Why does this rocket keep wobbling?

[HoloYolo]

http://imgur.com/a/Hq1TM Pics for clicks.

Some basic info, 1.2 Prerelease, No mods, Latest build. Forgot to put COM on, but it's right above the bottom middle fuel tank. It's obviously a rocket. I just want to know why it wobbles so much? I thought they fixed this problem in 0.23.5 (well they didn't because almost every update including then I had to install KJR)? Why is the reason it wobbles and how do I fix it? Some more stuff is that I tried putting an SAS part at the COM, but no cigar, and yes, I have struts.

[Superfluous J]

I would assume it's a mix of too-high TWR, and reaction wheels fighting engine gimbaling. Possibly with a side of engine oversteering due to minor flexing, causing a feedback loop.

[Tex_NL]

Have you tried to autostrut side boosters to the root part? Autostruts are incredibly strong.

Only one of the LV-N's is actually connecting the top to the bottom. Despite the struts this is still a relative weak section.

[Snark]

+1 to both of the previous posters.  A few more comments:

2 hours ago, HoloYolo said:

I tried putting an SAS part at the COM

1. It doesn't matter where you put reaction wheels on the ship, in terms of how effective they are.  They have equal effect no matter where they're located.  (It can matter in terms of wobble, if you have a very wobbly ship and the reaction wheels are the cause of the wobble.  I'd say that's not the case here; your wobble is due to other factors.)
2. Engine gimbal on a high-thrust ship, especially a very tall one, is so much stronger than reaction wheels that the reaction wheels have negligible effect, at least during the high-thrust portion of ascent..  You're not going to stabilize that way.

1 hour ago, 5thHorseman said:

Possibly with a side of engine oversteering due to minor flexing, causing a feedback loop.

^ This.  Untweaked engine gimbal tends to be way overpowered for very tall ships, since they have a big lever arm to work with.  I almost always end up reducing the gimbal range on Mammoth / Rhino / Mainsail / Skipper to around 30%.  Might be worth a try.

(Though admittedly, the SAS tuning in 1.2 is so much better now that this might not be so much of an issue anymore.  I haven't tested it specifically, but I have noticed that "hold prograde" during ascent with a gimbaled engine doesn't wildly spam the gimbal the way it used to.)

1 hour ago, Tex_NL said:

Only one of the LV-N's is actually connecting the top to the bottom.

^ This.  This right here.  I'd guess that this is overwhelmingly the biggest contributor to your problem.

KSP ships are a "tree" structure, which means you can't have closed loops of parts (except for struts and fuel lines).  It's not possible to mount four engines like you've shown and have them all connect at the bottom; only one will connect.  So basically, you have the entire upper part of your rocket (the whole part above the LV-N's) mounted on a single LV-N which is on a stack separator.

 

If you're not averse to using mods, then I highly recommend SpaceY, once it (along with ModuleManager) becomes available for 1.2:

Nice things about it for your use case:

• Mostly what it's about is adding big parts (ginormous 5m parts are a thing).  So if you're building a really big rocket, you can make it with a smaller number of really big parts, which makes the ship a lot stiffer.
• It also adds some really cool multi-node interstage adapters that will allow you to actually do what you're trying to do (i.e. have multiple LV-N's in-line with your bigger-diameter stack), while maintaining a stiff connection.  These are really cool parts, would love to see something like them in stock. Discussion of what they are and how they work here.

[HoloYolo]

2 hours ago, Snark said:

+1 to both of the previous posters.  A few more comments:

1. It doesn't matter where you put reaction wheels on the ship, in terms of how effective they are.  They have equal effect no matter where they're located.  (It can matter in terms of wobble, if you have a very wobbly ship and the reaction wheels are the cause of the wobble.  I'd say that's not the case here; your wobble is due to other factors.)
2. Engine gimbal on a high-thrust ship, especially a very tall one, is so much stronger than reaction wheels that the reaction wheels have negligible effect, at least during the high-thrust portion of ascent..  You're not going to stabilize that way.

^ This.  Untweaked engine gimbal tends to be way overpowered for very tall ships, since they have a big lever arm to work with.  I almost always end up reducing the gimbal range on Mammoth / Rhino / Mainsail / Skipper to around 30%.  Might be worth a try.

(Though admittedly, the SAS tuning in 1.2 is so much better now that this might not be so much of an issue anymore.  I haven't tested it specifically, but I have noticed that "hold prograde" during ascent with a gimbaled engine doesn't wildly spam the gimbal the way it used to.)

^ This.  This right here.  I'd guess that this is overwhelmingly the biggest contributor to your problem.

KSP ships are a "tree" structure, which means you can't have closed loops of parts (except for struts and fuel lines).  It's not possible to mount four engines like you've shown and have them all connect at the bottom; only one will connect.  So basically, you have the entire upper part of your rocket (the whole part above the LV-N's) mounted on a single LV-N which is on a stack separator.

 

If you're not averse to using mods, then I highly recommend SpaceY, once it (along with ModuleManager) becomes available for 1.2:

Nice things about it for your use case:

• Mostly what it's about is adding big parts (ginormous 5m parts are a thing).  So if you're building a really big rocket, you can make it with a smaller number of really big parts, which makes the ship a lot stiffer.
• It also adds some really cool multi-node interstage adapters that will allow you to actually do what you're trying to do (i.e. have multiple LV-N's in-line with your bigger-diameter stack), while maintaining a stiff connection.  These are really cool parts, would love to see something like them in stock. Discussion of what they are and how they work here.

Thank you very much Snark. Just tried it and the rocket works fine. Thanks.

[qoonpooka]

So, based on the answers in this thread, does flipping one of these over: http://wiki.kerbalspaceprogram.com/wiki/TVR-400L_Stack_Quad-Adapter

Not attach the four motors to the quad-side of things at all four points?

[Tex_NL]

22 minutes ago, qoonpooka said:

So, based on the answers in this thread, does flipping one of these over: http://wiki.kerbalspaceprogram.com/wiki/TVR-400L_Stack_Quad-Adapter

Not attach the four motors to the quad-side of things at all four points?

You can attach four parts to it. But you can not attach it to four parts.
Sounds contradictory but it is the way it is. A parent part can have multiple children. A child part can not have multiple parents.

[Norcalplanner]

7 hours ago, HoloYolo said:

Thank you very much Snark. Just tried it and the rocket works fine. Thanks.

Another handy-dandy mod which would get you out of this particular pickle is Modular Rocket Systems (MRS) also by NecroBones. It has the "quad-nuke" engine, which has precisely four times the weight and thrust of an LV-N in a single 2.5m part.

[foamyesque]

Life would be a lot easier if the damn adapters would take surface attachments for strutwork.

[Superfluous J]

I personally just don't use those adapters, except when they're at the bottom of the stack. Even then it's pretty rare, TBH.

I do this instead:

(parts and layout chosen for visibility, not function)

[qoonpooka]

3 minutes ago, 5thHorseman said:

I personally just don't use those adapters, except when they're at the bottom of the stack. Even then it's pretty rare, TBH.

I do this instead:

(parts and layout chosen for visibility, not function)

And that's not the wiggliest thing in the history of ever?  Iiiiiinteresting.

[Superfluous J]

1 hour ago, qoonpooka said:

And that's not the wiggliest thing in the history of ever?  Iiiiiinteresting.

In 1.x you can strut it. In 1.2 pre you can autostrut it

[Snark]

21 hours ago, qoonpooka said:

So, based on the answers in this thread, does flipping one of these over: http://wiki.kerbalspaceprogram.com/wiki/TVR-400L_Stack_Quad-Adapter

Not attach the four motors to the quad-side of things at all four points?

Correct, it won't.  Tex_NL already answered this, but just to provide some specific examples:

• You can put a quad adapter on the bottom of your rocket, and attach four things to it (e.g. engines).
• You can put an upside-down quad adapter on the top of your rocket, and attach four things to it.
• You can't do what you want, which is to have two quad adapters facing each other, with four things connecting them together.

It won't work.  They won't connect.  Can't be done.  Or rather... they will connect, but only on one of the four things.

It's because of the "tree" nature of KSP ships, as discussed above.

[qoonpooka]

1 minute ago, Snark said:

Correct, it won't.  Tex_NL already answered this, but just to provide some specific examples:

• You can put a quad adapter on the bottom of your rocket, and attach four things to it (e.g. engines).
• You can put an upside-down quad adapter on the top of your rocket, and attach four things to it.
• You can't do what you want, which is to have two quad adapters facing each other, with four things connecting them together.

It won't work.  They won't connect.  Can't be done.  Or rather... they will connect, but only on one of the four things.

It's because of the "tree" nature of KSP ships, as discussed above.

Does this mean that multiple docking ports won't provide a less-wobbly, more secure connection for parts of a large, interplanetary vessel?

[Snark]

5 minutes ago, qoonpooka said:

Does this mean that multiple docking ports won't provide a less-wobbly, more secure connection for parts of a large, interplanetary vessel?

Actually, multiple docking ports are somewhat of an exception, since they have that magnetized clamp effect.  Only one of them will actually be connected per se, but the others will be firmly stuck together in a way that's typically strong enough for interplanetary ships (which tend to be low-TWR).

Folks have been using the multiple-docking-port trick this way since forever.     (I hardly use it myself, since it makes docking kinda finicky.  I prefer to get by with just using a bigger-diameter docking port, and/or using a "tractor" configuration so that the docked parts are being pulled rather than pushed.  Or I use my favorite solution, which is the deployable struts from KAS.)

Note that autostrutting in KSP 1.2 can make it easier to dock stuff together in orbit and keep it rigid, if you like using autostrutting.