Spaceship wobbling with SAS

[chicken305]

Hi,

all of my Spaceships start wobbling when i activate SAS on accelerating

anyone knows how to fix that?

[Tracer]

I have the same problem with some of my rocket going wobbly with SAS on, until they reach a height where they are out of most of the atmosphere.

I've read that it can be caused by having a bendy rocket in the first place (MOAR STRUTS!!) and to many sources of attitude control, eg fins, reaction wheels and engines that gimble.

You tap, d to lean your rocket east, the rocket starts to bend, and all those sources of attitude control start to conflict with each other, leading to more wobble which exaggerates the problem.

If the rocket is still controllable without sas turned on, i just run with it anyway, Jebb likes a bit of wobble on his rockets

[Mister Dilsby]

You might try switching your engine gimbal off, if you have an engine that gimbals. The engine gimbal/thrust vector tends to 'fight' against aero control surfaces (fins) and reaction-wheel SAS. This goes double or triple for bendy rockets, as any or all of the aforementioned control methods will tend to produce an under-damped response to the bendy oscillations.

[Warzouz]

On take-off, don't activate SAS. You only need it at high altitude (around 40km).

If you have multiple controle devices, deactivate the most powerful : reduce or cut gimball, deactivate reaction wheels, use static wings.

Struts are usefull, but with new aero, they seem off-topic. Now I use the "Joint Reinforcement" mod which help to reduce wobble. Big structures still wobbles but much less. No more payload wobbeling out of fairing...

EDIT : also, MechJeb SAS (smart ASS) does a better job than Stock SAS.

[cephalo]

Jeb's reaction time is not great. He doesn't anticipate where he wants the rocket to point. If it's too far left he goes right, and if it's too far right he goes left. He doesn't slow down his rotation enough when approaching the desired heading. This causes terrible oscillation. Making your rocket more rigid makes it more Jeb proof.

[Warzouz]

Have you tried recent dev built ?

Also, I don't fly rocket in atmo with SAS or MJ (Ascent or smart ASS). I only activate SAS around 35/45km when I want to check the map to verify my apoapsis (I don't want the rocket to flip at that moment...).

[tbob]

Deactivate all but one control mechanism at the time: Aero controls can be switched off in the VAB but have to be switched one at the time in flight so I usually leave them on and turn off everything else (RCS, reaction wheels, gimbals) then switch on RCS or wheels at around 25-30km, where the aero controls fade out.

Mostly SAS will work with a rigid rocket and one control suite. The different vectors and impulse delays from competing controls seems to confuse it. A bendy rocket will also create waves but I find that either turning SAS off or spinning (rolling about the long axis rapidly) the rocket will help. Spinning, for whatever reason, tends to damp bendy oscillations when it doesn't make the craft break.

[Warzouz]

You don't need al lot of control to get a rocket to orbit. SAS is usually overkill and cause wobble. Wobble will make you quit the 5° safe angle and loose controle of you rocket much easily.

Not using SAS and "fine controls" (CAPS LOCK) helps a lot.

I also noticed that clamps can set you rocket off course at take-off. I don't use them anymore, even I launch a 230T payload to LKO. Big rockets are usually stable enough withour using them.

PS : I don't build asparagus pans anymore as in beta 0.9. I use reusable SSTO rockets. I send payload to LKO for a 470 to 650 funds per payload ton (from 12T to 236T). I find SSTO rockets to be easier as they don't have different flight controls. they behave mostly the same up to LKO. You only have to throttle back to keep TWR to reasonable level.

But again : if you have trouble with wobble, try "Joint Reinforcement" mod by Ferram4. Game feels more about rockets than noodles with it.

[KerBlammo]

Turn off (or limit) engine gimbal.

[Streetwind]

It's not only engine gimbals... winglets also do this on small rockets. The main reason is that all of the steerable winglets are way, way oversized for anything below 2.5m. If you install FAR and decrease the deflection of the AV-R8 winglets from 15° to 5°, suddenly 1.25m rockets too can fly under SAS without oversteering issues*.

* Engine gimbals were already mentioned.

[Warzouz]

Truthfully, when I tried the new aero, I had a hard time to fly rockets (as many). I added winglets in the middle to help (winglet at the bottom do harsh turns) That went quite well. I built many rocket with that general design, until I had some reentry heating problem on those winglets (it's a reusable SSTO LKO stage). I tried to remove them and noticed I flew the rocket the same way.

So general tip : be gentle on the command to avoid wobble and flipping. SAS is no gentle on controls

[DancesWithSquirrels]

Strut for stiffness, and limit your gimbals!

I built a near-Apollo mission (and my first try at fairings, which led to moar struts, without which the ship was an overcooked noodle) that shook itself to pieces, but setting my first stage gimbal limit (Mammoth) to 20% and my second stage (Rhino) to 30% made everything smooth. I left my third stage (Skipper) limit alone, that was small enough a stack to keep things manageable.

Edit: The SAS (with the exception of 'hold attitude') needs a serious revamp to have reasonable PID control values. As is, it's essentially a 'bang-bang' control that's giving full power corrections to any error, leading to major overshoots, and on a too-flexible ship that can break things.

Reducing the gimbal power reduces the overshoot, making it far more stable, and moar struts help keep the overcorrections from becoming ship flex. Make your ships rigid, limit control authority, and many more designs become flyable under SAS.

Now, if Squad would have just put in proper controller values for SAS modes other than hold attitude, like those we already have for 'hold attitude', these discussions would be far less common...

Edited May 26, 2015 by DancesWithSquirrels

[chicken305]

Thanks everyone i thought about something like that.. didnt think that too much control surfaces would be the problem

[A_name]

Honestly try Kerbal Joint Reinforcement. It changed my (Kerbal) life.

[Crzyrndm]

Edit: The SAS (with the exception of 'hold attitude') needs a serious revamp to have reasonable PID control values. As is, it's essentially a 'bang-bang' control that's giving full power corrections to any error, leading to major overshoots, and on a too-flexible ship that can break things.

Reducing the gimbal power reduces the overshoot, making it far more stable, and moar struts help keep the overcorrections from becoming ship flex. Make your ships rigid, limit control authority, and many more designs become flyable under SAS.

Now, if Squad would have just put in proper controller values for SAS modes other than hold attitude, like those we already have for 'hold attitude', these discussions would be far less common...

There is now a mod for that (this problem has just come up one too many times...). The main problem is not actually "bang bang" control in the typical sense, but a seriously overcooked derivative response causing vessels with high torque / MoI ratios to compulsively jiggle (the vessel in the video couldn't even move with the stock tuning it was vibrating so hard). Proportional response was high (and is reduced noticeably by the mod) but not enough to really cause the issues it had.

Edited May 27, 2015 by Crzyrndm

[DancesWithSquirrels]

"...a seriously overcooked derivative response ..."

Oh my - those stock proportional and derivative values are awfully high, no wonder there was oscillation.

[michaelsteele3]

Yeah, SAS Is notorious for making the ship wobble. SAS is basically like a robot using your keyboard, and the robot is running off of Windows 98 (Please quote me on that :3)

Edited May 27, 2015 by michaelsteele3

[Warzouz]

Honestly try Kerbal Joint Reinforcement. It changed my (Kerbal) life.

So do I !

[tg626]

SAS in all modes but "Attitude Hold" suffers from "Hunting" - So that's all I use. I then "self limit" myself to whatever modes are available in SAS and use those modes only in MechJeb's SmartASS.

[Astrofox]

Get Kerbal Joint Reinforcement - that'll stop the wobble on everything, and struts? YOU WON'T NEED THEM! That is, after you download one of the greatest mods ever made for KSP.

[A_name]

Get Kerbal Joint Reinforcement - that'll stop the wobble on everything, and struts? YOU WON'T NEED THEM! That is, after you download one of the greatest mods ever made for KSP.

Seconded.

I already posted about KJR in this thread but am doing so again to stress how awesome it is. It almost feels like cheating .

[quyxkh]

You're seeing what's called "pilot-induced oscillation" -- except here it's the autopilot, endlessly overcorrecting for its last mistake. There are a whole, whole lot of things that can confuse a ksp autopilot. Are you surprised? I mean, just _look_ at them. Would you really trust those guys to make an autopilot?

:-)

Keep as few controls active for it to twiddle as you can manage, make them not too weak (it overshoots when the acceleration is too slow, it expects a certain amount of authority), and make your craft stiff enough that it's not adding to the problem by wobbling in response to the controls you do have.

Edited May 28, 2015 by quyxkh