unstable rocket?

[deepspacecreeper]

hello, why does my rocket always tip-over to the right without my command? i've checked the center of mass and thrust. but it still tips over to east 90 degs. hdg. Always 90 hdg. y?? (i've tried using normal sas and even used mechjeb's smart a.$.$.)

Edited October 21, 2013 by deepspacecreeper

[Awaras]

Posting pics or a .craft file would help. It might be fuel drain imbalance...

[deepspacecreeper]

im using asparagus staging, i'll check my fuel lines. when i have time i will upload the craft file and the pic of the rocket.

[Awaras]

Also, when exactly does it start to turn by it self? As soon as you launch? At a certain altitude/velocity? After staging?

[deepspacecreeper]

few seconds after clearing the launchpad

[SRV Ron]

few seconds after clearing the launchpad

Check to make sure you don't have a second booster section clipped into an existing one. I had that happen which caused the imbalance that will flip a rocket over in seconds upon launch. Disassemble and reassemble to fix.

[UnthinkableThinker]

I don't know if it is too much to ask if it's simmetrically built.

[deepspacecreeper]

I don't know if it is too much to ask if it's simmetrically built.

i said in the beginning of the thread that i checked the centre of mass. of course i used symmetry to achieve this.

[SRV Ron]

i said in the beginning of the thread that i checked the centre of mass. of course i used symmetry to achieve this.

Check where the center of thrust is to make sure that it is in the center core and not off to one side. Rotate the camera view to make sure that the COT is not wobbling to one side or the other. If that is OK, the instability problem may be related to twisting of a booster during launch which will require bracing to fix.

Example, this design would spin out of control if it didn't have a SRB break off during flight. Once braced as shown, it flew properly with only the one SAS stabilizing it.

[hobbsyoyo]

If it isn't a fuel/tank imbalance, make sure you have enough control surfaces such as winglets to control your craft's attitude (i'm guessing it's too big for additional reaction wheels/rcs to make a differenc though fins will help a lot).

Also - turn off the thrust gimbling on your outer core of engines with action groups. If all the engines can gimbal, their combined thrust vectoring (particularly if using mainsails) can overwhelm the SAS's ability to hold your course. It will ask for a small course correction and get 12MegaNetwons of force applied at a 45 degree angle which it then has to correct with a massive swing of the engines the other way, which it then has to correct and so on and so forth. Leave the gimbal on in your central stack only.

[Gnipu]

Oh, it's so frustrating when you can't find the source.

Obviously it's not CoM. Stages isn't faulty (I usually just look underneath the vehicle after launch to see if everything is thrusting (symetrically)). Drag shouldn't be a problem since you built it symetrically.

Is the navball normal, i.e. facing upwards?

A few times I've built a probe rocket with uside down probe and launched with MechJeb ascent guide, and it automatically turned the whole thing upside down due to it thinking it was facing the wrong way. (to fix this, "control from" a part that is facing upwards)