A Year in Orbit Using Apollo Technology: Command and Service Module for Longevity (1966) (2)
During the Apollo 9 mission (3-13 March 1969) astronauts David Scott (pictured) and Russell Schweickart performed spacewalks outside the CSM Gumdrop (lower left) and the LM Spider (upper right) in low-Earth orbit. Had Hough's plan for a year-long CSML mission gone ahead, a scene similar to this might have taken place during crew exchange between a CSML/DESM combination and a newly arrived resupply CSML. Image credit: NASA.Much of Hough's report was devoted to determining the number of CSMLs needed for a one-year stay in space by at least one astronaut. Not surprisingly, this would depend on expected CSML endurance. At the "lower bound of technological sophistication" was a minimal CSML with an orbital endurance of just 35 to 40 days. This meant that NAA's XCSM, which was rated for 45 days, could easily do the job.
Using the XCSM would, however, mean that a one-year stay would require about 12 launches. Hough rejected this approach because it would need more Saturn rockets and Apollo spacecraft than NASA expected to have available each year for the AAP.
Hough described changes to the Block II Apollo CSM required to turn it into a CSML capable of operating in orbit without replacement for 94 days (in which case four CSMLs would enable a year-long stay) or 125 days (in which case three CSMLs would suffice). CM modifications would be relatively minor while SM modifications would be extensive.
The most significant CM modification in terms of weight impact would be replacement of the Block II Apollo lithium hydroxide carbon-dioxide removal system — except for a two-day emergency supply of canisters — with a "two bed, thermal swing, vacuum-dump molecular sieve" system. The twin chemical beds would alternate; that is, one bed would be opened to absorb carbon dioxide from the CSML cabin air while the other would be closed off, exposed to the vacuum of space, and heated to drive out the carbon dioxide it had absorbed.
Unlike the Apollo Block II CSM, the CSML would include nitrogen in its cabin air. Introduction of nitrogen was a concession to space life scientists who worried about long astronaut exposure to pure oxygen. Nitrogen would be stored in the SM, so CM weight changes resulting from the new air mix would be minimal.
Hough missed few details. He noted, for example, that the CM parachute compartment would gradually lose pressure during a long space stay, and that the vitally important parachutes it contained could be damaged if exposed to vacuum. He proposed placing nine kilograms (20 pounds) of solid "vaporizing material" of unspecified composition in the compartment. This would slowly turn to gas, keeping the pressure level in the compartment steady.
Most of Hough's study consisted of finding tradeoffs to keep CSML weight below the 17,100-kilogram (37,700-pound) limit. The most important of these tradeoffs was deletion of propulsion capability in favor of added electricity-generation capability.
He calculated that just 1633 kilograms (3600 pounds) of hydrazine fuel and nitrogen tetroxide oxidizer would be sufficient to carry out all major maneuvers required of the SPS main engine: specifically, boosting the CSML/DESM from its interim orbit to its operational orbit; resupply rendezvous with the CSML/DESM combination in operational orbit; and deorbiting the CSML at the end of its long stay in orbit. The amount of propellant required for these maneuvers would be the same regardless of the duration of the CSML mission.
This quantity of SPS propellants totaled less than 10% of the SPS propellant capacity of the Block II Apollo CSM. A pair of new, shorter SPS propellant tanks in sectors 2 and 5, measuring 1.3 meters (4.25 feet) in diameter by just 22.9 centimeters (9 inches) tall, would, Hough calculated, suffice to contain this quantity of propellants. That would free up most of sectors 2, 3, 5, and 6 and the central cylindrical compartment for fuel cell reactants and other consumables. Block II Apollo CSM sector layout. Image credit: NASA.The small amount of orbit maintenance propulsion required to avoid orbital decay during a long mission would, Hough wrote, be provided by the four Reaction Control System (RCS) thruster quads spaced evenly around exterior of the SM. The RCS would expend an average of about nine kilograms (20 pounds) of hydrazine fuel and nitrogen tetroxide oxidizer per day to maintain the CSML's orbital altitude and control its attitude, bringing the total RCS propellant load to about 846 kilograms (1880 pounds) for a 94-day CSML and about 1125 kilograms (2500 pounds) for a 125-day CSML. This would require expansion of the RCS tanks.
Hough proposed that four advanced "asbestos-membrane" fuel cells replace the three "Bacon-cell" fuel cells housed in sector 4 of the Block II Apollo SM. The latter were rated to operate for 400 hours (16.7 days), which was ample time to complete an Apollo lunar mission. He reported that a test version of the asbestos-membrane fuel cell had operated continuously for 1200 hours (50 days) and that it was expected to be capable of operating for up to 2500 hours (104.2 days).
Asbestos-membrane fuel cells featured a handy in-flight start capability, Hough explained, permitting them to be operated in shifts to extend CSML orbital lifetime and increase redundancy. He envisioned that one or two would remain on "cold standby" at any one time. He calculated that two could produce three kilowatts of electricity continuously if they consumed an average of 1.23 kilograms (2.72 pounds) of liquid hydrogen/liquid oxygen reactants per hour. Three kilowatts was approximately double the amount of electricity needed for routine CSML "housekeeping" functions, thus making available about 1.5 kilowatts for DESM experiments.
It is fair to ask why Hough did not consider systems other than fuel cells for generating CSML electricity. The Bellcomm engineer might have proposed that the CSML rely on solar arrays or an isotopic system, either of which would be less massive than fuel cells and heavily insulated tanks of cryogenic reactants. He explained that neither solar arrays nor a nuclear system had not been studied for use in XCSM missions, so they could not be considered to be within the bounds of Apollo technology as he defined them in his study.
Hough acknowledged that, in spite of careful tradeoffs, his year-long mission tended toward tight consumables margins. For example, he allotted just three days of overlap for each resupply mission. This meant that "a few days of hurricane watch at KSC at the time of a resupply launch would cause termination of the total mission."
Though he studied it carefully, Hough was not especially enthusiastic about the CSML/DESM approach to a one-year mission. He explained that "it is probable that the CSML/DESM is not the best approach when compared to the self-sufficient new module" approach, though he maintained that "it appears to be optimum if the constraint of use of Apollo technology. . .is imposed."
Hough argued that the main reason to settle for the CSML/DESM approach — aside from "a possible lean year or two of spacecraft launches" caused by AAP funding cuts — would be the appearance of new information concerning "man's compatibility with long-term spaceflight" that made the viability of long astronaut stays on board a self-sufficient module seem doubtful. In that case, attempting a one-year CSML/DESM mission to gain additional data ahead of a large investment in a new module might be seen as frugal.
He added that, if sufficient resources existed for both a one-year CSML/DESM mission and development of a self-sufficient module, then the CSML/DESM mission could be seen as a prudent step forward even if the viability of long-duration spaceflight were assured. Experiments in the DESM might include a prototype advanced power source independent of the CSML's fuel cells or test versions of long-duration life support systems.
In August 1966, NASA took a step toward a "self-sufficient new module" when it opted to focus its Earth-orbital AAP efforts on the SSESM/spent S-IVB stage laboratory option. The space agency renamed the SSESM the Airlock Module; the spent S-IVB stage became known as the Workshop. In the Airlock Module/Workshop scenario, the CSM would serve mainly as a crew transport; the Airlock Module/Workshop would include independent life support and electricity-generating systems.Apollo 9 CSM Gumdrop in low-Earth orbit as viewed from the LM Spider, March 1969. Image credit: NASA.Sources"Gemini 9 Underscores Knowledge Gaps," Aviation Week & Space Technology, 11 July 1966, p. 37.
"CSM Configuration Study for One Year Mission to be Achieved by Rendezvous and Resupply," W. W. Hough, Bellcomm, Inc., 21 July 1966.
"Washington Roundup — Apollo Roller Coaster," Aviation Week & Space Technology, 1 August 1966, p. 15.
"NASA Post-Apollo Plan Urged by Dec. 1," George C. Wilson, Aviation Week & Space Technology, 8 August 1966. pp. 26.
Skylab: A Chronology, NASA SP-4011, Roland W. Newkirk and Ivan D. Irtel with Courtney G. Brooks, NASA Scientific and Technical Information Office, 1977, p. 88.
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