Help fairing & strutting this behemoth

[Fwiffo]

Hi all,

Early on in my career game Lageranda Kerman managed to get herself stranded in low Kerbol orbit.  It took several failed rescue attempts to teach me just how far down the gravity well that is.  In fairness, there was no way the Terrier's and FLT-800's I had access to at the time were going to cut it.  I've come a long way since them, and am determined to bring that kerbal home no matter what the cost!  (And for my own gameplay reasons, preferably without resorting to Xenon tech).

I'm completing work on a huge, single-launch behemoth that will establish a research and mining base on Moho, then use it as a refueling depot for my low-Kerbol rendezvous.  This is roughly what it looks like so far:

Initially I attached my boosters like so, and am able to get this into orbit fairly reliably:

Spoiler

But for realism (and aesthetics) I'd like to wrap that equipment at the top up in a more aerodynamic fairing.  Unfortunately I kind of backed myself into a corner by not having everything coalesce down to a single convenient attachment from which to build the fairing.  I could stick it between the mining rig and tanker, but the tanker's un-aerodynamic reaction wheels would still be exposed.  Note the reaction wheels are welded into a single part with most of the tanks below, and I don't want to break them up.  Also don't want to stick anything between the rig's thrusters and the damage-protecting "thrust plates" below them (which eject after use).  I wish I could start my fairing from the top and close it to the surface of my tanker, but that doesn't seem to work.

Spoiler

So I figured, let's just throw more funds at the problem and encapsulate the whole shebang in Kerbin's biggest egg - with some assistance from Procedural Fairings (which lets you attach boosters directly to the fairings, helping me avoid a rocket too tall to fit in the VAB):

Launch could have gone better...

Spoiler

Liftoff was actually fine until separation of the first pair of boosters.  What's weird is the F3 Flight Results didn't report anything wrong until later in the RUD than I would have expected.  Slowing this down and going frame-by-frame, I discovered that as soon as I detached two of the boosters, all the struts holding the other ones broke as well!  And that happened before anything at all (other than "Separation of Stage 21 confirmed") was reported in Flight Results.

1 frame before separation:

1 frame after separation:

I played around with different AutoStruts settings on the boosters, fairings and fairing base, with varying results from a craft that wouldn't stand up on the launchpad to boosters that all fell off the egg before launch.  I suspect the problem is my boosters need moar struts to something on the inside of the fairing - but it's kind of hard to attach them with the fairing in the way.

Any suggestions?  Any radically different ideas on making this beast aerodynamic?

p.s. Note those boosters each consist of only 3 parts:

- Tanks, Separatrons and decoupler (1 weldment)
- Mammoth engine (stock)
- Parachute (Mk2-R scaled up 4x)

I can share my craft files and parts if anyone wants to play with this.

I don't want to split it up into separate launches so please don't suggest that :-).

Edit: Simply right-clicking one of the boosters while on the launchpad and decoupling it also causes my ship to fall apart.

Edit: The problem isn't specific to that pair of boosters.  If I remove them from my craft, it launches just the same and the struts all disappear when I stage next pair (which are now first to stage).

Edited October 21, 2016 by Fwiffo

[Vaporized Steel]

You say that you decouple the first pair of boosters? Are you really sure? Because it seems like your decoupling your fairing and thus everything else that is attached to it.

According to your animated gif picture (third one) you are not staging your boosters but you are jettisoning the fairings. Thats because on your gif it's not just the boosters that fall of, they actually stay somewhat in one place for a second while the fairing seems to fail as a first. On the last picture it is also visible that your fairings are being staged, not your boosters.

Remember that the procedural fairings use the same decoupler icons as radial decouplers.
The fairings fly inward and bulge out because your staging them according to the gif. Which is typical for procedural fairings as the jettison torque is applied at the procedural fairing base and the engine thrust sandwiches both halves inwards. After that your boosters go along for the ride or disconnect completely due to strutting failure.

You might probably be able to do it with auto struts only. Because if your staging is setup wrong which seems like it to me you probably don't even need any struts at all.
If you do need struts, you can take the fairings of and strut your boosters to the center cargo stack and then re apply the fairing.

In a nutshell: Check your stage 20 again to ensure you are really decoupling your boosters.

If you are actually decoupling the boosters then it is still strange why your fairing seems to decouple according to your last picture. It would then almost sound like a bug.

Take a look at your gif and the last picture in the OP. In the last you see the fairings decoupling, and consequently all struts failing because they're strutted to the mammoth which is the part your seperating from as the fairings are detached.

 

 

Edited October 21, 2016 by Vaporized Steel

[Fwiffo]

2 hours ago, Vaporized Steel said:

You say that you decouple the first pair of boosters? Are you really sure? Because it seems like your decoupling your fairing and thus everything else that is attached to it.

I know it seems that way - in fact at first I thought I screwed up my staging.  But I am 100% sure I am only decouping the first pair of boosters and their parachutes (which don't open until lower altitude)

2 hours ago, Vaporized Steel said:

According to your animated gif picture (third one) you are not staging your boosters but you are jettisoning the fairings. Thats because on your gif it's not just the boosters that fall of, they actually stay somewhat in one place for a second while the fairing seems to fail as a first.

That's not correct.  Sorry if it's not clear in the animation - the fairings actually stay stuck to my rocket (albeit a very wobbly manner) after all the boosters fall off and explode.  So I am 100% sure I am not staging the fairings.  See the "before" screenshot where you can clearly see there are only two decouplers and two parachutes in Stage 20.  The icons may look the same but they are not the ones for the fairings.  The fairings are not until Stage 14.  You can also see the Separatron engines are inactive on the "before" screenshot and active only on the two staged boosters in the "after" one (the Separatron icons aren't shown in staging because they are welded to the boosters).

Edited October 21, 2016 by Fwiffo

[Fwiffo]

Ok I made some progress.  If I dramatically increase the size of node_attach in the booster/decoupler part all goes well (or at least, much better).  Apparently the surface attachment node size, although invisible, influences joint strength of radially attached parts.

1 is too small.  3 is too small.  Tried 6, 12, 50; all too small.  250 seems to work.

A small seam does still open up between the two fairing halves after I drop the first two boosters.  Strutting the halves together helps a little, but I need to figure out why those boosters are apparently so key to the rigidity of my craft.

I also realize integrating the decoupler modules into my (very heavy) booster tanks is unorthodox and could be causing wonkiness (although the boosters work great on small test vehicles, even with the original node_attach size 1).

Edited October 21, 2016 by Fwiffo

[Plusck]

This might sound stupid but... isn't that actually quite normal?

It seems to me that the decouplers are attached through the fairing. While strutting through fairings isn't a problem (the strut simply ceases to exist when it blows), the decoupler is (I think) modelled as a part and can easily impact things. Here, it's violently blowing up inside the skin of the fairing.

Of course, I have no idea why making the node bigger helps - unless you're also offsetting them out so that they aren't actually touching.

Still, I'd bet that if you created a star-shaped beam structure around your Mammoth, put the decouplers and boosters on that, then had only struts going through the fairing, it wouldn't blow the whole thing up like that. You might need a few more struts to keep it together, but I suspect you'd hardly notice if you put a ton of them on. You already have the separatrons so the boosters should still peel off properly.

[Vaporized Steel]

@Fwiffo

Great that we can rule out the staging.

Procedural fairings of that size are very heavy and create alot of drag. On top of that, all the side boosters are hanging on the fairing.
 

So the sideboosters may be well strutted, but the fairing base fails to hold that weight.

So why does the disassembly happen when you stage the first boosters?
As far as I can tell it has 2 reasons.

One reason is the less obvious one which is change in G force by differentiating TWR when you drop the first side boosters. This might be the final trigger to shock the construction into disassembly.

The other more obvious reason: The tanks are also strutted to one another in a circle around the fairing. This actually strengthens the integrity of the fairing. By the looks of it you have 14 boosters attached that are strutted to each other in a 2x7 symmetrical setup. When you stage the first set you basically have 2 groups of 6 boosters that strut each 6 boosters on either side. But those 2 groups of 6 are not connected anymore because the tanks you dropped on the first stage held them together.

So now you have alot of weight hanging on both sides of the fairing, but both sides hang loose. So there is suddenly a large peak in centrifugal force on either side at the base of the fairing holding both halves because 1750tons is pulling on either side of the fairing. That in combination with the change in G force due to staging is what is causing the disassembly by the looks of it.

The solution to this.
You can procedurally increase the size of the fairing base up to a very large width. Stretching a procedural fairing base in it's width increases the joint attachment strength of the fairing base and may increase the bar to how much stress the fairings can take. So try to create a larger fairing base (as large as you can to rule out this posibility)
Maybe even strut the fairing walls to the fairing base, and the fairing base to a root or grandfather part (mammoth engine). Strutting the fairing walls should be possible with procedural fairings. But it's a long time since I used the mod as I'm rather pleased with stock fairings and not much into RSS lately which I mostly use this mod with.

As for the procedural fairings mod, it had and probably still has bugs. I can tell you I had problems with the procedural fairings mod a long while back in v0.25 causing parts ripping of when seperating them. This is another case entirely, but the point is that the mod did have bugs, and might still have bugs.

Sorry for the long and detailed input, hope it gave some ideas or better yet the solution:d





 

 

 

Edited October 21, 2016 by Vaporized Steel

[Capt. Hunt]

yeah, even worse, the two boosters you're ditching first are right on the seam between the two halves of the fairing.  Those two should probably be the last to go.

Also, I should point out that at this size, the faired payload is probably just as unaerodynamic as the un-faired payload.

[Fwiffo]

Ok I'm starting to chew on a theory of what's going on.  I think my decouplers (more accurately, my "self-decoupling boosters") were sunk too deep into the fairing.  Maybe when they split off, too much of the detaching part was still "inside" the fairing and exerted an outward shear force onto it causing the other boosters to break off but not causing the fairings themselves to detach.  (Not entirely sure, since no structural failures or collisions are recorded in the Flight Status log.  Do collisions get recorded if they originate inside a part's collider?).  Increasing node_attach probably made all the other booster joints strong enough to withstand this.

What I do know:  Pulling the boosters away from the fairing slightly to introduce a generous gap between the butt end of the decoupler standoffs and the outer surface of the eggshell seems to fix things (even with node_attach = 1).  Although the "floating decoupler" doesn't achieve the aesthetics I wanted.

Positioning a booster close to the clipping threshold (although visually not quite touching) causes a reduced effect where only boosters on the same fairing half fall off.  This incrementally changing behavior as you adjust the clearance seems to support my theory.

Using separate, "non-integrated" decouplers also alleviates the problem (even if their standoffs are sunk slightly into the fairing, and even when using a decoupler that's part of a smaller, lighter weldment).

In a nutshell, I suspect this is an issue in my booster/separator part (or using it with those huge fairings).  I've successfully included separators in weldments before, although never with the TT-70, and never in such a heavy weldment.

It's not a perfect theory and my gut feeling is I still don't 100% understand everything going on, but based on experiments I think I might be on the right track.  I've been trying to reproduce the behavior on smaller, simpler craft but haven't managed to achieve a more isolated demonstration yet.

I just noticed there have been a few replies since I started writing this.. will respond momentarily.

------

@Plusck I think you are bang-on.

@Vaporized Steel and @Capt. Hunt you guys also raise very excellent points.

I realize I might simply be asking too much of these poor procedural fairings ;-).  Will try some more things out based on your feedback.

I'm tremendously thankful to all of you for taking the time to have a hard look at my screenshots and posit ideas!

 

31 minutes ago, Capt. Hunt said:

I should point out that at this size, the faired payload is probably just as unaerodynamic as the un-faired payload.

LOL you're probably right.  But hey, they're aesthetic.  There's something fun about driving a bit egg up into the sky and cracking it open.

Edited October 21, 2016 by Fwiffo

[Fwiffo]

@Vaporized Steel, @Plusck, @Capt. Hunt

Found another way to skin the cat.

I had trouble closing the stock fairings to my tanker structure, but it turns out an upside-down Procedural Fairings interstage adapter was able to do the trick quite nicely!  Way less cost & mass, and none of the structural problems of mounting boosters to a fairing.  Even flew this thing at timewarp without it blowing apart.

Spoiler

 

[Fwiffo]

Picking my workaround as "best answer", but I wish I could mark all of the helpful posts here as great answers :-).