# Rovers and SAS - how does SAS work on terrain?

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Right now this is a question, but depending on the answer it may become a suggestion.

Let's say I'm driving a rover along a nice flat surface, with SAS on to assist with stability in turns. My heading is east, my pitch is zero, my roll is zero.

Then I reach the bottom of a hill and start driving the rover up a 20 degree incline. My heading is still 90, but my pitch is now up 20. When I turned on SAS my pitch was zero... so will SAS try to fight the pitch caused by the terrain incline?

Similarly, let's say I'm half way up the hill and turn right. As the turn is executed the rover would naturally roll further and further to the right until you were travelling along the face of the hill. Will SAS fight this as well?

In a worst case scenario, let's say I'm driving laterally along a hill, with the upslope to my left and the downslope to myright. Then I turn SAS on. Then I turn left (up the hill). If SAS tries to maintain my roll, then it will try to maintain the roll to the right, which would actually contribute to flipping the rover during the turn (since it will actually cause the rover to attempt to lean out of the turn) rather than maintaining stability through the turn.

If, right now, SAS does fight these orientation changes, then this would lead me to a suggestion: rover stability mode, in which SAS attempts to keep the wheels on the terrain (equivalently, match the rover pitch and roll to the orientation of the underlying terrain).

You could turn off SAS when first starting the climb, or while turning on the hill. But this would defeat the purpose of SAS in the first place, since turns are when it's needed.

Edited by allmhuran
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SAS has two modes - I call them "hold" and "dampening" (but I don't know what the actual names are).

"hold" - is what it does when you see the little "SAS" symbol on the navball. Basically, it will attempt to keep the craft pointing in the direction it was when the SAS was turned on. However, if you use the controls, deviate from the heading by too much (due to atmosphere for instance) or you are turning too fast at the pont you turn SAS on, then dampening mode will engage instead.

Dampening mode is engaged when the SAS symbol turns into two little arrows. At that point, the SAS will attempt to stop your crafts rotation/pitching. Once your craft is relatively steady - hold mode will be re-enabled, at the new heading.

Edited by severedsolo
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Cheers for the input, but that's not quite correct, nor quite what I'm asking.

The SAS light comes on whenever SAS is engaged. While SAS is engaged it is in one of two primary modes - locked to a predefined orientation, or stability assist.

Locking to an orientation is provided by high level probes or pilots (or always available in sandbox). This allows you to tell SAS to orient you to prograde, retrograde, etc. In this mode, SAS will return you to the locked target after any disturbance.

In stability assist mode, SAS holds your heading to a point that can be moved by control input. In this mode, if control input is provided, the SAS icon turns into the two arrows indicating that the control point is being moved by player input. In this mode SAS is still maintaining stability for any axes that are not subject to input. So if you provide yaw input, but do not provide pitch or roll input, then SAS will attempt to hold your pitch and roll while allowing you to yaw. Once the input is complete, the two arrows disappear and SAS will attempt to hold you to the new heading.

Now, none of this is actually relevant here, because we're not in a locked orientation, but neither is the player themself providing any control input. Rather, the terrain under the rover is changing, causing the natural orientation of the rover to change (because gravity). The question is this: since no player input has been provided, will SAS, in stability assist mode, fight the reorientation caused by the new terrian gradient?

Edited by allmhuran
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Now, none of this is actually relevant here, because we're not in a locked orientation, but neither is the player themself providing any control input. Rather, the terrain under the rover is changing, causing the natural orientation of the rover to change (because gravity). The question is this: since no player input has been provided, will SAS, in stability assist mode, fight the reorientation caused by the new terrian gradient?

Fair enough, I was actually referring to Stability Assist with my above post, I always forget that pilots can do that now, I never use it.

To actually answer your question: I wouldn't think so. If your craft is thrown off course due to (for instance) - atmospheric pressures, the SAS won't try and hold your original heading, it will hold the new one once everything calms down. I'm assuming there is a certain tolerance there, but the stability lock will be lost eventually.

If I'm wrong (and it's been known to happen occasionally), a better option may be to hold prograde? I presume your rover is going to be moving forward, so in theory that should do it (as long as the initial incline isn't too steep, otherwise you'd smack into the slope, as your prograde would be underground). I'm not sure if the prograde lock will hold your roll angle though.

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I'm not sure if holding prograde will allow you to turn, I'll look into that when I get home.

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Fair enough, I was actually referring to Stability Assist with my above post, I always forget that pilots can do that now, I never use it.

To actually answer your question: I wouldn't think so. If your craft is thrown off course due to (for instance) - atmospheric pressures, the SAS won't try and hold your original heading, it will hold the new one once everything calms down. I'm assuming there is a certain tolerance there, but the stability lock will be lost eventually.

If I'm wrong (and it's been known to happen occasionally), a better option may be to hold prograde? I presume your rover is going to be moving forward, so in theory that should do it (as long as the initial incline isn't too steep, otherwise you'd smack into the slope, as your prograde would be underground). I'm not sure if the prograde lock will hold your roll angle though.

Just my experience, but holding prograde in a rover that has reaction wheels (or torque from a probe core or cockpit for that matter), or RCS/downthrusters for stability, is a great way to make minor bumps into full blown rollovers. Any time prograde undergoes unexpected changes, such as when you leave the ground however briefly, the rover will of course attempt to follow that prograde change, which can be disastrous when what you really want is to just hold level and land on all wheels.

Stability assist gives good results on a lot of rover configurations. It tends to be surprisingly not-terrible at keeping wheels in contact with the ground and doesn't try to override your inputs. With rover wheels it seems to function almost like an ESC, if you drive along a slope it seems to adjust wheel speeds to "pull" you into the direction of best contact with the ground (i.e. the direction least likely to make you flip) if you let it; this can easily be overridden if you feel like you're going too far off course.

Keep in mind that if you introduce real thrusters, or control surfaces in an atmosphere, you'll face input from those too, so rovers built with those systems will need them adjusted properly (note: control surfaces can be excellent for stability in atmospheres!). I find these types benefit from stability assist even more though, since they tend to be pretty fast.

I've done all my circumnavigations with SAS enabled almost the entire time, I personally at least think it makes navigating things like hillsides a whole lot easier. YMMV.

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In my experience, the only thing that pulls rovers in any direction is gravity.

SAS will in fact attempt to hold your heading, including inclination, if you have reaction wheels installed. I've never experienced it automatically putting my wheels on the ground, but I have experienced the opposite. It also seems to resist turning when it can. I find that I frequently have to toggle SAS on and off as I drive around in certain cases. Holding prograde or whatever is just asking for disaster.

The effect of the reaction wheels feels more prominent in lower gravity.

Example mission with a Rover using reaction wheels

While crusing around on Dres using the same rover as above I was leaving the surface a lot, and had to roll the rover like an aircraft to ensure my wheels all hit the ground at the same time, so it doesn't do that for you either.

Edited by Randazzo
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Yep, this is what I thought happened and I confirmed that in testing tonight by creating a temporary custom SAS module with vastly excessive torque.

So, SAS for rovers has the following effects:

* On flat terrain it will do what you probably want it to do, keeping your wheels on the ground.

* On terrain of constant gradient it is only useful if you are not turning. It will actually be counterproductive if you are turning upslope, attempting to to flip the rover out of the turn. It will cause understeer when you turn downslope.

* On terrain with varying gradient it is never useful.

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At high speeds on flat terrain it's certainly useful. If I remember rightly my supersonic car would crash without SAS.

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I put SAS on 'docking mode', rather than staging mode, which makes your WASD keys turn rather than tumble. For varying terrain I tap 'F' every once in a while toggling SAS so that it resets to level with the local surface. Very helpful - every time I forget reaction wheels on a rover I end up flipping it at some point.

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Cruising around in a little rover with RCS enabled and no reaction wheel torque will let you see every control action that the SAS tries to make.

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At high speeds on flat terrain it's certainly useful. If I remember rightly my supersonic car would crash without SAS.

Yep, as I said on flat terrain it will do what you want it to do: by trying to keep your pitch and roll at zero it will be keeping the plane of your wheels aligned with the terrain. It's hills where it won't work, particularly if you try to turn while on a hill, where it may actually cause you to flip

Cruising around in a little rover with RCS enabled and no reaction wheel torque will let you see every control action that the SAS tries to make.

Good idea!

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Yep, as I said on flat terrain it will do what you want it to do: by trying to keep your pitch and roll at zero it will be keeping the plane of your wheels aligned with the terrain. It's hills where it won't work, particularly if you try to turn while on a hill, where it may actually cause you to flip

I wonder if the fact that I've never relied on reaction wheels at all on my rovers, and even disable torque on some cockpits and probe cores that go on them, is part of why I seem to have such a different experience. I hear about people adding enough torque to straight up flip their rovers; mine don't have enough torque to do more than very slightly adjust pitch if I get air.

Just a thought. Almost all my wrecks involve racking my undercarriage on terrain spikes, a hazard that I'm sure everyone is well familiar with; I almost never actually roll a rover over. I don't think that I'm nearly as good at this game as a lot of folks so I wonder what is making the difference. I sometimes suspect it is because I rely solely on directed force (dead weight, direct downforce, limited control surfaces, etc) rather than torque.

Could be that SAS with lots of torque is bad news. I have never driven one for any distance, I disliked the way torque-heavy rovers handled immediately on testing.*

*EDIT: Exception is playaround craft using anti-grav plates, I've used torque to steer those. But those obviously don't contact the ground.

Edited by Hagen von Tronje
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I expect so. There have been dozens (if not hundreds) of posts about how to prevent rovers from flipping. Most of the time SAS is recommended. But testing indicates that this is actually bad advice if you intend to use the rover anywhere other than entirely flat terrain.

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In my experience, SAS isn't great for surface rovering at all, except if you've already bounced badly and need to kill rotation before you hit the ground tumbling.

Here's the thing. When SAS is activated, the qewasd keys (in staging mode) basically have two effects.

1. While you're pressing them SAS does basically nothing - you're inputting maximum steering input in at least one direction, and the SAS more or less disables itself so you won't have to fight it.

2. When you release the key, SAS immediately tries to lock you to that attitude, and correctively steers to return you to it.

Now, SAS also self-adjusts to the mode in '1' if it's taking a beating from external things. I guess this is to avoid create feedback oscillations caused by a little bump making a big counter-steer moment. Hence why, if you're turning quickly, releasing a key won't instantly lock you to that fixed direction. (Most noticeably, in flight tapping an input key resets all the current SAS steering to zero, and then the SAS settles on a slightly different attitude once it's nulled out any natural drift.)

What I do is set up my rover keys to something else, like a joystick input or the numberpad let's say. When doing this, if it's active, the SAS will now try to resist changes to direction or attitude, whether it's input by you or by the surface topography, because you're not forcing it into 'mode 1' with your steering input anymore, you're just turning the wheels (this might not be the case anymore).

I mention the above because how you control the rover affects how the SAS will respond. Using qewasd in stage mode will have a very different effect to using the same in docking mode or using a completely separate control input. Not only that, but in the absence of that affect (from those keys in stage mode), SAS will attempt to stay locked to a fixed direction and attitude. In other words, if you have enough torque wheels and enough power, and you drive along a perfect sphere, by the time you get 1/4 of the way around, the SAS will have lifted the front wheels up from the surface to be perpendicular to it - you'll be standing on the back end of the rover. Imperfections in the surface may cause this to reset, as when pressing a qewasd key (discussed above), which is just as well, but over short, smooth distances, the SAS will continue to attempt to maintain it's attitude and direction. In other words, yes, it will try to stop the vehicle from returning to level after a hill climb, or it will hold the rover up on two or even one wheel if you drive along a slope and then reach flat ground. The SAS will only reorient it's 'locked' attitude if:

1. you disable and reactivate it

2. you press qewasd in staging mode

3. your rover hits a bump at speed, forcing the SAS to re-evaluate it's orientation.

Hope that's clear. This is based on extensive rovering a few version ago, so it's possible that some of the details have been altered, but in recent rovering on Minmus I was still using this conception of what's going on and it didn't seem to be any less accurate.

Edited by The_Rocketeer
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I expect so. There have been dozens (if not hundreds) of posts about how to prevent rovers from flipping. Most of the time SAS is recommended. But testing indicates that this is actually bad advice if you intend to use the rover anywhere other than entirely flat terrain.

You specify that you used a vehicle which exerted "vastly excessive torque". I submit that this test does not distinguish whether the bad advice is to use SAS, or to employ (probably centerline chassis) torque on rovers if your chief concern is not rolling over. SAS most certainly works on control methods other than reaction wheels; I believe it can act on wheels themselves, and I'm very sure it can work on RCS, engines, and control surfaces.

Also worth mentioning that I prefer to place stability controls such as downthrusters or control surfaces at the extremities near the wheels. I'd venture a guess that this behaves differently under SAS than centerline torque does.

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From my expierience, SAS on rovers is semi-helpful at best.

It does behave pretty much the same as if it was used in a plane.

In stability mode, when moving over the top of a steep hill at some speed, it will try to maintain the direction of the rover

and thus preventing the front part to sink down to the ground. If the reaction wheels/RCS is stong enough,

your rover will sink down on the other side horizontally. Bad for your front tires, as they will have no contact for a longer period of time than without SAS.

But good for your rear tires, as they will not be lifted as much as they would without SAS.

When turning on sloped terrain, it will also not adapt to the needs of a ground vehicle. It can even make thing worse.

It will always try to stop any externally inducted change of orientation, except for active player inputs.

The only real application on the ground is, that it will stabilize your current horizontal direction on fairly even ground to some extend

when driving over gentle humps that do not make your rover to take off.

SAS on the ground is not what you would need for a ground vehicle.

It's (sadly) not an ESC or a TC-like system.

A TC system will always try to maintain grip of your tires during turns or acceleration/breaking by reducing power of the engine or selective breaking of tires.

An ESC will try to maintain grip of your tires and additionally will try to counteract forces through every available control system that

would otherwise lead to a loss of the current driver-inducted "directional request".

I never expierienced SAS acting in such a way in stability mode.

So from what I can say, SAS has no appropriate control mode for rovers on the ground.

But I would really like to see such a feature in the future.

Edited by Cairol
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I didn't read the whole thread but, remap rover controls to I,J,K,L. (Why they haven't done this in the stock configuration is beyond my comprehension) Never have rover steering and torque on the same controls. For that matter, never have plane steering and yaw on the same controls either (planes use rover controls for steering). There is no need for SAS on a rover anyway, and should really just be turned off, but standard torque applies even when off. Since the normal reaction wheel torque applies, you should never use W to move forward anyway because that will torque forward. I,J,K,L is your solution.

It's interesting, if you have a flight stick with throttle control, you don't use W (or I) as forward anyway. Not sure if you can use Shift to increase speed of rover wheels or not.

Edited by Alshain
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I've used SAS on my rovers to actually increase the speed that they can turn at without flipping. The idea is somewhat like a motorbike... lean into the corner. First, you'd have to re-map your controls, as suggested by Alshain and others, but once you do it should be easy. Basically, when you're turning right, the top of the vehicle wants to tip the other way, so you counteract that with a little SAS rotation. This is the technique that I used for my circumnavigation back in 0.90, and it worked well enough then. The ability to rotate your vehicle so it lands smoothly on the next bit of terrain is surprisingly helpful. Much nicer than launching off a hill and not having any control in the air. Or if you hit a rough patch and find yourself two feet off the ground, going 50m/s sideways. With proper SAS, that's very easy to correct. Without... you'll prolly need a new rover.

It takes some getting used to, but with the proper input device and enough patience, it can get really easy.

Edited by Slam_Jones
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I didn't read the whole thread but, remap rover controls to I,J,K,L.

...

Since the normal reaction wheel torque applies, you should never use W to move forward anyway because that will torque forward. I,J,K,L is your solution.

...

Not sure if you can use Shift to increase speed of rover wheels or not.

Be aware that IJKL also fire RCS translate thrusters. Make sure RCS is off of you're using these. Better still, use something else completely.

Also, using shift-accelerate alt-w (dummy )does work the same as any other trim setting, but unless you're using shiftalt-w (or similar) and combining with pitch/roll/yaw/translate in docking mode you won't have a 'setting' indicator since it won't do anything to the pitch needle.

- - - Updated - - -

I've used SAS on my rovers to actually increase the speed that they can turn at without flipping... Basically, when you're turning right, the top of the vehicle wants to tip the other way, so you counteract that with a little SAS rotation.

You're talking about reaction wheels, not SAS. In older versions of KSP they were essentially the same thing/part, but they got overhauled sometime around 0.20 and SAS is now specifically the 'autopilot' that fixes your orientation, and reaction wheels are the rotation torque you're describing.

Edited by The_Rocketeer
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^^ Ah, I always forget that.

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Be aware that IJKL also fire RCS translate thrusters. Make sure RCS is off of you're using these. Better still, use something else completely.

Also, using shift-accelerate does work the same as any other trim setting, but unless you're using shift-w (or similar) and combining with pitch/roll/yaw/translate in docking mode you won't have a 'setting' indicator since it won't do anything to the pitch needle.

RCS is easy to turn off. Reaction wheels are harder to turn off. I wasn't talking about the trim control (that is Alt, not Shift), I was talking about throttle control. With my flight stick I use the throttle control to make my rover move forward or backward just like burning on a rocket. A big advantage of this is I do not have to keep holding it down. The disadvantage is you need to disable the wheels for transport, unless you just have an abundance of power. If your rover itself uses engines for something, they also need to be disabled when you don't want them.

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RCS is easy to turn off. Reaction wheels are harder to turn off.

True, but if you don't realise you're activating your RCS thrusters, best case you don't notice and have no monoprop left, and worst case you flip out of control and explode. Just offering fair warning to the uninitiated

I wasn't talking about the trim control (that is Alt, not Shift), I was talking about throttle control.

Yes of course! Got myself muddled there. The throttle setting of course doesn't affect rovers at all (unless you made a rocket car), but I guess you mean sharing a keybind for both controls. Personally for long haul rovering I find the trim setting very convenient. Driving around Kerbin or similar gravity worlds, I rely on low CoM and wide wheelbase to keep me upright, but I also tend to keep a reaction wheel active on low-g worlds just in case my rover loses interest in mortal life.

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Guys, the problem isn't with input bindings. Yes, those can also create a problem, but I'm not talking about that. And I'm not talking about the reaction wheels providing input. I'm talking very specifically about SAS itself.

SAS will use the reaction wheels (etc) to maintain an orientation of pitch yaw and roll. If you are rovering around on a hill, you do not want your pitch or roll orientation to be maintained, since stable values for pitch and roll on a hill depend on which way you are facing. On flat terrain, the stable values for pitch and roll are always zero, so SAS works as desired there.

Edited by allmhuran
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For that matter, never have plane steering and yaw on the same controls either (planes use rover controls for steering).
I'm the other way, I generally do like having my rudder and my wheel steering on the same control, so that I don't need to switch controls or worry about which to use during takeoff and landing rolls.

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