GBU-38/31 Flight dynamics

[howie87]

They are terrible! Flying online last night, lead dropped a GBU-38 and I dropped a GBU-12 a few seconds later. My GBU-12 reached a terminal velocity of around 550 knots and impacted BEFORE the GBU-38 which was floating down at around 250 knots.

 

Now, I know the aerodynamics for the GBU-38/31 use a simplified flight model but surely they could be improved? It can mean the difference between your bomb hitting a Stela or the Strela hitting you before the slow ass, gravity defying bomb reaches its target.

Edited February 20, 2014 by howie87

[Hansolo]

We noticed similar anomily yesterday. All drops where from 20000ft. My Lead dropped a GBU-10, I dropped a GBU-31 app. 5 seconds later, Lead element dropped a GBU-10 20 seconds later.

 

First impact is Lead's GBU-10 app. 33 seconds after release. My GBU-31 was 65 second. I know the release point isn't identically but Lead element was behind me, dropped his ordnance later yet mine was the last to impact.

 

I presume that observations is similar as the base for GBU-31/GBU-10 is the MK-84. Correct me if I am wrong on the facts.

 

cheers

Hans

[MTFDarkEagle]

GBU-10/12 = "AFM"

GBU-31/38 = "SFM"

[Hansolo]

Solid copy Lukas. Thanks

 

Cheers

Hans

[sobek]

Now, I know the aerodynamics for the GBU-38/31 use a simplified flight model but surely they could be improved?

 

The GBU-12 guidance is nothing compared to the advanced guidance of the 38. It's a different matter alltogether to design such an advanced control and have it behave like the real thing.

[howie87]

What I'm saying is couldn't the Mk82 AFM just be tweaked to approximate a GBU-38?

 

As it is currently, the JDAM's are massively hampered by their terrible performance.

 

Even adjusting the terminal velocity of the GBU-38's SFM to 500 knots would be a satisfactory fix from my point of view.

 

Just found this quote by Yo-Yo in another thread that has confused me even more...

 

In DCS JDAMs do have the same FM as free-fall bombs, by the way.

 

This is a quote from another source:

 

"Compared to the primitive analogue guidance in a baseline Paveway II, the JDAM achieves close to twice the glide range under similar launch conditions."

 

This is certainly not true for DCS, where trying to use a JDAM as a standoff weapon is tantamount to suicide.

Edited March 31, 2014 by howie87

[KaspeR32]

 

This is certainly not true for DCS, where trying to use a JDAM as a standoff weapon is tantamount to suicide.

 

I agree that the GBU-38 could have it's terminal velocity raised. Even with the "steering" mechanism, I would think the profile and weight of that bomb would travel a bit faster.

 

As for stand-off... 38s are essentially fire and forget. It's not like you have to lase it all the way to the target. Just release and get the heck out of there.

[howie87]

As for stand-off... 38s are essentially fire and forget. It's not like you have to lase it all the way to the target. Just release and get the heck out of there.

 

But it has less glide range than maybe even the paveway ll when it should have nearly twice as much...

[Aginor]

I remember at least two threads about that in the last two years, and yeah, it sucks.

 

My suggestion: Give the GBU-38 the flight model of the GBU-12. Still soo much closer to reality than the current thing, and presumably rather easy to do.

EDIT: Actually I wonder: Is it possible for a mod to assign a SFM or AFM to a weapon? I would volunteer to do it.

Edited April 1, 2014 by Aginor

[howie87]

I just did some testing in Tacview.

Here are the results:

 

GBU 38:

Release altitude - 14200 ft ASL

Impact altitude - 13 ft ASL

TAS at release - 312 kts

TAS at impact - 198 kts

TOF - 48 sec

 

GBU 12:

Release altitude - 16000 ft ASL

Impact altitude - 16 ft ASL

TAS at release - 356 kts

TAS at impact - 550 kts

TOF - 35 sec

 

Something is clearly not right here.

Edited May 10, 2014 by howie87

[sobek]

Something is clearly not right here.

 

You are preaching to the choire. The problems with GBU guidance are known, but there is no easy fix to this. To achieve a realistic flight profile, the bomb needs a sophisticated guidance that would need to be tuned over quite some time and it seems that that's not on the plate right now.

[Sceptre]

I always just assumed it was because the JDAM tends to

'glide' more than the Paveway and that the latter was 'skinnier' so it had less drag, but I guess I was wrong. But hey at least it's not the other way around, having the GBU-12 fall to slow.

[sobek]

The Paveway has a true flight model that is guided by an algorithm that works approximately like the real thing (which is comparatively simple, just bang bang guidance). Due to the much more complex guidance of the JDAM, ED didn't have the time to make it a true flight model that is steered by guidance, the bomb now has a somewhat artificial flight model that does not really follow the laws of physics.

[howie87]

To me this seems like it should be a priority. It doesn't have anywhere near the correct aerodynamics. Why not just give it the flight dynamics of a Mk82 and simple guidance of some sort? At least then it would fall like a bomb instead of a cardboard box.

[Sceptre]

Why not just copy + paste the GBU-12's flight model and put it into the GBU-38? At least that would be better than what it is currently.

[howie87]

The main issue is that it's just not as useful as ait should be. It should be gliding further and falling faster. Even without the advanced features of a real JDAM, this would be a huge improvement.

[sobek]

Why not just copy + paste the GBU-12's flight model and put it into the GBU-38? At least that would be better than what it is currently.

 

Because then you'd need to guide it by laser. Would that be more realistic?

[Aginor]

Actually.... why not?

If the flight model of the GBU-12 only works with a laser for some reason, change the code to the equivalent of "yeah, there's totally a magical laser pointing at the coordinates (insert coordinates that were given to the JDAM)"

[sobek]

I fail to see the improvement.

 

Also, i can tell you right now that this is not going to happen. The devs won't spend the time involved in making such a lackluster fix that will necessitate tackling the problem again.

[Aginor]

Well, at least the bomb would fall with a reasonable speed. Which it doesn't now. I would call that an improvement.

 

EDIT:

a lackluster fix that will necessitate tackling the problem again.

That's normal in software development. If a programmer always waits for the time to do a 100% proper solution he will (almost) never release something more complex than "Hello world!".

 

btw: Last time I asked whether a template exists for modders to create an AFM / SFM / whatever for a bomb and assign it as a mod. Does anyone know the answer?

Edited May 12, 2014 by Aginor

[howie87]

The issue with that is that the GBU-12 kind of flies in a straight line at its target, where as the JDAM should fly more of a ballistic arc.

 

Also, the GBU-12 only receives guidance when its nose is pointed at the laser, not as soon as it starts to fall.

 

The best thing to do would be to use the GBU-12 flight model and a tweaked version of the guidance from one of the AFM missiles in the 'coasting' stage of flight.

 

If ED can make an AFM missile that travels at Mach 3 and can hit a moving target from 40nm, they should be able to simulate a free falling bomb, guided towards a stationary target.

 

Proper modeling of JDAM's is desperately needed before DCS: F/A-18C. Missiles are getting a work over right now, JDAMS should be next.

Edited May 12, 2014 by howie87

[Aginor]

Speaking of which: Do the dumb bombs have AFM (or a simple one with a halfway decent behaviour)? I don't remember...

[howie87]

Speaking of which: Do the dumb bombs have AFM (or a simple one with a halfway decent behaviour)? I don't remember...

 

Yes

[Aginor]

Thanks. I was pretty sure, but not completely.

[sobek]

If ED can make an AFM missile that travels at Mach 3 and can hit a moving target from 40nm, they should be able to simulate a free falling bomb, guided towards a stationary target.

 

For the 3rd time, the problem is not the AFM but the guidance. JDAMs fly a ballistic (well they can fly a more complex path but nmd) that is just corrected for error. The JDAM needs to at all times steer towards the *ballistic* solution to its target (they can glide to some extent, but again that is a feature that makes the guidance even more complex). That is remarkably more complex than flying there in a straight line (like the paveway or PN missile guidance that is used currently). I'm not saying they aren't able to. I'm just saying that it is going to be a big project and they probably think that other things are more important right now.

 

EDIT: That's normal in software development. If a programmer always waits for the time to do a 100% proper solution he will (almost) never release something more complex than "Hello world!".

 

In software there are no 100% solutions. There are however solutions where the gain would not outweigh the loss. Especially since this wouldn't be a stopgap that can be built on later. Those are much more likely to happen.

Edited May 12, 2014 by sobek