Advice on how to avoid catastrophic wobble

[PTNLemay]

I noticed that when my rocket gets perticularly tall, I get an unavoidable wobble. It dances around like a piece of wet spaghetti.

I've used generous amounts of RCS thrusters and SAS modules. I even used structural reinforcement bars running from top to bottom to help alleviate the vertical strain, but I still get a lot of wobble... Is there another trick I could use? Or is there just a fundamental limit on how big rockets can be made in Kerbal? I'm only at 40 meters tall I think... I was hoping to go much much taller.

Edited August 4, 2012 by PTNLemay

[Atimed]

The rocket it self is unstable, the higher the engine stack the more wobble it will have..

- Not sure about moded parts but in stock it usually works to avoid stack higher than 4 tanks..

- To strut at least in 3x symmetry decouples to higher stages.. Because those are the weakest points in your rocket..

- Avoid thrust vectoring engines if possible..

- Test the rocket with some Winglets, they might make things Better but they might make it even worse..

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Hope it helps..

[PTNLemay]

It's the Nova Punch pack, just for the record.

[Endeavour]

For now, I think wider rockets are far better that taller ones because of the way drag works. So for the time being, don't use tall rockets.

[N3X15]

Brace it with struts. If all else fails, prop it up with more boosters.

[Randox]

These are my two main rockets at the moment, standing at 92 and 94 metres, and while they certainly have a bit of wobble during launch once they are down to the middle 3/middle engine, but it's not catastrophic. The 94 metre rocket does have an issue where without some fuel modulation once it's down to 4 engines, it will crumple under the high jerk forces that the gimbaled engines create (it can crush one of the fuel tanks, which normally destroys everything as the engines and remaining tanks fly through everything above them). It has even more struts that you cant see bracing the centre stack to try and combat all of this.

The shorter one looks flimsier, but all the orange units you see stuck to it are lander SAS units from a mod, each with 6 force available, and I suspect they help try to hold everything together.

If you can build your rocket in a pyrimid shape, so that it gets wider towards the base as it gets taller, that helps, though as you can see I've also shortened both rockets by strapping tanks to the side further up. Some iterations have seen wings on those tanks, which can also help.

Sorry, but I can't give you any specific points for nova parts, as the only nova part I have installed is the lander ASAS module.

[CrashnBurn]

Because drag isn't correctly implemented yet, I agree with Endeavour, wide staging is far better than tall. However if you look at my howto on tension/compression and mass damping, you CAN build tall rockets successfully, but it works best if you use the fixed large decouplers from the mods forum (the current large decoupler has exactly the same strength as the small decoupler, an oversight, I believe, on the developers part, as it needs about 8-10x the strength to work properly)

Overall, at this time, staging outward instead of downward is far more efficient, unless you use special techniques to support your rocket.

[Randox]

Because drag isn't correctly implemented yet, I agree with Endeavour, wide staging is far better than tall. However if you look at my howto on tension/compression and mass damping, you CAN build tall rockets successfully, but it works best if you use the fixed large decouplers from the mods forum (the current large decoupler has exactly the same strength as the small decoupler, an oversight, I believe, on the developers part, as it needs about 8-10x the strength to work properly)

Overall, at this time, staging outward instead of downward is far more efficient, unless you use special techniques to support your rocket.

Can you link us to the new decoupler. I tried and failed to find it, and I would certainly find this useful, since I tried looking in the part itself, and have no idea how to change the strength of it.

[togfox]

Wider rockets are easier to build but i prefer tall because they look like rockets.

Use verticle and horizontal struts plus symmetry all over and it should be fine.

[Clouds]

A shape that I've used to help against wobble is to put some long bracing pieces (eg. airplane tails) going out sideways, which gives a diamond shape when you put struts going from their tips to the top/bottom of the rocket, and gives the struts a much better angle for holding the rocket in line.

The symmetry function is very fiddly so you might have to attach it to a single piece (like the structural fuselage piece in my example picture) and then use THAT to attach it symmetrically.

As a bonus, this gives you a long arm for attaching some RCS nozzles where they will have higher torque.

[Fendleton]

Generally I have no trouble with my larger rockets unless I'm doing something seriously wrong....I build my rockets tall, rather than wide (otherwise my laggy computer tends to result in ASAS hurling them into the tower), and by using structures under tension you can make them perform pretty well.

Like Clouds, I personally use struts and "outriggers" to stabilise really long rockets, or keep the G-loads right down. Preferably both, since I'm using unmodded large decouplers. The idea is to have it wired up like old aircraft: Whichever way the rocket tries to bend, there's a strut/wire pulling it the other way. So have 4X, 6X or 8X fins at the midpoint of the wobble, attach struts to them from tanks well above and below, and make sure the fins themselves can't rip away. It's not brilliant as a solution if used excessively, the lift can destabilise rockets during gravity turns, but usually it works fine. Struts really don't work well when you try compressing them, or subjecting them to loads that are off the axis of the strut. By making the strut more horizontal, you convert more of the load to tension along the strut, and make it a more effective reinforcement.

[Randox]

Just in case anyone reading this was looking for the stronger decouplers, they can be found here: http://kerbalspaceprogram.com/forum/showthread.php/15491-16-Fix-for-weak-large-decouplers-plus-reduced-wobble