Stability Augmentation System

[Whirligig Girl]

The Probe Stabilizers: Stability Augmentation System

The Pilot Stabilizers: Sickness Avoidance Scheme

With the advent of higher-gimbal engines (which is good and realistic!), it seems that the SAS is no longer suited to the task of keeping rockets from wobbling to pieces in many cases. Along with that, the direction hold on the high-tech probes and high-skill pilots is still a little twitchy and wobbly on its own, even without active thrust power adding to the problems. One of the main things seems to be that SAS doesn't like slow gimbal response times. (EDIT: To be clear, I mean engines that actually HAVE slow response times, like the Vector or the Nuclear Lightbulb from Porkjet's Atomic Age mod)

I think it might be time for SAS to get another rework. (Not so major as the 0.21 one of course!)

Edited November 11, 2015 by GregroxMun

[Sanic]

The SAS is wobbly because the rocket gimballing overcorrects small wobble, which worsens the original wobble. You can turn it off for a sec, stop the wobble, and turn it back on. But that's just my experience, yours could be much different.

[BagelRabbit]

One of the main things seems to be that SAS doesn't like slow gimbal response times.

KSP's gimbal system isn't slow. Rather, it's practically instantaneous... which is where the problems happen. Sanic's exactly right when he says,

The SAS is wobbly because the rocket gimballing overcorrects small wobble, which worsens the original wobble.

Basically, here's the scoop.

Engines' gimbals are like Hobbes: they have no setting between 'off' and 'high.' So when the rocket is slightly off vertical, the engine gimbals to the side instantaneously to try to correct the problem.

With higher-gimbal engines, though, the gimbal overcorrects, pushing the craft over to the other side. As soon as the rocket passes vertical in the other direction, the same problem happens. And the larger the gimbal, the greater chance that these vibrations intensify until the craft shakes itself apart, as you described.

This problem would still occur with slower-gimbal engines, but it wouldn't happen as spontaneously, if my mind-physics is working properly today. Another way to fix this problem is simply to add fins, which do a good job of damping the vibrations. I think that while a SAS rework may be needed, it might be better to implement a slower gimbal (for realism's sake and to perhaps reduce the risk of the rocket shaking itself apart.) I also think that if you design craft carefully, the problem of things shaking apart is completely avoidable.

Hope this helps...

-Upsilon

[ferram4]

The engines gimbals are not like Hobbes. Under full control input, yes, they do go to full control, but under smaller in inputs they do not. Even though many of them do not have response speeds set up for them, the Vector engine does to try and damp things out. In fact, as any of the Realism Overhaul modders will point out, too slow a gimbal speed actually makes things worse with SAS as it is currently; it simply does not know how to handle it because it is too set on getting the problem fixed instantly. And that's before we get to handling wobble, which it has no idea it can cause.

The obvious solution is to make SAS less aggressive or to give it a better tuned / better self-tuning control system.

[Boris-Barboris]

PID is cancer.

[Alshain]

Another way to describe it is SAS is flying with a keyboard when it should be flying with an analog flight stick.

[Whirligig Girl]

KSP's gimbal system isn't slow. Rather, it's practically instantaneous...

The new SSME (and Porkjet's Nuclear Lightbulb mod engine) were what I was referring to.