SAS

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.

• Torque supplied by reaction wheels
• Control surfaces (only effective in atmosphere)
• RCS, if activated
• Engines' gimbaling, if throttled up

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.

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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”

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SAS capabilities

Name	Cost	SAS level & capabilities
		0	1	2	3
		Stability Assist ()	Prograde/ Retrograde (/)	Normal/Radial Hold (/)	Target Tracking (/)	Maneuver ()
Probodobodyne RoveMate	800	Yes	No	No	No	No
Probodobodyne Stayputnik	300	No	No	No	No	No
Probodobodyne QBE	360	Yes	No	No	No	No
Probodobodyne HECS	650	Yes	Yes	No	No	No
Probodobodyne OKTO	450	Yes	No	No	No	No
Probodobodyne OKTO2	1 480	Yes	Yes	Yes	No	No
Probodobodyne HECS2	7 500	Yes	Yes	Yes	Yes	Yes
RC-001S Remote Guidance Unit	2 250	Yes	Yes	Yes	Yes	Yes
RC-L01 Remote Guidance Unit	3 400	Yes	Yes	Yes	Yes	Yes
MK2 Drone Core	2 700	Yes	Yes	Yes	Yes	Yes
MPO Probe	9 900	Yes	Yes	Yes	Yes	Yes
MTM Stage	21 500	Yes	Yes	Yes	No	No
CH-J3 Fly-By-Wire Avionics Hub	5 200	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

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

• Updated Information on SAS in 0.21.1 blog entry by C7