Difference between revisions of "SAS"

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(ran some tests, I was wrong about some stuff!)
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SAS is a flight system that will help control your ship by countering rotation. Advanced SAS (ASAS) adds to this by also controlling heading.
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{{About|the SAS since 0.21|SAS in the [[demo]]|Stability Augmentation System (Demo)}}
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SAS is distinct from [[S.A.S._Module|S.A.S]], as the latter is a ship part (which exerts torque ). In order to use ASAS, you will need an [[Advanced_S.A.S._Module|ASAS module]].
 
  
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== Effect of SAS ==
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'''SAS''' is an automated [[craft]] stabilizing/orienting system. There are varied official descriptions of what “SAS” (or “S.A.S.”) stands for, including '''Stability Augmentation System''', '''Stability Assist (System)''', '''Sickness Avoidance Scheme''', and '''Sickness Avoidance Solution'''. Most often, it's simply called by its acronym.
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SAS units exert torque to stop vessel rotation. This torque is not available to the command unit for control authority, and is only active when the unit is turned on.
 
  
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== Effect of ASAS ==
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Prior to {{version|0.21}} there was a distinction between “SAS” and “ASAS” (Advanced S.A.S.). The old SAS was the "dumb" system providing torque but no automatic course correction while ASAS provided both as the "smart" system. After 0.21, SAS and ASAS were used interchangeably, and later the term “ASAS” largely ceased to be used.
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Although ASAS says that it does not exert any force, this is not true. Testing shows that ASAS units exert a small amount of torque. How much and why is not currently known. This torque appears to be under control of the pilot even when SAS is turned off.
 
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ASAS directly runs your ship controls as if it were pressing the "qweasd" keys, and thus affects control surfaces and engine gimbal. You can see it functioning by looking at the bottom left-hand corner and watching the pitch, and yaw twitch around. ASAS does not perform integration on roll, and thus will not attempt to correct for it. You can control the ship while ASAS is on, but the inputs are additive, so the SAS may fight you. This can be useful when trying to make slow movements or fine adjustments.
 
  
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Since ASAS also controls heading, it can be extremely useful for lander missions. Turn on SAS while facing straight up (relative to the surface) and then use the arrow keys to fight the ASAS and kill lateral motion. The ASAS will keep returning your vessel to upright so you can focus on steering without worrying about falling over.
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== Usage ==
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SAS is an automatic system toggled on or off during flight.  
  
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== Control ==
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For SAS to be available at all in career mode, the craft must include an unmanned pod (probe core) with SAS capability or, since {{version|0.90.0}}, a pilot. All robotic [[command module]]s except the [[Stayputnik Mk. 1]] have (varying) SAS capabilities. At least one [[kerbonaut]] manning a command module must have the Pilot specialization to use SAS. In [[Sandbox]] mode there is no restriction on SAS.
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You can turn the system on and off with the T key, and temporarily invert the state by holding the F key (For example, if SAS is on, holding F will turn it off, and releasing F will turn it back on). The current state of the SAS system is shown by a light on your [[Navball]]. The SAS and ASAS are controlled together, as ASAS adds heading control to the standard SAS rotation dampening.
 
  
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All command pods offer a small amount of SAS torque, and you can add to this by adding more SAS units to the craft. ASAS however is simply either there or not, you can not add multiple units to increase its effect.
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When active, SAS will provide input to stability systems to dampen craft rotation and, depending on capabilities, lock onto a specific orientation. The user can override the current rotation for any [[axis]]. Since it controls heading, it can be extremely useful for lander missions. When active, the SAS itself doesn't drain [[Electric charge|electricity]], though reaction wheels may continue fine-tuning even in space. It will utilize all active control systems to achieve this, like the user would by pressing the keys.
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* Torque supplied by [[reaction wheel]]s
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* [[Control surface]]s (only effective in [[atmosphere]])
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* [[RCS]], if activated
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* [[Engine]]s' [[gimbal]]ing, if throttled up
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=== Controls ===
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The system can be toggled with (by default) the {{Key press|T}} key and temporarily switched by holding down the {{Key press|F}} key (for example, if SAS is on, holding F will turn it off, and releasing F will turn it back on). The current state of the SAS system is shown by a blue light on the [[navball]].
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While SAS is turned on, if any capabilities are available beyond stability assist, a [[direction selector]] will appear to the left of the navball. If only "stability assistance" is available, no selectors will appear. By default "stability assistance" is selected and stabilizes on the current heading. But the user can select other vectors depending on the capabilities of the probe core/pilot such as prograde, retrograde, normal, antinormal, radially in/out, and target prograde/retrograde. Pilot kerbonauts have a levelled progression of directions they can point toward.
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Beware, having SAS activated while deploying a [[parachute]] may cause the parachute to rip off due to the SAS attempting to stabilize the craft's sudden rotation to orient in the wind.
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== Construction ==
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Placement should be considered while building craft in the [[editor]]. Though manned command modules will tend to be at the front of the craft for visibility in [[IVA]], SAS modules in general are best placed close to the [[center of mass]], especially those with more torque.
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{{quote|The placement does matter for [[reaction wheel]]s. Generally speaking they can cause some problems if placed far from the [[center of mass]]. Imagine you are grabbing that point and rotating it. That is what the reaction wheels will try to do. You'll get offcenter rotation anywhere other then near the [[Center of mass|COM]].|[http://forum.kerbalspaceprogram.com/entries/740-Updated-Information-on-SAS-in-0-21-1#comment_4254 comment] by C7, in his blog entry “[http://forum.kerbalspaceprogram.com/entries/740-Updated-Information-on-SAS-in-0-21-1 Updated Information on SAS in 0.21.1]”}}
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=== SAS capabilities ===
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{{Directions available}}
  
 
==Theory==
 
==Theory==
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{{Outdated}}
 
The S.A.S. modules use a P.I.D. system, which stands for “Proportional, Integral, Derivative”. The PID is applied to the vessel's angular velocity, not its heading.
 
The S.A.S. modules use a P.I.D. system, which stands for “Proportional, Integral, Derivative”. The PID is applied to the vessel's angular velocity, not its heading.
 
* ''Proportional'' means the S.A.S. module applies a turning-force that is proportional to the speed of rotation. In other words, the faster the ship is spinning, the harder the module tries to correct the spin.
 
* ''Proportional'' means the S.A.S. module applies a turning-force that is proportional to the speed of rotation. In other words, the faster the ship is spinning, the harder the module tries to correct the spin.
 
* ''Integral'' means the S.A.S. module increases the corrective force the longer the ship is off-target. Since the integral of speed is position, this corrects the vessel's heading (angle is the integral of angular velocity). Since the summed integral value is set to zero when SAS is turned on, the controller will attempt to lock the vessel's heading to whatever it was when SAS was turned on.
 
* ''Integral'' means the S.A.S. module increases the corrective force the longer the ship is off-target. Since the integral of speed is position, this corrects the vessel's heading (angle is the integral of angular velocity). Since the summed integral value is set to zero when SAS is turned on, the controller will attempt to lock the vessel's heading to whatever it was when SAS was turned on.
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*''Derivative'' means the S.A.S. module takes the angular acceleration of the ship into account, and tries to apply a force against it. So the faster the ship is going "into" the spin, the harder the S.A.S. module tries to stop it. This is supposed to dampen the action of the S.A.S module, preventing overshoot, as well as dampening any accelerating turn or roll.
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*''Derivative'' means the S.A.S. module takes the angular acceleration of the ship into account and tries to apply a force against it. So the faster the ship is going "into" the spin, the harder the S.A.S. module tries to stop it. This is supposed to dampen the action of the S.A.S module and prevent overshoot, as well as dampening any accelerating turn or roll.
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The SAS functionality found within command modules and standard SAS units use only PD, with no I, and thus do not lock the heading of the vessel.
 
  
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The PID control is applied to the vessel's rotational velocity '''at the command point'''. This means that large rockets that tend to wobble out of control during flight do so because the SAS sees the tip (where the command module usually is) rotating and assumes the entire vessel is spinning this fast. For this reason, "wobble" can be largely eliminated by controlling the vessel as close to the CG as possible. This can be accomplished by placing a [[Command_Module|command module]] or [[Parts#Docking|docking port]] in the middle of the rocket. Right click the unit and select "Control From Here" to change the control point. If you end up controlling the "dead" end of the rocket after stage seperation, simply press the "[" or "]" keys to select the appropriate section.
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The PID control is applied to the vessel's rotational velocity '''at the command point'''. This means that large rockets that tend to wobble out of control during flight do so because the SAS sees the tip (where the command module usually is) rotating and assumes the entire vessel is spinning this fast. For this reason, "wobble" can be largely eliminated by controlling the vessel as close to the CG as possible. This can be accomplished by placing a [[command module]] or [[docking port]] in the middle of the rocket. Right click the unit and select "Control From Here" to change the control point. If you end up controlling the "dead" end of the rocket after stage separation, simply press the "[" or "]" keys to select the appropriate section.
  
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[[Advanced S.A.S. Module|Advanced S.A.S.]] does not produce torque itself (or if it does, the forces are small), all it does is alter the PID values.
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== See also ==
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* [http://forum.kerbalspaceprogram.com/entry.php/740-Updated-Information-on-SAS-in-0-21-1 Updated Information on SAS in 0.21.1] blog entry by C7
  
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== SAS/ASAS Parts ==
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[[Category:Control]]
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The game currently offers four SAS/ASAS units; a small radius SAS, a small radius ASAS, a large radius ASAS and an 'avionics package'. You also get some SAS functionality from command modules.
 
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===Dedicated Units===
 
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{{Template:Stats Table SAS|{{Template:SUBPAGENAME}}}}
 
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===Command Pods===
 
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{{Template:Stats Table Commands Pods|{{Template:SUBPAGENAME}}}}
 

Latest revision as of 10:18, 30 April 2015

This article is about the SAS since 0.21. For the SAS in the demo, see Stability Augmentation System (Demo).

SAS is an automated craft stabilizing/orienting system. There are varied official descriptions of what “SAS” (or “S.A.S.”) stands for, including Stability Augmentation System, Stability Assist (System), Sickness Avoidance Scheme, and Sickness Avoidance Solution. Most often, it's simply called by its acronym.

Prior to version 0.21 there was a distinction between “SAS” and “ASAS” (Advanced S.A.S.). The old SAS was the "dumb" system providing torque but no automatic course correction while ASAS provided both as the "smart" system. After 0.21, SAS and ASAS were used interchangeably, and later the term “ASAS” largely ceased to be used.

Usage

SAS is an automatic system toggled on or off during flight.

For SAS to be available at all in career mode, the craft must include an unmanned pod (probe core) with SAS capability or, since version 0.90.0, a pilot. All robotic command modules except the Stayputnik Mk. 1 have (varying) SAS capabilities. At least one kerbonaut manning a command module must have the Pilot specialization to use SAS. In Sandbox mode there is no restriction on SAS.

When active, SAS will provide input to stability systems to dampen craft rotation and, depending on capabilities, lock onto a specific orientation. The user can override the current rotation for any axis. Since it controls heading, it can be extremely useful for lander missions. When active, the SAS itself doesn't drain electricity, though reaction wheels may continue fine-tuning even in space. It will utilize all active control systems to achieve this, like the user would by pressing the keys.

Controls

The system can be toggled with (by default) the T key and temporarily switched by holding down the F key (for example, if SAS is on, holding F will turn it off, and releasing F will turn it back on). The current state of the SAS system is shown by a blue light on the navball.

While SAS is turned on, if any capabilities are available beyond stability assist, a direction selector will appear to the left of the navball. If only "stability assistance" is available, no selectors will appear. By default "stability assistance" is selected and stabilizes on the current heading. But the user can select other vectors depending on the capabilities of the probe core/pilot such as prograde, retrograde, normal, antinormal, radially in/out, and target prograde/retrograde. Pilot kerbonauts have a levelled progression of directions they can point toward.

Beware, having SAS activated while deploying a parachute may cause the parachute to rip off due to the SAS attempting to stabilize the craft's sudden rotation to orient in the wind.

Construction

Placement should be considered while building craft in the editor. Though manned command modules will tend to be at the front of the craft for visibility in IVA, SAS modules in general are best placed close to the center of mass, especially those with more torque.

“ The placement does matter for reaction wheels. Generally speaking they can cause some problems if placed far from the center of mass. Imagine you are grabbing that point and rotating it. That is what the reaction wheels will try to do. You'll get offcenter rotation anywhere other then near the COM.

— comment by C7, in his blog entry “Updated Information on SAS in 0.21.1”

 ”

SAS capabilities

Name Cost SAS level & capabilities
0 1 2 3
Stability
Assist
(Level) 		Prograde/
Retrograde
(Prograde/Retrograde) 		Normal/Radial
Hold
(Normal/Anti-normal) 		Target
Tracking
(Target prograde/Target retrograde) 		Maneuver
(Maneuver prograde)
Probodobodyne RoveMate 800 Funds Yes No No No No
Probodobodyne Stayputnik 300 Funds No No No No No
Probodobodyne QBE 360 Funds Yes No No No No
Probodobodyne HECS 650 Funds Yes Yes No No No
Probodobodyne OKTO 450 Funds Yes No No No No
Probodobodyne OKTO2 1 480 Funds Yes Yes Yes No No
Probodobodyne HECS2 7 500 Funds Yes Yes Yes Yes Yes
RC-001S Remote Guidance Unit 2 250 Funds Yes Yes Yes Yes Yes
RC-L01 Remote Guidance Unit 3 400 Funds Yes Yes Yes Yes Yes
MK2 Drone Core 2 700 Funds Yes Yes Yes Yes Yes
MPO Probe 9 900 Funds Yes Yes Yes Yes Yes
MTM Stage 21 500 Funds Yes Yes Yes No No
CH-J3 Fly-By-Wire Avionics Hub[Note 1] 5 200 Funds Yes Yes Yes Yes Yes
Pilot level 0 0 XP Yes No No No No
Pilot level 1 2 XP Yes Yes No No No
Pilot level 2 8 XP Yes Yes Yes No No
Pilot level 3 16 XP Yes Yes Yes Yes Yes
Pilot level 4 32 XP Yes Yes Yes Yes Yes
Pilot level 5 64 XP Yes Yes Yes Yes Yes
  1. ↑ dedicated SAS units don't work without command modules


Theory

The S.A.S. modules use a P.I.D. system, which stands for “Proportional, Integral, Derivative”. The PID is applied to the vessel's angular velocity, not its heading.

  • Proportional means the S.A.S. module applies a turning-force that is proportional to the speed of rotation. In other words, the faster the ship is spinning, the harder the module tries to correct the spin.
  • Integral means the S.A.S. module increases the corrective force the longer the ship is off-target. Since the integral of speed is position, this corrects the vessel's heading (angle is the integral of angular velocity). Since the summed integral value is set to zero when SAS is turned on, the controller will attempt to lock the vessel's heading to whatever it was when SAS was turned on.
  • Derivative means the S.A.S. module takes the angular acceleration of the ship into account and tries to apply a force against it. So the faster the ship is going "into" the spin, the harder the S.A.S. module tries to stop it. This is supposed to dampen the action of the S.A.S module and prevent overshoot, as well as dampening any accelerating turn or roll.

The PID control is applied to the vessel's rotational velocity at the command point. This means that large rockets that tend to wobble out of control during flight do so because the SAS sees the tip (where the command module usually is) rotating and assumes the entire vessel is spinning this fast. For this reason, "wobble" can be largely eliminated by controlling the vessel as close to the CG as possible. This can be accomplished by placing a command module or docking port in the middle of the rocket. Right click the unit and select "Control From Here" to change the control point. If you end up controlling the "dead" end of the rocket after stage separation, simply press the "[" or "]" keys to select the appropriate section.

See also

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