Bomb fall speed

[Nealius]

I'm trying to find it, but somewhere in "Vipers in the Storm" the author mentions a manual with ballistics tables with bomb range, fall time, etc. that was locked away in a safe somewhere on base and only pilots had access to it.

[Eddie]

I thought all the ballistics data on bombs was classified? In some -34 manual or something. How'd they get it?

 

I don't know if ED did, but lets just say that having tested DCS performance against such data, they match more than closely enough for a sim. Close enough that you can use real world weapon delivery calculations/data with the sim.

 

There is no such thing as ballistic tables, and hasn't been for a long time. It's all done using electronic mission/weapon delivery planning software now.

 

And while it isn't publicly available, don't assume that doesn't mean people around here don't have access to it, or the raw data it is based on.

 

As for the JDAM in DCS. No, it does not fly like it should. To be honest, apart from how it looks, it shares very little in common with the real JDAM. JDAM modelling is one of the areas that DCS A-10C does not match reality. Hopefully that'll change one day, as the JDAM and weapons like it are so much more than bomb that you pickle and then forget about.

[Yellonet]

Here's some interesting info about the Mk-82 from Australia.

There's some info on the drag coefficients for F/A-18 with Mk-82 in there too.

http://www.dsto.defence.gov.au/publications/1989/DSTO-TR-0554.pdf

[Exorcet]

I'm a little confused now. Is Yo-Yo saying that the drag from the small apparatus that guides the JDAM high enough to slow the bomb down? If that's the case, why is the GBU-12 (that also has wings to guide it) so much faster?

The aspect ratio on the JDAM is pitiful, which equates to high drag. The LGB's have higher AR wings.

 

A bomb is just an oval with fins. Shouldn't a high school physics student be able to derive all the relevant data?

Drag coefficients for most things are very hard to predict. They need to be measured; wind tunnel or CFD. They also aren't necessarily constant.

 

The can also have an AoA, which will have an effect on drag, and lift.

[howie87]

The aspect ratio on the JDAM is pitiful, which equates to high drag. The LGB's have higher AR wings.

 

Could you please elaborate on this? I am not sure what bomb aspect ratio is or AR wings.

[Spudman]

This is a simplified explanation. The free-fall bomb dropped from high altitude counteracted air drag dependant on actual Mach and bomb IAS. This parametres change as the altitude changes so TAS you see in F2 view bar can vary as the bomb enters high density air.

 

The second note is for guided bombs that flies using their lift. Lift creates induced drag that is high for the low-aspect wings of these bombs. The effect I mentioned for free-fall bombs is even more noticable.

 

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

 

All of this may be true, but unless you're dropping a high drag bomb, free fall bombs are going to hit their targets a hell of a lot faster than 138 kts. As far as "guided bombs that fly using their lift", there aren't any flying bombs written into DCS that I've seen, even so glide bombs still need to maintain enough forward speed to maintain lift. As for lift and drag , a free fall bomb doesn't need to worry about lift, it's in free fall, not flight. With fall speed and air density, we're talking about a a stream lined object that weighs between 500 and 2000 lbs, air density is going to have little effect on the velocity in Earths atmosphere.

[Speed]

All of this may be true, but unless you're dropping a high drag bomb, free fall bombs are going to hit their targets a hell of a lot faster than 138 kts. As far as "guided bombs that fly using their lift", there aren't any flying bombs written into DCS that I've seen, even so glide bombs still need to maintain enough forward speed to maintain lift. As for lift and drag , a free fall bomb doesn't need to worry about lift, it's in free fall, not flight. With fall speed and air density, we're talking about a a stream lined object that weighs between 500 and 2000 lbs, air density is going to have little effect on the velocity in Earths atmosphere.

 

Where does 138 knots come from?

 

Anyway, air density will have a measurable effect on ball fall velocity. Even before an object hits terminal velocity, drag counteracts some of the gravitational force accelerating the object, making the object accelerate at less than 9.8 m/s^2. At terminal velocity, the force of drag is equal to the force of gravity, and the object accelerates no more. Needless to say, the force of drag is dependent on air density.

Edited May 16, 2013 by Speed

[Exorcet]

Could you please elaborate on this? I am not sure what bomb aspect ratio is or AR wings.

 

Aspect ratio (AR) is the wingspan divided by the chord line, or how wide the wing is compared to how long it is. So a glider has a high AR wing, the EF-2000 has a low AR wing. AR has a large influence on induced drag, which is energy lost to air rolling up into a vortex at the wingtips due to the creation of lift. The smaller the AR, the stronger the vortex, and the more energy lost to it.

 

Basically, it costs the JDAM more of a drag penalty to generate the lift it needs to maneuver on target than it does for the LGB's.

 

All of this may be true, but unless you're dropping a high drag bomb, free fall bombs are going to hit their targets a hell of a lot faster than 138 kts.

Mine tend to exceed 500.

 

As far as "guided bombs that fly using their lift", there aren't any flying bombs written into DCS that I've seen, even so glide bombs still need to maintain enough forward speed to maintain lift. As for lift and drag , a free fall bomb doesn't need to worry about lift, it's in free fall, not flight. With fall speed and air density, we're talking about a a stream lined object that weighs between 500 and 2000 lbs, air density is going to have little effect on the velocity in Earths atmosphere.

Density will have a major effect. You can measure it with the ballistic coefficient.

 

http://en.wikipedia.org/wiki/Ballistic_coefficient

 

Which density is an important part of. Any guided bomb will also have to worry about lift, because lift is how it guides itself to the target. Now if you drop a bomb absolutely perfectly, such that it simply follows a ballistic path to the target, lift won't matter. However, any error in trajectory required correction by lift. Wind would be a big one, as the bomb would need to trim itself for the entire flight to stay on course.