GBU-54 impact angle

[Khallimero]

Hello

 

I've experienced some "issue" when using the GBU-54 as a JDAM (i.e. without laser designation), especially from high altitudes.

As far as I have tested, the default impact angle (0, degree I guess) seems to ask the bomb to hit the target from above (= vertically).

Because of the odd trajectory, it can make the bomb to go backward, and sometimes it even makes it to miss the target, 

Althought the "impact angle" weapon profile button tooltip says it has "no function", as for the GBU-38, it seems to work for the GBU-38, but not for the GBU-54.

 

Attached is a track where I, from a high altitude :

- first I drop a GBU-38 on a target, with the default 0° impact angle. You will notice the odd trajectory of the bomb.

- then I change the impact angle to 45° in the weapon profile ; then I drop another GBU-38, the bomb trajectory looks good, with a 45° terminal angle. So it works fine.

- then I drop a GBU-54 with the default settings (0° impact angle) ; the trajectory is the same as the first GBU-38, with a vertical terminal angle.

- then I change the impact angle to 45° for the GBU-54 weapon profile ; the trajectory is the same as with a 0° impact angle.

 

As a remediation, is it possble to :

- make the default impact angle to 60° for all GPS-guided bombs, as for the F-16 JDAM, so it won't be necessary to change it before each flight

- maybe edit the tooltip to say it works, at least for the GBU-38

- make the weapon profile impact angle configuration to work for the GBU-54 too ?

 

(All tests done with the 2.7 stable release)

 

Regards

 

GBU-54.trk

Edited June 17, 2021 by Khallimero
typo

[Frederf]

0 degree is definitely disabled at least in DCS. Real JDAM is only impact angle 0-90 but I don't know if A-10 software allows the full range. Asking for a low angle impact takes a lot of energy to maneuver. I set two bombs angles 0 and 1 and the 1 flew minimum angle (about 20-30?) and the 0 flew very steep. I did a few GBU-54 impact angles and here are the terminal pitch angles. Was from 25kft, 6.0nm slant, 250 KIAS.

0 -57

1 -57

15 -58

30 -57

60 -57

90 -57

 

Each one seemed to go through a hard pitch gate of -60 and then did a little pull out in the terminal seconds. Looks like GBU-54 (GBU_54_V_1B should be GBU_54B_V_1) doesn't respect impact angle preference. I don't know why.

 

[Khallimero]

13 hours ago, Frederf said:

Asking for a low angle impact takes a lot of energy to maneuver.

 

Agreed. My first suggestion is only to make 60° the default value for the A-10, as it is for the F-18 and the F-16, instead of 0°.

 

13 hours ago, Frederf said:

Looks like GBU-54 (.......) doesn't respect impact angle preference. I don't know why.

 

Agreed too. That's my second point. (strange, especially because it works fine for other GPS guided bombs)

[Khallimero]

On 6/19/2021 at 10:05 PM, Frederf said:

I did a few GBU-54 impact angles and here are the terminal pitch angles. Was from 25kft, 6.0nm slant, 250 KIAS.

 

I tried to reproduce your results with the GBU-54. Looks like it also depends on when you release the bombs.

 

If you release the GBU-54 as soon as you are in range, the bomb will not have enough energy to reach the vertical of the target, and it will actually hit the target with an angle smaller than 90°.

But if you wait and drop the GBU-54 from the middle of the Dynamic Launch Zone (as I did in the track I provided with my first post), the bomb will go above the target, and then dive vertically. Regardless of the impact angle in the weapon profile.

[Khallimero]

Just feeling beeing ignored  

This looks like a serious issue to me.

Here is another track where the bomb completely misses the target, due to its odd trajectory. (I strugled locking the target that time, but I wanted to be sure it was locked before droping the bomb)

GBU-54-2.miz

[jaylw314]

I've been watching this issue with some measure of interest since I have no idea what's supposed to happen in real life.

It SORT OF makes sense that the GBU-54 would try to first try extend its glide close to the target and come in as steep as possible.  After all, there is the possibility the operator may switch targets, and gliding above the target would retain the most energy to move to a different target, even past the original target.  If it came in shallow and the target changed to a larger slant distance, the bomb wouldn't have the energy to adjust.

On the other hand, it does seem pretty outlandish that it would or could come in from straight above or even retrograde, so I'm guessing there is some measure of bug still, but I've recalled seeing such behavior since the GBU-54 first came out

[Khallimero]

That's why you can (and should) change the desired impact angle in the weapon's profile, and it should work accordingly.

That works for the GBU-38, but the GBU-54 doesn't care about it.

At least it should use a less steep angle by default, so it wouldn't miss a stationnary target with the default value.

F-16 and F-18 use 60° angle by default (edit : for GBU-38), and there is no issues with them (as far as I know).

Edited November 8, 2021 by Khallimero

[jaylw314]

46 minutes ago, Khallimero said:

That's why you can (and should) change the desired impact angle in the weapon's profile, and it should work accordingly.

That works for the GBU-38, but the GBU-54 doesn't care about it.

At least it should use a less steep angle by default, so it wouldn't miss a stationnary target with the default value.

F-16 and F-18 use 60° angle by default (edit : for GBU-38), and there is no issues with them (as far as I know).

 

I was just speaking specifically to the GBU-54.  The GBU-38 would never have to shift target mid-flight unless you ride along and hot-wire it mid-flight a la Slim Pickens   But, of course, who knows how the GBU-54 works IRL...

[WobblyFlops]

On 11/9/2021 at 1:00 AM, jaylw314 said:

I was just speaking specifically to the GBU-54.  The GBU-38 would never have to shift target mid-flight unless you ride along and hot-wire it mid-flight a la Slim Pickens   But, of course, who knows how the GBU-54 works IRL...

This is what I've been told so I'm telling this as an interesting anecdote, I don't have any docs. 

 

The 54 is a pretty interesting weapon, with many additional advantages over both a traditional JDAM and a traditional LGB. The advantages of a JDAM are pretty well given; off axis attacks, preprogrammable imapct parameters, pretty diverse fuzing options, all weather guidance and autonomous point targeting. The advantages of a traditional LGB is the ability to change its trajectory dynamically. The 54 combines both of these into a single weapon with many additional advantages over the 12.

 

The 12 is limited by the bang bang guidance and the fact that if the laser energy is disrupted by equipment failure, improper geometry or atmoshperic effects it will simply fall ballistically. This can be mitigated by the basic tactic that the wingman should also track the target, be set up with their own laser and in case the bomb loses the laser, he can also take over and laze it in but this is an inherent limitation regardless. 

The 54 employs a much smarter guidance scheme which obviously results in better kinematics for the bomb (and you don't really need that good laser marksmanship to hit a mover anymore) while also respecting the terminal parameters. Depending on the target and the mission, even regular LGBs may require specific impact parameters (velocity, angle) to be met for the desired effects, this is precalculated and manually controlled by the pilot with carefully following the proper release point, altitude, airspeed and the exact laze delay timing. For a 54, you just need to have enough energy and be in the LAR for the specific parameters and the bomb will do all of this on its own. 

The way it can achieve both updated, continuous guidance and adherence to terminal parameters is the way laser guidance is integrated into the weapon. It's a proper JDAM with INS/GPS guidance and it doesn't simply follow the laser energy like a PW2 or even 3 but rather it has an encoded terrain mesh and it will use the depression angle and azimuth of the laser spot corroborrated with its own position and the terrain mesh to update the coordinates. So you shouldn't think about it like an traditional PW 2 at all, but rather a more advanced JDAM that can use a lased spot in addition to GPS signals for guidance. Once the coordinated is update, the missile autopilot will take care of hitting the DPI while following the release parameters.

This is all unclassified info. Anything deeper than this is probably going to run into more issues, like knowing what (if any) limitations it has for updating the coordinate source (like how quickly it can use the laser spot to do the math) how it prioritizes the different impact parameters if there's not enough energy to meet everything and so on.

[Yurgon]

4 hours ago, WobblyFlops said:

It's a proper JDAM with INS/GPS guidance and it doesn't simply follow the laser energy like a PW2 or even 3 but rather it has an encoded terrain mesh [...]

Oh wow, great info!

I'd heard before that the GBU-54 can be "retasked" in flight by lasing the new target with the proper lasercode, and then the weapon would guide to that target even if lasing stopped before impact. I've been scratching my head ever since, trying to figure how the bomb could possibly do that. Having its own terrain elevation information certainly explains it.

Now the follow-up question is, do you know if it has a global terrain database? Or does it have like a memory card slot or a thumb drive that's inserted before each mission with the data for the terrain it's supposed to go to? Do techies attach a laptop and update the weapon's internal database for the target area? Still trying to wrap my head around the practical aspects of this weapon.

[WobblyFlops]

1 hour ago, Yurgon said:

Now the follow-up question is, do you know if it has a global terrain database?

I know that it's not unlimited but I don't know how they load it exactly or how big the limit is. I'd imagine that since the jet itself has its own DTSAS and the relevant region is loaded for the mission, the database can be downloaded during the alignment. Since all JDAMs have to talk to the aircraft (to use the aircrafts INS to align its own INS, get mission data, GPS cryptos and almanac, ZTOD, etc.) it makes sense to simply utilize this connection and have the database downloaded for the selected region, but I'm just spitballing here.