Arguments on Earth had raged for months over the necessities—or lack of them—for the huge mass of water aboard, but the fluid mass served many purposes better than anything else could serve those purposes.
As a radiation shield, it provided sufficient safety against cosmic radiations of space and from solar radiations, except for solar flare conditions, to provide a margin of safety for the crew over the three months in which they would do their jobs before being rotated back to Earth for the fifteen-month recovery period.
The margin was nearly enough for permanent duty—and there were those who claimed it was sufficient—but the claim had not been substantiated, and the three months maximum for tour was mandatory.
Originally, shielding had not been considered of vital importance, but experience had proven the necessity. The first construction personnel had been driven back to Earth after two weeks, dosimeters in the red. The third crew didn't make it. All five died of radiation exposure from a solar flare. An original two weeks' limit was raised as more shielding arrived—three weeks, four, five—now the shadowy edge of the theoretic ninety-day recovery rate from radiation damage and the ninety days required to get the maximum safe dosage overlapped—but safety procedures still dictated that a red dosimeter meant a quick return to Earth whether the rate of recovery overlapped or not.
The question was still open whether more shielding would be brought up to make the overlap certain, or whether it would be best to maintain a personnel rotation policy indefinitely. Some factions on Earth seemed determined that rotation must remain not only a procedural but an actual requirement—their voices spoke plainly through the directives and edicts of U.N. Budget Control—but from what source behind this bureaucratic smokescreen it would have been difficult to say.
As a heat sink, the water provided stability of temperature that would have been difficult to achieve without it. Bathed in the tenuous solar atmosphere that extends well beyond the orbit of Earth, and with a temperature over 100,000 C, maintenance of a livable temperature on board the big wheel was not the straight-forward balancing of radiation intercepted/radiation outgoing that had been originally anticipated by early writers on the subject.
True, the percentage of energy received by convection was small compared to that received by radiation; but it was also wildly variable.
As a biological cultural medium, the hydraulic system provided a basis for both air restoration and food supplies. When the proper balance of plankton and algae was achieved, the air jets that gave the ship its spin would also purify the ship's air, giving it back in a natural manner the oxygen it was now fed from tanks.