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 Post subject: AA missile attitude.
PostPosted: 22 Feb 2020, 05:39 
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Joined: 07 Aug 2014, 19:18
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Dear All.
I hope this is not a terribly naive question but can someone enlighted me on this please: most air-to-air missiles are fairly symmetrical (4 similar looking control surfaces at 90 degree intervals; back and centre or back and front). Most (but not all) missiles are carried in an "x" configuration on the launching aircraft. So what happens after launch? Does the missile continue with "v" surfaces like the tail end of a beech bonanza, or does it make a quick 45 degree roll and continue with regular horizontal and vertical surfaces? And if the latter is the case, why are the horizontal and vertical surfaces superficially identical in many cases? To switch roles as required?


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 Post subject: Re: AA missile attitude.
PostPosted: 23 Feb 2020, 23:11 
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Joined: 07 Apr 2008, 11:50
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Just speculation, no actual knowledge: Would such a missile even need to reference the horizon at all? Does "vertical/horizontal" have any meaning within its control logic, which is "single mindedly" focussed on steering "towards target"?


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 Post subject: Re: AA missile attitude.
PostPosted: 24 Feb 2020, 13:02 
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Joined: 23 Jul 2004, 15:11
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Roger, your question is relevant and therefor provide the answer as well. The missile does not have a up, down left or right side, horizontal or vertical reference. It is an object in tree dimensional space, using its own position and that of the target as the only relative positions in space and trying its utmost best to get the missile to the same three dimensional position in space, thereby theoretically occupying the same space at the same time and bang!

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 Post subject: Re: AA missile attitude.
PostPosted: 24 Feb 2020, 13:07 
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What actually happen is the seeker head of the missile follows the target and the missile tries to zero the off boresight angle of the seeker head versus that of the missile body, thereby steering the missile to the target...
I hope i have that right #-o

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 Post subject: Re: AA missile attitude.
PostPosted: 25 Feb 2020, 10:34 
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Joined: 15 Feb 2010, 02:11
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boertjie wrote:
Roger, your question is relevant and therefor provide the answer as well. The missile does not have a up, down left or right side, horizontal or vertical reference. It is an object in tree dimensional space, using its own position and that of the target as the only relative positions in space and trying its utmost best to get the missile to the same three dimensional position in space, thereby theoretically occupying the same space at the same time and bang!
This may be true of short-range missiles like V3B, V3C, V3S, (I forget details of the V3S intercept algorithm, if I ever knew it) but the V4 very definitely knows where "up" is.

In contrast to the simple short-range missiles, when launched against a target at medium to long range the V4 climbs to an optimal altitude after launch then descends once it is "close" to the target. Drag is far higher at low altitudes than at high altitudes so range in a Low-High-Low flight path is much higher than if the missile doesn't climb. So V4 (and presumably all other medium to long range AAM) knows exactly where up and down are, as well as where the target was last seen and what direction it was moving in.


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 Post subject: Re: AA missile attitude.
PostPosted: 25 Feb 2020, 10:54 
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boertjie wrote:
What actually happen is the seeker head of the missile follows the target and the missile tries to zero the off boresight angle of the seeker head versus that of the missile body, thereby steering the missile to the target...
I hope i have that right #-o
Hmmm. Vague memories tell me it isn't that simple except for a target flying straight towards or directly away from the launch aircraft. If you follow this method (continuously pointing the missile at the target), the missile typically follows a curved flight path which requires constant direction changes and is inefficient.

Assuming the missile is fired with the target straight ahead of the launch aircraft but moving from Left to Right, initially the missile flies straight ahead, but as the target moves to the right, the missile corrects to fly slightly right of aircraft boresight. As the missile gets closer to the aircraft, the corrections get more and more drastic, eventually in the worst case the missile is close to doing a tail-chase on the target which is a good way to run out of energy.

The preferred algorithm is predictive - make an estimate of where the target will be by the time the missile gets to the target range assuming the target keeps on moving the way it is moving at the time of firing. Direct the missile to this theoretical intercept point so it makes one turn after launch to point at intercept point then flies straight. Once "close" to the target, intercept it - the algorithm can be as simple as "if the angle between missile and target starts increasing, steer towards the target". I guess that this fully optimised algorithm is only be available for the newer (V4 and later) missiles since it needs information on target motion (which V4 radar can supply once it is within about 10km of the target) as well as a good guidance system, but I guess even V3S and V3C have some element of "aim ahead of the target" built into their steering computers.


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 Post subject: Re: AA missile attitude.
PostPosted: 25 Feb 2020, 13:17 
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Hi Cheetahsupporter,
I was simplifying the tracking part of my answer as that was not really what the question was all about.
You are quite correct with your assessment/answer.
I would add some detail here:
The flightpath of the target is not followed sheepishly, as the missile updates the position of the target literally hundreds of times per second and thus will always follow the shortest route to the target, keeping in mind that the missile has a much shorter turn radius than most maneuvering targets.
You are correct about the "predictive" nature of the missiles flight path as well. This used to be called "leed-bias", if I'm not mistaken, in the tracking algorithms per lance. This entails that the missile's flight computer progressively predicts where the target would be ones they cross paths. Thus it is always flying an even shorter route to the target's predictive intercept point and is thus not really following the target, per say.

So the seeker head tracks the target, even at some considerable angles off boresight, and the flight computer always try to keep the difference in the off boresight angle of the seeker and the missile body as close to 0-deg as possible in all the axis, then adds leed-bias through some calculation of relative speeds, angles and tempo of change of these and some other parameters, in order to reach the target in the shortest possible time and distance.

For short range missiles, this can be from the time the missile departs the launch rail, but at longer ranges, or for medium and long range missiles launched from longer distances, this may happen only during the terminal stage (when the missile reaches a predetermined distance from the target), with the initial flight path following a predetermined route, but still keeping the target's relative angle in consideration.

Also concerning the "up" and "down" issue, this might be that the missile flight computer does not use up and down relative to itself but makes use of barometric pressure and/or gps/inertial Nav data to change altitude???
Will have to get input from someone working with these flightpath calculations or missile flight computers, to help us out here. I used to know, quite well, one of the senior mathematicians at Denel dynamics and would like to tap his mind on these matters???

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 Post subject: Re: AA missile attitude.
PostPosted: 26 Feb 2020, 10:43 
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boertjie wrote:
I was simplifying the tracking part of my answer as that was not really what the question was all about.
Hey Boertjie - I take a different approach. I have been accused of providing too much information at times. . . .

boertjie wrote:
Also concerning the "up" and "down" issue, this might be that the missile flight computer does not use up and down relative to itself but makes use of barometric pressure and/or gps/inertial Nav data to change altitude???
In the case of the V4, I know it had an inertial nav unit.


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 Post subject: Re: AA missile attitude.
PostPosted: 26 Feb 2020, 15:31 
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Great insight, thanks for the replies. :smt023

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