Understanding the drag box format

[GoSlash27]

All,

From what I see, each part has 24 values to define it's drag box for each state (deployed/ retracted and so on)

I notice a definite pattern in the values that makes me think they're grouped in 3s. This suggests a group of spline curves to me, but I can't reconcile 8 curves for 3 axes or 6 faces.

OTOH, they might be in an order that's more useful for the game engine, or something else entirely *shrug*

Does anybody know the definition for the values in a drag box by position?

Thanks,

-Slashy

Edited September 8, 2015 by GoSlash27

[The_Rocketeer]

Slashy, I expect you to answer these questions, not ask them...

[Val]

Slashy, I expect you to answer these questions, not ask them...

Agreed

But then again, everyone has to have asked the question before they can answer it.

[Red Iron Crown]

If the hollow parts are cylindrical and of a normal size, you're in luck. You'll want to replace the third and fourth triplets. Find a part of the same stack radius and copy its numbers.

The the numbers in each of the first six triplets are area, drag coefficient, and depth, and the triplets are X+, X-, Y+, Y-, Z+, Z-; the last two triplets are the bounds center and the bounds extents.

For stack parts, those areas will be roughly 1.227 for 1.25m parts (and 1/4 for .625, and 4x for 2.5, and 9x for 3.75m).

If your part is not the same shape as a regular old stack part, you'll have to calculate its face area yourself.

NathanKell knows things.

[GoSlash27]

NathanKell knows things.

Outstanding! This is exactly what I needed to know! "Depth, bounds center, and bounds extent" I don't quite get. I assume they have something to do with stacked occlusion...

This is enough for me to start with.

Thanks,

-Slashy

[OhioBob]

Each set of three numbers are for a different face of the drag cube, ordered X +/-, Y +/-, Z +/-. Within each set of three numbers, the values are area, drag coefficient, and depth of the part.

The drag cube looks something like this:

Using the Mk1 Command Pod as an example, we have:

 PART
 {
                 url = Squad/Parts/Command/mk1pod/mk1Pod/mk1pod
                 DRAG_CUBE
                 {
                                 cube = Default, 1.015811,0.6656196,0.7373285, 1.015811,0.6769236,0.7315681, 1.250829,0.4787524,1.097382, 1.250829,0.9474788,0.3448916, 1.013055,0.6620117,0.7325699, 1.013055,0.6601633,0.7441065, 0,0.104412,-0.001005709, 1.268897,1.132536,1.270908
                 }

 }

Therefore, tabulating the data into a more easy to read format, we have:

[TABLE=class: outer_border, width: 500]

[TR]

[TD=align: center]Surface[/TD]

[TD=align: center]Area[/TD]

[TD=align: center]Cd[/TD]

[TD=align: center]Depth[/TD]

[/TR]

[TR]

[TD=align: center]X+[/TD]

[TD=align: center]1.015811[/TD]

[TD=align: center]0.6656196[/TD]

[TD=align: center]0.7373285[/TD]

[/TR]

[TR]

[TD=align: center]X–[/TD]

[TD=align: center]1.015811[/TD]

[TD=align: center]0.6769236[/TD]

[TD=align: center]0.7315681[/TD]

[/TR]

[TR]

[TD=align: center]Y+[/TD]

[TD=align: center]1.250829[/TD]

[TD=align: center]0.4787524[/TD]

[TD=align: center]1.097382[/TD]

[/TR]

[TR]

[TD=align: center]Y–[/TD]

[TD=align: center]1.250829[/TD]

[TD=align: center]0.9474788[/TD]

[TD=align: center]0.3448916[/TD]

[/TR]

[TR]

[TD=align: center]Z+[/TD]

[TD=align: center]1.013055[/TD]

[TD=align: center]0.6620117[/TD]

[TD=align: center]0.7325699[/TD]

[/TR]

[TR]

[TD=align: center]Z–[/TD]

[TD=align: center]1.013055[/TD]

[TD=align: center]0.6601633[/TD]

[TD=align: center]0.7441065[/TD]

[/TR]

[/TABLE]

[GoSlash27]

Interesting...

A lot of the information we've been operating with is incorrect. The Y+ direction is towards the attachment point, not necessarily forward.

Several parts have different drag coefficients than are shown on this forum. For example, the structural intakes are actually *very* aerodynamic.

I'm going to have to compile a new list.

Best,

-Slashy

[NathanKell]

Yep, the depth and the bounds are used for occlusion. (Depth is the depth in meters from the "front" of the part, looking backwards along the given axis, to the widest point of the part).

The axes are part-local. If you place a part in the VAB without attaching it to something, you'll see what the local axes are (Y is up, X is towards you, and Z is to your left, in the default camera view IIRC; if looking towards the wall so, say, the window of the Mk1 pod faces you, X+ is right and Z+ is away from you).

[GoSlash27]

Okay, so as I'm digging through the drag box numbers, I'm noticing something odd:

The tail cone has a much lower drag coefficient than the ram air intake and they have the same frontal area, yet the ram air intake exhibits less drag in testing. Anyone know what's up with this?

Thanks,

-Slashy

[OhioBob]

Y is up, X is towards you, and Z is to your left, in the default camera view IIRC

I assume you mean Y+ is up, X+ is towards you, and Z+ is to your left (you didn't specific + or -).

if looking towards the wall so, say, the window of the Mk1 pod faces you, X+ is right and Z+ is away from you

That doesn't agree with the above. If X+ goes from "towards you" to "to the right", then Z+ goes from "to the left" to "towards you". Your first description are right-handed coordinates and your second description are left-handed coordinates.

My diagram above is a the right-handed system. I knew Y+ was up, but I was guessing at the other axes.

[Claw]

Slashy,

Go read this thread. I mean all of it, not just the OP.

To answer your cone vs. intake question, (without looking) I'm going to guess that some of it has to do with the fact that the tail cone has a lot more "side" surface length than the intake does. Hence, it'll have more skin friction drag.

Cheers,

~Claw

[GoSlash27]

Claw,

Aye aye

Thanks,

-Slashy

[GoSlash27]

Claw,

I've been through the whole thing (good stuff) but I'm afraid it doesn't explain what I'm seeing.

I'll carry it on over there.

Thanks,

-Slashy

[Red Iron Crown]

Okay, so as I'm digging through the drag box numbers, I'm noticing something odd:

The tail cone has a much lower drag coefficient than the ram air intake and they have the same frontal area, yet the ram air intake exhibits less drag in testing. Anyone know what's up with this?

I thought the tail cone was shown to be less draggy than the ram intake in testing. Let me find the test. Edit: Found it: http://forum.kerbalspaceprogram.com/threads/119198

Edited September 9, 2015 by Red Iron Crown

[GoSlash27]

I thought the tail cone was shown to be less draggy than the ram intake in testing. Let me find the test. Edit: Found it: http://forum.kerbalspaceprogram.com/threads/119198

Apologies, I misspoke (mistyped?)

I meant the shock cone. On paper, the tail connecter should be better.

Best,

-Slashy

[Red Iron Crown]

I seem to recall another test where the tail connector outperformed the shock cone too (dragwise at least). It's bulky and heavy though, which I guess is why the shock cone gets used more.

[GoSlash27]

I seem to recall another test where the tail connector outperformed the shock cone too (dragwise at least). It's bulky and heavy though, which I guess is why the shock cone gets used more.

Possibly, but it's not outperforming it in my tests. I tried both balance beaming and vertical ascent at high acceleration (both with compensation for the mass difference) and the shock cone outperforms the tail connecter in both.

Best,

-Slashy

[NathanKell]

You're using the DRAG_CUBE in the shock cone cfg, yes? It has an override, so the one in PartDatabase isn't used.

[GoSlash27]

NathanKell,

As a matter of fact, I wasn't; I hadn't noticed that.

But after checking the drag box in the .cfg, they're the same for the "Pointy end forward" values.

Best,

-Slashy

[Red Iron Crown]

Slashy, I had some time tonight to test tail connector drag versus shock cone drag. I built two nearly identical rockets, differing only in nose treatment and payload adjusted to make them mass within 6kg of each other on 8t total mass. My repeated observation is that the very slightly lighter shock cone rocket very slightly outpaces the tail connector rocket until the sound barrier, then the tail connector rocket quite rapidly overtakes the shock cone one. It would seem that their drag is very similar when subsonic but the tail connector has less drag when supersonic.

However, if I tweak the payload to be equal on both (which gives the shock cone version a better TWR and more closely resembles the actual choice between nose treatments a player faces), the additional mass of the tail connector is too much of a penalty for the lower drag to overcome and it doesn't catch up.

I tentatively conclude that while the tail cone has less drag it is almost never the better choice. Maybe, maybe if the vessel is intended to spend a long, long time at high supersonic speeds the drag reduction might be worthwhile.

I'd appreciate it if you could examine the test rig ((craft file, just press T and Space) and point out any errors I may have made or offer suggestions to improve it.

[GoSlash27]

RIC,

Thanks, I'll do that!

Best,

-Slashy