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| The Skylab 2 Command and Service Module sits atop its Saturn IB launch car on Pad 39B at NASA Kennedy Area Heart, Florida. The spacecraft, with astronauts Charles Conrad, Paul Weitz, and Joseph Kerwin on board, lifted off on 25 Might 1973, docked with the Skylab area station, and returned to Earth on 22 June 1973. Picture credit score: NASA. |
The Apollo Command and Service Module (CSM) was the work-horse U.S. piloted spacecraft from its first launch in October 1968 till its final Earth-atmosphere reentry and splashdown in July 1975. Launched atop two-stage Saturn IB and three-stage Saturn V launch autos, it carried six three-man crews to low Earth orbit and 9 to lunar orbit. These missions noticed it function twice by itself, 9 instances with Apollo Lunar Module (LM) landers, 3 times with the Skylab Orbital Workshop, and as soon as with a Docking Module and a Soviet Soyuz spacecraft.
At mission’s finish, simply previous to reentry, the CSM break up into two components. The conical Command Module (CM) included a bowl-shaped reentry warmth defend, a pressurized crew compartment with couches and controls, lithium hydroxide canisters for eradicating from its pure oxygen cabin air carbon dioxide exhaled by its crew, a nose-mounted docking system, reentry response management rocket engines, reentry batteries, and parachutes in a nose-mounted compartment.
The most important a part of the CSM, the drum-shaped Service Module (SM), included in six inner “sectors” and a cylindrical central part three gas cells for making electrical energy and water, radiators, 4 clusters of response management rocket engines, the Service Propulsion System (SPS) primary engine, and tanks containing cryogenic liquid hydrogen/liquid oxygen gas cell reactants, helium pressurant, and hypergolic (ignite-on-contact) propellants. The SM offered the CM with electrical energy, oxygen, water, thermal management, propulsion, angle management, and (when required) a long-range radio hyperlink to Earth.
After the 2 modules separated, the SM was destroyed in Earth’s environment. The CM, in the meantime, descended on its deployed parachutes to an ocean splashdown.
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| Block II Apollo CM cutaway. Picture credit score: NASA. |
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| Block II Apollo SM cutaway. Picture credit score: NASA. |
Apollo CSMs might need flown many extra missions had NASA’s Apollo Purposes Program (AAP) gone forward as deliberate in 1965-1966. As described elsewhere on this weblog (see “Extra Info” under), AAP aimed to take advantage of Apollo spacecraft and Saturn rocket {hardware} developed for the Moon program to perform new issues in area at decreased price. It emphasised three main themes: science experiments, together with a lot of potential profit to folks on Earth; superior lunar exploration that includes lengthy lunar floor stays with enhanced mobility; and long-duration Earth-orbital flights.
AAP, which was initially supposed to span from 1968 by 1972, was proposed by NASA and endorsed by President Lyndon Baines Johnson, however some inside Congress, NASA, and the aerospace business had combined emotions about it. Some noticed it as a “make-work” program; a subset of these believed that NASA ought to aspire to goals higher than mere repurposing of Apollo and Saturn {hardware}. Olin Teague, the chair of the Home Area Subcommittee and a champion of the NASA Manned Spacecraft Heart (MSC) in Houston, Texas, went as far as to name the Johnson Administration “derelict” in establishing a post-Apollo aim for NASA.
On the similar time, the U.S. army dedication in Indochina was increasing quickly, making many members of Congress uneasy about funding new area tasks — even when these tasks aimed to economize by repurposing area know-how already developed.
By June 1966, it had develop into abundantly clear that Congress wouldn’t fund AAP in Fiscal 12 months 1967 on the degree the Johnson Administration had requested. In opposition to that inauspicious backdrop, William Hough, an engineer with Bellcomm, NASA’s Washington DC-based planning contractor, commenced a research of a low-cost, low-complexity long-duration AAP mission.
Hough aimed to find out whether or not NASA might, by minimal upgrades of Apollo lunar program {hardware}, maintain one, two, or three astronauts in low-Earth orbit constantly for a 12 months. A one-year keep would lay the biomedical groundwork for extra formidable area missions — a big everlasting Earth-orbital laboratory was excessive on the checklist — starting within the mid-to-late Seventies.
In a 21 July 1966 technical memorandum, Hough described a spacecraft he known as the CSM for Longevity (CSML), which might be derived from the Block II CSM deliberate for Apollo missions to the Moon. The CSML would faucet superior Apollo know-how that engineers at North American Aviation (NAA), the CSM prime contractor, had studied to be used in an Prolonged CSM (XCSM) design. NAA described its XCSM within the multi-volume Remaining Report, Preliminary Definition Section: Apollo Extension System, which they accomplished in December 1965-January 1966. The corporate ready the XCSM report on contract to NASA MSC.
The CSML would function with a Dependent Experiment Assist Module (DESM), which could, Hough wrote, be based mostly on any of the a number of Apollo-derived laboratory modules proposed for AAP or “a module as but undefined.” In mid-1966, candidate AAP lab modules included a stripped-down Apollo LM with or with no Descent Stage (the “LEM Lab”), a refurbished flown Apollo CM, and a drum-shaped Spent Stage Experiment Assist Module (SSESM) hooked up to an S-IVB Saturn rocket stage. Whatever the type the DESM took, it will depend on the CSML for electrical energy, life assist, thermal management, and propulsion. This is able to, Hough defined, allow the lab module to be devoted completely to experiments.
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| Candidate DESM: LEM Lab. A CSM is proven docked to offer a way of scale. Picture credit score: NASA. |
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| Candidate DESM: two designs for a refurbished flown CSM. Picture credit score: NASA. |
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| Candidate DESM: Spent Stage Experiment Assist Module (SSESM) and S-IVB stage. A CSM is proven docked to offer a way of scale. Picture credit score: NASA. |
In the beginning of the one-year mission a CSML bearing a crew of three would raise off from Cape Kennedy, Florida, and ascend to a 148.2-kilometer (80-nautical-mile) low-inclination interim Earth orbit both 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 can be launched individually to interim orbit atop a second Saturn IB.
Drawing on July 1966 information, Hough estimated that the utmost weight a Saturn IB might ship to interim orbit was 17,100 kilograms (37,700 kilos). He set this because the higher boundary of CSML weight at launch.
Within the Saturn IB-launched case, the CSML would rendezvous with the DESM hooked up to the highest of the spent S-IVB second stage of the Saturn IB that launched it then would dock with the DESM. Within the Saturn V-launched case, the CSML would detach from the Saturn V’s spent S-IVB third stage, flip finish for finish, and dock with the DESM hooked up to the highest of S-IVB.
If the Saturn V-launched DESM have been based mostly on the LM or a refurbished CM, the CSML crew would detach it from the spent S-IVB. If the DESM have been an SSESM/S-IVB stage, then again, the CSML crew would enter the SSESM and vent leftover liquid hydrogen and liquid oxygen propellants from the S-IVB stage in order that its 6.7-meter-diameter (21-foot-diameter) hydrogen tank might function a laboratory. In any case, after they checked out and ready the DESM the crew would hearth the CSML SPS primary engine to spice up the CSML/DESM mixture to a 370.4-kilometer (200-nautical-mile) low-inclination operational orbit.
A single CSML couldn’t carry sufficient consumables to assist a three-man crew in orbit for a 12 months, so resupply CSMLs an identical to the primary CSML can be launched periodically atop Saturn IB rockets. “Resupply” was one thing of a misnomer, for no provides can be transferred to the CSML/DESM mixture already in orbit.
As an alternative, as few as one or as many as three astronauts on board the CSML/DESM would spacewalk to swap locations with an equal variety of astronauts newly arrived within the resupply CSML. After the swap, the astronauts within the almost spent CSML hooked up to the DESM would undock to return to Earth whereas these within the contemporary resupply CSML would dock with the DESM in order that the astronauts who transferred from the almost spent CSML might proceed their one-year mission.
Whilst Hough started his research, NASA launched Gemini IX (3-6 June 1966). A day into the mission astronaut Eugene Cernan carried out the second U.S. spacewalk. As a result of he lacked enough handholds and footholds and needed to battle his go well with’s inner strain to bend his legs and arms, Cernan grew to become dangerously overheated. He was unable to check a U.S. Air Power-built Astronaut Maneuvering Unit backpack as deliberate. NASA was quickly compelled to rethink its strategy to spacewalking. Although his one-year mission plan would rely closely on spacewalks, Hough made no reference to Gemini IX in his memorandum.
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| Throughout the Apollo 9 mission (3-13 March 1969) astronauts David Scott (pictured) and Russell Schweickart carried out spacewalks exterior the CSM Gumdrop (decrease left) and the LM Spider (higher proper) in low-Earth orbit. Had Hough’s plan for a year-long CSML mission gone forward, a scene much like this might need taken place throughout crew trade between a CSML/DESM mixture and a newly arrived resupply CSML. Picture credit score: NASA. |
A lot of Hough’s report was dedicated to figuring out the variety of CSMLs wanted for a one-year keep in area by no less than one astronaut. Not surprisingly, this may rely on anticipated CSML endurance. On the “decrease sure of technological sophistication” was a minimal CSML with an orbital endurance of simply 35 to 40 days. This meant that NAA’s XCSM, which was rated for 45 days, might simply do the job.
Utilizing the XCSM would, nevertheless, imply {that a} one-year keep would require about 12 launches. Hough rejected this strategy as a result of it will want extra Saturn rockets and Apollo spacecraft than NASA anticipated to have out there annually for the AAP.
Hough described adjustments to the Block II Apollo CSM required to show it right into a CSML able to working in orbit with out alternative for 94 days (during which case 4 CSMLs would allow a year-long keep) or 125 days (during which case three CSMLs would suffice). CM modifications can be comparatively minor whereas SM modifications can be in depth.
Essentially the most important CM modification when it comes to weight affect can be alternative of the Block II Apollo lithium hydroxide carbon-dioxide removing system — apart from a two-day emergency provide of canisters — with a “two mattress, thermal swing, vacuum-dump molecular sieve” system. The dual chemical beds would alternate; that’s, one mattress can be opened to soak up carbon dioxide from the CSML cabin air whereas the opposite can be closed off, uncovered to the vacuum of area, and heated to drive out the carbon dioxide it had absorbed.
Not like the Apollo Block II CSM, the CSML would come with nitrogen in its cabin air. Introduction of nitrogen was a concession to area life scientists who anxious about lengthy astronaut publicity to pure oxygen. Nitrogen can be saved within the SM, so CM weight adjustments ensuing from the brand new air combine can be minimal.
Hough missed few particulars. He famous, for instance, that the CM parachute compartment would steadily lose strain throughout an extended area keep, and that the vitally vital parachutes it contained could possibly be broken if uncovered to hoover. He proposed putting 9 kilograms (20 kilos) of strong “vaporizing materials” of unspecified composition within the compartment. This is able to slowly flip to fuel, conserving the strain degree within the compartment regular.
Most of Hough’s research consisted of discovering tradeoffs to maintain CSML weight under the 17,100-kilogram (37,700-pound) restrict. Crucial of those tradeoffs was deletion of propulsion functionality in favor of added electricity-generation functionality.
He calculated that simply 1633 kilograms (3600 kilos) of hydrazine gas and nitrogen tetroxide oxidizer can be adequate to hold out all main maneuvers required of the SPS primary engine: particularly, boosting the CSML/DESM from its interim orbit to its operational orbit; resupply rendezvous with the CSML/DESM mixture in operational orbit; and deorbiting the CSML on the finish of its lengthy keep in orbit. The quantity of propellant required for these maneuvers can be the identical whatever the period of the CSML mission.
This amount of SPS propellants totaled lower than 10% of the SPS propellant capability of the Block II Apollo CSM. A pair of latest, shorter SPS propellant tanks in sectors 2 and 5, measuring 1.3 meters (4.25 toes) in diameter by simply 22.9 centimeters (9 inches) tall, would, Hough calculated, suffice to include this amount of propellants. That may unlock most of sectors 2, 3, 5, and 6 and the central cylindrical compartment for gas cell reactants and different consumables.
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| Block II Apollo CSM sector structure. Picture credit score: NASA. |
The small quantity of orbit upkeep propulsion required to keep away from orbital decay throughout an extended mission would, Hough wrote, be offered by the 4 Response Management System (RCS) thruster quads spaced evenly round exterior of the SM. The RCS would expend a mean of about 9 kilograms (20 kilos) of hydrazine gas and nitrogen tetroxide oxidizer per day to keep up the CSML’s orbital altitude and management its angle, bringing the whole RCS propellant load to about 846 kilograms (1880 kilos) for a 94-day CSML and about 1125 kilograms (2500 kilos) for a 125-day CSML. This is able to require growth of the RCS tanks.
Hough proposed that 4 superior “asbestos-membrane” gas cells substitute the three “Bacon-cell” gas cells housed in sector 4 of the Block II Apollo SM. The latter have been rated to function for 400 hours (16.7 days), which was ample time to finish an Apollo lunar mission. He reported {that a} take a look at model of the asbestos-membrane gas cell had operated constantly for 1200 hours (50 days) and that it was anticipated to be able to working for as much as 2500 hours (104.2 days).
Asbestos-membrane gas cells featured a useful in-flight begin functionality, Hough defined, letting them be operated in shifts to increase CSML orbital lifetime and enhance redundancy. He envisioned that one or two would stay on “chilly standby” at anyone time. He calculated that two might produce three kilowatts of electrical energy constantly in the event that they consumed a mean of 1.23 kilograms (2.72 kilos) of liquid hydrogen/liquid oxygen reactants per hour. Three kilowatts was roughly double the quantity of electrical energy wanted for routine CSML “housekeeping” capabilities, thus making out there about 1.5 kilowatts for DESM experiments.
It’s honest to ask why Hough didn’t think about programs apart from gas cells for producing CSML electrical energy. The Bellcomm engineer might need proposed that the CSML depend on photo voltaic arrays or an isotopic system, both of which might be much less large than gas cells and closely insulated tanks of cryogenic reactants. He defined that neither photo voltaic arrays nor a nuclear system had not been studied to be used in XCSM missions, so they may not be thought-about to be throughout the bounds of Apollo know-how as he outlined them in his research.
Hough acknowledged that, regardless of cautious tradeoffs, his year-long mission tended towards tight consumables margins. For instance, he allotted simply three days of overlap for every resupply mission. This meant that “a couple of days of hurricane watch at KSC on the time of a resupply launch would trigger termination of the whole mission.”
Although he studied it rigorously, Hough was not particularly enthusiastic concerning the CSML/DESM strategy to a one-year mission. He defined that “it’s possible that the CSML/DESM shouldn’t be one of the best strategy when in comparison with the self-sufficient new module” strategy, although he maintained that “it seems to be optimum if the constraint of use of Apollo know-how. . .is imposed.”
Hough argued that the principle motive to accept the CSML/DESM strategy — apart from “a potential lean 12 months or two of spacecraft launches” attributable to AAP funding cuts — can be the looks of latest info regarding “man’s compatibility with long-term spaceflight” that made the viability of lengthy astronaut stays on board a self-sufficient module appear uncertain. In that case, trying a one-year CSML/DESM mission to achieve further information forward of a giant funding in a brand new module could be seen as frugal.
He added that, if adequate assets existed for each a one-year CSML/DESM mission and improvement of a self-sufficient module, then the CSML/DESM mission could possibly be seen as a prudent step ahead even when the viability of long-duration spaceflight have been assured. Experiments within the DESM may embrace a prototype superior energy supply impartial of the CSML’s gas cells or take a look at variations of long-duration life assist programs.
In August 1966, NASA took a step towards a “self-sufficient new module” when it opted to focus its Earth-orbital AAP efforts on the SSESM/spent S-IVB stage laboratory possibility. The area company renamed the SSESM the Airlock Module; the spent S-IVB stage grew to become referred to as the Workshop. Within the Airlock Module/Workshop state of affairs, the CSM would serve primarily as a crew transport; the Airlock Module/Workshop would come with impartial life assist and electricity-generating programs.
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| Apollo 9 CSM Gumdrop in low-Earth orbit as considered from the LM Spider, March 1969. Picture credit score: NASA. |
Sources
“Gemini 9 Underscores Information Gaps,” Aviation Week & Area Know-how, 11 July 1966, p. 37.
“CSM Configuration Research for One 12 months Mission to be Achieved by Rendezvous and Resupply,” W. W. Hough, Bellcomm, Inc., 21 July 1966.
“Washington Roundup — Apollo Curler Coaster,” Aviation Week & Area Know-how, 1 August 1966, p. 15.
“NASA Put up-Apollo Plan Urged by Dec. 1,” George C. Wilson, Aviation Week & Area Know-how, 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 Info Workplace, 1977, p. 88.
Extra Info
Apollo Extension System Flight Mission Task Plan (1965)
Apollo Purposes Program: Lunar Module Relay Experiment Laboratory (1966)
“With out Hiatus”: The Apollo Purposes Program in June 1966
