know how a sidewinder missile hits these objects (whatever they are)? My missile knowledge is non-existent but movies show me it has something to do with heat signature - which these don’t seem to have.

View on HN

Not much is known of the AIM-9X but it is probably similar to the AIM-9L.

The AIM-9L doesn't lock on to the hottest thing, it takes an IR picture of the scene and follows the targeted "blob" in the image that's generated. Additionally it has laser proximity fusing so it knows when it is close to the shape it is tracking. By not simply picking the hottest thing in a scene you can't hide in front of the sun anymore, and the effectiveness of flares is reduced.

The only thing needed to generate a "blob" good enough to target is sufficient ΔT between the scene and the targeted object.

The objects, and they have been curiously consistent with calling them "objects", definitely have sufficient ΔT compared to the background sky if they are made of any solid material that has been exposed to sunlight for any period of time.

Not to sound like a Redditor, but this username checks out.

The AIM-9X is infrared imaging based rather than heat signature based. The pilot resolves that contrast between the object and sky before firing the missile and missile targets the object based on the image.

Do you know what sets it off? Seems like it wouldn't be from impact as these balloon materials couldn't be comparable to say hitting another plane, right?

All modern missiles use a proximity fuse that selects the optimum position relative to the target to detonate, it isn't based on contact. One thing to consider is that the warhead is not in the nose -- that is where the sensors are -- but further back on the missile. These warheads often us an annular (ring-like) blast pattern, hence why positioning and orientation relative to the target at detonation time is important to maximize damage.

Adding to janderwogs response, air intercept missiles don't really deal damage with the explosion. The missiles are built with shrapnel payloads propelled by the explosion. These are what actually damage the aircraft on the receiving end.

https://en.wikipedia.org/wiki/Proximity_fuze

The AIM-9X uses an IR proximity fuse.

These payloads are duds - all you need to do is to pierce the balloon.

A little research on the Canadian Ballon shootdown a number of years back show that isn't exactly the case. A larger balloon was shot with a (20mm I believe) cannon and continued to float a long distance.

‘Duds’ is an odd way of putting it. Sidewinders usually have an ‘exploding ring of metal rod’ warhead, I believe - and that was replaced by something else better suited to destroying the balloon while leaving the payload intact enough for technical analysis.

> The AIM-9X is infrared imaging based rather than heat signature based.

Isn't heat detected by infrared? What's the difference?

Cloudless sky is cold, and at 30,000 feet extremely cold. Anything not-sky is going to contrast against the sky, even if it has no internal heat source (and remember: if it has electric things in it, it produces heat).

Yes warm bodies put off infrared, but so does the sun, and the suns IR rays are reflected by objects just as they are in the visible range. These reflections used for imaging

The balloon reflects IR, just as it reflects visible light

The missiles seek and track based on heat differences. To an IR sensor the sky is typically pretty "cold" and just about anything in the sky is relatively "hot". Once the missile is locked onto the relatively hot signature against the cold background the sensor stares at it and adjusts the missile's trajectory to intercept it (and blow up).

Countermeasures like flares try to trick a missile's sensor into thinking the hot object it was staring at was the flare going that way and not the maneuvering aircraft going the other way. Modern IR tracking missiles try to stay locked on the object by ignoring hotter or cooler objects that might end up in its view.

At FL400+, anything not the earth is basically space – if you point an IR camera in the sky outwards, it really does look very cold as the pressure is basically zero and scattered IR photons are few and far between. Objects that are not vacuum have a signature in the IR – both a black-body temperature (which, even at ~ -55 ºC, is a hell of a lot higher than single-digit kelvin…) and possibly the opportunity to reflect any incident light.

Some modern imaging-based guidance packages for anti-aircraft missiles use multiple spectra to defeat countermeasures like flares. While a flare and a plane sometimes look similar in IR, they are easy to discriminate in the UV spectrum.

Well it goes real noisy like NEERRRRRERERRRRRERRRRERRRRwarblewarblewarble and something gets hit. I thought it knew where it wasn't but that's the Tomahawk. The Sidewinder just knows where thing is.

That's the extent of my knowledge about the AIM-9 Sidewinder.

Reply on news.ycombinator.com