Unsual pulling

[technion]

Hey guys,

I'm having some trouble with my transporter. The bottom part of this is my "traditional" transport. The top part is a booster I built (because 40 minutes burns were a problem).

The odd thing is, if I burn just the three bottom rockets (as below) it works fine. If I burn just the top three, it also works fine. But fire all six, and it pulls to one side quite strongly (even with three ASAS modules there). This contradicts my understanding of weight or thrust, or whatever is causing this. Any advice appreciated - I've pulled three nukes on this mission I don't seem able to use.

[Sirrobert]

I have no idea on the pulling, but I can suggest you to add a 4th engine to your bottom stack, using this part http://wiki.kerbalspaceprogram.com/wiki/TVR-2160C_Mk2_Stack_Quad-Coupler

That should help with the long burns a little bit

[Dogface]

I had a similar problem in a completely different set of facts. And I never figured it out. So, I sympathize.

I will say, in your case, when you light the second set of rockets you are completely changing the Center of Thrust. Which is probably contributing to the problem, whatever that may be. How many SAS/Reaction Wheels do you have on there and where? It looks like there is one between the two rocket clusters (and where the new CoT would be).

Edited November 23, 2013 by Dogface

[technion]

Thanks guys, but I think we can chalk this up to a bug. Specifically, I made the burn the "long" way, then when all remaining fuel was in the top section, I went to undock the bottom one. Only, I got the "can't undock" bug. After undocking the top section, then redocking it just to fix said bug, suddenly this problem went away...

Dogface, answer your question, I have one ASAS between the two orange tanks, one right under the lander at the top, and one on top of that monopropellant tank you see. I realise these were probably overkill, but each section that went up and docked had its own SAS and I never planned a method of removing them.

[breakthrough]

Solar panels blocking the thrust of some of the upper engines causing it to be asymmetric. If the thrust is 'blocked' it provides no acceleration at all and it can be blocked by a surprisingly small obstacle that is quite a distance from the nozzle.

[Tex_NL]

This is a VERY common problem and it has a equally simple answer.

Your three upper engines are place in front of the center of mass. Control surfaces reverse their behavior when they are in front, engines should reverse their gimbal too but they don't. As a result they steer left when they should steer right and up when they should go down.

Since you can not reverse the gimbal your only remaining option is to lock them; right click the engine and select 'Lock gimbal'

[DeMatt]

Yeah, I'm pretty sure it's both those issues - from what I can see, you have two solar panels attached to the upper orange tank instead of three, so I think the third got blown away by the nuke above it. And then you have the off-center thrust from that being further amplified by the reversed gimbals. Using just the upper nukes, I think it blows all three panels away before the bad gimbaling kicks in.

[technion]

After finding the top Eve ascent vehicle wasn't able to ascend from Eve, I'm about to launch this whole thing again. I'll shift the panels around and lock the gimbals, and report back on the next trip.

[cr055H41rz]

Post close ups of problematic engine(s).