SAS

The Stability Augmentation System (abbr. SAS and also known as the Sickness Avoidance Scheme or Sickness Avoidance Solution) 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.

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:

• Torque supplied by reaction wheels
• Engines' gimbal
• Control surfaces
• RCS, if activated

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

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 3 dedicated parts as well as being included in all Command Pods and Probe bodies.

Dedicated Units

Image	Part	Radial size	Cost ()	Mass (t)	Max. Temp. (K)	Tolerance (m/s)	Tolerance (g)	Torque (kN·m)	Electricity (⚡/s)
	Small Inline Reaction Wheel	Tiny	600	0.05	2 000	9	50	5	0.25 (15 ⚡/min)
	Advanced Inline Stabilizer	Small	1 200	0.1	2 000	9	50	15	0.45 (27 ⚡/min)
	Advanced Reaction Wheel Module, Large	Large	2 100	0.2	2 000	9	50	30	0.6 (36 ⚡/min)

Command modules

Image	Part	Radial size	Cost ()	Mass (t)	Max. Temp. (K)	Tolerance (m/s)	Tolerance (g)	Torque (kN · m)	S.A.S. level	Required Crew/ Power	Capacity (⚡)
	Mk1 Cockpit	Small	1 250 (1 241)	1.28 (1.25)	2 000 (1 100)	40	50	10			50 ⚡ 7.5 MP
	Mk1 Inline Cockpit	Small	1 600 (1 591)	1.03 (1.00)	2 000 (1 100)	40	50	10			50 ⚡ 7.5 MP
	Mk2 Cockpit	Mk2	3 500 (3 482)	2.06 (2.00)	2 500 (1 400)	45	50	15		+	150 ⚡ 15 MP
	Mk2 Inline Cockpit	Mk2	3 500 (3 470)	2.10 (2.00)	2 500 (1 400)	45	50	15		+	150 ⚡ 25 MP
	Mk3 Cockpit	Mk3, Small	10 000 (9 880)	3.90 (3.50)	2 700 (1 500)	50	50	40/40/20		+	500 ⚡ 100 MP
	Mk1 Command Pod	Small, Tiny	600 (588)	0.84 (0.80)	2 200 (1 200)	14	50	5			50 ⚡ 10 MP
	Mk1-3 Command Pod	Large, Small	3 800 (3 764)	2.72 (2.60)	2 400 (1 400)	45	50	15		+	150 ⚡ 30 MP
	Mk1 Lander Can	Small	1 500 (1 482)	0.66 (0.60)	2 000 (1 000)	8	50	3			50 ⚡ 15 MP
	Mk2 Lander Can	Large	3 250 (3 202)	1.515 (1.355)	2 000 (1 200)	8	50	15		+	100 ⚡ 40 MP
	PPD-12 Cupola Module	Large, Small	3 200 (3 188)	1.80 (1.76)	2 000 (1 000)	8	50	9			200 ⚡ 10 MP
	Probodobodyne RoveMate	Small	800	0.15	1 200	12	50	—	0	2.4 ⚡/min (144 ⚡/h)	120 ⚡
	Probodobodyne QBE	Tiny	360	0.07	1 200	15	50	—	0	1.5 ⚡/min (90 ⚡/h)	5 ⚡
	Probodobodyne HECS	Tiny	650	0.1	1 200	12	50	0.5	1	1.5 ⚡/min (90 ⚡/h)	10 ⚡
	Probodobodyne OKTO	Tiny	450	0.1	1 200	12	50	0.3	0	1.2 ⚡/min (72 ⚡/h)	10 ⚡
	Probodobodyne OKTO2	Tiny	1 480	0.04	1 200	12	50	—	2	1.8 ⚡/min (108 ⚡/h)	5 ⚡
	Probodobodyne HECS2	Small	7 500	0.2	2 000	8	50	10	3	3.0 ⚡/min (180 ⚡/h)	1 000 ⚡
	RC-001S Remote Guidance Unit	Small	2 250	0.1	2 000	9	50	0.5	3	3.0 ⚡/min (180 ⚡/h)	15 ⚡
	RC-L01 Remote Guidance Unit	Large	3 400	0.5	2 000	9	50	1.5	3	4.8 ⚡/min (288 ⚡/h)	30 ⚡
	MK2 Drone Core	Mk2	2 700	0.2	2 500	20	50	15/3/3	3	3.0 ⚡/min (180 ⚡/h)	250 ⚡
	MPO Probe	Small	9 900 (9 854)	0.895 (0.395)	2 200	9	50	6	3	3.0 ⚡/min (180 ⚡/h)	1 000 ⚡ 45 LF 55 O
	MTM Stage	Small	21 500 (6 300)	0.795 (0.415)	2 200	12	50	12	2	1.8 ⚡/min (108 ⚡/h)	4 000 ⚡ 3 800 XE

See also

• Updated Information on SAS in 0.21.1 blog entry by C7