Long ships start to wobble until breaking apart... help!

[Wurmfist]

I'm sure this has been answered and maybe I'm just a little searching-unsavvy but I could not find anything here on it, although I'm certain I'm not the only that has experienced this. When building long ships in orbit as soon as I attach the front finished module to the build the entire thing start to wobble and jerk catastrophically until the ship breaks apart. Even with everything turned off (SAS, torque, etc.) it still does it. Suggestions? Seems odd that this happens because when I build large spacestations, the station does not exhibit this issue.

Edited February 27, 2014 by Vanamonde

[Starstrider42]

I haven't seen this specific issue, but KSP is known to be pretty buggy with large ships. Have you installed Kerbal Joint Reinforcement (http://forum.kerbalspaceprogram.com/threads/55657-0-23-Kerbal-Joint-Reinforcement-v2-0-2-4-14)?

[Dangerous_Beans]

turn off SAS or Smart ASS just before you dock. SAS/Smart ASS is trying to keep the ship still, but because the ship is so long the torque causes it to bend, and the SAS/Smart ASS reacts by applying more SAS torque. if you hit the right length, weight, and SAS/RCS/control surface distribution you get ships that just wobble themselves apart.

for long, micro gravity ships i suggest you design them so each part has RCS thrusters, and use them to move it around. also make sure you take your time, taking a minute to turn 180 degrees is entirely reasonable.

[Jouni]

I've never had problems with large ships when not using physics warp, so these are just generic questions.

- Are you using part clipping? It can sometimes confuse the physics engine.

- Are you using any other mods than MechJeb? They may be the source of the trouble.

- I've never had any problems with SAS during docking, but Smart ASS may behave worse. Have you tried disabling it?

- What was your final approach speed during docking? I usually try to keep it below 0.1 m/s.

- Do you have a larger picture of the ship where we can see the individual parts?

[Wurmfist]

Thanks for all the help. The pic isn't entirely accurate because I was trying to turn the ship in the pic, but it lasted about 20 seconds before it flew apart. 2 out of my 3 attempts, as soon as I docked the front module, it immediately started wobbling and broke apart, and again, that is with everything turned off, everything, mechjeb, SAS, gymbals, reaction wheels, everything. Other than a couple KAS winches and the mechjeb, everything is stock. I currently don't have any other pictures because I got frustrated and just de-orbited the entire thing.

I have not tried the kerbal joint reinforcement, I will look into that, thanks.

I know the long ships can be done, LOL_Lobster on KSP TV build very long ships and seems to have 0 problems with them, and he plays completely vanilla.

[Kasuha]

My experience with long ships: put engines in the front and disable gimbal on them. KSP works much better with pulling than with pushing.

Also large docking ports (Sr) are more stable than small ones.

The example below is my ship for the Jool-5 challenge (main engines somewhere around the middle) and I did not have any wobbling issues until I decomposed (and lost struts on) the science module at the top which was built using some docking port Jrs. But turning it around was kinda pain.

Edited February 26, 2014 by Kasuha

[MarvinKitFox]

With a long ship, you want *precise* radial symmetry of mass, and as stiff as possible. Because rockets are under very high stresses, but these stresses are applied and removed slowly, and virtually *all* along the mass axis. You turn ships using SAS.

With a long station, you *dont* want stiff. You need reasonable radial mass balance around the docking port.

Let me repeat.. not stiff! You *want* squishy parts to absorb and dissapate the shock-stresses of docking.

(remember, you are shoving a 120-ton hunk of steel into another hunk of steel. If both are rigid, you are making one almighty CLANG)

Stations are subject to very low sustained stresses, but with short sharp thumps from dockings.

You turn stations, and ships attached to stations, using RCS not SAS.

In your sample picture, you have smart ass switched on.. That is not only SAS, but SAS with Parkinson's!!

Lose the sas, dock, let it settle down all the way. Timewarp to get rid of any further wobbling. THEN switch sas back on

Edited February 26, 2014 by MarvinKitFox

[Kryxal]

Here I was just thinking he was cranking everything hard over. Roll is all the way to the right, yaw all the way to the left, and there's a moderate pitch-down. Might this be from a sudden change in the "control from" point's bearing with Smart ASS active?

[Wurmfist]

Thanks all for the suggestions. I was just experimenting with the Smart ASS, it would still wobble and break apart even with everything, and I mean everthing, turned off. It seems to not be a problem if only one side is connected so I will just have to rethink a bit in my design. Thanks again for all the suggestions, much appreciated!

[Wurmfist]

Also, I would change this thread to "answered" if I knew how to do that.

[Vanamonde]

I would change this thread to "answered" if I knew how to do that.

I did it for you, but if you need to do it again, go to edit the first post in the thread, choose "go advanced" options, and you'll see you can change it there.

[snepel]

I think using the broomstick analogy might help you out:

Consider a broom that you try to balance on the palm of your hand.

The palm of your hand being the engines of your craft.

If the broom brush is at the top, it's easy to balance: so the center of mass must be far away from the center of propulsion.

Other way around, it will start to wobble.

This is a very helpful analogy and in my experience it really does behave this way in KSP.

Hope it helps!

source: http://wiki.kerbalspaceprogram.com/wiki/Tutorial:Intermediate_Rocket_Design

Edited February 28, 2014 by snepel
added source