The Skylab mission was performed in order to gather scientific data regarding the prolonged exposure of mankind in the Low Earth Orbit environment. It was first proposed in 1965. At that time, it went under the name of the Advanced Apollo Project (AAP). This program was oriented towards examining uses of the Saturn V/ Apollo launch system for practical uses after the Apollo missions were finished. What came out of the AAP was the first American effort to place a space station in orbit. The first configuration considered was to place a solar telescope assembly mounted to an Apollo capsule in orbit. Solar readings would then take place and the assembly would return to Earth along with the crew. From out of this effort grew the idea of putting a laboratory in orbit that was to be created out of a spent fourth stage of the Saturn V launch vehicle. A final design for this configuration was decided on in 1967. The project name was changed to Skylab in 1970, with lauches following in 1973.
Three missions were conducted during that year, of 28, 59, and 87 days duration. Sincere to the earliest concepts of the AAP, solar astronomy was to remain a major goal of the space station effort. The small solar telescopes originally considered were upgraded into the Apollo Telescope Mount. Earth resource and materials experiments were added to the design as was medical experiments to be conducted on the crew. These experiments revealed a plethora of valuable data and continue to contribute to our knowledge of the space environment that is used in spacecraft design today. The Skylab saga came to a close in 1979 when, after innovative attempts to save the station failed, America's first space station experienced enough deterioration in its orbit that it reentered the atmosphere and burned up.
Propulsion/ Booster
The Skylab cluster carried no propulsion, except for its attitude thrusters, so its orbit was not maintained. It was launched into a 431.5 by 433.7 kilometer orbit, with a 50 deg inclination.
Skylab was launched on the last Saturn V booster. For Skylab, the Saturn V was used in a two stage configuration, consisting of the S-IC lower stage and the S-II second stage. The S-IC had five F-1 engines, fueled with LO2 and RP-1, developing a total of 34.4 megaNewtons (MN). of thrust. The S-II had five J-2 engines, fueled with LO2 and LH2, and a total thrust of 5.02 MN. The Skylab cluster was mounted above the S-II stage with the Orbital Workshop (OWS) placed in the position of the S-IVB of the Saturn V Apollo rockets. The remaining sections of the cluster were positioned atop the OWS, and were covered with an enormous shroud for environmental and aerodynamic protection. The shroud split apart and separated on orbit. The Skylab cluster mounted on the Saturn V is shown in Figure 1.
The crew were delivered to orbit in a Apollo Command/Service Module (CSM), launched on a Saturn IB booster. The Saturn IB was a two stage rocket, composed of a S-IB lower stage and a S-IVB upper stage. The S-IB had eight H-1 engines, fueled with LO2 and RP-1, and a total thrust of 7.30 MN. The S-IVB had one J-2 engine and a total thrust of 1.02 MN. Four Saturn IBÕs were committed to Skylab, one for each crew, and one to launch a rescue mission. The Apollo CSM had one main engine, mounted in the Service Module. This engine, fueled with UDMH (unsymmetrical dimethylhydrazine) and N2O4 (nitrogen tetroxide), produced 91.2 kN. of thrust.
Figure 1. Skylab at Launch (ref 3)
Structure
Skylab's structure was divided into four major sections. They were, starting from the position of the Apollo command/service module, the Apollo Telescope Mount/solar observatory (ATM), the Multiple Docking Assembly (MDA), the Airlock Module (AM), and the Orbital Workshop (OWS). The cluster was approximately 28 meters (m) long, and had a mass of 84,900 kilograms (kg). (Figure 2)
Figure 2. Skylab Cluster on Orbit (ref 3)
The ATM was initially intended as a independent structure, to be used as a development of the Apollo system. As the Skylab concept grew, the ATM was incorporated into the design of Skylab, and its power, communications and attitude control systems were primary components of the overall Skylab subsystems. The structure of the ATM consisted of two major parts. The 'rack' was an octagonally shaped truss structure, 3.3 m. in diameter and 3.6 m. long. On the rack were mounted the ATM power conditioners, the attitude control gyroscopes and the ATM communications system. Inside the rack was a cylindrical canister, 2.1 m. in diameter and 3 m. long, which housed the solar telescopes. The canister was able to rotate 240 deg about its axis and 2 deg normal to it. Internally, it was supported by a cruciform spar for rigidity. During launch, the ATM was mounted axially with the rest of the cluster, attached to the MDA. After orbit was achieved, the entire ATM was rotated 90 deg , to point normal to the main Skylab axis on the sunward side.
The MDA was a cylinder, sealed at one end, 3.04 m. in diameter and 5.27 m. long. There were two docking ports mounted on it, the primary docking port on the forward end, and the alternate port on the anti-sunward side. The primary port was exposed when the ATM was rotated into position.
The AM was connected to the aft end of the MDA, and to the forward end of the OWS. It consisted of three concentric cylindrical sections, the forward one 3.04 m. in diameter, the middle one 1.67 m. in diameter, and the aft one 6.70 m. in diameter. It had an overall length of 5.36 m. The middle cylinder had airlocks at either end where it joined the forward cylinder and the OWS. There was also an EVA hatch on the side of that cylinder. The annular space between the middle and aft cylinder held controls and the pressurized containers that held the workshop's atmosphere. This cylinder was called the Instrument Unit (IU).
The OWS was constructed from an unused Saturn S-IVB third stage. Consideration had been given to refitting a 'wet' stage after it had reached orbit, but the 'dry' option was used because of its greater simplicity. The upper, or forward, fuel tank, which was intended to hold H2, was converted into the main workshop, the lower, O2 one, was retained as a waste disposal container, with a pass-through airlock connecting it to the workshop. The workshop was divided by a grid floor into two compartments. The forward compartment was primarily the workspace, for experiments and observations, while the aft compartment was principally the crew quarters. Around the outside of the OWS, there was a thin metal shield for micrometeoroid protection. It was to be held tightly against the outside of the OWS during launch, and on orbit, swung out to a 0.15 m. separation by torsion arms. The OWS was 6.70 m. in diameter, 14.66 m. long, and had a mass of 35,380 Kg.
The crew were delivered in standard Apollo Command /Service modules. A fourth CM was modified to perform as a rescue vehicle by the replacement of its aft storage lockers with two additional acceleration couches.
The most significant failure in the Skylab mission occured in the structural system. Sixty three seconds after launch, the micrometeoroid shield ripped off of the OWS. The probable cause of this was aerodynamic pressure build up between the shield and the OWS.
Skylab Experiments and Sensors
The experiments on Skylab consisted of almost 300 different investigations, but can be divided into four main areas consistent with the Skylab mission objectives: space science, life science, Earth resources, and space technology projects. Skylab also offered for the first time the opportunity for high school students from across the country to participate in the United States space program through the Skylab Student Project. Through the Project, 17 experiments were selected from over 3400 proposals from high school students to fly on Skylab. These experiments were highly techinical ranging from studies of quasars to behavior of bacteria to even a search for the supposed planet of Vulcan, thought to reside inside the orbit of Mercury. These experiments returned a wealth of information for the young investigators and established a tradition that continues to this day of student involvement under the Getaway Specials flown on Shuttle flights.
The space science experiments were further subdivided into three areas: solar physics, stellar astronomy, and space physics. In the area of solar physics, all but one of the instruments were mounted in the Apollo Telescope Mount (ATM). The lone exception was experiment S020, Ultraviolet and X-Ray Solar Photography. This instrument consisted of a spectrograph through which long term exposure photographs were taken in the 1-20 nanometer wavelength range. In the ATM, eight telescopes were arranged in an assembly to point together at the Sun. (Figure 3) These telescopes were generic in nature, oriented towards gathering a broad spectrum of data. The collected data would then be handed over to the investigators who would pick and choose the specific data they required. Among the general data gathered was chromospheric network and supergranulation, solar flares, and center-to-limb data on the quiet Sun.
The telescopes that carried out this mission consisted of: a white light coronagraph that obtained high resolution photographs of the solar corona from 1.5 to 6 solar radii, an X-Ray Spectrographic Telescope that recored x-ray images of the Sun over the 0.2-6 nm wavelength region, and an X-Ray Telescope that photographed the SunÕs disk in the 0.6-3.3 nm range and monitored the x-ray flux in the 0.2-0.8 nm region. Three ultraviolet examining telescopes were contained in the ATM as well. The Ultraviolet Scanning Polychromator/Spectroheliometer obtained data along six spectral lines and the Lyman continuum from 30 to 140 nm from 5 x 5 arcsec
Figure 3. Apollo Telescope Mount Experiments. (ref 3)
surface elements of the Sun. The Extreme Ultraviolet Spectroheliograph recorded monochromatic images of the Sun in the spectral range of 15 to 62.5 nm, and observed the chromosphere and lower corona. The Ultraviolet Spectrograph acquired spectra of solar flares and active areas on the Sun as well as obtained UV spectra of small regions of the Sun in the 97-394 nm range. The last two telescopes used in the ATM were the Hydrogen-Alpha Telescopes. They operated in the red light region of the Balmer series (hydrogen-alpha lines) as bore sights for the other equipment in acquiring regions of the Sun undergoing activity. This allowed the crew to point the telescope assembly manually at those areas of the Sun where unexpected activity was seen.
In the fields of stellar astronomy and space physics, many experiments were conducted. The most prominent of these experiments concerned the observations of the newly discovered comet Kohoutek. These observations used eleven different sensors that covered the electromagnetic range from visible light to x-rays. A electronographic camera with an image converter tube was specially prepared by scientists to photograph the comet and sent up on the third mission. This was the first space based observation of a cometary body in the history of mankind.
Increased biomedical knowledge was another major objective of Skylab. It was deemed necessary to acquire data regarding biological long term exposure to a zero-G enviroment. To accomplish this, many experiments were performed that are too numerous to examine each of them in this document, but they can be divided into three main areas of investigation. Medical experiments monitoring the crewsÕ physiological reaction to the zero-G environment and reaffirming similar effects seen in previous manned missions were conducted. Biology experiments were utilized that examined biological phenomena and their chemical response to zero-G. Biotechnology experiments investigated the effeciency of man-machine interfaces in zero-G and looked at ways to improve bioinstrumentation used in space. These experiments provided a wealth of information that still represents to this day the best bioknowledge that NASA has regarding long-term exposure to a zero-G environment.
The Earth Resources Experiment Program (EREP) was the first remote sensing effort mounted that did not operate exclusively in the visual or infrared regions of the electromagnetic spectrum. It covered these regions and, in addition, the microwave region. Under EREP, 146 investigations representing 32 distinct applications areas were carried out. To support these investigations, six sensors were used, with coverage of the Earth being limited to the ground track of Skylab's orbit as shown in Figure 4:
Figure 4. EREP Earth Coverage from Skylab (ref. 2)
To photograph the surface of the Earth, the Multispectral Photographic Cameras and Earth Terrain Camera obtained high resolution (30 ft and 11 ft respectively) pictures of the surface when cloud cover allowed. The infrared observations were conducted by the Infrared Spectrometer and Multispectral Scanner. These infrared instruments were designed to find out if infrared observations from space could prove useful to scientists studying geographic phenomena. They collected data on atmospheric attenuation, surface temperature, and vegatation mapping. Microwave radiation was measured by the Microwave Radiometer/Scatterometer and Altimeter, and the L-Band Radiometer. The Microwave Radiometer measured reflected radar waves in the 13.9 GHz band as well as emmited radiation at the same frequency. Comparisons could then be made with respect to various terrain types on Earth. The L-Band Radiometer was a passive instrument operating at 1.43 GHz in support of the MRSA experiment. Cloud cover effects were examined by comparing the two experiments results obtained at different frequencies. Unlike the ATM, which only had an astronaut override capability, EREP was largely manually controlled by the crew. Real-time decisions had to be made regarding where to point the instruments to avoid cloud cover and weather phenomena. The wealth of data returned by EREP in remote sensing proved the use of remote sensing from space and once again showed the usefulness of having astronauts conducting on-the-spot scientific investigation.
Space technology represents the last of the main areas of investigation on the Skylab mission. Many of the tools used on Shuttle by the astronauts (and in the near future on Space Station Freedom (SSF) ) were first developed and prototyped on Skylab. In addition to this development, observations were carried out to examine how the space environment affected the Skylab vehicle and exposed experimental sensors. Radiation was measured inside the spacecraft to determine the effectiveness of the thermal and radiation protection for the crew, and zero-gravity system studies were done to find out the effects of crew motion on experiments as well as to examine different ways for crew on EVAs to maneuver around the vehicle. Also, many experiments were conducted in the field of materials science which proved that materials procesing on-orbit created purer forms of materials than those that could be formed on Earth. Purer materials processing first shown on Skylab has proven to be one of the main economic motivations for SSF.
Environmental Control & Life Support (ECLSS)
At the time of Skylab's design, there still existed concern over preventing an oxygen fires such as the fire that occurred while Apollo 1 was on the pad. To provide a safer environment, Skylab's life support was constructed with a 74% oxygen and 26% nitrogen mixed atmosphere on an open-cycle system. An open-cycle system is one which materials used in the system are not reclaimed for reuse. This atmosphere provided enough inert gas to preclude a catastrophic fire while still providing the astronauts with the same oxygen as they would breathe on Earth. The weight of the oxygen and nitrogen in the atmosphere was about 350 lbs.
Due to the use of the Apollo modules to deliver the Skylab astronauts, Skylab could not be pressurized to full atmospheric pressure. Thus, the Skylab module was pressurized to about one-third of full atmospheric level to facilitate docking, which also had the benefit of lowering some of the structural overhead. With the total atomospheric pressure limitted to one-third of Earth normal, the composition of the Skylab atmosphere resulted in a partial pressure of oxygen that was actually Earth normal. This would facilitate breathing for the astronauts as well as making the transition back to Earth living easier. If full atmospheric pressure would have been used, the weight of the contents in the crew atmosphere would have increased from 350 lbs to approximately 1000 lbs. Thus, using a lower pressure resulted in the net savings of thousands of pounds of oxygen and nitrogen during the course of the entire Skylab mission.
Carbon dioxide (CO2) removal was another critical concern for the designers. On all previous manned missions, a lithium hydroxide system was used to remove carbon dioxide from the spacecraft's environment. The lithium hydroxide in this type of CO2 removal system is an expendable quantity. It is used as a filter and, once used, cannot be reused. Therefore, for Skylab to use the lithium hydroxide filters, a large supply had to be carried on board Skylab or delivered with each crew, which, since keeping weight to a minimum is always a desired effect for spacecraft, was not possible. Therefore, another system had to be developed.
What came of this design effort was a molecular sieve. It consisted of two zeolite beds in each of two units. These units functioned by passing the cabin air through one of the zeolite beds for 15 minutes where the CO2 would be collected. After 15 minutes of absorption had passed, the one bed would switch off and the other bed would switch on. While the one bed was off, the CO2 that had been collected by that bed would be vented overboard. These units were reusable and provided a low weight cost alternative to the lithium hydroxide canisters. (Currently, the Space Shuttle also uses similar lithium hydroxide filters. These filters are also being replaced in STS by molecular sieves for the Extended Duration Orbiter that will be launched next spring.)
Temperature and humidity control of the crew cabin was another major concern. The effect of solar heating (or lack of solar heating for the dark side) influenced the internal temperature of Skylab in a very limitted fashion due to the thermal coating and insulation around the areas of crew habitation. Electric heaters mounted throughout the modules provided heat when necessary. The heaters also would prevent excess moisture from forming on and damaging equpiment. Heat irradiation for the crew module was routed to the Skylab radiators mounted externally which will be covered under the Thermal Analysis section. Temperature was maintained at a value between 55 and 90 degrees Fahrenheit.
Relative humidity was desired to be around 26% at 86 degrees F , which for anyone who has lived in the Houston area can tell you, is a rather comfortable working environment. Humidity control was conducted by passing the cabin air through water removal condensors. Thus, along with the electric heaters, excess moisture in the atmosphere would be removed. These condensors also had the effect of lowering the temperature of the crew cabin. It was not deemed necessary to have a way to increase the humidity, since low humidity would pose no problem to neither the crew nor the experiments.
Attitude & Control
Several types of attitude sensors were used on Skylab. Many of the experimental instruments had their own fine attitude sensing and control apparatus that was designed to meet that experiment's needs. For example, the scientific cannister in the Apollo Telescope Mount (ATM) needed to point at the Sun with extreme accuracy. Provided with a course alignment towards the Sun by Skylab, the experiment pointing control system used redundant precise Sun sensors and four rate-integrating gyros to sense and update the experiment attitude. These were mated through an analog computer to control actuators consisting of a manual pointing controller, pitch-and-yaw flex pivot actuators, and a roll positioning mechanism to correct experiment pointing. These actuators provided 120 degrees of roll motion and 2 degree motion in the pitch and yaw axes. Stability of up to 1 arc-second of drift in 15 minutes was provided by this system.
Skylab itself had a Sun seeker mounted in the ATM that was designed to remain pointing towards the center of the Sun, giving the Skylab module a reference vector to the Sun. This location was chosen since one of the principle objectives of Skylab was solar study. Thus, near-continuous pointing at the Sun was not only a requirement of the mission but could provide an excellent source of attitude data. But to completely determine attitude, at least two reference vectors were needed. This other vector was given by a star tracker. The star tracker sought a reference vector much as the Sun seeker did but was oriented towards keeping track of one of three candidate stars. In the beginning and for most of the mission, the primary candidate was the star Canopus.
During each of the missions, however, the star tracker faced problems. On the first mission, the tracker proved to be too sensitive, often tracking contanimant particles from Skylab that were reflecting Earth or Sun light. On the second mission, the shutter stuck in the open position several times. On one of these times, direct Earth light reached the tracker and significantly degraded its performance. Alternate stars had then to be chosen for tracking. On the third mission, the tracker suffered the loss of its outer gimbal position encoder and was rendered useless. The crew then had to return to the days of the Columbus and use a sextant to determine star positions.
To measure the moments about Skylab's principle axis, rate gyroscopes were used. Rate gyros measure the rate about the principle axis about which they are oriented. These values can then be integrated to obtain resultant angular changes from the reference attitude provided by the Sun seeker and star tracker. On Skylab, nine rate gyros were mounted in the ATM independent of the experiment assembly, three for each of the principle axis. Ideally, one gyro was used for standby and the other two were averaged for control. But this system had problems as well. After initial deployment of Skylab, overheating caused the rate gyros to drift randomly with large amplitudes which made attitude control of Skylab difficult. Eventually, six of the nine rate gyros had to be replaced on orbit during extravehiclular activities. These new rate gyros functioned properly for the rest of the missions.
Control moment gyros (CMG) were used for the first time on Skylab. Prior to Skylab, all manned missions had been of short enough duration that thrusters could be used for attitude manuveuring. Though Skylab did have thrusters, not enough fuel could be carried on board to cover all the manuveurs that were to be required during the three Skylab missions. Thus another method had to be found to conduct the majority of Skylab attitude manuveurs.
This method consisted of mounting three CMGs mutually perpendicular to each other (Figure 5). CMGs work by storing momentum in their spinning rotors. If a manuveur was desired, the spinning gyro was rotated. This caused an equal and opposite rotation (as per Newton) in the spacecraft. To stop the manuveur, momentum was transferred back to the CMG rotor and the vehicle would stop rotating. Inertially, the gyro would now be back in the same position as it started.
Figure 5. Skylab Control Moment Gyros
The Skylab CMGs were made up of the rotor, a power inverter assembly, and an electronics assembly. The rotor rotated at about 9000 revolutions per minute, weighed 155 pounds and was 21 inches in diameter. Any two of the CMGs were enough to control Skylab attitude motion, with the third CMG providing redundancy. This redundancy was needed during the third mission when ground controllers noticed that one of the CMGs was experiencing irregularity in the rotor speed due to an increase in bearing temperature. The CMG was shut down, and, while it did not come to pass, early termination of the mission was considered.
The Thruster Attitude Control System (TACS) on Skylab consisted of six nitrogen gas expulsion nozzles mounted on the aft ent of the Orbital Workshop arranged in two 3 engine clusters on opposite sides of the Workshop. A schematic for the TACS is shown in Figure 6. The TACS was used to control Skylab during spinup of the CMG rotors during the first 10 hours of each mission, docking with the CSM, and as a backup system. In addition, 50 lbs of thrust was needed from TACS during seperation from Saturn V after launch. The only problem that occurred with the TACS was caused by the rate gyros that behaved erratically during the first half of the Skylab mission. Spurious signals were sent to the CMGs causing frequent saturation. Thruster manuveurs were then required to desaturate the CMGs. This used up much of the propellant before the first crew even arrived. Luckily, conditions favorable to Skylab allowed a 25% excess of nitrogen to be carried to orbit at launch. This offset much of the problem, but the amount of used propellant still made the CMG problem during the third mission more critical than it should have been.
Figure 6. Thruster Attitude Control System (TACS) (ref 2)
Communication System
Communications for the Skylab mission were handled through the Spacecraft Tracking and Data Network (STDN). This ground based system consisted of 13 sites during the Skylab mission. Currently, a modified STDN is still being used for STS missions with many of the same sites. Due to the site locations and Skylab's orbital inclination (59 degrees), real-time telemetry was limited to about 32% of the total time with a contact time averaging 6.5 minutes per site. This non-continuous communication facilitated the need for Skylab to have the capacity to dump large amounts of data to the ground in a short time. A playback system was used for this that allowed dumping of two hours of recorded data in 5.45 minutes, thus providing a compfortable margin under the 6.5 minute average communication time with a STDN site. Dumping was performed with much of the data on Skylab, from biomedical data from the astronauts, to ECLSS information on pressure, temperature, and humidity, to some scientific data. Other scientific data was brought physically back onboard the Command module when a crew would return to Earth.
The communications hardware was focused in three systems to provide redundancy in case of failure. One system was in the Command and Service Module that brought the astronauts up to Skylab. This system was composed of a unified S-band transponder with a Pulse Code Modulating (PCM) system. Voice communications with the ground were carried out through the CSM communications system. Periodic television transmission from the 5 Apollo Telescope Module cameras and the portable cameras were routed through the S-band as well. Data storage for return to Earth and for some dumping was also conducted through the CSM. For rendezvous with Skylab, a VHF system was used to transmit a tone-modulated signal to Skylab where a corresponding transponder would receive and then retransmit the signal back to the CSM. The phase difference would then be measured to compute the relative distance and closing rate between Skylab and the CSM.
Another system was mounted in the Apollo Telescope Mount and consisted of a VHF transmitter and UHF command receiver/decoder. Again, a PCM system was used. Data storage and subsequent dumping was carried out through this system as well as total control of the ATM by the crew. Mission Control could also conduct limited control of the ATM through the system even while a crew was not on board Skylab. Some scientific observations could thus be performed when Skylab was unmanned. This greatly increased Skylab's value to the scientific community.
The third system was located in the Airlock Module (AM). It also consisted of a VHF transmitter and UHF digital command receiver with a PCM. Data storage and dump could also be carried out through here. One of the innovations of Skylab was the use of a teleprinter to communicate with the crew. This teleprinter was a dot matrix device that allowed instructions from Mission Control to be sent to the astronauts even if the astronauts were involved in some activity such as sleep that precluded direct communications. Communications from the ground to this teleprinter were conducted through the Airlock Module system. Finally, the AM system allowed ground control over Skylab systems functions when a crew was not present. Thus, commands such as the command for Skylab to pressurize before a crew arrived were routed through this system as were the atttitude commands for Skylab to change orientation prior to reentry in 1979.
Power
Skylab's power was supplied from solar radiation, converted by solar cell arrays. Keeping the array surfaces normal to the sun vector for maximum exposure was a principle cause of Skylab's constant sun orientation. The power generation was performed by two independent subsystems, one based on the ATM and one on the OWS. Each subsystems cell arrays had an area of approximately 110 square meters and had a gross production of about 12 kiloWatts (kW). The conditioning and battery systems reduced the output to about 4 kW each.
The ATM power subsystem cell array contained approximately 165,000 silicon cells, which were arranged in four wings, projecting out obliquely from the ATM. The wings were folded against the ATM structure during launch and then were deployed on orbit, following the rotatation of the ATM into position. The cells were divided into 18 groups, each of which supplied a separate power conditioning system. These systems, located on the ATM rack, included a nickel-cadmium battery, a battery charger and a load regulator. The power conditioners fed into the power distributor, where power was routed to the operating systems through 11 load distributors. The batteries were held in their operating temperature range by electrical heaters and radiant cooling methods. To insure adequate battery life, they were limited to a depth of discharge of 30% from their 6 ampere-hour capacity. The average output of the ATM power system was 3.7 kW.
The OWS power subsystem cell array contained approximately 74,000 cells, which were arranged on two wings, one on either side of the OWS. The wings were folded against the OWS, over the micrometeoroid shield, during launch and were deployed on orbit. The cells were split into eight groups, each normally feeding one of the eight power conditioners, which were placed in the AM. In the OWS system, each cell group could be switched to one alternate conditioner to optimize power usage. Metering and control of this system was possible through panels in the OWS. The batteries for this system had a capacity of 9.9 ampere-hours, and were also limited to a 30% depth of discharge. Temperature control for the OWS power system was provided by the primary Skylab coolant system. The average output of the OWS system was 3.8 kW.
The two power systems were interconnected in parallel to allow maximal utilization of available power. In addition to the hard wired connection to the major power consuming systems, there were 28-volt utility outlets in the OWS, the AM and the MDA. The total demands on the system ranged from 3.2 KW during unmanned periods to an average of 5.8 KW during occupation, so there was significant excess capacity designed into the system.
The Apollo CSM was powered by fuel cells and batteries in the Service Module when separated from the Skylab cluster, and drew power from the Skylab power system when attached.
When the micrometeoroid shield was lost during launch, it took one of the OWS array wings with it, and also jammed the remaining wing in its closed position. This prevented the OWS system for being activated and left the ATM system as the sole power source. The need to limit the temperatures in the spacecraft required that it be rotated out of its full sun orientation, sharply reducing the exposure of the ATM array, and until repairs were made the power system was operated at a 100% consumption level. The strain from this, and from the temperature problems, eventually led to the failure of several of the batteries, which reduced the power system capability.
After the first crew deployed solar shielding, the spacecraft was rotated to full sun exposure, increasing the power from the ATM to its full capability. The astronauts were then able to free the remaining OWS wing and when this deployed, power output was raised to 7 KW, which fulfilled all of the demands on the system.
Thermal
The thermal control subsystem used in Skylab was required to maintain the equipment within its operating temperature range and to maintain the interior at a comfortable temperature for the crew. It was also required to provide refrigeration for certain supplies and specimens.
Heating of the air was performed by a system of eight heat exchanger fans and duct heater elements. There were also eight radiant wall heaters to maintain a 4 deg C minimum temperature during the unmanned phases of the mission. Thermostatically controlled heaters were provided for the specific areas of the spacecraft that might require active heating.
Radiative transfer (both heating from solar irradiation and losses to space) was controlled by a precise selection of coatings and insulation. The micrometeoroid shield also was to function as a sunshield, minimizing the irradiation onto the OWS.
The primary Skylab coolant system was located in the AM. It was used to provide cooling to the air and to the equipment systems. It also provided the cooling for the spacesuit temperature control systems via an umbilical connection. The air was cooled by thermal transfer through heat exchangers into the primary coolant system. Most equipment was cooled by both of two independent and redundant coolant loops.
The primary coolant system rejected its heat to space through a radiator mounted on the MDA and the forward portion of the AM. This radiator was buffered by a pair of thermal capacitors, which consisted of aluminum honeycomb boxes filled with a wax having a melting point of -6 deg C. These capacitors allowed the system to handle the thermal load variations (principally driven by orbitally induced variations in solar heating) without requiring excess radiator size.
The refrigeration system in the OWS was cooled by a separate coolant system consisting of two independent loops with a capacitor buffered octagonal plate radiator mounted on the aft end of the OWS.
The ATM/Solar Observatory was designed with an independent cooling system. It was separate from the primary system in part due to the independent design concept of the ATM, and in part because of the demands imposed by its constant full sun exposure. In order to minimize thermal loads, all the external surfaces were provided with low absorption coatings and a substantial sunshield was provided to protect instruments from direct solar irradiation. Equipment with low levels of heat generation were shielded to prevent excessive radiative cooling and high heat generating components were arranged and insulated so as to provide essentially uniform heat distribution. The eight solar telescopes were mounted in an insulated canister, which had an active cooling system to maintain the stable internal temperature required for instrument calibration. This system used interior cold plates which transfered heat to an external radiator through a fluid loop.
The loss of the micrometeoroid shield during the launch created significant thermal control problems. The absorption of the unshielded OWS drove the internal temperature over 65 deg C. Attempts to reduce the solar heating by changing the orientation of the spacecraft caused major power losses and also allowed the temperature in shadowed areas to drop near freezing. Eventually, a 40 deg pitch down orientation was used, which lowered the temperature to an acceptable 54 deg C, while providing adequate power and preventing freezing.
The first Skylab crew deployed a seven meter square, parasol-style fabric sunshield from the experiment airlock on the sunward side of the OWS. This shield covered much of the surface, allowing the spacecraft to be returned to its normal orientation with a tolerable internal temperature of 26 deg C (Figure 7).
The parasol sunshield had been chosen for this deployment as the easiest option to use. Another shield design involved rigging two poles from the ATM truss to the aft end of the OWS and suspending a fabric awning for them. As this option provided better coverage, and used a more stable fabric, it was decided to have the second crew deploy a shield of this type over the parasol shield. The added coverage from this shield allowed the internal temperature to fall to the 21 deg C design level.
Figure 7. Parasol Shield on Skylab (ref. 4)
Summary
The Skylab missions have contributed greatly to our knowledge of space and the Earth. The results from the many investigations conducted continue to impact our lives in such diverse fields as medicine, materials science, astronomy, and verified the feasability of remote sensing in examining the Earth. The Skylab vehicle itself remains to this day a remarkable feat of engineering. Numerous new technologies were used in its contruction and many advances were made in other technical fields that were necessary for Skylab to fly. These advances have led to the improvement of the United States space program and have given many aspiring young engineers inspiration in their difficult course work. It is certainly probable that Skylab would be in use today if it had been possible to rescue it from the fiery death it experienced in 1979.
Sunday, 01-Aug-2004 00:36:47 CDT
CSR/TSGC Team Web
