SPH mirror symmetry, unequal drag on cargo bays

[Clouds]

Sorry that I don't have a more minimal example/reduced test case. This is the first time I've NOTICED a problem.

This (ugly, utilitarian, unspectacular) SSTO was pulling to the right, and also having trouble breaking the sound barrier. Well...I found out why. Check the drag values. (You can also see the SAS beginning to try left-yaw to hold it true.)

It was built with symmetry. If I alt-click to copy each side, then place them back without using symmetry, then both sides have equal, low drag.

Craft file: https://www.sendspace.com/file/az9t10

Edited September 9, 2015 by Clouds

[Gaarst]

Cargo bays are known to cause problem with drag: something like parts behind them being exposed to airflow and causing drag. But I didn't know there were issues with symmetry...

There's not much you can do I'm afraid. I don't think Claw's Stock Bug Fix Modules has a fix for this issue (to be confirmed) so you either have to wait for a fix from Squad or redesign your craft. This issue might also be somewhat random, so if you launch you ship several times there might be a chance that this doesn't occur.

I hope someone else will be able to help you better than I can

[Clouds]

I hope someone else will be able to help you better than I can

Well, I did say that it was "fixable" by placing the parts without symmetry after the fact, but, that's a workaround. It doesn't keep it from happening in the first place.

[Padishar]

I suppose the most important question is whether it only happens with cargo bays (and service bays) or with other parts when applied with symmetry too...

Some similar sounding bugs have been reported in the official bug tracker, e.g. http://bugs.kerbalspaceprogram.com/issues/5260, though about lift rather than drag. They may well be different effects of the same underlying bug.

[Stoney3K]

Noticed the same thing. Mk3 space plane with Mk2 side tanks and utility/cargo bays, causes uneven drag even though the tanks and bays are exactly identical. Ultimately this leads to the ship yawing to one side and losing control when supersonic.

I noticed the phantom lift issue as well, which would cause a similar problem on the roll axis.

[sal_vager]

This is interesting and I think I know what's going on, this shouldn't be reproducible with other parts if I'm right.

Cargobays have two nodes at each end so they can be serially attached and the payload can be fitted inside, when a cargobay is normally attached the "out" facing node is connected and drag is calculated correctly, but if the bay is a symmetry clone it may be attaching via the wrong node, ignoring the strict part alignment rules.

This would cause KSP to treat the bay as if it was not occluded at all, you get a lot of drag from the flat face of a Mk3 part.

Also, because of the strict attachment rules you should find that you cannot fit a part to the front inner node of the bay, it will instead try to fit to the front outer node, ie: inside the part in front of the cargobay.

This does not seem to occur all the time though, the craft file Clouds posted has equal drag on both sides but was made with symmetry, the issue only arises when the cargobays themselves are removed and refitted in symmetry.

I can see from the part hierarchy that he placed other parts of the nacelle first, then created a symmetric pair, so the first cargobay would attach normally, its clone would also have correct attachment and the craft would be fine.

So rather than having to place all cargobays singularly to avoid this issue, just place the parent part of the cargobay in symmetry instead.

Edit:

Further testing shows I was right, marking as solved.

Edited September 16, 2015 by sal_vager

[Stoney3K]

Further testing shows I was right, marking as solved.

Negative on that. I have a craft with a Mk3 main fuselage and Mk2 fuel tanks used as nacelles, attached in symmetry, which have cargo bays attached to them (via their node). This seems to cause unequal drag and body lift issues.

Will provide craft file if required.

[sal_vager]

Please provide it.

[Stoney3K]

Just something I quickly slapped together:

Craft file:

https://drive.google.com/file/d/0BxraMdXm6-GLejJ1N2k0V2ZJajg/view?usp=sharing

Bay on the left has a drag of 14, while the bay on the right has a drag of 7 just like the fuel tanks.

Bays are attached to the forward fuel tanks which are attached to the center cargo bay fuselage. The fuel tanks (and wings and everything attached to that) were placed with mirror symmetry.

Issue is not Windows-specific since I built this craft on Linux.

[sal_vager]

Okay, so knowing what I know from my previous testing I poked your craft a bit, and sure enough it was not possible to attach a part to the front inner node of the starboard cargobay, it was already connected to the fuel tank in front of it.

And flying the craft showed excessive drag on one side, just as you say.

But it was fixable in the same way I outlined above, resulting in a low drag craft, further testing showed the drag from the bad bay to be just as high as other cargobays with no streamlining at all.

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Here's the fixed craft [Dropbox link]

The issue with drag can be solved, and is only a symptom of another issue, that being the strict part attachment rules ignored by symmetry clones and can only cause an issue with bays as they are the only parts with dual nodes.

But that is an issue for the official bug tracker, this is not the bug tracker, this is the support forum

[Stoney3K]

So how did you fix the issue now? Did you attach each nacelle one by one? Because that would work fine when the nacelles are perfectly level and centered on the middle fuselage, but it would throw the craft out of balance if you try to shift or rotate the wings, as their symmetrical clones won't move or rotate along with it. And there is no way to align them numerically, so you would need to eyeball it.

I already tried removing the bays and re-attaching them without any symmetry, but that will break the symmetry for the rest of the parts in the nacelle. It's fine and dandy when the nacelles are straight and level, but impossible to balance when the nacelle and the wings have an angle of incidence.

My suspicion is that the problem is caused by the inner and outer nodes of the cargo bay being so close together that the symmetry rules will pick the closest one nearby, which is the wrong one. Adding a bulkhead and shifting the node slightly inward would solve that issue, but it would cause a new issue with the cargo bays no longer being capable of carrying long payloads.

Alternatively, you would have the inner node floating in mid-air, but this would cause a cosmetic ugliness.

[sal_vager]

I just used normal 2x symmetry, if you look at the craft I fixed you will see the nacelles are not separate parts, one is the symmetry clone of the other.

If the strict part attachment rule was being honoured by symmetry clones this issue would not occur, if you read through the Imgur gallery you'll see what I mean, so next time this happens to you just rettach the nacelles to get it to work correctly.

It would be useful to know exactly how you and Clouds are able to trigger this issue when most other players are not, so if you can consistently reproduce the effect with a simpler craft please post step by step instructions.