Difference between revisions of "SAS"

Revision as of 22:54, 5 March 2015

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

This page or section is in need of being brought up to date. Please help Kerbal Space Program Wiki by fixing inaccurate or outdated information. The SAS went through an overhaul and the new features need to be explained here.

SAS is an automated craft stabilizing 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, 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 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 finally 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, 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.

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.

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 ()	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	[Note 1]		50 ⚡ 7.5 MP
	Mk1 Inline Cockpit	Small	1 600 (1 591)	1.03 (1.00)	2 000 (1 100)	40	50	10	[Note 1]		50 ⚡ 7.5 MP
	Mk2 Cockpit	Mk2	3 500 (3 482)	2.06 (2.00)	2 500 (1 400)	45	50	15	[Note 1]	+	150 ⚡ 15 MP
	Mk2 Inline Cockpit	Mk2	3 500 (3 470)	2.10 (2.00)	2 500 (1 400)	45	50	15	[Note 1]	+	150 ⚡ 25 MP
	Mk3 Cockpit	Mk3, Small	10 000 (9 880)	3.90 (3.50)	2 700 (1 500)	50	50	40/40/20[Note 2]	[Note 1]	+	500 ⚡ 100 MP
	Mk1 Command Pod	Small, Tiny	600 (588)	0.84 (0.80)	2 200 (1 200)	14	50	5	[Note 1]		50 ⚡ 10 MP
	Mk1-3 Command Pod	Large, Small	3 800 (3 764)	2.72 (2.60)	2 400 (1 400)	45	50	15	[Note 1]	+	150 ⚡ 30 MP
	Mk1 Lander Can	Small	1 500 (1 482)	0.66 (0.60)	2 000 (1 000)	8	50	3	[Note 1]		50 ⚡ 15 MP
	Mk2 Lander Can	Large	3 250 (3 202)	1.515 (1.355)	2 000 (1 200)	8	50	15	[Note 1]	+	100 ⚡ 40 MP
	PPD-12 Cupola Module	Large, Small	3 200 (3 188)	1.80 (1.76)	2 000 (1 000)	8	50	9	[Note 1]		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[Note 2]	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

Theory

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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