Well, once again Wednesday has rolled around. This week, we turn our attention to the preparations for Spacelab and take an in-depth look at the modifications made to it in preparation for its flight.
Eyes Turned Skyward, Post #11:
Assembly work on Spacelab resumed at an almost indecent haste once it became clear that Skylab had been repaired and therefore there was no longer any potential need for a Skylab B to replace the wounded station. Since construction had been stopped at an early stage--little more had been done than removing the engine and most of the hardware needed for it--a considerable amount of work would be needed for completion, and it was obvious it would be at least several years before launch. However, despite this and the lack of data from Skylab to support particular designs, the engineers and designers working on the project already had clear ideas about the most major changes needed for Spacelab compared to Skylab.
First and foremost, of course, was correcting the failure of the micrometeroid/solar shield deployment mechanism that had nearly caused the loss of Skylab. Since the same basic shield idea would be used on Spacelab, this was obviously a very high priority. Further, since the failure had been the result of what had been believed to be a non-safety-critical part, a thorough program of safety scrubbing--rigorously analyzing the entire spacecraft for potential failure modes in all components--was begun. While this energetic approach to ensuring the correct and safe functioning of the station did divert resources from the main program, the legacy of nearly a decade of highly successful service indicates that this diversion paid off handsomely.
Next was expanding the available pressurized volume. While the hydrogen tank that had been used by Skylab for the vast majority of its pressurized volume was very large and more than adequate for that station, NASA believed that Spacelab might be in use (indeed much more active use) for considerably longer. A permanently manned duration of at least several years was believed likely, and in conjunction with the ASTP II program more volume was desired for habitation. Adding additional volume would allow the gradual extension of Spacelab capabilities as necessary, whether that was more habitat volume for extra crew or more laboratory space for materials science experiments. The most obvious way to increase the pressurized volume of the station was to use the SIVB’s oxygen tank, with over 2,500 cubic feet of volume (similar to a 40-foot shipping container). Doing so would increase the pressurized volume of the Orbital Workshop section of the station by over 25%, and would be relatively easy to accomplish on the ground. Skylab and previous "wet workshop" studies had left the tank open to vacuum due to limited resources and planned to use of the tank as a kind of "dumpster" to store garbage, but the newly developed AARDVark could supply whatever might be needed to use the tank over time and be used for trash disposal by incineration during reentry.
Another aspect of Skylab's design considered decidedly inferior by most of the engineers working on Spacelab was the arrangement for the airlock "module". Inserted between the Orbital Workshop and Multiple Docking Adapter, use of the airlock prevented anyone inside the Orbital Workshop from reaching the CSM in case of an emergency, forcing anyone not going on a spacewalk to wait it out inside the CSM. Engineers on Spacelab had designed a new Airlock Module, fitted to the emergency docking port for use. At this point, however, the politics of the station began to interfere in the design process. One of the major goals of Spacelab and a significant factor in maintaining the station's development funding through the decade was the ASTP II mission with the Soviet Union. However, the Soyuz spacecraft used by the Soviets naturally used a very different (and completely incompatible) docking system from the Apollo spacecraft, in addition to having a different internal pressure and atmospheric composition, mandating the use of a docking adapter from Soyuz to Spacelab to allow cosmonauts to pass from one to another, much like on ASTP I. Since the emergency docking port was the only one free for mounting of this adapter, and since the adapter could neither be launched on a later Apollo flight (not the last time the probe-and-drogue system would cause operational difficulties for NASA) nor with the Soyuz (the weight would prevent the Soyuz from being able to reach Spacelab at all) and thus had to be launched with Spacelab, the airlock module could not be launched with Spacelab itself. If it was, the resulting airlock-docking module stack would protrude beyond the edge of the aerodynamic fairing covering the Multiple Docking Adapter. Instead, it would have to be launched with the third Spacelab flight, after the ASTP II flight. Until then, the CSM could be temporarily used as an airlock "module"; since there were no scheduled spacewalks before the third flight, this was considered an acceptable emergency substitute.
The development of the airlock module opened up a new line of thought about how to expand Spacelab's pressurized volume--perhaps some kind of similar but larger module could be launched and maneuvered into place with the AARDV? European scientists and engineers suggested the creation of a Research Module--a small "add on," massing perhaps 14-15 metric tons, which could be launched and docked to the station in the same fashion as an AARDVark. Such a module might be able to add additional capabilities to the station even better than AARDVark flights could, and might be useful to gather information about future modular constructions (which seemed increasingly likely with the demise of US heavy lift capability). After significant study, development of the Research Module was approved by NASA to be carried out by the ESA, with a launch sometime after ASTP II. This meant that a basically European project was being subjected to the whims of the US political process, one of the first whiffs of the conflict that would slowly build between the two agencies and indeed between the United States and Europe on the subject of space flight.
Finally, near the end of the design and development process, the first elements of actual use data from Skylab began filtering in. In the main, this data confirmed the direction taken--Skylab was quite usable and most of its systems functioned well, and Spacelab would be more of the same--but it did lead to some small changes around the edges. For example, the bicycle ergometer design, with its elaborate (and as events proved, entirely counterproductive) tether system was completely changed, while a treadmill (useful both for aerobic exercise and, more importantly, for maintaining lower body strength) was added. The shower, which had proved largely useless and superfluous in orbit, where sponge baths were both easier and just as good for getting clean, would be removed. And, in a move which would be much lamented by the Astronaut Corps, the freezer (which had allowed Skylab astronauts to enjoy such delicacies as real ice cream and filet mignon) would be removed. Supplying frozen or refrigerated food would impose too large a payload penalty on the AARDVarks carrying the food, and the capability was removed early in the design process.
However, not every change to Spacelab was adding capabilities over Skylab. In a controversial move, the Apollo Telescope Mount, one of the scientific centerpieces of Skylab and a highly productive instrument, had been totally deleted from Spacelab's design. Due to the need to accommodate CSM, AARDVark, and Soyuz capsules simultaneously, Spacelab required at least three docking ports. Even in routine operations, it would be normal to have a CSM and AARDVark docked at least some of the time, and a third port was desired for emergency operations. Given a Multiple Docking Adapter design similar to Skylab's (in order to save on development costs), the only way to accommodate three ports was to delete the ATM. Since Spacelab had been sustained partially on the need to accommodate ASTP II, and with the beginnings of significant European involvement in the station, retaining the ATM was never seriously considered as an option, despite the pleas from solar physicists. Observations would simply have to revert to automated and ground-based platforms, whether or not Skylab had been more effective.