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Stability Augmentation System

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

The Stability Augmentation System (abbr. SAS and also known as the Sickness Avoidance Scheme) is an automatic flight control system that employs various sources of torque, including reaction wheels, to control the current rotation of a spacecraft. All command modules, with the exception of the EAS-1 External Command Seat, include SAS capabilities. It is also available in dedicated SAS modules.

Prior to version 0.21 there was a distinction between SAS and ASAS. While the previous ASAS acted like the new SAS, the previous SAS acted like new reaction wheels.

Contents

Effect

When the craft is equipped with at least one SAS unit, it can utilize this unit to dampen the rotation and finally lock onto a specific orientation. This system will utilize all available control systems, like the user would with pressing the keys:

It will first dampen the rotation and reaching a specific value and then will try to lock onto its current 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.

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.

Control

The system can be turned 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 blue light on the navball.

Theory

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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.

SAS Parts

SAS has 4 dedicated parts as well as being included in all Command Pods and Probe bodies.

Dedicated Units

Image Part Radial size Cost (¤) Mass (t) Drag Torque (kN · m) Electricity (e/s)
Sas module.jpg
Inline Reaction Wheel Small 600 0.3 0.2 20 0.3
Advanced sas module.jpg
Inline Advanced Stabilizer Small 1100 0.5 0.2 20 0.3
Advanced sas module large.jpg
Advanced S.A.S Module, Large Large 2200 0.2 0.2 20 0.3

Command modules

Image Part Radial size Cost (¤) Mass (t) Drag Max. Temp. (K) Tolerance Torque (kN · m) Required Crew/Power
MK1COCKPIT.png
Mk1 Cockpit Small 1800 1.25 0.1 3400 45 3.75 1
Mk2cockpit.png
Mk2 Cockpit Small 1600 1 0.08 3400 40 3 1
Mk3cockpit.png
Mk3 Cockpit Small 1600 3.5 0.2-0.15 3400 50 10.5 3
Command Pod Mk1.png
Command Pod Mk1 Small, Tiny 600 0.8 0.2 3400 14 2.4 1
Mk1-2commandpod.png
Mk1-2 Command Pod Large, Small 1800 4 0.2 3400 45 12 3
LanderCanMK1.png
Mk1 Lander Can Small 600 0.6 0.2 3400 8 3 1
Can.png
Mk2 Lander-can Large 950 2.5 0.2 3400 8 7.5 2
Cupola.PNG
PPD-12 Cupola Module Large, Small 2000 4.5 0.4 3400 8 9 1
Cube1.png
Probodobodyne QBE Tiny 500 0.08 0.15-0.2 3200 40 0.25 1.7 e/m
Probodobodyne HECS.png
Probodobodyne HECS Tiny 450 0.1 0.2 1200 12 0.3 1.7 e/m
Octo.png
Probodobodyne OKTO Tiny 450 0.1 0.15-0.2 1200 12 0.3 1.7 e/m
ProbeFlatty.png
Probodobodyne OKTO2 Tiny 230 0.04 0.15-0.2 1200 12 0.15 1.7 e/m
Sput.png
Stayputnik Mk. 1 Tiny 300 0.05 0.15-0.2 1200 12 0.15 1.7 e/m
Probestack.png
RC-001S Remote Guidance Unit Small 750 0.1 0.2 3400 9 0.3 3.0 e/m
Probehuge.png
RC-L01 Remote Guidance Unit Large 2000 0.5 0.2 3400 9 1.5 3.0 e/m

See also