Why is it "wobbly"?

[strider3]

One thing I have noticed since upgrading to 1.1.3 is a lot of "wobble" in ships which, in the past, were very stable. What changed?

Anywho, IMHO whatever was changed is a bit...aggravating? I'm no rocket construction engineer (as I've proven to you over and over LOL) but let me give you an example of the latest problem. The screenshot below is of a craft I've built to, eventually, bring fuel tanks for attachment to my Sunoco Minmus refueling station, in orbit around...guess (I crack myself up!). This is not a new "layout" for me as I've used many similar builds in the past...but now they have a pronounced "wobble" problem. I don't understand why...they have, what I assume are, solid attachments at the decoupler/separator points. If the "long, skinny" at the top is a problem, I don't see how going all "Rockomax" would help (I want to bring a Rockomax Jumbo64 to the station on each flight)...it would only be "skinnier"? ? Anywho...this one only wobbles in one spot...right at the Rockomax/Kerbodyne connection. I can alleviate the wobble problem by going Kerbodyne all the way to the one Jumbo I'm trying to bring to the station, but that craft has so much mass it is ungainly.

What am I doing wrong now?

Edited September 10, 2016 by strider3

[Gaarst]

Tank-sas-decoupler-engine junctions are known to be wobbly (a lot).

Magical solution to wobbling is struts. (Also KJR but it's a mod)

Your rocket is tall for KSP standards, so even if a real rocket with the same proportions (which actually seem reasonable) would have no problem flying, in KSP it's going to be extremely wobbly. So either use struts, or go for a wider design.

Edited September 11, 2016 by Gaarst

[Fearless leader]

Try using the EAS-4 Strut Connector I think that would decrease the wobbliness a bit.

[ForScience6686]

I'm gonna guess your sas is fighting itself.  You shouldn't need much torque once you've reached speed, and the sas is a little over zealous.  I would try that before any other adjustments.  Maybe engine gimbal as well.

[Magzimum]

Attach 4 struts to the Kerbodyne ADTP-2-3 at its widest point, and attach them to the bottom of the lowest orange tank. That should remove any wobbliness from that particular part of the rocket.

I am surprised that the SRBs at the base of the rocket don't give you any problems. I usually use one strut for each SRB to attach their nose cone to the central stage.

[FyunchClick]

19 hours ago, ForScience6686 said:

 Maybe engine gimbal as well.

^ this. Gimbals are all or nothing, and with SAS on they'll have your rocket shimmying right out from under you. If your missile is aerodynamically stable, once you nudge it into the right angle for a gravity turn you I find you need very little authority (until you get out of the atmosphere you probably can even dispense with sas stabilization too). If you do need some gimballing you can always dial them down and see if that helps.

[ForScience6686]

15 hours ago, FyunchClick said:

^ this. Gimbals are all or nothing, and with SAS on they'll have your rocket shimmying right out from under you. If your missile is aerodynamically stable, once you nudge it into the right angle for a gravity turn you I find you need very little authority (until you get out of the atmosphere you probably can even dispense with sas stabilization too). If you do need some gimballing you can always dial them down and see if that helps.

I tend to reduce our eliminate gimbal once I'm up and the boosters have separated.  That's also my first move if I start noticing an oscillation.

[strider3]

I do have struts on the bottom of each booster with the radial attachment near the top. I will add some between the ADTP-2-3 and the lower Jumbo 64 tank and play with the gimbals on the 1st stage.

Thanks!

[ForScience6686]

I would avoid struts unless you don't have another option.  They are quite a, drag...

[cubinator]

On 9/10/2016 at 7:01 PM, Gaarst said:

Tank-sas-decoupler-engine junctions are known to be wobbly (a lot).

^this. I'd try going directly adapter-decoupler-engine. That one SAS module probably doesn't help a whole lot anyways with the Mammoth at the bottom gimballing.

[Chiron0224]

In the new aero SAS is more liability than help during ascent.  You tend to get buffeted by the air, SAS corrects, the air pushes back, SAS corrects more etc.  It's a feedback loop that can shake your rocket to pieces.  For one I use KJR because I imagine that my Kerbals don't make their rockets out of toilet paper rolls but also I don't use SAS anymore on ascent.  Just get a TWR between 1 and 2 (I like 1.5 although mine tends to go up and up throughout the launch but that can't be helped) and give it a slight nudge when it reaches 10m/s.  At that point just watch the rocket nose itself over gradually.  By 10k you'll want it to be between 60 and 45 degrees.  If it's pitching over too fast accelerate, if not fast enough decelerate but don't use SAS or wasd.  Let the rocket fly ballistically.  This should give you a nice clean ascent profile and it won't have as much tendency to wobble.  Of course, you can also add struts.