Computers in Spaceflight: The NASA Experience

- Chapter Three -
The Skylab Computer System


[66] Skylab, America's first orbital workshop, carried a highly successful computer system. For much of the operating life of the space station, the computer was not just the fourth crew member but the only crew member. It made a large contribution to saving the mission during the 2 weeks after the troubled launch and later helped control Skylab during the last year before re-entry. The entire system functioned without error or failure for over 600 days of operation, even after a 4-year and 30-day interruption. It is significant as the first spaceborne computer system to have redundancy management software. The software development for the system followed strict engineering principles, producing a fully verified and reliable real-time program.
The record of the computer system stands in contrast to that of the workshop itself. NASA launched Skylab on May 14, 1973 on a Saturn V booster. The first two stages put the modified S-IVB third stage into orbit. The S-IVB contained the workshop, which included a solar telescope mount and living and working quarters. The plan was to launch the first crew the next day aboard a Saturn IB carrying an Apollo command and service module. However, shortly after achieving orbit, telemetry from the unmanned Skylab indicated that one of the two wings of solar panels was missing and the other had not deployed. The panels on the Apollo Telescope Mount (ATM) had opened properly but they were too small to supply power for the whole workshop. In addition, the gyros were drifting and the thermal shield was damaged. These failures caused concern that the interior of the space station would overheat and destroy the equipment. The damage was so serious that for the first 3 or 4 hours the ground controllers felt that NASA would be fortunate if the systems were to function for 1 day1. However, by using the computer system that controlled the workshop's attitude, the ground controllers were able to keep the Skylab at angles to the sun such that the equipment would be exposed to tolerable temperatures in the laboratory in concert with generating adequate power from the remaining solar panels. At times these were conflicting requirements. This had to be done for 2 weeks while engineers prepared repair materials for the crew to fix the workshop. Controller Steven Bales remembered that time as "the hardest 2 weeks I have ever spent," since a 24-hour watch had to be maintained on the attitude and temperature2.
The computer system again served as "captain" during the entire Skylab reactivation. The workshop systems were shut down on February 9, 1974, after the last crew left. NASA expected that the Skylab would stay in orbit until the mid-1980s. By that time the Space Shuttle would be operational and, it was thought, could be used to bring up rockets to boost the laboratory into a higher orbit. However, unexpected solar activity in the mid-1970s resulted in an increase in the density of the atmosphere, so the Skylab's orbit decayed at a much faster rate than projected3.

Figure 3-1.
Figure 3-1. Skylab in orbit. Note the foil sun shield above the center section and the missing large solar panel. The Apollo Telescope Mount is the section with the "windmill" solar panels. (NASA photo 74-H-98)

By 1978, the predicted re-entry time was to be late that year or in early 1979. NASA decided to attempt to change the attitude of the workshop so that minimal drag would ensue. In this way, the orbit might be maintained until the Shuttle could rescue the space station. Engineers reactivated and reprogrammed the computer to maintain the proper attitude and, later, to control the re-entry when NASA abandoned the attempt to maintain orbit. They accomplished this over 4 years after the computer was shutdown.
The need for the computer system that served Skylab so well was not apparent until the original "wet workshop" concept (the laboratory to be assembled in space inside of the empty propellant tanks of the last stage of the launch vehicle) had progressed through more sophisticated designs to the eventual "dry workshop"4. In December 1968, NASA decided to acquire a dual computer system to help control attitude while in orbit5. Attitude control was crucial to the success of the solar experiments. In fact, the name of the computer reflects this: Apollo Telescope Mount Digital Computer (ATMDC). Two of these computers were a part of the Skylab Attitude and Pointing Control System (APCS), which consisted of a number of other components, [68] such as an interface unit, magnetic tape memory, control moment gyros, the thrustor attitude control system, sun sensors, a star tracker, and nine rate gyros6.
Marshall Space Flight Center devised this complex system-a pioneering effort because it represents the first fully digital control system on a manned spacecraft7. Its mission-critical status led to the use of extensive redundancy in its design, in both hardware and software. The computer system not only managed its own redundancy, but all redundant hardware on the spacecraft8. The uniqueness and complexity of the control laws associated with the control moment gyro attitude system led one NASA engineer to refer to it as "a crazy animal"9. It was up to the Skylab computer system to tame it.

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