Stability Augmentation System
The Stability Augmentation System (abbr. SAS) 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.
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.
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.
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 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.
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.
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.
SAS has 4 dedicated parts as well as being included in all Command Pods and Probe bodies.
|Image||Part||Radial size||Cost (¤)||Mass (t)||Drag||Torque||Electricity (e/s)|
|Inline Reaction Wheel||Small||600||0.3||0.2||20||0.3|
|Inline Advanced Stabilizer||Small||1100||0.5||0.2||20||0.3|
|Advanced S.A.S Module, Large||Large||2200||0.2||0.2||20||0.3|
|Image||Part||Radial size||Cost (¤)||Mass (t)||Drag||Max. Temp. (K)||Tolerance||Torque||Required Crew/Power|
|Command Pod Mk1||Small, Tiny||600||0.8||0.2||3400||14||2.4||1|
|Mk1-2 Command Pod||Large, Small||1800||4||0.2||3400||45||12||3|
|Mk1 Lander Can||Small||600||0.6||0.2||3400||8||3||1|
|PPD-12 Cupola Module||Large, Small||2000||4.5||0.4||3400||8||9||1|
|Probodobodyne QBE||Tiny||500||0.08||0.15-0.2||3200||40||0.25||1.7 e/m|
|Probodobodyne HECS||Tiny||450||0.1||0.2||1200||12||0.3||1.7 e/m|
|Probodobodyne OKTO||Tiny||450||0.1||0.15-0.2||1200||12||0.3||1.7 e/m|
|Probodobodyne OKTO2||Tiny||230||0.04||0.15-0.2||1200||12||0.15||1.7 e/m|
|Stayputnik Mk. 1||Tiny||300||0.05||0.15-0.2||1200||12||0.15||1.7 e/m|
|RC-001S Remote Guidance Unit||Small||750||0.1||0.2||3400||9||0.3||3.0 e/m|
|RC-L01 Remote Guidance Unit||Large||2000||0.5||0.2||3400||9||1.5||3.0 e/m|
- Updated Information on SAS in 0.21.1 blog entry by C7