At mission's end, just prior to reentry, the CSM split into two parts. The conical Command Module (CM) included a bowl-shaped reentry heat shield, a pressurized crew compartment with couches and controls, lithium hydroxide canisters for removing from its pure oxygen cabin air carbon dioxide exhaled by its crew, a nose-mounted docking system, reentry reaction control rocket engines, reentry batteries, and parachutes in a nose-mounted compartment.
The largest part of the CSM, the drum-shaped Service Module (SM), included in six internal "sectors" and a cylindrical central section three fuel cells for making electricity and water, radiators, four clusters of reaction control rocket engines, the Service Propulsion System (SPS) main engine, and tanks containing cryogenic liquid hydrogen/liquid oxygen fuel cell reactants, helium pressurant, and hypergolic (ignite-on-contact) propellants. The SM provided the CM with electricity, oxygen, water, thermal control, propulsion, attitude control, and (when required) a long-range radio link to Earth.
After the two modules separated, the SM was destroyed in Earth's atmosphere. The CM, meanwhile, descended on its deployed parachutes to an ocean splashdown.
|Block II Apollo CM cutaway. Image credit: NASA.|
|Block II Apollo SM cutaway. Image credit: NASA.|
Apollo CSMs might have flown many more missions had NASA's Apollo Applications Program (AAP) gone ahead as planned in 1965-1966. As described elsewhere in this blog (see "More Information" below), AAP aimed to exploit Apollo spacecraft and Saturn rocket hardware developed for the Moon program to accomplish new things in space at reduced cost. It emphasized three major themes: science experiments, including many of potential benefit to people on Earth; advanced lunar exploration featuring long lunar surface stays with enhanced mobility; and long-duration Earth-orbital flights.
AAP, which was originally intended to span from 1968 through 1972, was proposed by NASA and endorsed by President Lyndon Baines Johnson, but some within Congress, NASA, and the aerospace industry had mixed feelings about it. Some saw it as a "make-work" program; a subset of those believed that NASA should aspire to objectives greater than mere repurposing of Apollo and Saturn hardware. Olin Teague, the chair of the House Space Subcommittee and a champion of the NASA Manned Spacecraft Center (MSC) in Houston, Texas, went so far as to call the Johnson Administration "derelict" in establishing a post-Apollo goal for NASA.
At the same time, the U.S. military commitment in Indochina was expanding rapidly, making many members of Congress uneasy about funding new space projects — even when those projects aimed to economize by repurposing space technology already developed.
By June 1966, it had become abundantly clear that Congress would not fund AAP in Fiscal Year 1967 at the level the Johnson Administration had requested. Against that inauspicious backdrop, William Hough, an engineer with Bellcomm, NASA's Washington DC-based planning contractor, commenced a study of a low-cost, low-complexity long-duration AAP mission.
Hough aimed to determine whether NASA could, through minimal upgrades of Apollo lunar program hardware, keep one, two, or three astronauts in low-Earth orbit continuously for a year. A one-year stay would lay the biomedical groundwork for more ambitious space missions — a large permanent Earth-orbital laboratory was high on the list — beginning in the mid-to-late 1970s.
In a 21 July 1966 technical memorandum, Hough described a spacecraft he called the CSM for Longevity (CSML), which would be derived from the Block II CSM planned for Apollo missions to the Moon. The CSML would tap advanced Apollo technology that engineers at North American Aviation (NAA), the CSM prime contractor, had studied for use in an Extended CSM (XCSM) design. NAA described its XCSM in the multi-volume Final Report, Preliminary Definition Phase: Apollo Extension System, which they completed in December 1965-January 1966. The company prepared the XCSM report on contract to NASA MSC.
The CSML would operate with a Dependent Experiment Support Module (DESM), which might, Hough wrote, be based on any of the several Apollo-derived laboratory modules proposed for AAP or "a module as yet undefined." In mid-1966, candidate AAP lab modules included a stripped-down Apollo LM with or without a Descent Stage (the "LEM Lab"), a refurbished flown Apollo CM, and a drum-shaped Spent Stage Experiment Support Module (SSESM) attached to an S-IVB Saturn rocket stage. Regardless of the form the DESM took, it would rely on the CSML for electricity, life support, thermal control, and propulsion. This would, Hough explained, permit the lab module to be devoted entirely to experiments.
|Candidate DESM: LEM Lab. A CSM is shown docked to provide a sense of scale. Image credit: NASA.|
|Candidate DESM: two designs for a refurbished flown CSM. Image credit: NASA.|
|Candidate DESM: Spent Stage Experiment Support Module (SSESM) and S-IVB stage. A CSM is shown docked to provide a sense of scale. Image credit: NASA.|
At the start of the one-year mission a CSML bearing a crew of three would lift off from Cape Kennedy, Florida, and ascend to a 148.2-kilometer (80-nautical-mile) low-inclination interim Earth orbit either by itself atop a Saturn IB or atop a Saturn V with the DESM. If the CSML reached Earth orbit on a Saturn IB, the DESM would be launched separately to interim orbit atop a second Saturn IB.
Drawing on July 1966 data, Hough estimated that the maximum weight a Saturn IB could deliver to interim orbit was 17,100 kilograms (37,700 pounds). He set this as the upper boundary of CSML weight at launch.
In the Saturn IB-launched case, the CSML would rendezvous with the DESM attached to the top of the spent S-IVB second stage of the Saturn IB that launched it then would dock with the DESM. In the Saturn V-launched case, the CSML would detach from the Saturn V's spent S-IVB third stage, turn end for end, and dock with the DESM attached to the top of S-IVB.
If the Saturn V-launched DESM were based on the LM or a refurbished CM, the CSML crew would detach it from the spent S-IVB. If the DESM were an SSESM/S-IVB stage, on the other hand, the CSML crew would enter the SSESM and vent leftover liquid hydrogen and liquid oxygen propellants from the S-IVB stage so that its 6.7-meter-diameter (21-foot-diameter) hydrogen tank could serve as a laboratory. In any case, after they checked out and prepared the DESM the crew would fire the CSML SPS main engine to boost the CSML/DESM combination to a 370.4-kilometer (200-nautical-mile) low-inclination operational orbit.
A single CSML could not carry enough consumables to support a three-man crew in orbit for a year, so resupply CSMLs identical to the first CSML would be launched periodically atop Saturn IB rockets. "Resupply" was something of a misnomer, for no supplies would be transferred to the CSML/DESM combination already in orbit.
Instead, as few as one or as many as three astronauts on board the CSML/DESM would spacewalk to swap places with an equal number of astronauts newly arrived in the resupply CSML. After the swap, the astronauts in the nearly spent CSML attached to the DESM would undock to return to Earth while those in the fresh resupply CSML would dock with the DESM so that the astronauts who transferred from the nearly spent CSML could continue their one-year mission.
Even as Hough began his study, NASA launched Gemini IX (3-6 June 1966). A day into the mission astronaut Eugene Cernan performed the second U.S. spacewalk. Because he lacked adequate handholds and footholds and had to fight his suit's internal pressure to bend his arms and legs, Cernan became dangerously overheated. He was unable to test a U.S. Air Force-built Astronaut Maneuvering Unit backpack as planned. NASA was soon forced to rethink its approach to spacewalking. Though his one-year mission plan would rely heavily on spacewalks, Hough made no reference to Gemini IX in his memorandum.
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.|
"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.