The radiators were, in essence, a gigantic exploit in zero-gravity plumbing. The excess heat had to be collected from where it was produced (mostly, the inhabited and pressurized parts of Izzy) and transported to where it could be gotten rid of (the “empennage” growing to aft). The only plausible way of doing this was by using a fluid, pumping it around a loop, heating it up at one end and cooling it off at the other. At the hot end they used heat exchangers and so-called cold plates that just soaked up heat from wherever it was a problem. At the cold end the fluid fanned out through networks of thin tubes, like capillaries, sandwiched between flat panels whose sole purpose was to become slightly warm and shine infrared light into deep space, cooling down Izzy by warming up faraway galaxies. Joining the hot and cold ends of the loop was a system of pumps and pipes that got bigger every day and that was prone to many of the same kinds of trouble as bedeviled earthbound plumbing. Making it twice as complicated was that some of the loops used anhydrous ammonia and others used water. Ammonia worked better, but it was dangerous, and you couldn’t easily get more of it in space. If the Cloud Ark survived, it would survive on a water-based economy. A hundred years from now everything in space would be cooled by circulating water systems. But for now they had to keep the ammonia-based equipment running as well.
Further complications, as if any were wanted, came from the fact that the systems had to be fault tolerant. If one of them got bashed by a hurtling piece of moon shrapnel and began to leak, it needed to be isolated from the rest of the system before too much of the precious water, or ammonia, leaked into space. So, the system as a whole possessed vast hierarchies of check valves, crossover switches, and redundancies that had saturated even Ivy’s brain, normally an infinite sink for detail. She’d had to delegate all cooling-related matters to a working group that was about three-quarters Russian and one-quarter American. The majority of all space walk activity was related to the expansion and maintenance of the cooling system and, uncharacteristically for her, she was content just to get a report on it once a day.
All of that plumbing, and all of those radiators, needed to be supported by Izzy’s structure just like anything else—they were especially prone to troubles under the general heading of “too floppy to survive reboost.” So, proceeding in the same general putting-out-fires mode, Ivy and the engineers on the ground next had to steer the program in the general direction of “consol,” or, as Ivy put it privately, “defloppification,” of the space station’s overall structure. And since it was out of the question to take apart what the Scouts and Pioneers had put in place, this took the form of building what amounted to external scaffolding around what was there. Viewed from a kilometer away, it looked quite similar to what one saw when some old and treasured building was being renovated: a latticework of structure, ugly but serviceable, grew around the underlying object, enveloping it and strengthening it without actually penetrating it.
In the early going, sections of truss were assembled on the ground, launched up whole, and slammed into place by teams of spacewalkers, buying large increases in structural integrity quickly and expensively. That approach soon fell prey to the law of diminishing returns and it became clear that the Arkers, as they’d started to be known, couldn’t be forever dependent on ground-based engineers custom-building structures.
The ground-based engineers didn’t even really know what was going on with Izzy anymore. Their CAD models had fallen behind. Dinah knew it because of a sudden surge in messages from exasperated engineers requesting that she send a robot out to such-and-such a place and aim its camera at such-and-such a module so that they could see what was actually there.