How to prevent gapping under TR-18A decoupler?

[kfsone]

KSP 1.0.4.861, Windows 10 x64 via Steam. First time install. No mods.

If you put weight under a TR-18A, you get a weird hole in your rocket that causes strange behavior. If you put less weight, the TR-18A seems to be a major cause of bending/toppling of rockets. It's easy to repro:

. Command pod,

. TR-18A decoupler,

. FL-800 tank,

. Reliant engine, <--

. TR-18A decoupler <-- problem,

. FL-800 tank, <--

. FL-800 tank,

. Tricoupler,

. 3 Reliant engines,

. 4x Launch Stab Enhancer attached to middle of upper FL-800.

. Ensure rocket is several meters off the ground

--> Launch

Pan the camera and you'll see a clear, visible gap between the lower decoupler and the 800 tank below it.

.craft file

And here's a video of the problem (137Mb mp4), notice also that once the craft takes off, there is a weird "pump" action at the seam when I kill the throttle and restart it...

Depending on how bad the hole is on a given rocket, various different problems occur from the pump shown to tipping or I even had one rocket where the top slid off during launch.

I'm fairly sure it's a bug but I thought I'd enquire first. I didn't see anything in the known issues list that seemed to match this description, but I'm fairly new so maybe I just don't know the right terminology?

Edited November 8, 2015 by kfsone

[Mikki]

Hi OP

Consider this a the normal gameengine behaviour like bending or wobbling... nothing unusual here i think:wink:

And welcome to the forums!

Joint strength is a main feature of this physics based non-rigid body sandbox simulation, a mod called Kerbaljoint reinforcement applies stronger jointstrenghts for very large builds if desired...

[kfsone]

Hi OP

Consider this a the normal gameengine behaviour like bending or wobbling... nothing unusual here i think:wink:

And welcome to the forums!

Joint strength is a main feature of this physics based non-rigid body sandbox simulation, a mod called Kerbaljoint reinforcement applies stronger jointstrenghts for very large builds if desired...

My first instinct was that the gap was just a visual anomaly, but then I've seen too many different behaviors that aren't consistent with how other couplers work.

I found this because every rocket build I tried was tipping, trying to launch a slimline rocket with a scanner into kerbin-polar orbit; "OK, I'll try a different CoM" I thought, same result. Etc, etc. Until I replaced it with the Rockomax decoupler (which is oversized for the rocket). When I switched back, that's when I noticed the gap.

It doesn't *always* happen, but when it does, the rocket fails during takeoff - tipping, wobbling or slipping. When it doesn't (rare), then several of the rockets I'd previously written off work fine, even if I am less than delicate with them during takeoff.

Edited November 7, 2015 by kfsone

[Alshain]

It's not a visual anomaly, it's a normal physics behavior. You have 3 large LFO engines under that thing plus 3 fuel tanks and your support clamps are above it. It is quite literally being pulled apart by it's own weight. KSP is quite forgiving that it doesn't break. I usually mount my clamps to the top of the first stage as decouplers and docking ports are a bit weaker than tank nodes. Now perhaps the gap is not the best way for the game to illustrate this, but I'm certain that is the cause.

You can actually simulate this with engines too, I've seen that gap occur because engine thrust pulled away too strongly. Despite the gap, it probably will not break. I know docking ports have an abnormally strong magnetic hold even after this gap starts to appear.

Edited November 7, 2015 by Alshain

[kfsone]

It's not a visual anomaly, it's a normal physics behavior. You have 3 large LFO engines under that thing plus 3 fuel tanks and your support clamps are above it. It is quite literally being pulled apart by it's own weight. KSP is quite forgiving that it doesn't break. I usually mount my clamps to the top of the first stage as decouplers and docking ports are a bit weaker than tank nodes. Now perhaps the gap is not the best way for the game to illustrate this, but I'm certain that is the cause.

You can actually simulate this with engines too, I've seen that gap occur because engine thrust pulled away too strongly. Despite the gap, it probably will not break. I know docking ports have an abnormally strong magnetic hold even after this gap starts to appear.

Great info, thank you!