Jump to content

Dealing With SAS Oscillations


Recommended Posts

Oscillations caused by SAS seem to be the bane of my existence right now. Whenever I use ASAS during launch for particularly large rockets (using NovaPunch, so "large" here means the bottom stage might have a 5m tank and a bunch of booters), the system introduces huge oscillations in the rocket; the whole thing tends to bend around the center of mass. Without enough struts, these oscillations can tear the whole thing in half (usually at the decouplers between stages).

So...how does one go about dealing with this? It seems obvious to me that the ASAS code is over-correcting, then over-correcting its over-correction, and back and forth it goes. Should I slap a ton of RCS thrusters and tanks on as well to keep things steady? Or...am I just missing something entirely? I've kinda been using real-world rockets as a rough guide, like the Saturn V and SLS concepts, but maybe this is just totally wrong-headed for KSP.

Link to comment
Share on other sites

You could try and use mechjeb, it has a setting which keeps the rocket on a defined course and works better than ASA IMHO. Alternatively, make sure your main engine has thrust vectoring and that you are not at full throttle, it will give you more control authority. (or adjust the settings of the ASAS, but that requires some math)

Link to comment
Share on other sites

To mitigate the effect, you'll want to get it as close to immobile as possible before engaging ASAS. You can also disable it before it crosses the center of oscillation and reenable it just as it crosses the centre. This usually minimises the oscillation, but requires a bit of practice and repetition. One hopes they manage to fix it soon...

Link to comment
Share on other sites

overall the asas and mechjeb need to fine tune the multiplier going to the controls..even the slightest wobble with high g's can tear a fairly sturdy rocket to pieces... you can help it by using less controls.. eg less rcs thrusters... an engine with lower gimbal range... but this makes maneuvering harder

Link to comment
Share on other sites

You could try and use mechjeb, it has a setting which keeps the rocket on a defined course and works better than ASA IMHO. Alternatively, make sure your main engine has thrust vectoring and that you are not at full throttle, it will give you more control authority. (or adjust the settings of the ASAS, but that requires some math)

Indeed, I do use MechJeb. For any craft that experiences these significant oscillations caused by ASAS, MechJeb's ascent autopilot tends to flip the craft on its nose. (This is rather hilarious, but I'm afraid the hilarity of this problem is starting to wear-off on me. It's probably over-throttling it.) Using Smart A.S.S. with SURF HDG: 90 PIT: 90 holds it on course a little better, but still ends-up being about as wobbly as ASAS. And then it pitches over end-over-end again when I start a gravity turn by adjusting the pitch in Smart A.S.S.

To mitigate the effect, you'll want to get it as close to immobile as possible before engaging ASAS. You can also disable it before it crosses the center of oscillation and reenable it just as it crosses the centre. This usually minimises the oscillation, but requires a bit of practice and repetition. One hopes they manage to fix it soon...

That sounds like a nifty trick. I'll have to try that with my next launch. Maybe it'll help. Downside of course being that I have to go all-manual with mental fixation on the ASAS. I'm not good a task-switching on the best of days.

Perhaps I should expect a lot of pain and suffering though when the payload I'm trying to launch is about 100-tons. I'm considering an almost ludicrous design where the payload is actually at the bottom and center of the rocket. Not sure of/how that'll work. I guess until they fix ASAS to be less aggressive, long and pointy rockets are the wrong way to go.

Try retuning the ASAS? It requires editing the Ki/Kd/Kp values in the CFG, and restarting KSP, but it might give a smoother ascent.

Well, I've seen those values...but I haven't the faintest idea what they represent. Perhaps you could help in that regard.

Edited by phoenix_ca
Link to comment
Share on other sites

  • 2 weeks later...

The three values are constants associated with a controls method know as PID control (Proportional, Integral, Derivative). It has been quite some time since I have done any controls-type work (yes I have actually take classes in controls), but my guess is that one, or a combination of all three, of the constants are set too high, causing over compensations. You can read a bit more about PID control on Wikipedia:

http://en.wikipedia.org/wiki/PID_controller

gosnold is correct in assuming that it would take some fairly complex math to recalculate these values. However, you may be able to just "play" with a one value at a time and try to get a better control set up.

Link to comment
Share on other sites

This thread is quite old. Please consider starting a new thread rather than reviving this one.

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

×
×
  • Create New...