GBU-38's inaccurate or is it me?

[camsr]

OK. This would indicate that the bomb's flight control surfaces cannot counteract the wind's forces and gravity sufficiently. Or that the body in general is still suffering from bare model as it is built to free-fall and not to actually fly. However, I'm still sure the GPS module is aware of the fact that the bomb is off course and headed for the wrong coords.

 

It has to be aware of it's course obviously, but it cannot predict the wind or changes of.

[JayPee]

GBU-38's inaccurate or is it me?

 

The bomb's nose is always aiming for the target point

Whereas common sense would suggest it should over compensate and point the nose to an off set point if it detects heavy drift.

 

 

This way would lead to the principle of always reducing whatever distance is left by half... You'll never reach the finish.

[camsr]

Whereas common sense would suggest it should over compensate and point the nose to an off set point if it detects heavy drift.

 

 

This way would lead to the principle of always reducing whatever distance is left by half... You'll never reach the finish.

 

Right, converging on the target to compensate for drift. But maybe the flight dynamics prevent this if the windage is too high.

[JayPee]

The JDAM kit should ideally have a way to calculate a TVV so it could place the TVV over the target coords instead of the nose.

[Supersheep]

That sounds pretty expensive for a generally disposable item.

[Flagrum]

That sounds pretty expensive for a generally disposable item.

War is always expensive ...

 

Just for comparasation: a simple Mk-82 costs something about 500 USD whereas the very same bomb fittet with a JDAM kit (i.e. GBU-38) costs about 60,000 USD :o)

 

http://forums.eagle.ru/showthread.php?t=112188

[JayPee]

Yeah, insane isn't...

 

500 bucks for a bomb and another 59 and a half grand for a guidance tail kit. But the problem is likely to be R&D costs which you're done covering for on a 40 or so year old dumb bomb design.

[Scrim]

Well, let's put it like this: During the Vietnam war, the US Air Force and Navy spent several years attempting to put an important bridge in North Vietnam out of action. Several planes were downed with their pilots taken captive or killed, and hundreds if not thousands of bombs were dropped against the bridge, with accuracy down to nill on account of AAA engaging the low flying planes. Not until an early laser guided bomb was dropped was the bridge taken out, in the first attempt. Do guided bombs still sound expensive?

 

It's one thing to cram explosives into a metal shell, make it fly stable, put a fuze on it and various other small things. It's a completely different thing to make it hit something pretty much just because you're shining what is essentially an invisible light at it.

[Supersheep]

That's not what I wanted to say. #108 is correct in illustrating why a $$$ for the kit is acceptable, but I was referring to the notion that a kit should compute a TVV. That makes the whole thing even more complicated, complex, expensive, failure-prone, you get the point. While I have no idea how the actual thing works, I can think of multiple ways to implement guidance, some are nice, some are pragmatic, some are overdone.

[Eddie]

The (real) JDAM does compute its VV and AoA and a lot else besides. It also doesn't have any issue at all hitting its target with wind.

 

Again, don't have time right now to type a lengthy post about the operation of JDAM, but II will summarise by swaying that the DCS JDAM shares its appearance with the real JDAM, and that's pretty much it.

 

If I remember I'll address a few other points that have been brought up later on when I'm at home on the PC with an actual keyboard.

[PondLife]

The main error I've seen when most people use JDAM"s is they don't lase whilst creating the markpoint or they lase too far out. I'm not going to draw diagrams but basically the assumed point in space can be off if you are not within 15 miles or so and lasing when the MP is created.

[Flagrum]

The main error I've seen when most people use JDAM"s is they don't lase whilst creating the markpoint or they lase too far out. I'm not going to draw diagrams but basically the assumed point in space can be off if you are not within 15 miles or so and lasing when the MP is created.

That was discussed in this thread already ... :o)

[Eddie]

A bit of JDAM background for anyone who's interested. Hopefully it'll illustrate how different the DCS JDAM and real JDAM are.

 

JDAM employment uses what is known as a LAR (Launch Acceptability Region), not max/min range. The LAR is entirely dynamic and varies depending on things such as launch aircraft speed, altitude, heading, winds, desired impact heading, desired impact angle, and desired impact speed.

 

The LAR surrounds the target and is essentially a circle in which if the weapon is released under the current flight conditions it will be able to hit the target with a minim impact angle of 35 degrees and a minimum impact speed of 300 ft/sec. Being in the LAR doesn't imply that the weapon can meet any specific chosen impact parameters.

 

You then have a smaller area of sky within the LAR in which releasing the weapon will enable it to satisfy any chosen impact parameters. Parameters that can be specified include impact heading, impact angle, and impact speed. If for any reason the weapon determines it cannot hit the target and meet the specified parameters it will sacrifice impact speed in the first instance, followed by impact angle and heading.

 

The release aircraft also doesn't need to be pointing at the target, you can release a JDAM on a target 90 degrees off your nose if you want to, you'll just see a significant reduction in range. You can release a JDAM at any airspeed between 165 knots and mach 1.5 depending on the variant, with it being recommended to release at as high and altitude and airspeed as the tactical situation will allow. Winds are taken into account by the launch aircraft when determining the LAR.

 

After release the weapon performs a clearing manoeuvre to ensure safe separation from the launch aircraft and then begins flying to the desired impact position. For approx. the first 10 seconds of flight a JDAM is running purely on INS guidance and after that time and as it begins to acquire GPS satellites its accuracy increases, with the CEP dropping to 15 metres after about 30 seconds of flight. If the GPS system is being jammed the weapon will guide to target using INS only. In the last second before impact the weapon reduces its angle of attack to zero.

 

While you can release a JDAM using a dive or a loft, doing so doesn't really offer any big advantages and in most cases actually reduces range.

 

There are several other features as well such as pattern releases, pre-plan targets, and others. Some stuff is used by the A-10C, some stuff isn't.

 

But as far as DCS goes, not even the cockpit symbology matches the real thing. JDAM is one of the areas where DCS differs most from reality, in fact aside from countermeasures, RWR, & MWS, I'd say it's the most different.

[Yurgon]

I'm not going to draw diagrams [...]

 

No need to, either. I already have. :)

 

Inspired by ExcessiveHeadspace's video that was already referred to here.

 

A bit of JDAM background for anyone who's interested. Hopefully it'll illustrate how different the DCS JDAM and real JDAM are.

 

Cool, thanks for sharing! :)

[JayPee]

GBU-38's inaccurate or is it me?

 

Eddie's JDAM 101

And all this was present in early alpha builds? Nevertheless, are you aware of ways to mod DCS into having JDAMs work more like the real thing?

 

Thanks for the lesson.

 

PS for those interested, an explanation of the LAR in general: http://www.aerospaceweb.org/question/weapons/q0250.shtml

Edited February 13, 2014 by JayPee

[Guest Izoul123]

After release the weapon performs a clearing manoeuvre to ensure safe separation from the launch aircraft and then begins flying to the desired impact position. For approx. the first 10 seconds of flight a JDAM is running purely on INS guidance and after that time and as it begins to acquire GPS satellites its accuracy increases, with the CEP dropping to 15 metres after about 30 seconds of flight. If the GPS system is being jammed the weapon will guide to target using INS only. In the last second before impact the weapon reduces its angle of attack to zero.

 

Isn't that kinda a catch 22 though on this whole topic depending on what/who you are attacking with a JDAM? Most things I've read have basically said than 'in reality/ actual combat flight/drop time' these are basically INS only weapons 'in combat use in real life' for the uses they've had in actual combat. IE: Lower altitude (Afghan), and by the time GPS acquisition is done it's to late/already impacted in most documented..er...discussed combat instances, as they are often running very light for longer flight time to support close combat on the ground troop vs troop scenario vs an ugly situation where they'd be at higher altitude fighting a mechanized war vs say a Soviet equivalent? Therefore why would you not try to quick lase/upload data, then release vs no lase at all? I'm not talking 'continuous lase to impact' like a LGB. I'm just saying as a general practice, it seems to be more sensible to lase quick, upload and go from there in instances where you are striking basic targets. How many caves/trucks have LASE sensors on them in these instances in areas like Afghanistan? I don't think there is any 'penalty' modeled in DCS that alerts a 'ground unit' they have been lased, so why not? For now? Again, this applies more to "THE SIM" of DCS than real world practice. My 'guess' is that in real life we have far more accurate ways for JTAC to upload far more precise perimeters for this, but they won't be discussed here or implemented in 'our sim' for obvious reasons.

 

For argument sake:

I could see if you were doing a vs situation on equivalent 1st world items like Russian arsenal being operated by Russians, then it might not be in the best interest to lase/drop. Don't other countries like Russia use laser sensing technology/sensors as well to give a warning of 'Holy hell, we are being lased by 'something'?' I guess it all depends on the theater and target, either that or I'm still on the 'I'm not fully on the understanding it boat'. By the way, don't turn this into a pissing match of "VS Russia"...I have friends there, and I respect the Russian/Soviet military very much, only when operated by those vodka slamming friends of mine. :)

 

In DCS:

I've always lased quick first before my TMS up long, and have had really good results with 38's. Honestly they seem like a 'I win' bomb to me, with LGB being the next, and dumb bombs being the real challenge. I still am not sure if wind correction entry even works in the CDU with the latest patch? I guess I need to start researching/test more, hence why I love this sim, I might be wrong and love learning.

[Eddie]

Well I would respond, but frankly even after reading your post six times, I still can't work out what on earth you are talking about or what point(s) you are trying to make.

[ENO]

I think it's one of those new bots that just takes a word from every post in the thread and mashes it into one.

[ralfidude]

I can concurr that since two patches ago, the 38's have somehow decreased in accuracy. I'm able to get 8/10 spot on, but then there are the odd 2/10 hits that are just a few feet off, and we all know that a bomb landing next to a tank means it's still fine.

 

Obviously I know how to deploy JDAMs, so I can say without a shadow of a doubt that something changed not too long ago. I started to designate my SPIs just before drop point to maximize the SPI accuracy, and it's worked out the same exact way. There are still misses, they are seldom, but they do happen. I just took it as GBU 38 inaccuracy that got implemented. I mean I have been dropping GBUs since the beta without any problems, I don't see how I would start doing something wrong now.

[ENO]

And I'm all for applying some inaccuracy too but not before some accuracy to bomb blast damage is modeled. I guess it may be a chicken / egg thing and one may have had to come first...

[NoCarrier]

And I'm all for applying some inaccuracy too but not before some accuracy to bomb blast damage is modeled. I guess it may be a chicken / egg thing and one may have had to come first...

Well, even an accurate bomb blast damage model won't do much good with all the ground units in DCS relying on a simple hitpoint damage model. In real life, a 500 pound bomb dropping two meters from a tank would cause no end of problems—smashed optics, snapped antennas and disabled radios, broken tracks. Unless the crew was completely buttoned up, the overpressure would stun them, if not outright kill them. The tank would've been a mission kill. As it is now, it's "subtract 20 hitpoints and Bob's your uncle."

 

I realize that trying to implement Steel Beasts-like penetration and damage modeling is perhaps too much of a good thing, performance and complexity wise, but I think even a simple adjustment like a "half hitpoints mission kill" vehicle state would immensely improve the sim.

[Flagrum]

Blast damage is modelled, but what isn't sufficiently modelled is the effect of shrapnell.

[JayPee]

I'm not sure about blast damage.. Atleast for MK5 and 151 it sucks. You need to score a direct hit or it doesn't do shit, not even to an unarmoured truck..

[Flagrum]

I'm not sure about blast damage.. Atleast for MK5 and 151 it sucks. You need to score a direct hit or it doesn't do shit, not even to an unarmoured truck..

MK5 is shaped charge of 1 kg. It punches small holes into things. If it does that to the engine, the truck might be rendered useless. If the tank is hit, it might even start to burn. Everywhere else it more or less only adds an additional air vent hole ...

For closed, (lightly) armored vehicles the effects could(should?) be more serious as pressure effects and molten metal within a crew compartment probably cause some more serious damage. But there is close to none area effect, so, yes, only direct hits can do anything.

 

Mk151 has a fragmentation warhead. The blast itself should be more pronounced than that of the MK1 - especially as it is not directional only. But considering the size of the warhead, I would suspect anything farther away than 1-2 meters from the impact point might be left unharmed. Maybe a shattered windshield or so for unarmored vehicles. The fragments is here what damages and kills. Your truck should be shattered with lots of tiny holes - and also your truck drivers and crew. But THIS is not really modelled in DCS and therefore here also only direct hits might show some considerable effect on your target.

[camsr]

Rcokets are only good on large groups of infantry, otherwise they are just deadweight.