Twitchy SAS Direction Hold

[Sir_Robert]

I have recently started playing KSP again after a long hiatus (pre release). After a few landings on the Mun, I have noticed a few things.

Retrograde hold SAS is extremely useful for landing. And Retrograde hold SAS (and prograde) are extremely twitchy, to the point of rapidly draining power by constantly running the reaction wheels at full. Is this normal? And is there anything you can do to reduce it?

 

Thanks

[Snark]

31 minutes ago, Sir_Robert said:

Retrograde hold SAS is extremely useful for landing. And Retrograde hold SAS (and prograde) are extremely twitchy, to the point of rapidly draining power by constantly running the reaction wheels at full. Is this normal? And is there anything you can do to reduce it?

Yes, this is a common problem; the PID tuning isn't great for SAS.  When you're seeing this, it's basically because you have way too much reaction torque on your ship.  Put fewer reaction wheels on, or disable some of the ones that are on there already.  (For very small/light craft with no command pod, even the smallest reaction wheel is too much-- best to just leave off entirely, and rely on the probe core's built-in reaction wheels.)

If you're willing to go the mod route, one of the Tweakable Everything components allows you to set the amount of torque from your reaction wheels (i.e. not just "on" or "off").  Also, I seem to recall that Claw's "stock bug fix" mod includes something to tone down the SAS jitter.

[Sir_Robert]

Thanks, so it was as I suspected. Sounds roughly the same as the super twitchy regular SAS from way back during alpha. The problem is that regular SAS is fine, so it's probably specifically a problem with the constantly changing nature of the directions.

For now I'm just using MJ smart ASS to hold direction instead, which is as silky smooth. I'll take a look at that stock bug fix collection. I've been seeing it quite a bit lately

[Geschosskopf]

1 hour ago, Sir_Robert said:

Thanks, so it was as I suspected. Sounds roughly the same as the super twitchy regular SAS from way back during alpha. The problem is that regular SAS is fine, so it's probably specifically a problem with the constantly changing nature of the directions.

For now I'm just using MJ smart ASS to hold direction instead, which is as silky smooth. I'll take a look at that stock bug fix collection. I've been seeing it quite a bit lately

The stock PID SAS things?  Never use them.  They've been totally bugged since the get-go.  Squad should really look at MJ's SmartASS for how to do that sort of thing correctly, or at least give the stock version a thorough revamp.

[Snark]

1 hour ago, Geschosskopf said:

The stock PID SAS things?  Never use them.  They've been totally bugged since the get-go.  Squad should really look at MJ's SmartASS for how to do that sort of thing correctly, or at least give the stock version a thorough revamp.

That's a little harsher than I'd say.  I use them all the time, it's the only thing I've used since starting KSP a couple of years ago, and they generally work just fine for me.  Maybe I have a lower bar than you do for what constitutes "fine". 

They do have some problems (the jitter when a ship has lots of torque), but I don't get bitten by that very often-- it's just a matter of not putting too many reaction wheels on the ship.

Yes, they have issues, but far from the point of being unusable.

[Sir_Robert]

24 minutes ago, Snark said:

That's a little harsher than I'd say.  I use them all the time, it's the only thing I've used since starting KSP a couple of years ago, and they generally work just fine for me.  Maybe I have a lower bar than you do for what constitutes "fine". 

They do have some problems (the jitter when a ship has lots of torque), but I don't get bitten by that very often-- it's just a matter of not putting too many reaction wheels on the ship.

Yes, they have issues, but far from the point of being unusable.

The thing is that 'to much torque' is surprisingly little. A simple early game orbiter (MK1 pod, fuel tank, engine) has WAY to much torque for directional SAS, and goes completely nuts when I ask it to hold retrograde during re-entry. Only the other hand, it's rock steady during normal SAS. Same problem with an early game Mun lander, which is the one thing it'd actually be useful for (pod, science, tank, engine, legs)

[Geschosskopf]

25 minutes ago, Snark said:

Yes, they have issues, but far from the point of being unusable.

The stock PIDs burn RCS in the direction of the new heading the ENTIRE TIME of the change and only stop when they get there.  This results in massive overshoots, repeated many times.  This is a hideous bug.  So if your ship relies on RCS for helm changes, you must NEVER hit the PID button prior to getting the ship pointing that way yourself.  Get the swing started, let it coast, then stop it bang on the desired heading all manually.  Only then hit the PID button to keep it there.  But OTOH, if you built the ship correctly so it's not wobbly and has adequate (but not excessive) reaction wheels, it'll stay there by itself anyway without the PID button.

So basically, if you don't have RCS, the PID buttons are useless because they're twitchy--they make the ship move when it won't move without them.  And if you have RCS, the PID buttons are decidedly counterproductive and equally unnecessary once you're pointed in the desired direction.  Therefore, there is no point in having them.  The only reason they exist is to give some point in leveling up pilots, and thus just another example of the failed Kerbal class system that should be buried in an unmarked grave at midnight anyway.

[HebaruSan]

The directional controls introduce an extra design decision: am I going to use the directional controls with this craft? If yes, include extra (like x4 or more) monopropellant and/or electriccharge for it to waste. As long as you have plenty of margin in those resources, and plenty of time to wait for overshoots to overcorrect, the job they do can be tolerated. I find it can also help to look away from the screen while they're operating.

What I've been doing lately (generally with heavier craft) is activating the direction I want for a few seconds, just till the craft starts to drift in the right direction, and then I turn SAS off completely and handle the rest manually. All the directional SAS feature does in this case is show me the approximate optimal direction to turn, and they're decent at doing that.

[Waxing_Kibbous]

7 hours ago, Sir_Robert said:

I have recently started playing KSP again after a long hiatus (pre release). After a few landings on the Mun, I have noticed a few things.

Retrograde hold SAS is extremely useful for landing. And Retrograde hold SAS (and prograde) are extremely twitchy, to the point of rapidly draining power by constantly running the reaction wheels at full. Is this normal? And is there anything you can do to reduce it?

 

Thanks

From my experience for Mun/Minmus retrograde hold works fine, Kerbin reentry on the other hand can really suck batteries dry, so I try to make sure I have full power before going in. Also, generally I don't start the hold until around 52,000m, or when things start glowing, whichever comes first. Time acceleration (I assume physical time acceleration, doesn't happen in space) when holding can REALLY drain batteries fast, so be careful- it doesn't  when in the default SAS mode however (the upper left icon in the array is active).

[Snark]

5 hours ago, Geschosskopf said:

The stock PIDs burn RCS in the direction of the new heading the ENTIRE TIME of the change and only stop when they get there.  This results in massive overshoots, repeated many times.  This is a hideous bug.  So if your ship relies on RCS for helm changes, you must NEVER hit the PID button prior to getting the ship pointing that way yourself.  Get the swing started, let it coast, then stop it bang on the desired heading all manually.  Only then hit the PID button to keep it there.  But OTOH, if you built the ship correctly so it's not wobbly and has adequate (but not excessive) reaction wheels, it'll stay there by itself anyway without the PID button.

So basically, if you don't have RCS, the PID buttons are useless because they're twitchy--they make the ship move when it won't move without them.  And if you have RCS, the PID buttons are decidedly counterproductive and equally unnecessary once you're pointed in the desired direction.  Therefore, there is no point in having them.  The only reason they exist is to give some point in leveling up pilots, and thus just another example of the failed Kerbal class system that should be buried in an unmarked grave at midnight anyway.

Again:  not my experience.

I can't speak to the RCS case, because I never, ever use RCS for rotation, only for translation during docking.  I rely on reaction wheels for that.

And I just don't have a problem with it.  I just don't.  It works just fine.  I'm careful not to over-supply my craft with reaction wheels, and it does fine.  And I use the "hold prograde", "hold retrograde", "hold maneuver node", etc. functions all the time, with no trouble at all.

Yes, it's annoying that the PID isn't tuned better and that I have to think about over-torquing when designing the ship, and I really wish they'd fix that.  But it works fine for me and is a major component of the game that I use all the time.