SAS

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


SAS is an automated craft stabilizing system relying on gyroscopic torque from reaction wheels powered by electricity. This is in contrast to the Reaction Control System or RCS which stabilizes craft using thrust from reaction engines expending monopropellant or other fuel. There are varied official descriptions of what “SAS” stands for, including Stability Augmentation System, Stability Assist (System), Sickness Avoidance Scheme, and Sickness Avoidance Solution.

Usage

SAS is an automatic system toggled on or off during flight. For SAS to be available at all, craft must include a part with SAS capability and, since version 0.90.0, a pilot. All “probe cores” satisfy both requirements. Most manned command modules, except the EAS-1 External Command Seat, and also SAS-enabled reaction wheel modules satisfy the capability requirement. At least one kerbonaut manning an SAS-enabled command module must have the Pilot specialization to use SAS. In Sandbox mode there is no restriction on SAS.

Prior to version 0.21 there was a distinction between “SAS” and “ASAS” (Advanced SAS). 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.

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, a direction selector will appear to the left of the navball. By default "stability assistance" is selected and stabilizes on the current heading. But the user can select other vectors 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.

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.

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.

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

SAS Parts

SAS has 3 dedicated parts as well as being included in nearly all manned command modules and all probe cores.

Dedicated Units

Image Part Radial size Cost
(Funds)
Mass
(t)
Max. Temp.
(K)
Tolerance
(m/s)
Tolerance
(g)
Torque
(kN·m)
Electricity
(⚡/s)
Inline reaction wheel.png
Small Inline Reaction Wheel Tiny 600 0.05 2 000 9 50 5 0.25
(15 ⚡/min)
Inline advanced stabilizer.png
Advanced Inline Stabilizer Small 1 200 0.1 2 000 9 50 15 0.45
(27 ⚡/min)
ASAS large.png
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
(Funds)
Mass
(t)
Max. Temp.
(K)
Tolerance
(m/s)
Tolerance
(g)
Torque
(kN · m)
S.A.S.
level
Required Crew/
Power
Capacity
(⚡)
MK1COCKPIT.png
Mk1 Cockpit Small 1 250
(1 241)
1.28
(1.25)
2 000
(1 100)
40 50 10 Crew Crew 50 ⚡
7.5 Units of fuel MP
Mk1 Inline Cockpit.png
Mk1 Inline Cockpit Small 1 600
(1 591)
1.03
(1.00)
2 000
(1 100)
40 50 10 Crew Crew 50 ⚡
7.5 Units of fuel MP
Mk2 Cockpit.png
Mk2 Cockpit Mk2 3 500
(3 482)
2.06
(2.00)
2 500
(1 400)
45 50 15 Crew Crew + Crew 150 ⚡
15 Units of fuel MP
Mk2 InlineCockpit.png
Mk2 Inline Cockpit Mk2 3 500
(3 470)
2.10
(2.00)
2 500
(1 400)
45 50 15 Crew Crew + Crew 150 ⚡
25 Units of fuel MP
Mk3 Cockpit.png
Mk3 Cockpit Mk3, Small 10 000
(9 880)
3.90
(3.50)
2 700
(1 500)
50 50 40/40/20[Note 1] Crew Crew + CrewCrewCrew 500 ⚡
100 Units of fuel MP
Mk1CommandPod Dark.png
Mk1 Command Pod Small, Tiny 600
(588)
0.84
(0.80)
2 200
(1 200)
14 50 5 Crew Crew 50 ⚡
10 Units of fuel MP
Mk1-3CommandPod.png
Mk1-3 Command Pod Large, Small 3 800
(3 764)
2.72
(2.60)
2 400
(1 400)
45 50 15 Crew Crew + CrewCrew 150 ⚡
30 Units of fuel MP
LanderCanMK1.png
Mk1 Lander Can Small 1 500
(1 482)
0.66
(0.60)
2 000
(1 000)
8 50 3 Crew Crew 50 ⚡
15 Units of fuel MP
Mk2canN.png
Mk2 Lander Can Large 3 250
(3 202)
1.515
(1.355)
2 000
(1 200)
8 50 15 Crew Crew + Crew 100 ⚡
40 Units of fuel MP
Cupola.PNG
PPD-12 Cupola Module Large, Small 3 200
(3 188)
1.80
(1.76)
2 000
(1 000)
8 50 9 Crew Crew 200 ⚡
10 Units of fuel MP
RoveMate White.png
Probodobodyne RoveMate Small 800 0.15 1 200 12 50 0 2.4 ⚡/min
(144 ⚡/h)
120 ⚡
QBE.png
Probodobodyne QBE Tiny 360 0.07 1 200 15 50 0 1.5 ⚡/min
(90 ⚡/h)
5 ⚡
HECS.png
Probodobodyne HECS Tiny 650 0.1 1 200 12 50 0.5 1 1.5 ⚡/min
(90 ⚡/h)
10 ⚡
OKTO.png
Probodobodyne OKTO Tiny 450 0.1 1 200 12 50 0.3 0 1.2 ⚡/min
(72 ⚡/h)
10 ⚡
OKTO2.png
Probodobodyne OKTO2 Tiny 1 480 0.04 1 200 12 50 2 1.8 ⚡/min
(108 ⚡/h)
5 ⚡
HECS2.png
Probodobodyne HECS2 Small 7 500 0.2 2 000 8 50 10 3 3.0 ⚡/min
(180 ⚡/h)
1 000 ⚡
Probestack.png
RC-001S Remote Guidance Unit Small 2 250 0.1 2 000 9 50 0.5 3 3.0 ⚡/min
(180 ⚡/h)
15 ⚡
Probehuge.png
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.png
MK2 Drone Core Mk2 2 700 0.2 2 500 20 50 15/3/3[Note 1] 3 3.0 ⚡/min
(180 ⚡/h)
250 ⚡
  1. 1.0 1.1 Torque differs between axes. These numbers are for pitch/yaw/roll respectively.



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