Large ships become unsteerable

[4l4b4m4m4n]

Hey Forum,

i have a problem: when i build a orbiter/lander docked combination, the ship gets impossible to steer. in the launch phase i cant steer the rocket, after the launch i cant steer into an orbit. doesnt matter if i try to steer per rcs or rocket vector. sometimes it moves slowly back and forth, but i cant move it into a specific direction. i had this problem with a previous ship too. i have the feeling no matter how much struts i place, it doesnt change the fact. the ship gets somewhat less wobbly, still, i cant steer it. maybe someone can point me into a direction how i can solve the problem?

[unwisemonk]

It would help if we had a picture of said rocket.

[agentexeider]

use SAS and ASAS rocket parts, one ASAS and several SAS parts, what these do is add gyroscopes to your rocket allowing you to rotate without using RCS, ASAS will allow you to stabilize and lock your orientation. which you can turn on and off with the T key.

regarding RCS placement, you need to place the RCS around the center of gravity, but make sure it's the center of graviy that the rocket will have when it gets to orbit. The position of the CoG changes as you add or remove mass to the rocket, and RCS thruster towards the ends of the rocket have less turn rate then those on the CoG. Ideally, you should design every rocket stage to it's CoG and turn on and off the RCS thrusters quads based on which stage and CoG you have.

you can organize your thrusters in the Action Groups menu in the VAB.

Edited May 11, 2013 by agentexeider

[4l4b4m4m4n]

thanks for the quick reply. i know about asas, sas and rcs thruster. i will try the trick with the rcs thrusters in actiongroups. the problem still remains, that i cant steer my rocket launch stage. its a standard rocket i designed for lifting various parts and 15 tons more mass then my comboship needs and its perfectly steerable in all stages. its just strange, its this shaking back and forth, when i have this type of combo ship going. i even have a spacestation with sort of random placed rcs thrusters and multiple docked parts and it can rotate just fine (with a little wobble of course).

[Uberick]

Images might help in analyzing the problem, but without that I would recommend:

ADD MOAR STRUTS! if your rocket is just unsteerable it might be because it isn't stable enough, and the rocket is wobbling itself to death. To check, when you put it on the launch pad, does it wobble around or does it stay stable?(watch the nav ball) If it's wobbling on the pad, try adding some more struts.

Try adding some canards to it, these can give you some control of your rocket in the ascent stage.

Check your rocket balance, make sure that everything on the rocket is symmetrical. . anything that is imbalanced in even the slightest way is going to make your rocket torque one way or the other.

Hope some of this helps man, happy launching

[falofonos]

Is the control pod in the thrust section or the lander section?

If lander, is it facing down?

[4l4b4m4m4n]

each has its own control pod. the ships a coupled at their noses and the lander pod is facing against the orbiter pod. maybe i should turn one pod around?

[Mandarani]

Kinda sounds like you forget to turn off your ASAS before trying to turn?

[Crown]

What I experienced is that sometimes struts itself can cause uncontrollable tumbling.

I launched a station into munar orbit and left the injection tank+engine attached. Attached with a docking port and struts. The station tumbled. After decoupling the injection stage and with it mostly all the struts, the station stayed stable where I had put it.

[falofonos]

each has its own control pod. the ships a coupled at their noses and the lander pod is facing against the orbiter pod. maybe i should turn one pod around?

Just make sure you select "control from here" on the pod that's pointing up. I had a long session of banging my head on the desk trying to figure out why my rocket on mechjeb ascent assist kept turning and crashing itself. Eventually realized the primary control pod was facing the wrong way.

Winglets near the bottom of the lifter may give more control in the first 30km of ascent. After that, RCS will have to do.

[4l4b4m4m4n]

Just make sure you select "control from here" on the pod that's pointing up. I had a long session of banging my head on the desk trying to figure out why my rocket on mechjeb ascent assist kept turning and crashing itself. Eventually realized the primary control pod was facing the wrong way.

Winglets near the bottom of the lifter may give more control in the first 30km of ascent. After that, RCS will have to do.

ok, i'll check out to control from the different possible points. if that doesn't work, i will make a video to better visualize the problem.