It was now obvious that man could live and work in space for very long periods of time, if he were given the proper support. So plans were changed. The third manned period would be extended to take advantage of this capability and to perform many more scientific experiments.
Although a number of technical difficulties had been experienced, each had been overcome by hard work and a spirit of cooperation and determination that characterized the entire Skylab effort. Much more had been accomplished by the first two crews than the planners had anticipated.
Earlier in 1973, a Czechoslovakian astronomer named Lubos Kohoutek, working with his telescope in Hamburg, West Germany, had discovered a new comet. According to his calculations, it would be quite near the Sun in December. Delay of the launch and extension of the manned period would permit the third crew to get a close look at this comet, and perhaps to make valuable observations of it.
Preparation for Third Manned Period
During the second manned period when an apparent leakage was detected in the coolant loop, the crew had examined the entire system very carefully, but could find no evidence of leakage. Nevertheless, the coolant pressure continued to drop slowly. So work continued in the technical laboratories to fabricate a kit by which the coolant loop could be reserviced. The kit contained a tank and panel assembly filled with coolant, three short hoses and adapters, repair seals, and valves for connecting to the spacecraft coolant lines.
Two weeks before launch, one of the control-moment gyroscope's speed decreased slightly, while the current in one motor winding increased. But, alter about an hour, the wheel speed and current returned to the normal reading. Although ground engineers did not know it at the time, this was an indication of a problem which was to threaten the third manned mission early in the period.
Unmanned solar observatory experiments continued to be performed as scheduled until just before launch of the third crew, when an orbital lock on the pointing control system failed to release. Observations were discontinued until the third manned period began.
At the beginning of the unmanned period, the workshop had again been depressurized to the 2 pounds per square inch level required to lower the dewpoint. Immediately thereafter, it was repressurized with nitrogen to 5 pounds per square inch to aid in cooling the six gyroscopes located in the docking adapter. This resulted in an imbalanced mixture of nitrogen and oxygen. Shortly before the third crew was to enter the workshop, the pressure was lowered to purge it of the unwelcome mixture and then repressurized with the prescribed mixture of nitrogen and oxygen.
 Skylab's Scientific Mission Supported by Ground-Based Teams
Many of the experimental programs on Skylab were an extension of studies which had been underway on Earth for some time. Skylab added a new capability by which the results of many of these studies could be verified, or through which much new data could be obtained. Experimenters found in Skylab a means of comparing and correlating the results of Skylab experiments with data gained from ground-based observations. And they designed additional ground-based study programs to provide data in support of Skylab experiments.
Such ground-based supporting projects supplemented Skylab studies in several ways. By comparing ground observations and Skylab observations of the same object, an experiment could determine the kind and the degree of superiority which space observations might have over ground observations. By observing a given object, such as a group of sunspots, over an extended period of time before and after the Skylab mission, insight into evolutionary processes could be gained which would greatly enhance the value of Skylab observations. By observing at close range certain features of the Earth's surface, such as tectonic formations, ocean currents, or plant-growth patterns, a calibration table for the interpretation of Skylab pictures of the same features was established. Finally, in the case of biological and medical experiments, the ground-based observations enabled the investigator to isolate the effects of weightlessness on living organisms by comparing Skylab data with the results of observations under similar conditions on the ground.
The Skylab ground-based astronomy program was designed to obtain solar data from observatories around the world at the same time that the solar observatory instruments were viewing the Sun from orbit. Data gathered on the ground supported and supplemented the space-gathered data.
As a result of a request by NASA to solar astronomers for ground-based observations that would support and extend solar observations on Skylab, there were a number of agencies and companies which participated actively in the program. The University of Hawaii's Institute of Astronomy at Haleakala constructed a photometer for observations of active regions in the corona.
This instrument measured simultaneously the intensities of several visible coronal lines to determine the rates of energy loss and gain from the active regions and the effects of flare events on the corona. The Lockheed Missiles & Space Co. at Palo Alto, Calif., operated a spectroheliograph at the Kitt Peak's McMath Solar Observatory, which mapped physical parameters of the solar atmosphere. The California Institute of Technology installed a large photoheliograph at its observatory at Big Bear Lake, in California, a location where observing conditions are exceptionally good. Many others participated in this program, including the National Bureau of Standards, the University of California in San Diego, the Uttar Pradesh State Observatory in Naini Tal, India, and among others, the Applied Physics Laboratory of Johns Hopkins University in Baltimore.
The National Oceanic and Atmospheric Administration coordinated a solar data collection network among observatories in the United States and foreign countries. They also stationed representatives at Mission Control to provide NASA with real-time space environment data, analyses, and forecasts and to coordinate the operation of the solar data network.
"Ground truth" data became very important to the Skylab mission. Such data were obtained by direct observations on the ground of those areas, objects, and phenomena which were also being observed by Skylab. Nearly simultaneous observations were made of weather, lighting conditions, and other environmental factors which might influence the data gathered from Skylab. By comparing ground-truth observations with orbital observations of a particular test site, scientists were able to establish calibration factors which allowed the proper interpretation of orbital data from many sites.
Finally, NASA-operated and private aircraft were used to obtain data over the sites being observed by Skylab. These aircraft were equipped with a variety of cameras and imaging devices which generally approximated the capabilities of instruments aboard Skylab. Data acquired in this fashion were used to analyze and understand the space-acquired data.
Thus, while Skylab's crews operated scientific instruments, made observations, and conducted experiments, thousands of people all over the world worked in cooperation with them.
Materials Processing Studies
Skylab offered a unique opportunity for materials processing specialists. Drop tower experiments had proved that the elimination of the influence of gravity profoundly affected some materials processes. Some limited experiments conducted on Apollo flights verified these early results, so when the opportunity for more elaborate and better controlled experiments on Skylab arose, experimenters prepared an extensive program to study the processing of materials under prolonged weightlessness. Melting and mixing without the contaminating effects of containers, the suppression of thermal convection and buoyancy in fluids, and the ability to take advantage of electrostatic and magnetic forces and otherwise masked by gravitation opened the way to new knowledge of material properties and processes. Ultimately this beginning will lead to the production of valuable new materials for use on Earth.
The materials processing facility developed for Skylab accommodated 14 different experiments carried out during the three manned periods. The facility, located in the docking adapter, contained a spherical work chamber that could be evacuated by opening a vent toward space, a 1.6-kilowatt electron beam gun for intense local heating, and a furnace for the uniform heating, or heating with a temperature gradient, of samples in three separate cartridges. The flight crews operated the facility by selecting the experiment, loading the experiment  into the furnace, and applying the desired heating. After cooling, the samples were stowed for return to Earth where detailed analyses were performed by the experimenters.
Each of the experiments demonstrated some decisive influence of gravity upon processes that are essential in the formation of materials.
Simulators Considered To Be Valuable Tools
Innovation and improvisation characterized the Skylab mission. But this was often the result of extensive training under simulated space conditions.
A number of simulators were used before and during the mission for procedures development, problem analysis, crew training, and other functions. These proved extremely valuable to the success of the mission, especially with the problems related to loss of the micrometeoroid shield.
The Skylab simulator proved to be a very valuable device in which the crewmen familiarized themselves with workshop operating systems. It was especially useful in learning to operate the solar observatory and in understanding the attitude control system, since these systems were operated in orbit by the flight crews. The simulator was designed so that one crewman could operate the control system while the other crewman operated the other workshop systems and the solar observatory. Failures in the control system were then introduced, and the system was operated while the solar observatory was kept pointing at the Sun.
The neutral buoyancy simulator was a very effective training device. Through its use, the crews developed the skills and procedures needed for extravehicular activity. The second crew reported that, "Once basic extravehicular activity skills are acquired in the water tank, a crewman can perform extravehicular activity tasks for which he is not specifically trained if adequately detailed instructions are given. The techniques and skills developed underwater are almost identical to those used during the extravehicular activity, with the actual zero-g task being slightly easier."
Full-scale simulated flight components were always used for practicing extravehicular activity procedures underwater.
The crew further stated: "Underwater simulations and training are not needed for tasks to be performed inside the spacecraft, unless the crewman will be performing the tasks in a pressurized suit. Anything that can be done on Earth in a l-g environment in shirtsleeves can be accomplished in zero-g. A slightly different body position may be required in zero-g."
Other simulators were used for training in use of the Earth resources experiment package, in stowage procedures for the command module, in rendezvous and entry procedures, and in familiarization with Skylab's operating systems.
Thus, simulators were valuable tools for training flight crews. But they were also extremely valuable to engineers and technicians supporting Skylab when troubles occurred.
A simulator which reproduced the workshop instrumentation and communications systems, located in St. Louis, and a test unit and a solar observatory instrumentation and communications simulator located at the Marshall Space Flight Center were used to reproduce problems in that system and develop means for correcting them.
Simulators were also used to support analysis of the Skylab electrical power system. A computer program simulated electrical system performance over a wide range of operating conditions and environments. This program was used each day during the mission to analyze proposed attitudes of the space station. It was especially valuable during the first 10 days of the mission. Throughout the mission, however, it proved its value in power management.
Many simulators were used in the design and verification of the attitude and pointing control system. Such simulators ranged from those which were computer models of the overall system to those employing full-size system components. One such model, used extensively late in the mission, simulated thruster bursts for given maneuvers and recorded the quantity of nitrogen gas used. With the aid of this model, engineers planned thruster use to keep gas consumption to a minimum.
The glamour of manned spaceflight has often overshadowed the enormous amount of work done by ground crews during the flight and by the many engineers and technicians at the NASA centers, aerospace contractor plants, universities, and by other groups in preparation for these flights.
Skylab demonstrated dramatically the great contribution to mission success made by those thousands on Earth who supported the flight crews throughout each manned period and who kept...
...Skylab operating properly during the periods in which it was unoccupied.
Thousands of interdependent tasks had to be performed correctly and on time for the mission to succeed. Countless hours of preparation went into the execution of each task, and thousands of people, each working at his own specialty, made up a team whose spirit of dedication and cooperation assured success. Skylab was a team effort, and no one was more aware of this than the third flight crew which was being prepared for man's longest voyage into space.