SAS is crazy

[TheJoolian]

For god's sake i'am angry with SAS. Everytime i build a rocket it always get out of control. Can this have a solution? 

[Geonovast]

You're going to have to really, really, really elaborate.  SAS is NOT an autopilot.

[TheJoolian]

Just now, Geonovast said:

You're going to have to really, really, really elaborate.  SAS is NOT an autopilot.

So what is it anyway

[Geonovast]

It's stability assist that will try to keep your ship pointing in a single direction.  Its current orientation, prograde, retrograde, normal, antinormal, radial out, radial in, target, antitarget, or at a manuever.

I can't infer what you mean by "crazy".  If you're having issues launching a rocket, there's about a 99.98% chance that SAS isn't the problem.  Can you be more specific, preferably with screenshots as to the problems you're having?  I bet we can help you out with whatever's going on.

 

[Jestersage]

My guess is you try to go to orbit by pressing prograde only.

Best use of SAS is stability assist, since in space, when you start rotating, you keep the rotation rate (roll, pitch, yaw) even when you let go. However, with SAS, once you let go, it kept the last pointed position liek modem space sims.

[Serenity]

2 hours ago, TheJoolian said:

For god's sake i'am angry with SAS. Everytime i build a rocket it always get out of control. Can this have a solution? 

You need to use SAS as a guide and not lock to prograde as the post above me said.

Launching and following the prograde is not a good idea, you will eventually loose control.Just stay a bit above it and adjust slightly.

Make the SAS follow your guidance and not the other way.

Edited August 21, 2019 by Boyster

[TheJoolian]

11 minutes ago, Boyster said:

You need to use SAS as a guide and not lock to prograde as the post above me said.

Launching and following the prograde is not a good idea, you will eventually loose control.Just stay a bit above it and adjust slightly.

Make the SAS follow your guidance and not the other way.

Thanks for the help. But i need to turn off SAS when i start to launch the vessel?

[IncongruousGoat]

35 minutes ago, TheJoolian said:

Thanks for the help. But i need to turn off SAS when i start to launch the vessel?

Not at all. You can still steer while SAS is on (when it's in stability assist mode, in any case) - it just tries to keep the ship steady while you're not actively steering. During launch, this is really useful, especially if your rocket isn't aerodynamically stable and needs to be actively stabilized to avoid cartwheeling.

[Superfluous J]

Note that SAS isn't magic. It can't hold your ship still if your ship is incapable of holding still. It's like steering a car that's skidding across a sheet of ice. It's not the car's fault that the steering isn't doing what you expect.

[Pds314]

On 8/21/2019 at 4:54 PM, Boyster said:

You need to use SAS as a guide and not lock to prograde as the post above me said.

Launching and following the prograde is not a good idea, you will eventually loose control.Just stay a bit above it and adjust slightly.

Make the SAS follow your guidance and not the other way.

I mean, following prograde after like 50-150 m/s and a 5 degree turn depending on the design is not only a good idea. It's literally what a gravity turn is. It is one of the most efficient flight paths. Granted, you need to actually be able to stick to prograde, and SAS doesn't know how your vehicle is designed and what its envelope of safe, controllable flight is under any specific conditions. So some rockets need manual adjustments to stay on course.

 

I do find that certain types of vibrational or torque loads really mess with SAS's functionality.

Edited August 23, 2019 by Pds314

[Incarnation of Chaos]

Also be advised; if you have a lander with a probe core attached on top of your stack it will often cause your stack to think it's upside down when launching. So if you can't figure out why your rockets are going kamikaze for no reason that's likely the culprit; and pretty easily resolved by changing the orientation with the context menu or just reverting to VAB and physically flipping the core (This will mean you have to remember your lander is reversed though).

[AVaughan]

1 hour ago, Incarnation of Chaos said:

 pretty easily resolved by changing the orientation with the context menu or just reverting to VAB and physically flipping the core (This will mean you have to remember your lander is reversed though).

Assuming the root part is a probe core or a command pod that is oriented the right way, none of that should be necessary.  If it is, it is normally easier to right click and appropriated probe core/command pod/docking port and "control from here".  I certainly wouldn't recommend flipping the the probe core of a lander, that is just asking for problems when you try to use it later.  

Most like OP's rocket is simply aerodynamically unstable, and flipping due to aerodynamic effects as it's speed and aero loading increase after takeoff.

@TheJoolian Try adding some fines (or for very large rockets maybe even wings) to the bottom of your rocket.  If that doesn't work post a picture of the rocket in the VAB, and in flight when you start to lose control.  (Make sure you leave the UI visible, so we can see things like your airspeed, and the control positions).

 

[Incarnation of Chaos]

Just now, AVaughan said:

Assuming the root part is a probe core or a command pod that is oriented the right way, none of that should be necessary.  If it is, it is normally easier to right click and appropriated probe core/command pod/docking port and "control from here".  I certainly wouldn't recommend flipping the the probe core of a lander, that is just asking for problems when you try to use it later.  

Most like OP's rocket is simply aerodynamically unstable, and flipping due to aerodynamic effects as it's speed and aero loading increase after takeoff.

@TheJoolian Try adding some fines (or for very large rockets maybe even wings) to the bottom of your rocket.  If that doesn't work post a picture of the rocket in the VAB, and in flight when you start to lose control.  (Make sure you leave the UI visible, so we can see things like your airspeed, and the control positions).

 

If the probe core is pointing down selecting "Control from here" won't fix it; you would have to have another command unit (Like a pod etc.). Also i seriously doubt his rocket is that unstable; it takes some really silly geometry to make aero forces flip a stack in KSP (Even with FAR) and most times just throttling down until you get higher up will be all that's needed. 

[AVaughan]

9 minutes ago, Incarnation of Chaos said:

you would have to have another command unit (Like a pod etc.).

I did say

14 minutes ago, AVaughan said:

right click an appropriate probe core/command pod/docking port and "control from here".

So yeah, the probe core that is upside down is not an appropriate command part.    (Very minor typo.  I actually said "right click and appropriated probe core/command pod/docking port", but I'm sure you understood what I meant).

The way some people build a big fairing up front, being aerodynamically unstable when you hit Mach 1 is often a possibility, especially without fins.  (Indeed I'm in the habit of always adding fins, because I've seen it happen too many times with my own designs.  It could also be a shuttle style design with asymmetrical thrust that becomes unstable as the centre of mass changes.  But there is no way to tell if that is the problem, without more information).

[AHHans]

1 hour ago, AVaughan said:

Try adding some fines [...] to the bottom of your rocket.

OMG. The puns, the PUNS!!!

• You can't park that rocket here!
• Do you have a license for that engine?
• There is a mandatory inspection sticker missing on the fins.
• Your rocket is overweight, that costs extra.

P.S. Yes, I've also been screwed over by autocorrect myself. But I just couldn't resist.

[Serenity]

8 hours ago, Pds314 said:

I mean, following prograde after like 50-150 m/s and a 5 degree turn depending on the design is not only a good idea. It's literally what a gravity turn is. It is one of the most efficient flight paths. Granted, you need to actually be able to stick to prograde, and SAS doesn't know how your vehicle is designed and what its envelope of safe, controllable flight is under any specific conditions. So some rockets need manual adjustments to stay on course.

 

I do find that certain types of vibrational or torque loads really mess with SAS's functionality.

Some rockets?You mean most rockets, cause anything else than a basic design will fail to follow the best gravity turn path with locked prograde.As you said too many factors in place.

Edited August 23, 2019 by Boyster

[esmenard]

Once when I was reentering a spaceplane with SAS on prograde, il was oscillating weirdly, and stopping the SAS stopped the oscillations. Except that time, I never had any problem with it (or the problem came from a lack of control, not the SAS itself).

[Serenity]

3 hours ago, esmenard said:

Once when I was reentering a spaceplane with SAS on prograde, il was oscillating weirdly, and stopping the SAS stopped the oscillations. Except that time, I never had any problem with it (or the problem came from a lack of control, not the SAS itself).

The problem became probably with SAS overworking itself and messing around with aero forces in constant level.

Sometimes you can also Hold F button(the default key) to turn temporaly SAS to give it a small breather.

I find this very useful when i try to land and the velocity comes close to 10-20 and the retrogade marker goes crazy.

Which doesnt result in good landings if you have the retrogade locked.

 

[esmenard]

7 hours ago, Boyster said:

I find this very useful when i try to land and the velocity comes close to 10-20 and the retrogade marker goes crazy.

Which doesnt result in good landings if you have the retrogade locked.

 

This shouldn't be a problem if you keep the retrograde straight down (ie. don't overburn and start going up again).

[Serenity]

21 minutes ago, esmenard said:

This shouldn't be a problem if you keep the retrograde straight down (ie. don't overburn and start going up again).

The problem is at 10-20ms the retrogade starts changing/moving to prograde, which is normal cause it will change IF you keep accelerating.

But if you want to soft land and reach close to zero velocities the vessel will flip instantly when you have locked SAS to retrogade.

So i am guessing this is more of a gameplay mechanic so the markers wont instantly change/appear after you hit Zero velocity.

Edited August 24, 2019 by Boyster

[Superfluous J]

1 hour ago, Boyster said:

The problem is at 10-20ms the retrogade starts changing/moving to prograde, which is normal cause it will change IF you keep accelerating.

This does not happen to me. I frequently lock to retrograde all the way down to 1 m/s (where it goes to regular SAS).

[Pds314]

6 hours ago, 5thHorseman said:

This does not happen to me. I frequently lock to retrograde all the way down to 1 m/s (where it goes to regular SAS).

A lot of landings with landers I've only flown one or two times before bounce back up.

[Serenity]

9 hours ago, 5thHorseman said:

This does not happen to me. I frequently lock to retrograde all the way down to 1 m/s (where it goes to regular SAS).

You are correct, in normal sitiuations it isnt really a problem but when you need to burn fast and big to land then it becomes one.

I should have clarified that this happens with very high powered landers.Like Boosters landing back to Kerbin or Landers with well quite larger engines than they should really need to land on planets like Mun.

The problem there is everything happens so fast and the marker moves like almost instantly, just enough to flip your lander.

 

Edited August 24, 2019 by Boyster

[Geonovast]

15 hours ago, Boyster said:

The problem is at 10-20ms the retrogade starts changing/moving to prograde, which is normal cause it will change IF you keep accelerating.

I've only experience this in target mode during docking in low alt, high drift orbits.

 

While it may frequently work, keeping retrograde during landing isn't always the best idea.  You want to zero out your horizontal velocity just above the surface, then switch to radial out for landing.  You'll never flip around with radial out.  A few days ago I was doing some hopper tests with landing a rocket back on its landing clamps, and it was radial out the whole way, even before launch.

[Serenity]

2 hours ago, Geonovast said:

I've only experience this in target mode during docking in low alt, high drift orbits.

 

While it may frequently work, keeping retrograde during landing isn't always the best idea.  You want to zero out your horizontal velocity just above the surface, then switch to radial out for landing.  You'll never flip around with radial out.  A few days ago I was doing some hopper tests with landing a rocket back on its landing clamps, and it was radial out the whole way, even before launch.

HUH, radial you say?Thats really interesting...i gotta try it.Tnx ").