AIM-120 Automatic lofting performance degraded

[Default774]

Intro

The lofting algorithm of the AIM-120(C, B not tested) is very inefficient. The auto-lofting consistently under lofts the missile at ranges further than ~20nm, and seems to get progressively worse as speed increases.

This will be outlined in a series of tests comparing the automatic lofting with manually lofting the missile 30deg nose up. Lofting higher might result in better performance in some cases, but the AIM-120 pitches down to 30deg when it is manually lofted more.

All tests are performed at 30,000ft with the target aircraft flying in a straight line at Mach 1.0.

BLUE/RED - automatic lofting
GREEN/PURPLE - manual 30deg loft

Tests

Ownship speed mach 1

20nm

Spoiler

120_loft_20nm_m1.acmi120_loft_20nm_m1.trk

30nm

Spoiler

120_loft_30nm_m1.acmi120_loft_30nm_m1.trk

40nm

Spoiler

120_loft_40nm_m1.acmi120_loft_40nm_m1.trk

50nm

Spoiler

120_loft_50nm_m1.acmi120_loft_50nm_m1.trk

Summary

The 20nm test is the only test where manually lofting the missile resulted in similar performance to the automatic lofting. The manually lofted missile has a higher speed on impact, but falls behind in time on target slightly. From 30nm onwards, the manually lofted begins decisively outperforming the auto lofted missile, achieving both better time on target and higher speed on impact. At 50nm the manually lofted missile has achieved almost 1 mach higher terminal speed, which results in a significant improvement in Pk, especially against players and defending targets.

Ownship speed mach1.5

20nm

Spoiler

120_loft_20nm_m1p5.acmi120_loft_20nm_m1p5.trk

30nm

Spoiler

120_loft_30nm_m1p5.acmi 120_loft_30nm_m1p5.trk

40nm

Spoiler

120_loft_40nm_m1p5.acmi120_loft_40nm_m1p5.trk

50nm

Spoiler

120_loft_50nm_m1p5.acmi120_loft_50nm_m1p5.trk

Summary

A similar situation compared to the mach 1 ownship shots exists where the 20nm shot gets outperformed in time to target when compared to auto loft. There is probably a happy middle ground to be found here somewhere in between the auto loft angle of ~7.5deg and the 30deg shot, but this is outside of my current testing scope. The difference in performance in shots 30nm and beyond becomes increasingly large when at higher speeds. In the 50nm shot, a difference of almost 1.5mach can be observed coupled with the time on target being almost 10 seconds faster.

Ownship speed mach ~1.9

20nm

Spoiler

120_loft_20nm_m2.acmi120_loft_20nm_m2.trk

30nm

Spoiler

120_loft_30nm_m2.acmi120_loft_30nm_m2.trk

40nm

Spoiler

120_loft_40nm_m2.acmi120_loft_40nm_m2.trk

50nm

Spoiler

120_loft_50nm_m2.acmi120_loft_50nm_m2.trk

Summary

Once again very similar results when compared to the slower ownship shots. Beyond 20nm the missile has severely degraded performance from the inferior lofting profile. 

Ownship speed effect on auto lofting

Spoiler

120_loft_auto.trk120_loft_auto.acmi

Summary

Ownship speed has a very significant effect on the behaviour of the auto lofting when assuming a similar launch range. Launching at 40nm with ownship speeds of mach 0.75 and mach 1.9. When taking into account that there was a mach 1.2 difference of launch speeds between the two shots, the terminal speed of both shots is very similar, with the mach1.9 shot very inefficiently turning this extra energy into a reduced time on target.

Observations

• The missile consistently does not loft high enough at ranges greater than ~20nm(30000ft, this will be different at differing altitudes), resulting in significantly degraded performance.
• The auto lofting profile has both inferior time on target, terminal speed and consequently range when compared to manually lofting the missile higher.
• This is not a minor performance decrease
• This problem gets progressively worse the higher your ownship speed, with the missile lofting less and lower.

Notes

• Manually lofted shots are often shot either slower and/or lower than the auto lofted shot. Redoing these tests to do this more precisely would only result in the manually lofted shots being even better than they are now.
• The missile is limited to 30deg manual loft, a higher loft could result in even more improved performance in some shots.
• Please do point out any errors I made!

120_loft_auto.acmi 120_loft_auto.trk

Edited January 26, 2023 by Default774

[MARLAN_]

I don't understand the issue, is it that you think the loft angle for the missile should be greater than 30 degrees?

As for manual lofting, that is something done in the real world and is expected to improve the performance of a launch.

[Default774]

2 hours ago, MARLAN_ said:

I don't understand the issue, is it that you think the loft angle for the missile should be greater than 30 degrees?

As for manual lofting, that is something done in the real world and is expected to improve the performance of a launch.

The current automatic lofting of the missile gets insanely inefficient at longer ranges and higher speeds. Im saying that there is a lot of performance on the table in terms of optimising the missile lofting algorithm.

If there was a hypothetical issue with the proportional navigation causing the missile to lose ~1 mach of speed over the course of flight unnecessarily, I think you would agree with me that that is a problem. Why not approach the lofting behaviour with the same line of thinking?

Edited January 27, 2023 by Default774

[MARLAN_]

Is your evidence saying that the auto loft doesn't reach 30 degrees but if you manually loft to 30 degrees it stays there and improves performance?

[Default774]

2 hours ago, MARLAN_ said:

Is your evidence saying that the auto loft doesn't reach 30 degrees but if you manually loft to 30 degrees it stays there and improves performance?

Please read my post.

My point is that the current auto lofting algorithm is very inefficient in most circumstances and does not loft the missile as much as it should to maximise performance. I'm illustrating this point by showing what the the performance deficit is when compared to a more suitable lofting angle. In this case, I am achieving this differing loft angle by manually lofting the missile up to 30 degrees. I am proposing that the automatic lofting algorithm of the missile is adjusted so that this lost performance can be recovered. This is not a situations of trade offs where the automatic lofting algorithm is optimised for a different end result(Improved time on target, speed or range), the automatic lofting algorithm is straight up inferior to manually lofting the missile higher when at ranges beyond ~25nm. This is also not a minor performance deficit. 

[MARLAN_]

48 minutes ago, Default774 said:

Please read my post.

My point is that the current auto lofting algorithm is very inefficient in most circumstances and does not loft the missile as much as it should to maximise performance. I'm illustrating this point by showing what the the performance deficit is when compared to a more suitable lofting angle. In this case, I am achieving this differing loft angle by manually lofting the missile up to 30 degrees. I am proposing that the automatic lofting algorithm of the missile is adjusted so that this lost performance can be recovered. This is not a situations of trade offs where the automatic lofting algorithm is optimised for a different end result(Improved time on target, speed or range), the automatic lofting algorithm is straight up inferior to manually lofting the missile higher when at ranges beyond ~25nm. This is also not a minor performance deficit. 

I did read your post, I was trying to clarify, no need to get hostile.

Do you know what angles it currently lofts at besides the one case of ~7.5 degrees at 20nm? Does it ever automatically loft to 30 degrees or close?

Edited January 27, 2023 by MARLAN_

[Default774]

42 minutes ago, MARLAN_ said:

I did read your post, I was trying to clarify, no need to get hostile.

Do you know what angles it currently lofts at besides the one case of ~7.5 degrees at 20nm? Does it ever automatically loft to 30 degrees or close?

 

The angle it lofts at depends on a few factors. Generally, the slower you are the higher it will loft, and the further away the higher it will loft. For example, a 30nm shot fired from mach 1 automatically lofts it self to 13deg, and a 30nm shot fired at mach 1.9 lofts itself to 7deg(120_loft_30nm_m1&m2.acmi). Its worth noting that despite the fact that the mach 1.9 shot has 0.9mach more energy available to it at launch, the missiles both hit the target with around the same speed; The mach 1 missile hitting the target at mach 1.71, and the mach 1.9 missile at mach 2.01. The difference in these two shots is in time on target with the mach 1 shot taking 55 seconds to hit, and the mach 1.9 shot hitting about 10 seconds earlier at 45s. It is reasonable to assume that as speeds get higher, the lofting algorithm reduces to angle added to attempt to reduce time on target, however, it does this very inefficiently.

When comparing the manually lofted shot fired at mach 1 to the automatic shot, the manually lofted shot reaches the target 3 seconds faster while also being 0.30 mach faster at impact. With the mach 2 shot, the difference is even larger, with the missile also reaching the target 3 seconds earlier, but now also 0.7 mach faster. A manually lofted shot fired from mach 1 reaches the target with the same speed as a shot fired at mach 1.9 with auto loft, only being around 7 seconds slower!

You can compare the lofting angles with eachother by reading out the pitch displayed in the tacview for each shot if you want to look into more examples, but in short, this difference in performance gets larger the further away the shot is and the faster the missile is fired.

You can get the missile to loft to near to 30deg by flying comically slow and launching your missile out of range. For example, here is a track of me launching a 120 from mach 0.4 at 50nm. The missile in this case lofts itself to 26deg nose up. The missile is obviously fired out of range, so you end up with a wonky chart as the missile desperately attempts to make it to the target. It would actually hit the target if it had not run out of battery.

120_example_1.trk 120_loft_example_1.acmi 

In short, you can look at your DLZ. The closer to the NEZ, the less the missile will loft. The worse the shot is, the more the missile will loft.

You could probably double, if not triple, the current added pitch by the auto loft in most cases and you would get drastically improved performance. 

Edited January 27, 2023 by Default774

[MARLAN_]

38 minutes ago, Default774 said:

The angle it lofts at depends on a few factors. Generally, the slower you are the higher it will loft, and the further away the higher it will loft. For example, a 30nm shot fired from mach 1 automatically lofts it self to 13deg, and a 30nm shot fired at mach 1.9 lofts itself to 7deg(120_loft_30nm_m1&m2.acmi). Its worth noting that despite the fact that the mach 1.9 shot has 0.9mach more energy available to it at launch, the missiles both hit the target with around the same speed; The mach 1 missile hitting the target at mach 1.71, and the mach 1.9 missile at mach 2.01. The difference in these two shots is in time on target with the mach 1 shot taking 55 seconds to hit, and the mach 1.9 shot hitting about 10 seconds earlier at 45s. It is reasonable to assume that as speeds get higher, the lofting algorithm reduces to angle added to attempt to reduce time on target, however, it does this very inefficiently.

When comparing the manually lofted shot fired at mach 1 to the automatic shot, the manually lofted shot reaches the target 3 seconds faster while also being 0.30 mach faster at impact. With the mach 2 shot, the difference is even larger, with the missile also reaching the target 3 seconds earlier, but now also 0.7 mach faster. A manually lofted shot fired from mach 1 reaches the target with the same speed as a shot fired at mach 1.9 with auto loft, only being around 7 seconds slower!

You can compare the lofting angles with eachother by reading out the pitch displayed in the tacview for each shot if you want to look into more examples, but in short, this difference in performance gets larger the further away the shot is and the faster the missile is fired.

You can get the missile to loft to near to 30deg by flying comically slow and launching your missile out of range. For example, here is a track of me launching a 120 from mach 0.4 at 50nm. The missile in this case lofts itself to 26deg nose up. The missile is obviously fired out of range, so you end up with a wonky chart as the missile desperately attempts to make it to the target. It would actually hit the target if it had not run out of battery.

120_example_1.trk 119.5 kB · 0 downloads   120_loft_example_1.acmi 89.55 kB · 0 downloads  

In short, you can look at your DLZ. The closer to the NEZ, the less the missile will loft. The worse the shot is, the more the missile will loft.

You could probably double, if not triple, the current added pitch by the auto loft in most cases and you would get drastically improved performance. 

 

It seems like the current algorithm is probably okay since as you said it is factoring in range/speed/etc. however it's not as aggressive as it should be, maybe some multiplier in the algorithm could be tweaked to increase the auto loft angle.

[Prez]

I don't believe this to be a problem. Just because you get the manually lofted missile to loft at a higher angle than the one shot at the horizon does not mean there is a problem with the lofting algorithm. Should it loft higher? I'm not entirely sure because I don't work at Raytheon, however, if the automatic loft was increased for non-manually lofted shots then you would not necessarily see an increase in performance. This is because the missile would just bleed more energy in the initial lofting maneuver than it does already. I believe that energy bleed from G is factored in the loft procedure, and will therefore reject a more aggressive auto-loft in favor of retaining more initial energy from the motor burn. You can actually see this exact problem in Heatblur's AIM-54 loft where the auto-loft is so aggressive it actually hinders the missile's ability to accelerate in the early stages of flight. Another example of this problem can be seen in the AIM-7MH and P variants' automatic loft.

You should be manually lofting the AIM-120 at any range above 20NM anyways. It's a "medium" range missile, it needs the help. The manual loft gives it that extra range by allowing the missile to retain more energy since it doesn't have to maneuver as well as it being thrown higher in the air from the more aggressive lofting angle.  

TL;DR - I don't think this is a bug.

[Маэстро]

On 1/27/2023 at 7:10 PM, Default774 said:

The current automatic lofting of the missile gets insanely inefficient at longer ranges and higher speeds. Im saying that there is a lot of performance on the table in terms of optimising the missile lofting algorithm.

Thank you for your interest and effort in missile testing. We aware that current loft implementation is far from ideal and can be improved. Unfortunately, real-time computation of optimal solutions for missile lofting is a quite hard non-linear problem which require lots of time to solve. It's already included in weapons wishlist and will be solved with time.

[Py]

On 2/1/2023 at 11:07 PM, Маэстро said:

Thank you for your interest and effort in missile testing. We aware that current loft implementation is far from ideal and can be improved. Unfortunately, real-time computation of optimal solutions for missile lofting is a quite hard non-linear problem which require lots of time to solve. It's already included in weapons wishlist and will be solved with time.

Could tables be precalculated and interpolated for a realtime approximation?
There are many variables, but even a fairly coarse precalculated set of solutions might give significant improvements.