23 September 2013

Cones

Or getting the damn sidewinder to do something besides make pretty smoke trails.

The AIM-9B was commissioned by the US Navy and adopted by the US Air Force to arm its fighters and interceptors to attack bombers.

Theses were the perceived threats of the day and fighters had guns to deal with other fighters, just like WW1, WW2 and Korea.

The problem begins some time after Korea when speed becomes ever more important in the design and an attitude that ACM (air combat maneuvering) or dogfighting will be supplanted by missiles and this whole thing will culminate with unmanned aircraft anyways so why make traditional fighters?

Yeah, about that.  It seems the Soviets didn't get the memo.  They kept making traditional fighters.

We didn't worry much about it since we really didn't expect to meet those short ranged things while defending ourselves agains the looming bomber threat.

Then there was Vietnam.  There our optimized for an entirely different role and theater planes met aircraft being used precisely in the role they were intended and designed for.

Oops!

A side note about one fighter we sent to Vietnam that got a bad shake.  The F-104C didn't get much air-to-air time.  It seems (now) that the Soviets had fully compromised our defense industry and had very accurate performance numbers on the Starfighter and they apparently told the NVAF to avoid them.  It's an unsubstantiated claim from a normally solid author.  Update: Found my notes, but not my references.  Project Feather Duster was a study on dissimilar ACM with all of the various planes the USAF could muster.  It showed that the F-104 was nigh unbeatable in ACM, despite the relatively short range and small weapons carriage.  IIRC the author in question named names about whom got hold of the Feather Duster report and how it got into Soviet hands.

At any rate cones.

The seeker looks forward in a cone that gets wider to the front of the missile.  Because of limited sensitivity, it can't see things that are colder than a certain temperature.  This means that it needs to see the tail pipe of the target directly.  This projects behind the target aircraft in a cone widening to its rear.

With the AIM-9B this is a 4˚ cone looking forward.  This cone must be inside an approximately 60˚ cone behind the target for the seeker to begin tracking.  The good news is the seeker is gimbaled so that it can point the center of the 4˚ cone 12.5˚ off the centerline of the missile.  The bad news is that this is still a very narrow cone!  Even worse that 12.5˚ is constrained by a 11˚ per second tracking rate.  So if a target can get out of the 4˚ cone faster than the head can slew, then missile loses lock and he escapes even it it's still inside the nominal 25˚ cone of the gimbaled head.

The missile also turns, adding its turn rate to the speed of the tracking head.  Because the missile is going much faster than the target, it needs to turn at a much higher g to follow.

In fighter terms this is a 3g break to get out of that cone and get away.  A Cessna can perform such maneuvers!  (A Cessna wouldn't need to since its internal combustion engine isn't hot enough for an AIM-9B to see).

How do you score kills with an AIM-9B?

Option A: fight like Baron Richthofen.  Sneak up behind him while he's not looking and shoot him before he realizes your shooting.  The AIM-9B is excellent if the target is not wagging its ass around.

Option B: because if they know you're there they don't sit still for it you get him into a situation where he can't put 3g on the plane anymore and has to helplessly look over his shoulder as the missile relentlessly bores in on his fragile pink body.  That is all about airspeed.

And this is why you don't wanna be in an F-4B or C over Vietnam and armed with AIM-9B.  By the time the fight slows down to the point where a MiG-17F can't pull 3g, you can't keep your nose on him and thus can't get the initial lock, assuming you don't stall or depart for trying to maneuver at such low speeds.  But there's another airspeed where a MiG-17 can't pull 3g!  Above a certain speed (500 kias or so) the Fresco's controls get too heavy for the muscles of the pilot to deflect the control surfaces enough to pull serious g!  The bad news about this is that it takes serious work to get 500+ knots on in a dogfight and remember what we said before, "don't fight how your enemy fights!"  High speed fights are where the Phantom wants to be, so don't go fast!

And they didn't.

We never did come up with effective tactics to employ the AIM-9B against fighters.  The Navy had a Come-To-Jeebus moment and founded TOPGUN to teach some ACM tactics to its interceptor thinking aviators, but another change preceded those students entering the fight: this was a new version of the Sidewinder; the AIM-9D.

The Navy looked at the problem and came away with the notion that the missile just wasn't intended to do what they were trying to use it for.  A smaller group in the fighter community also acknowledged that they didn't really know what they were doing either.  This led to a two prong attack on the problem with improved missiles reaching the fleet first.

The D model 'Winder has a greatly improved seeker head.  At first glance it doesn't seem much better.  The field of view is reduced to 2.5˚.  The limit is extended to 15˚ off center and the tracking rate is just a bit better at 12˚ per second.  Sounds like it's just as easy to defeat as the B model, huh?  It probably would be, except that it can see the target pretty much as long as it can see the back of the plane at all.  It took 4-5g to escape which was still readily available to most MiGs, the fight could be dragged to a point where a Phantom could still fly and the MiG didn't have enough energy to get it's rear arc out of the seeker's view.  Additionally the increased sensitivity of the new head also allowed it to be fired from farther away, more than three times as far!  The biggest improvement from a firing point of view was increasing the maximum launch g from 2 to 4g and the missile was also able to turn harder increasing from 14 to 24g.  The AIM-9D was a lot more effective than the AIM-9B.

Plus, by the time that TOPGUN was bearing fruit, a further improved Sidewinder was reaching the fleet.  The AIM-9G.  What the AIM-9G had that the D didn't was SEAM (Sidewinder Enhanced Acquisition Mode), this slaved the seeker head to the firing aircraft's radar and got it to begin tracking farther from "boresight".  Better tactics, better missiles, better kill ratios!

Something else worth mentioning, the D and G were also more reliable as solid state electronics replaced tube and both benefitted from a more than doubled warhead with improved fragmentation performance.  It's all win!

One would think that the Air Force would have been paying attention to what the Navy was up to and implementing a similar plan, right?

Not really.

It took them a lot longer to realize that no amount of improved procedure was going to change the reality of the cone situation.  Then, rather than adopt the Navy's improved missile, they opted for a new version of their own.  This is the AIM-9E.  Like the D, the seeker was made more sensitive and the FoV was reduced to 2.5˚; but the limit was increased to 20˚ off center and the tracking rate was cranked up to 16.5˚/sec.  The range where you could get a lock was almost doubled and the motor burned longer giving a much longer range than the B.  The E wasn't as maneuverable as the D and was more limited in its launch conditions though.  2.5g max launch g and it only pulled 18g following the target.  It was still a vast improvement and kill ratios did get better, but not as much as the Navy so the Air Force grudgingly emulated their sister service.

Still, it's all about cones and making the cones bigger makes it easier to hit.

3 comments:

  1. What's the name of this author (the one making the claims about the air industry being penetrated)?

    ReplyDelete
  2. I can't recall off the top of my head. He's written a lot of defense industry stuff over the years, you might even have a book or two. He was dead on about how big the Walker ring was when others were dismissive.

    The crux of the paper was how fully infiltrated the DoD was by Soviet agents and he suggested that the reason the MiG's all seemed to evaporate when the Starfighters were around was that if head to head fights started then the NATO nations buying the F-104G would have scored valuable confirmation they'd made a good decision at a time when there were about ten scandals floating about how Lockeed flat out bribed generals and defense departments of foreign governments to buy them in the first place.

    ReplyDelete
    Replies
    1. There was a lot of doubt about the F-104 among the allies "tricked" into buying them. The USAF didn't seem to want theirs and the accident rate was appalling. Keeping the design from being vindicated in combat to preserve that doubt is a solid intel coup.

      Delete

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