[Tutorial] SAS or ASAS?

[sodopro]

OUTDATED. Still here for demo users, but this is mostly useless for 0.21+.

I previously wrote a different version before the great DDOS attack. Since there is no tutorial sub section, I'll post it here for now.

SAS, or ASAS? That is the question. Which one do YOU need for YOUR spacecraft? In this mini-guide, I'll be teaching you the differences, as well as which one to use on specific aircraft/rockets.

First of all, to use any one of these parts, press T to toggle (on/off) or F to trigger (holding F will keep SAS on).

SAS

SAS is your basic rotational stabilizer. Usually used on rockets, SAS stops your rocket from spinning all over the place. SAS has rotational torque only, making it useless for vertical and horizontal stability. However, SAS can be useful when launching large rockets, to prevent them from entering death spirals.

http://wiki.kerbalspaceprogram.com/wiki/S.A.S._Module

ASAS

ASAS stands for Advanced SAS, and keeps the entire aircraft/rocket stable. It does this by using other controllable parts. ASAS, unlike SAS does not have any torque itself, so by itself, it is useless. However, ASAS utilizes gimbal engines, winglets/canards, RCS and other parts with torque (Command pods and SAS) to keep the craft stable.

http://wiki.kerbalspaceprogram.com/wiki/Advanced_S.A.S._Module

Avionics Package

The avionics package is basically the same as ASAS; it utilizes other parts to stop your craft from moving. The AvPa is usually mounted on a nosecone.

http://wiki.kerbalspaceprogram.com/wiki/Avionics_Package

Edited September 12, 2013 by sodopro

[sodopro]

So which one are you going to use? Well, it depends on your spacecraft.

Most of the time, you'll find yourself using ASAS, just for generally everything. ASAS can apply stability to all planes of motion, whereas normal SAS only does this for rotational force. If you do find yourself needing torque, an empty command pod is probably the better choice, as they can be used by ASAS and they apply torque to all planes. If using a large rocket, and you just want it to stop spinning, SAS may be your choice.

On planes, the Avionics Package is a better choice.

Edited April 21, 2013 by sodopro

[Fourjays]

I'll ask this here as I suspect the answer may help others in future.

With either an SAS or an ASAS, I'm having issues with a simple rocket rotating/spinning (always clockwise) as soon as it leaves the pad. As I understand it, ASAS should be using the thrust vectoring and SAS the torque. Why aren't either of them having an effect?

The design is very simple. Probe sphere and an SAS/ASAS, T400 tank below it with an LVT30 engine. Then 3 T800 tanks with LVT30 engines strapped to the side with separators. Here is a screenshot, and you can see it is a long way off after just 13 seconds (no player input, just turned SAS on and launched).

[Vanamonde]

SAS mode is TRYING to keep you steady, but the mass of your ship is distributed to the sides, and it's going to have a far amount of rotational inertia for the guidance to fight. Also, gimbaled engines currently do nothing to fight spin, so the guidance has very little to work with. Some canards or fins with control surfaces, though, should fix that little ship right up.

[Specialist290]

sodopro: Nicely done. Adding this to the Drawing Board.

Fourjays: The simplest answer is likely that the torque produced by the thrust from your engines is enough to overpower the combined torque from your probe body and your SAS units. The LVT-30 is also a non-gimbaled engine, so your ASAS can't use any sort of thrust vectoring on the radial-mounted engines to counter the rotation like it could with gimbaled engines such as the LVT-45.

EDIT: Didn't know that engine gimbaling isn't used to counter rotation. Still, Vanamonde's advice is sound. Add a set of winglets, and you'll have much better control, at least in atmosphere.

[purpletarget]

With either an SAS or an ASAS, I'm having issues with a simple rocket rotating/spinning (always clockwise) as soon as it leaves the pad. As I understand it, ASAS should be using the thrust vectoring and SAS the torque. Why aren't either of them having an effect?

The design is very simple. Probe sphere and an SAS/ASAS, T400 tank below it with an LVT30 engine. Then 3 T800 tanks with LVT30 engines strapped to the side with separators. Here is a screenshot, and you can see it is a long way off after just 13 seconds (no player input, just turned SAS on and launched).

And here lies the confusion with the names and what each part does/doesn't do.

SAS has some muscle for killing rotation, but no brains.

ASAS is a flight computer...no muscle, but it tells all the other controls (including pod Torque) what to do.

If I'm reading your picture right, the problem is that you have a SAS for kill Rot, but no ASAS to tell it to do anything. Without ASAS or Avionics, clicking on the SAS button in flight basically does the square root of SFA.

Try adding an ASAS module to the design above, and nothing else...and see what you get. It should get the SAS module to kill the rotation.

[Eric S]

SAS mode is TRYING to keep you steady, but the mass of your ship is distributed to the sides, and it's going to have a far amount of rotational inertia for the guidance to fight. Also, gimbaled engines currently do nothing to fight spin, so the guidance has very little to work with. Some canards or fins with control surfaces, though, should fix that little ship right up.

Correct. The only thing that ship has to keep it from spinning is the rotPower of the probe pod, and the stayputnik has the lowest rotPower of all command pods. Yes, even the other probe pods have much more rotPower than the stayputnik. I recommend avoiding that pod for exactly that reason unless you are willing to put up with the lack of rotPower for aesthetic reasons.

EDIT: Missed the SAS. That helps kill spin/rotation, with or without an ASAS, but won't help create spin/rotation.

Edited May 16, 2013 by Eric S

[Jokurr]

So which one are you going to use? Well, it depends on your spacecraft.

Most of the time, you'll find yourself using ASAS, just for generally everything. ASAS can apply stability to all planes of motion, whereas normal SAS only does this for rotational force. If you do find yourself needing torque, an empty command pod is probably the better choice, as they can be used by ASAS and they apply torque to all planes. If using a large rocket, and you just want it to stop spinning, SAS may be your choice.

On planes, the Avionics Package is a better choice.

Could you please explain why exactly the avionics package is better for planes?

[Finnerty]

Another thank you for this explanation.

[Rhomphaia]

Could you please explain why exactly the avionics package is better for planes?

Essentially the A.S.A.S will attempt to counteract any rotoation while the Avionics package will simply attempt to stop it. Neither of them can tell whether the rotation caused by instability or steering input

So when flying a plane you can have the avionics running and still steer, if you had A.S.A.S it would have to be switched off before maneuvering or it would fight the controls constantly trying to return to its original heading.

Also the bit in the OP about S.A.S only applying torque to one plane is wrong.

[Sorcie]

Your twist is from unsupported radial design. Without struts on the lower half of your radial tanks, when thrust is applied, some of any flex in the decouplers, tanks, gimbal from engines, will be applied in a circular motion around the craft. Eliminate this with at least struts on the bottom of the tank.

Looks like even with 4 sets of perfectly symmetric struts the same thing occurs at a slower rate. In any case, use ASAS vs. SAS on that.

I know this was SAS/ASAS discussion, but that craft doesn't need a SAS torque unit, it's too small. ASAS or avionics only.

I'll ask this here as I suspect the answer may help others in future.

With either an SAS or an ASAS, I'm having issues with a simple rocket rotating/spinning (always clockwise) as soon as it leaves the pad. As I understand it, ASAS should be using the thrust vectoring and SAS the torque. Why aren't either of them having an effect?

The design is very simple. Probe sphere and an SAS/ASAS, T400 tank below it with an LVT30 engine. Then 3 T800 tanks with LVT30 engines strapped to the side with separators. Here is a screenshot, and you can see it is a long way off after just 13 seconds (no player input, just turned SAS on and launched).

Edited May 16, 2013 by Sorcie

[georgTF]

The Avionics package attempts to hold course just like ASAS does. The difference is that ASAS resists your control input to the point of allways returning to the same heading/angle, while Avionics resist your input but make no attempt to return to the heading/angle the craft had when you engaged it.

This makes very agile planes easier to fly and land because control is smoother but you can still steer. ASAS would "steer back" making your control input useless.

[Fourjays]

Thanks for all the tips. Seems there are several areas I can look at.

Was quite a shock though when this simple design kept going into death spirals (been playing with mods for a while). Despite that I did manage to launch two probes with it by jettisoning the boosters when things got too out of control (heading down instead of up) and using the second stage to regain control. Needless to say a new rocket design is going to be needed before I send Kerbals up again.

[XerZin]

So if you place both ASAS and SAS on your ship.. what happens? Since the Stock part of the space station have both..

[sodopro]

Well, the ASAS would take control of the SAS.

[Kerbol Prime]

So the SAS would be sort of a gyroscope, while ASAS is more of a computer type-majigy?

[Eric S]

That would be correct.

[Promii]

My understanding is that the SAS module is anti-rotational only. It'll use its gyros to stop you spinning, but won't help you turn like the gyros in a pod or probe body will.

[WhiteWeasel]

Does ASAS stack?

[Specialist290]

ASAS doesn't stack; an ASAS module is probably best thought of as a sort of Master Control Computer that governs all other forms of attitude control (engine gimbals, SAS, pod torque gyros, stabilizing fins, RCS, etc.) in maintaining a given heading. The only reason to have multiple ASAS units on a craft is if you plan on having two modules capable of independent operation from one another.

[WhiteWeasel]

The only reason to have multiple ASAS units on a craft is if you plan on having two modules capable of independent operation from one another.

Unless if you're using small nose cones, turns out the avionics package is slightly lighter and looks cooler, and it won't do any harm if you already have an ASAS on board .

[haltux]

My understanding is that the SAS module is anti-rotational only. It'll use its gyros to stop you spinning, but won't help you turn like the gyros in a pod or probe body will.

Yes, correct, unless KSP wiki is wrong. That's explicitely stated here: http://wiki.kerbalspaceprogram.com/wiki/SAS

In my opinion, it's a pity. There is not any kind of rational behind this, neither from the physical point of view, nor from the gameplay point of view. It means that the only way to efficiently rotate a rocket without RCS is to put Kerbals in it :-(

Edited June 14, 2013 by haltux

[John FX]

The Avionics package attempts to hold course just like ASAS does. The difference is that ASAS resists your control input to the point of allways returning to the same heading/angle, while Avionics resist your input but make no attempt to return to the heading/angle the craft had when you engaged it.

This makes very agile planes easier to fly and land because control is smoother but you can still steer. ASAS would "steer back" making your control input useless.

So for the times in space where I want the ASAS to hold me to a course but let me make adjustments as I go then I should have an avionics package instead of an ASAS which needs turning on and off for the same effect?

Can you turn them on and off individually? (switch from avionics to ASAS and back again)

[entaran]

I'll ask this here as I suspect the answer may help others in future.

With either an SAS or an ASAS, I'm having issues with a simple rocket rotating/spinning (always clockwise) as soon as it leaves the pad. As I understand it, ASAS should be using the thrust vectoring and SAS the torque. Why aren't either of them having an effect?

The design is very simple. Probe sphere and an SAS/ASAS, T400 tank below it with an LVT30 engine. Then 3 T800 tanks with LVT30 engines strapped to the side with separators. Here is a screenshot, and you can see it is a long way off after just 13 seconds (no player input, just turned SAS on and launched).

Necro Response but...

The issue here isn't that SAS/ASAS isn't holding the rocket. It's that the outer boosters aren't strutted so they're bending on the decouplers and applying force that isn't parallel to the centre pylon. Your rocket will never stop spinning until you strut those outer stages correctly.

[sodopro]

Well, the new ASAS in 0.21 will help with that more than the avionics package.

And no, you can only toggle them all on/off together.