Stability Augmentation System (Demo)

The Stability Augmentation System (abbr. SAS) is a flight system that will help control your ship by countering rotation. Advanced SAS (ASAS) adds to this by also controlling heading, which requires an ASAS module. In the demo this is only supplied by the Advanced S.A.S Module.

Effect of SAS

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.

Effect of ASAS

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.

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.

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.

Control

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.

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.

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 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/ASAS Parts

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.

Dedicated Units

Image	Part	Radial size	Cost ()	Mass (t)	Drag	Max. Temp. (K)	Tolerance (m/s)	Torque (kN·m)
	Small Inline Reaction Wheel	Small	600	0.05	0.2	2 000	9	5

Command Pods

Image	Part	Radial size	Cost ()	Mass (t)	Drag	Max. Temp. (K)	Tolerance (m/s)	rotPower (kN·m)	Crew
	Command Pod Mk1	Small, Tiny	600	0.8	0.2	3 400	14	5	1
	Stayputnik Mk. 1	Tiny	300	0.05	0.15-0.2	1800	12	0	N/A