CBU-99 Height Release Issue

[Renko]

I think there are some issue with the CBU-99 related to the height at which it release the submunitions.

I paired them with the FMU140, and i tried different avaiable options for the Airburst Altitude (height ?).
And none of them releases at the set height.
For example the default 1500ft releases consistently at around 900ft.

I included some tracks with different fuzes, but its easy to test within this mission.


BTW. I noticed too a discrepancy.
When i set the Fuze 500 in the editor, in game in a HotPlane the weapon page VT1 shows HT 500
Fuze 2200 in editor, in game will be HT 1800
Fuze 2600 in editor, in game will be HT 2200

On another Note:
i noticed the effect the submunitions does, the brown dust clouds, are the same even with differen dispersion options.
It will be worth that its updated to change with the dispersion, that will give better feedback to the users and avoid confusion with it.

CBU_Height_Issue_500_centerHit.trk CBU_Height_Issue_900_centerHit.trk CBU_Height_Issue_1500_centerHit.trk

CBU_Height_Issue_3000.trk

CBU_Height_Issue_3000_AUTO.trk

Edited Wednesday at 07:18 PM by Renko

[SloppyDog]

@Renko sir, I think you've found something truly interesting.

And I think it is modeled correctly in DCS (not a common occurence)

My theory is this: the bombs are bursting open in a height below to what is set on them in the Mission Editor and in the cockpit because the FMU-140 sensor does not measure distance to the ground, but to the target.

The FMU-140 fuze sensor information I took from this web page (https://navyaviation.tpub.com/14023/css/Fmu-140-B-Dispenser-Proximity-Fuze-32.htm). It states that the FMU-140 is a doppler radar. It does not show the radar pattern, but I believe it projects forward of the fuze, similar to the M20/M20A1 main lobe projection, forward of the bomb axis, as shown in the picture below, taken from the same web page. (see pictures below)

So, my hypothesis is this: what the bomb fuze is seeing is the slant range to target, not the height above ground, as one would expect. Let's take the first example, HOF set at 1,500 feet.

From the track, we gather that the bomb pitch angle is - 35 degrees at the moment of the burst. The burst altitude AGL is around 900 feet, as stated before. 

The bomb scheme can be simplified as a right triangle. The bomb pitch angle is -35º at the moment of the burst. Let's disregard the negative sign. So if the external angle to a right triangle is 35º, in order to both sum up 90º,  the internal has to be 55º (let's call it Alpha). In order for the angles of a right triangle to sum 180º we have: 90º (from the right angle) + 55º (Alpha) + internal angle 2 (Beta) = 180º. Thus, the Internal angle 2 (Beta) is 35º.

Using sine, we have that: sine of 35º is equal to the burst height h (ft AGL), divided by the hypotenuse, in this case, the HOF (same as the Slant Range). 

Adjusting the equation we end up with h = sin 35º x S.R. (Slant Range) 

Doing the math, we have: h =  0,57 x 1,500. Thus h = 855 ft. Which is consistent to the results we see on the tracks provided.

For the other drop, with a HOF set at 3,000 ft, we can use the same logic:

In this case, h = sin 44º x 3,000 . Thus h = 2,000 ft AGL. Again, consistent to what can be extracted from the track files.

The solution? Either drop from a steeper angle, or from level flight at a higher altitude. I usually use CBU-99s with the FMU-140 from 10,000 feet with a HOF of 3,000 feet, and they work most of the time.

 

 

Edited Friday at 04:08 AM by SloppyDog

[Renko]

5 hours ago, SloppyDog said:

@Renko sir, I think you've found something truly interesting.

And I think it is modeled correctly in DCS (not a common occurence)

My theory is this: the bombs are bursting open in a height below to what is set on them in the Mission Editor and in the cockpit because the FMU-140 sensor does not measure distance to the ground, but to the target.

The FMU-140 fuze sensor information I took from this web page (https://navyaviation.tpub.com/14023/css/Fmu-140-B-Dispenser-Proximity-Fuze-32.htm). It states that the FMU-140 is a doppler radar. It does not show the radar pattern, but I believe it projects forward of the fuze, similar to the M20/M20A1 main lobe projection, forward of the bomb axis, as shown in the picture below, taken from the same web page. (see pictures below)

So, my hypothesis is this: what the bomb fuze is seeing is the slant range to target, not the height above ground, as one would expect. Let's take the first example, HOF set at 1,500 feet.

From the track, we gather that the bomb pitch angle is - 35 degrees at the moment of the burst. The burst altitude AGL is around 900 feet, as stated before. 

The bomb scheme can be simplified as a right triangle. The bomb pitch angle is -35º at the moment of the burst. Let's disregard the negative sign. So if the external angle to a right triangle is 35º, in order to both sum up 90º,  the internal has to be 55º (let's call it Alpha). In order for the angles of a right triangle to sum 180º we have: 90º (from the right angle) + 55º (Alpha) + internal angle 2 (Beta) = 180º. Thus, the Internal angle 2 (Beta) is 35º.

Using sine, we have that: sine of 35º is equal to the burst height h (ft AGL), divided by the hypotenuse, in this case, the HOF (same as the Slant Range). 

Adjusting the equation we end up with h = sin 35º x S.R. (Slant Range) 

Doing the math, we have: h =  0,57 x 1,500. Thus h = 855 ft. Which is consistent to the results we see on the tracks provided.

For the other drop, with a HOF set at 3,000 ft, we can use the same logic:

In this case, h = sin 44º x 3,000 . Thus h = 2,000 ft AGL. Again, consistent to what can be extracted from the track files.

The solution? Either drop from a steeper angle, or from level flight at a higher altitude. I usually use CBU-99s with the FMU-140 from 10,000 feet with a HOF of 3,000 feet, and they work most of the time.

 

 

Well i think you are into something. Maybe thats the issue there

I think what you said is possible to be how in DCS the FMU-140 calculates HoB (Height of Burst), it uses the SlantRange.
But i dont think that is correct, if it does that its an error.

First of all, if we compare to other cluster bombs like the CBU-87. That has the FZU-39/B that works with setting a HoB too.
If you use it on DCS set at 1500, it releases at 1500ft AGL.
(if you have the F16, I added a track i did that day to see if there was any difference with the CBU87 Radar Fuze)

And i dont think the FM-140 is different.
As you can see cleary here it has in the picture a setting for HoB. So i dont think that works with SlantRange, but as any other fuze with Height
https://cat-uxo.com/explosive-hazards/fuzes/fmu-140-fuze

But thank you, because i think you pointed the devs where the issue could be.

VIPER_CBU_Height_Issue_1500_centerHit.trk

[Kah0tika]

As far as I understand the FMU-140 radio alt antena does correct the slat range error within normal release parameters.
Errors should only arise outside of standard parameters (extreme angles).

 

This seems to be a bug.

[Renko]

It is a bug.

I just tested a vertical angle drop, very near to 90º falling down and it opens at the correct HoB.

For some reason the CBU-87 work with HoB, but the CBU-99 with the FMU-140 works with Slant Range

This should be corrected

Edit: i forgot to add the  track
CBU_Height_Issue_1500_90degrees.trk

Edited Friday at 07:43 PM by Renko