2011, which was designed to protect much of the uncrewed spacecraft's electronics, using 1 cm thick walls of titanium. The Skylab film vault was used for storing film from various sources including the Apollo Telescope Mount solar instruments. Six ATM experiments used film to record data, and over the course of the missions over 150,000 successful exposures were recorded. The film canister had to be manually retrieved on crewed spacewalks to the instruments during the missions. The film canisters were returned to Earth aboard the Apollo capsules when each mission ended, and were among the heaviest items that had to be returned at the end of each mission. The heaviest canisters weighed 40 kg and could hold up to 16,000 frames of film. Gyroscopes There were two types of gyroscopes on Skylab. Control-moment gyroscopes (CMG) could physically move the station, and rate gyroscopes measured the rate of rotation to find its orientation. 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. Some of the forces acting on Skylab that the pointing system needed to resist: Gravity gradient Aerodynamic disturbance Internal movements of crew. Skylab was the first large spacecraft to use big gyroscopes, capable of controlling its attitude. The control could also be used to help point the instruments. The gyroscopes took about ten hours to get spun up if they were turned off. There was also a thruster system to control Skylab's attitude. There were 9 rate-gyroscope sensors, 3 for each axis. These were sensors that fed their output to the Skylab digital computer. Two of three were active and their input was averaged, while the third was a backup. 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 21-inch diameter rotor weighed and rotated at approximately 8950 revolutions per minute". There were three control moment gyroscopes on Skylab, but only two were required to maintain pointing. The control and sensor gyroscopes were part of a system that help detect and control the orientation of the station in space. Other sensors that helped with this were a Sun tracker and a star tracker. 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.