Editing Skylab (section)

== Gyroscopes ==
[[File:P46bskylabgyros.jpg|thumb|Skylab could [[attitude control|change its attitude]] without using propellant by changing the spin of large gyroscopes.]]

There were two types of [[gyroscope]]s on Skylab. [[Control moment gyroscope|Control-moment gyroscopes]] (CMG) could physically move the station, and rate gyroscopes measured the rate of rotation to find its orientation.<ref>{{cite news |url=https://www.nytimes.com/1974/01/23/archives/skylabs-gyroscope-has-worst-seizure-breakdown-feared-working-pair.html|title=Skylab's Gyroscope Has Worst Seizure; Breakdown Feared |newspaper=The New York Times|issn=0362-4331|date=January 23, 1974|access-date=11 January 2017}}</ref> The CMG helped provide the fine pointing needed by the Apollo Telescope Mount, and to resist various forces that can change the station's orientation.<ref name="nasa.gov3">{{cite report |url=https://ntrs.nasa.gov/citations/19710007043 |title=SKYLAB ATTITUDE AND POINTING CONTROL SYSTEM |last1=Chzlbb |first1=W. B. |last2=Seltzer |first2=S. M. |date=February 1, 1971 |publisher=NASA |id=NASA-TN-D-6068}} {{cite web |title=PDF |url=https://ntrs.nasa.gov/api/citations/19710007043/downloads/19710007043.pdf}} {{PD-notice}}</ref>

Some of the forces acting on Skylab that the pointing system needed to resist:<ref name="nasa.gov3"/>
* [[Gravity-gradient stabilization|Gravity gradient]]
* Aerodynamic disturbance
* Internal movements of crew.

{{blockquote|The Skylab-A [[Spacecraft attitude control|attitude and pointing control]] system has been developed to meet the high accuracy requirements established by the desired experiment conditions. Conditions must be maintained by the control system under the influence of external and internal disturbance torques, such as gravity gradient and aerodynamic disturbances and onboard astronaut motion.|''Skylab Attitude and Pointing Control System'' (NASA Technical Note D-6068) {{PD-notice}}<ref name="nasa.gov3"/>}}

Skylab was the first large spacecraft to use big gyroscopes, capable of controlling its attitude.<ref name="nasa.gov4">{{cite web |url=https://history.nasa.gov/SP-400/p46.htm|title=p46}} {{PD-notice}}</ref> The control could also be used to help point the instruments.<ref name="nasa.gov4"/> The gyroscopes took about ten hours to get spun up if they were turned off.<ref name="nasa.gov5">{{cite web |url=https://history.nasa.gov/SP-400/ch3.htm#46|title=ch3}} {{PD-notice}}</ref> There was also a thruster system to control Skylab's attitude.<ref name="nasa.gov5"/> There were 9 rate-gyroscope sensors, 3 for each axis.<ref name="nasa.gov5"/> These were sensors that fed their output to the Skylab digital computer.<ref name="nasa.gov5"/> Two of three were active and their input was averaged, while the third was a backup.<ref name="nasa.gov5"/> From NASA SP-400 ''Skylab, Our First Space Station'', "each Skylab control-moment gyroscope consisted of a motor-driven rotor, electronics assembly, and power inverter assembly. The {{convert|21|in|mm|adj=mid|-diameter}} rotor weighed {{convert|155|lb}} and rotated at approximately 8950 revolutions per minute".<ref name="nasa.gov6">{{cite web |url=https://history.nasa.gov/SP-400/ch3.htm|title=Chapter 3, We Can Fix Anything.|publisher=NASA.gov}} {{PD-notice}}</ref>

There were three control moment gyroscopes on Skylab, but only two were required to maintain pointing.<ref name="nasa.gov6"/> The control and sensor gyroscopes were part of a system that help detect and control the orientation of the station in space.<ref name="nasa.gov6"/> Other sensors that helped with this were a [[Solar tracker|Sun tracker]] and a [[star tracker]].<ref name="nasa.gov6"/> The sensors fed data to the main computer, which could then use the control gyroscopes and or the thruster system to keep Skylab pointed as desired.<ref name="nasa.gov6"/>