Question: How to make my rocket less wobbly

[cmheisel]

I've got a rocket (see the craft file below) that seems to have a terrible case of the wobblies.

During ascent when it's four boosters are attached it wobbles about it's center of thrust. The center engine gimbals around in a circle and the top of the stack wobbles around.

I've set action group one to disable the outer 4 engine gimbals during ascent, but even with that it's still so wobbly it sometimes goes way off course even with mech jeb piloting it.

Any ideas how to counteract the wobblies? I'm using mechjeb, the spherical fuel tank, and KW Rocketry mods.

https://www.dropbox.com/s/72ltb55v5489dct/Hercules%20Pegasus%20IIc.craft

Edited June 22, 2013 by cmheisel

[Nalin_Airheart]

Have you tried a minimum of two struts on anything that isn't the center? Also secure the spherical tank with four struts on the top and bottom because that might be rolling around. Sometimes designs are just wobbly by nature so try a different setup if you have the patience.

[DChurchill]

Struts are your friend. Especially when you have a top-heavy monstrosity and don't feel like redesigning it.

[Thrundar]

I concur / agree with Dchurchill more struts (space tape).

[Spyritdragon]

Struts are like duct tape for KSP :-)

[Sirrobert]

Struts are your friend. Especially when you have a top-heavy monstrosity and don't feel like redesigning it.

http://i.imgur.com/5TveiWK.jpg

Damn that's a monster

[Uberick]

I highly recommend struts. There are very few structural problems that a liberal application of struts cannot solve.

Also, sometimes if you add some RCS or fins to give you more control in addition to engine gimbal, that can help(just put RCS on your droptanks so you don't waste too much energy carrying it into space.)

[cmheisel]

Thanks for the tips, I'll try struts and winglets next.

Would center of mass or something similar be an issue because my orbital stage up top has three NERVA engines hanging off the side _and_ my strapon liquid boosters are 4 tanks around the core. Didn't know having mass on the sides at the bottom, and mass on the sides up top but the middle being "bare" might have anything to do with it?

[cmheisel]

Thanks for the tips everyone. Struts were the answer! Turns out I didn't have near enough struttage between the core stage and the strap on boosters. After I switched it to asparagus staging I saw the problem.

Without the struts the center stage engine was gimbaling and in turn the entire center tank was picking up that oscillation.

Hooked it up at 4 places to each strap on booster, added winglets and it flies like a charm!

[DChurchill]

Damn that's a monster

Yeah, I need to start over with it. It does it's job, getting Kerbals to Mun and Minimus with a car for them to drive, but it's not very efficient or elegant. Too top-heavy. Even with all those struts, it flexes like crazy.