[FIXED] How to use AIM-9L properly

[maverick3320]

Hello everyone. Just started Northern Fury 1.

My F-16A is firing an AIM-9L Sidewinder at a range of 2000m, directly behind an SU-24 Fencer D. The F-16 is on auto-attack flying at 550kts at an altitude of 26,000 feet. The Su-24 is flying at 480 knots at an altitude of 27000 feet. While not textbook-perfect, this would seem to be pretty close to the ideal firing environment for an AIM-9L.

When I fire, the base PH of the weapon is 85%. However, immediately after firing, the PH drops to 58% due to "PH adjusted for weapons speed 58%" (pure-aerodynamic weapons control). Final PH: 58%.

A previous AIM-9L fired almost under the exact same parameters had the same adjustment take the 85% initial PH down to 46%.

Note again that the F-16s are on auto-attack and would theoretically be firing at peak condition. Is there a better way that I can manually direct the firing of heat seeking missiles at short range so they aren't losing 50% of their hit chance immediately?

Save attached.

[Dimitris]

The thing with a tail-chase is that, by definition, you are hunting down a target that is running away from you. This moves the intercept point far away, which means that the missile spends a lot of time "in the air", and without a lofting profile this will most likely shed a lot of energy; hence the pH reduction that you see. This is especially true in edge-of-envelope shots.

The only thing you can do to increase the pH in these circumstances is to delay launching (either manually or by adjusting the WRA firing range) until you're almost on top of them, but this is a risk tradeoff; what if someone else whacks you in the meantime, for example.

Another way (with all-aspect weapons like the AIM-9L+) is to engage head-on or beam-on. This moves the intercept point closer to the launch point so the missile sheds less speed in flight. Of course these too have their disadvantages; a head-on engagement means giving the other guy the chance to shoot you too if he can, and a beam-on intercept reduces the pH in another manner.

Choices...

[Tcao]

interesting, I am wondering why the F-16 reduce the speed to 350kts before release the missile and why the max speed of AIM-9L is only 650kt?

aim-9L.jpg (19.03 KiB) Viewed 5880 times

[Tcao]

You need to turn the afterburner on
Here is the PoH when the F-16's speed set at military power 540kt, missile's max speed is 880kt

2/13/1994 1:25:54 PM - Weapon: AIM-9L Sidewinder #4768 is attacking 722 Bmbr Regt (Su-24M Fencer D) with a base PH of 85%. PH adjusted for weapon speed: 67% (pure-aerodynamic attitude control). Final PH: 67%. Result: 45 - HIT

and here is the result when I put the F-16 at AB, (it released the missile when it was at 740kt, and Sidewinder's max speed is about 1100kt, still 300kt slower than its max speed 1400kt)

2/13/1994 1:25:50 PM - Weapon: AIM-9L Sidewinder #4767 is attacking 722 Bmbr Regt (Su-24M Fencer D) with a base PH of 85%. Final PH: 85%. Result: 68 - HIT

[thewood1]

I assume the F-16 slowed so as to not overshoot. I've seen that before in a couple engagements up close against slow targets.

[thewood1]

So built a small test scenario. F-16A with AIM-9L tail chasing an Su-24. F-16 is at a standard cruise of 480 and the Su-24 is at 350. I wanted a small closing distance. Because of the DLZ logic, the F-16 wouldn't fire until within 700m. When the AIM-9 left the rails, its motor burn pushed it to 690 knots. It had about a 60-70% chance to hit the Su-24 over multiple runs. If I went to AB with the F-16, the AIM-9 left the rails and accelerated to 1300 knots or so. Nothing shocking about that.

But the interesting thing is if I forced the AIM-9 to fire at 1.2nm, the AIM-9's 1 sec burn time pushed it to almost 1000 knots. I tried variations on this and it seems that the 1 sec. burn time of the AIM-9's motor is consistent and matches some of the sources. But what is puzzling is that the max speed of the missile seems to be dependent on the range at which it is launched. Even up close, the AIM-9 has plenty of time to get to max velocity. And that max velocity is more dependent on range to target than motor burn time. It is dependent on launcher speed, but you still see that consistent max velocity depending on range to the target.

I picked up a little bit of this issue in the BVR testing I was doing, but couldn't seem to make it happen consistently. I added the test scenario. The pieces are all in place, but you might have to move the F-15 around to get it lined up on the Su-24.

[Tcao]

something is not right, here in my testing, the F-16 fire missile at 2.5nm, and max speed of missile is 840kt ?

f16vssu24.jpg (232.58 KiB) Viewed 5689 times

Baseline AA Testing F16 Su24 ready 480 fire.zip

(11.1 KiB) Downloaded 47 times

[maverick3320]

The thing with a tail-chase is that, by definition, you are hunting down a target that is running away from you. This moves the intercept point far away, which means that the missile spends a lot of time "in the air", and without a lofting profile this will most likely shed a lot of energy; hence the pH reduction that you see. This is especially true in edge-of-envelope shots.

The only thing you can do to increase the pH in these circumstances is to delay launching (either manually or by adjusting the WRA firing range) until you're almost on top of them, but this is a risk tradeoff; what if someone else whacks you in the meantime, for example.

Another way (with all-aspect weapons like the AIM-9L+) is to engage head-on or beam-on. This moves the intercept point closer to the launch point so the missile sheds less speed in flight. Of course these too have their disadvantages; a head-on engagement means giving the other guy the chance to shoot you too if he can, and a beam-on intercept reduces the pH in another manner.

Choices...

Thanks. I think I understand the basic physics. I guess I'll ask it in a different way: what does the "base" Ph of the AIM-9L actually represent, if it doesn't represent the chance to hit from essentially perfect firing condition? I suppose I could engage the afterburner, but the missile has plenty of fuel, so would a faster launch speed actually increase the chance to hit?

[Dimitris]

The nominal Ph represents the "perfect day, big fat cooperative target, right after burnout (ie. max kinetic energy), easiest shot in the universe" condition which almost never happens "in the field".

The only thing you can do in this case is to reduce the time/distance gap between burnout and impact; therefore, to delay firing until you are point-blank.

[Tcao]

The only thing you can do in this case is to reduce the time/distance gap between burnout and impact; therefore, to delay firing until you are point-blank.

I am afraid highlight part is not very practical
The tests show that the launching speed matters too, at least in WVR battle, although that falls into "reduce the time gap between burnout and impact" conclusions . High launching speed gives a significant boost to missile max speed after rocket engine finish boosts phase.

Honest speaking, I never thought the launching speed can make too much difference in the final PoH calculation
I remember when Tiny beta roll out, people did some tests on launching speed and BVR, got conclusion that the launching speed only gives a very small benefit to the BVR battle. It can , extend the NEZ range 1nm further at best.
So, something changed between Tiny Beta and new official version? You guys kept a secret recipes from us ?

[Dimitris]

Honest speaking, I never thought the launching speed can make too much difference in the final PoH calculation
I remember when Tiny beta roll out, people did some tests on launching speed and BVR, got conclusion that the launching speed only gives a very small benefit to the BVR battle. It can , extend the NEZ range 1nm further at best.
So, something changed between Tiny Beta and new official version? You guys kept a secret recipes from us ?

I have explained in detail, here, the two ways that a high-speed launch helps IRL, and how Command currently models one of them but not the other.

[amizaur]

The 650kts AIM-9L burnout speed bothers me too.

Later model Sidewinders have all motors with burn time of around 5 seconds. But burn time (and trust) would change depending on temperature, so in cold freezing conditions it would burn longer (like 6-7s but producing less thrust), in very hot conditions (like after supersonic flight) it would burn shorter (like 4s), giving higher thrust. But the total impulse should remain more or less the same (slightly higher with shorter burn due to higher chamber pressure).

Data for AIM-9L rocket motor:

AIM-9L SMC, 1974:



It says 2660lbf for 5.23s and it's given as a nominal data.

Missile weights 86kg and the motor (from different source) weights 40kgs full and 18kg empty (not sure this empty value, some sources are for 30kg of fuel, but that would only make the missile faster). Total impulse would be around 13900lbf*s from 22kg of fuel, that's 6.1830kN*s, which gives specific impulse of about 286. Little high, but we don't use this value anyway.
[edit: if we assume for motor 99lb loaded mass, that's 45kg and 18kg empty mass, we'll get 27kg of fuel and specific impulse of 233 which sounds about right]
[edit2: I found another source witch says the motor weights 99lb full and contains 60lb of fuel - that's 27kg].

More important is that the missile weight is 86kg at launch and 64kg [59kg] at burnout. So the initial acceleration is 14g, and increases to 19g [20.3g] before burnout. Without taking air drag into account, the missile after launch would gain 828m/s over the launching aircraft speed (that's pure physics). When we include air drag, the values would be about 780m/s at high altitude and about 700m/s at low altitude [estimated roughly using the old "MiniZaP" program].

AIM-9L launched from a plane going 350kts (180m/s) it should accelerate to about 913m/s (this value is from MiniZAP so it less than 780m/s+180m/s, as at higher missile burnout speeds the air drag is higher and is limiting the max speed somewhat), so 1774kts or Mach 3,04. For a low altitide launch (100m) from a plane going 350kts the AIM-9L terminal speed would be around Mach 2.36.

Of course those are not exact values, some assumptions as air drag coefficient or fuel mass may be little different, but it's general ballpark. So 650kts seems a little low. If AIM-9L launched from 350kts gets accelerated only to 650kts, then what would be burn-out speed of Chapparal launched AIM-9L ? Around 350kts only ?

edit2: Maybe burn time as low as 1s is too short for simulation engine to work reliably, and this is the reason for inconsistent Sidewindwer burnout speed in different conditions from post 8018 ?

[amizaur]

So built a small test scenario. F-16A with AIM-9L tail chasing an Su-24. F-16 is at a standard cruise of 480 and the Su-24 is at 350. I wanted a small closing distance. Because of the DLZ logic, the F-16 wouldn't fire until within 700m. When the AIM-9 left the rails, its motor burn pushed it to 690 knots. It had about a 60-70% chance to hit the Su-24 over multiple runs. If I went to AB with the F-16, the AIM-9 left the rails and accelerated to 1300 knots or so. Nothing shocking about that."

For me a 700kts burnout speed for AIM-9L is waaay too low. Even if a Sidewinder (to be precise, a MIM-72 Chapparal which is based on Sidewinder with very similar rocket motor) was launched from the ground as a SAM, with zero initial speed and in dense air, it's burnout speed is given officially as 1.5 Ma, which is 990kts.

According to this, the missile like AIM-9D (it had similar weight and configuration as AIM-9L and almost identical rocket motor - according to some data a bit more powerfull) the burnout velocity is 2.5Ma above the speed of launching aircraft.



Of course the "2.5Ma above" is not precise at every speed and alitude but just some rule of thumb to remember, but in case of 480kts (0.75 Ma) launch the rule would give us1600kts and 3.24 Ma.

I run your test scenario in CMO and also set it up (10192m, 480kts, 350kts) in MiniZAP. In CMO F-16 fired at 2.5nm (target was manually marked as hostile, so it had no knowledge of it's max speed) and AIM-9L burnout speed was 840-842kts (Mach 1.43), the terminal (attack) speed was about 650kts.

The MiniZAP result was burnout speed of Mach 3.31 (1900kts). It may be a little high, because the MiniZAP estimation of missile Cx doesn't take into account the drag generated by constantly working steering surfaces, but I already increased drag of the Sidewinder missile in MiniZAP x1.2 to calibrate it with known performance from this graph:



It shows boundaries of high Pk (80-90%) shot from rear aspect, target and shooter both 0.9Ma at altitude of 3000ft, 5g target) and the range is 4000m (13000ft).

After calibration the missile "hit" the target with speed of about 1.4Ma. But if I increased the launch range by 10% to 4400m, then the missile speed at "hit" dropped already to about 1Ma, which is only little more than speed ot the target and is hardly allowing reliable intercept. It is consistent with info from British Jaguar (plane) manual, which mentioned that max launch ranges given in tables and graph are for high-pk (80-90%) shot against not maneuvering (or maneuvering with given G value) target, but after that range the Pk drops quickly and if the missile was fired at 110% of given range the Pk would drop to somethng like 10-20%.

So I believe that with this setup MiniZAP is not overperforming by much and quite consistent with "2.5 Mach above speed of launching aircraft". If the burnout speed of Sidewinder in CMO was somewhere between Mach 2.5 and Mach 3.3 I would be ok with that, but definitely not Mach 1.43.

Anyway, with current modeling AIM-9L should just accelerate in 1s to 1400kts if I understand correctly. But maybe my argumentation can be helpfull in the future, when real burn-out speed dependant on speed of launchung aircraft are implemented. The data for Sidewinder is ready .

But the interesting thing is if I forced the AIM-9 to fire at 1.2nm, the AIM-9's 1 sec burn time pushed it to almost 1000 knots. I tried variations on this and it seems that the 1 sec. burn time of the AIM-9's motor is consistent and matches some of the sources.

Early Sidewinders (AIM-9B, E) had motor with short 2.2s burn time with high 4200lbs thrust (total impulse 9240lb/s). Later Sidewinders (D, G, H, L, M) had better motor with somewhat less thrust (2880-2660lbs) but twice longer burn time (nominally 5s or 5.23s) and higher total impulse (13900-14400lb/s) which means higher speed. The acceleration of a Sidewinder D...M with working motor is in order of 15-20g so the air drag is not going to be limiting factor, but at low altitudes it reduces the burnout speed considerably.

But what is puzzling is that the max speed of the missile seems to be dependent on the range at which it is launched. Even up close, the AIM-9 has plenty of time to get to max velocity. And that max velocity is more dependent on range to target than motor burn time. It is dependent on launcher speed, but you still see that consistent max velocity depending on range to the target.

I have suspicion that maybe 1s of burn time is too short to work reliably in the game engine. Or it can be some other reason. I would change the burn time to 5s and see if it works correctly then. But other missiles with very short burn times may be affected as well.

[Dimitris]

something is not right, here in my testing, the F-16 fire missile at 2.5nm, and max speed of missile is 840kt ?
f16vssu24.jpg

Baseline AA Testing F16 Su24 ready 480 fire.zip

Thanks for the save! That was very helpful in uncovering the problem.

The root cause was that missiles with a burn time less than 5 seconds (in this case the AIM-9) were not accelerating fast enough to reach the maximum speed-at-altitude at burnout time.

This has now been fixed for the next update.