I have a stack symmetry issue with this part.

[Galane]

What I've figured out is that something in the cfg is not allowing decouplers hung off the bottom to correctly all trigger at once. It's an upsize of the same part in PolecatEZ's original ReStock pack. This one is for mounting 4 Kerbodyne, 1 Rockomax and 4 LV engines to the bottom of my upsized ThunderMaximus tanks.

What happens is if four Kerbodyne engines are on the bottom of this, then four Kerbodyne inline decouplers are on the bottom of those and all put in the same stage they will not all trigger together. They go one at a time and things get messy and explody. Even worse is if more Kerbodyne tanks and engines are asparagus mounted to the sides of the tanks below the engines on the bottom of this adapter, they inherit the not going all together problem.

I noticed a similar "round robin" effect in August 2013 with a stock quad coupler (these jumbo ones are just upsized massively from it) and used it as a feature for staging my KETM kinetic kill vehicles. To get the desired sequential staging I had to adjust the individual staging of the KETM decouplers to match whatever the numbering scheme is.

The problem now is I *do not want* any such effect. There should not be such an effect when all the decouplers in a stage below this part are all put in the same stage - but it is acting as if each decoupler is in its own separate stage.

Hmmm. Now that I think on it, I have had issues doing quad stack stages off the underside of this part in its stock form and in the Rockomax upsized version in original ReStock. Very explosive because only one decoupler triggers, the next stage's engines quickly destroy the other three decouplers and blow the lower tanks etc away.

But on no other rockets has this trickled down to other decouplers on stacks mounted to the sides of stacks directly below the quad coupler. That's only shown up with the heavy Kerbodyne parts and the stock quad coupler upsized to work with them.

Looks like a stock KSP bug that gets worse with larger and larger parts. Now how to fix it?

Edit: Another thought is that it might have something to do with the additional five nodes on the bottom. I'll have to build a test rocket using the original quad coupler and see if it exhibits the same one at a time release of four decouplers in the same stage.

PART
{
name = thunderBiQuatro-2
module = Part
author = Squad, PolecatEZ, Galane

MODEL
{
   model = Squad/Parts/Structural/adapterLargeSmallQuad/model
   position = 0, 0, 0
   scale = 3.25, 3.2, 3.25
   rotation = 0, 0, 0
// parent = anotherModelTransform	<---------Not necessary unless Second or subsequent part.
// texture = model000 , Squad/Parts/Command/probeCoreOcto/model000
// texture = model001 , Squad/Parts/FuelTank/fuelTank2-2/model001
}

scale = 1.72
rescaleFactor = 1.0

// --- node definitions ---
// definition format is Position X, Position Y, Position Z, Up X, Up Y, Up Z
node_stack_top = 0.0, 0.0, 0.0, 0.0, 1.0, 0.0, 3
node_stack_bottom01 = 1.25, -1.4, 1.25, 0.0, 1.0, 0.0, 3
node_stack_bottom02 = 1.25, -1.4, -1.25, 0.0, 1.0, 0.0, 3
node_stack_bottom03 = -1.25, -1.4, 1.25, 0.0, 1.0, 0.0, 3
node_stack_bottom04 = -1.25, -1.4, -1.25, 0.0, 1.0, 0.0, 3
node_stack_bottom05 = 0, -1.4, 0, 0.0, 1.0, 0.0, 2
node_stack_bottom11 = 0, -1.15, 2, 0.0, 1.0, 0, 2
node_stack_bottom12 = 0, -1.15, -2, 0.0, 1.0, 0, 2
node_stack_bottom13 = 2, -1.15, 0, 0.0, 1.0, 0, 2
node_stack_bottom14 = -2, -1.15, 0, 0.0, 1.0, 0, 2

stackSymmetry = 3

TechRequired = veryHeavyRocketry
entryCost = 8800
cost = 26600
category = Propulsion
subcategory = 0
title = ThunderMaximus-Rocko 4x4 Adapter
manufacturer = AeroKerbin Thunder Division
description = Ideally, designed for 4 Kerbodybe-sized engines and 4 smaller gimballing engines plus one Rockomax size engine in the middle in close quarters. Includes built-in torque support at a lot of extra weight. The engineers (and janitors) at every other aerospace firm on Kerbin had the same thing to say about this component, but what they said is mostly un-mentionable in polite company.

attachRules = 1,0,1,1,0

mass = 7.5
dragModelType = default
maximum_drag = 0.2
minimum_drag = 0.3
angularDrag = 2
crashTolerance = 600
maxTemp = 2900
fuelCrossFeed = True
breakingForce = 1e37
breakingTorque = 1e37


MODULE
{
	name = ModuleReactionWheel

	PitchTorque = 38
	YawTorque = 38
	RollTorque = 38

	RESOURCE
	{
		name = ElectricCharge
		rate = 0.6
	}
}
}

Edited June 18, 2014 by Galane

[Galane]

After various tests using all stock parts and with the 9 engine mount - the problem is definitely the 9 engine mount in both the ThunderMax and ThunderMaximus sizes. Somehow it causes the Kerbodyne inline decouplers to not function correctly and when mounted upside down so they can stay with top stacked tanks that get dumped, they act like stack separators, coming loose from both sides but putting a lot of force downward from their normally top side.

For one test I put the tank drop stage as the first stage (so it wouldn't launch) and when triggered only one of the four Kerbodyne decouplers triggered, coming loose from the tank above it, but not activating the separatrons.

So I hit the spacebar again and the separatrons on *just that tank* activated and flew the tank away. Three more hits on the spacebar triggered each of the other three Kerbodyne decouplers in turn *and* activated the separatrons attached to each tank.

So, what the heck is with the cfg file in the first post that is causing this sequential triggering of four decouplers in a single stage, which ought to all go at the same time?

[Galane]

Forgot to mention I found the issue with using the Kerbodyne parts. Squad has made the big decouplers physics-less in flight. Find the CFG file for the part and delete the Physics Significance line, or change the 1 to a 0 or put // in front of the line to comment it out.

When I found that in .24 I told Squad about it but they've kept that in there. still there in .90. Rather cheaty to have those large decouplers having no mass! It also causes plenty of problems if you're using Kerbodyne parts in anything but single stack rockets or as a single core stack of an asparagus rocket. Don't even think about top-staging drop tanks with those upside down until you fix the Physics Significance line.