"There's quite a variation in the preferred temperatures of different parts of such a ship, incidentally. Think, for instance, of the liquid oxygen tanks for cabin pressurization.
The oxygen in them shouldn't boil — despite its low boiling point of -183 °Centigrade — in order not to produce any undesired pressure in the storage tank or leak losses through the safety valve above it. Then we have the food storage bays in which the food should lie deep frozen. And of course, we should like to spare ourselves the weight of refrigeration equipment with a capacity for almost three year's store of comestibles. That can all be done with this method of blinds operated by thermostats. It's a simple and relatively trouble-free expedient and involves a low weight penalty."
"All this seems to me most intelligible," remarked Woolf, "but I'm somewhat surprised that the Mars vessels will have this bright, mirror finish, when all the pictures I've seen of the Sinus ferries to Lunetta show them as black as ink."
Peyton smiled tolerantly. "There's quite another reason for that," said he. "Don't forget that the Siriuses have to traverse Earth's atmosphere at such high flight speeds as to cause great heating of their skins by air friction. By coloring that skin black, the friction-generated heat is dissipated by better radiation, and the skin does not reach such high temperatures as would otherwise be the case.
"The pilot's compartment and the disposable interior of a Sirius vessel are temperature insulated within the outer shell, and their temperature regulation is exclusively taken care of by the air conditioning system. But the Mars vessels have, as you know, no outer sheathing. That means that radiation balance must play the premier role in temperature control."
At this point Holt injected himself into the conversation. "If you'll excuse me, Sam, perhaps I can add a little illustrative touch to this Mars vessel temperature subject," he said. "Lunetta remains in the umbra of the Earth for almost one half of each of her revolutions around it, during the equinoxes at least. She too is painted silver and uses the same method of temperature regulation which Dick has just described. Now we've noticed that the regulating blinds hardly change their angle even when passing through the umbra, when there is no sunlight at all. That happens because the heat capacity of the station is so great compared to the slight amount of reradiation through the usually halfopened blinds, that a single hour is hardly enough for the interior temperatures to react to the absence of solar radiation and to affect the thermostats.
"Dick also forgot to mention another factor, namely the heat which is constantly being liberated in the operating spaces. In the case of Lunetta, there's a continuous utilization of some 15 kilowatts of electrical energy. This is converted into heat via lighting units, hot plates, motors, and the like. Then too, there are some 80 men in Lunetta's peacetime complement. You'd hardly believe what a lot of heat 80 men, each eating 4,000 calories of food per day, can put out! They alone are a very efficient heating plant! In point of fact, the blinds on the shady side are almost constantly wide open in order to expel all that heat. Otherwise I can assure you that it would gradually get uncomfortably warm."
John Wiegand decided to put his oar in at this point. "Speaking of electricity, what are you going to do about that in these Mars ships? Will it be the same setup as in Lunetta?" John had been intimately associated with assembling the power station in Lunetta and was vaguely remembering a lot of almost forgotten difficulties.
"No indeed, Johnny," answered Peyton. "Not only the problem, but the whole buildup of our Mars ship power plants differs in several respects from those of Lunetta. Her current requirement is a relatively steady 15 kilowatts, more or less. When she's in the umbra, her steam generator naturally cuts out because there's no solar radiation reaching her reflector, and that's about half the time at the equinoxes. The load is taken over by accumulators, so the generator, when it's operating, has to furnish 15 kilowatts to the mains and another fifteen to recharge the batteries so that they can carry the load during the next passage through the umbra.
"The picture on the Mars ships is quite different, for there are no repeated passages through the shadow of the Earth. On the other hand, when power maneuvers are in effect, each ship calls for some 70 kW for steering purposes. The earlier propulsive maneuvers last much longer than the later ones, so we shall draw the major part of the electric current from batteries and jettison a part of the batteries afterwards, which will economize on propellants by decreasing the mass of the ship when the next maneuver comes up.
"The intervals between maneuvers will be very long, so that a trickle charge can easily bring back batteries pretty well run down by a power maneuver. All in all, it means that, even with the heavy currents called for during the relatively short periods when the rocket motors run, we can get by with generators of moderate capacity.
"Lunetta needs a minimum generator capacity of 20 kW and we have actually installed 35 kW; but on the Mars ships, 16 kW generators will do. During the long, unpowered flight periods, most of the current will be used by the blowers and pumps for water and air regeneration and recuperation. All of these together will call for about 6 kW, leaving ample current available for such auxiliaries as electric stoves, washing machines, remote annunciators, thermostatic controls and the like. We shall not need, except to a small extent, electric lighting; for unlike Lunetta, our ships will almost always fly in sunlight. Of course the batteries will stand an occasional peak load of 40 or 50 kW if it is not applied just prior to a power maneuver.
"The power-plants of the Mars vessels will be operated by turbogenerators fed by sun-heated boilers. We shall require solar reflectors of 94 square meters surface to produce the required 16 kW, and oddly enough, that's more surface than is used for Lunetta's 35 kW. The apparent discrepancy is explained by the need to keep up our 16 kW even when near Mars, where the solar energy caught by the reflector is less than half of that near the Earth's orbit.
"After the vessels have been drawn in by Mars's gravitation, they will circle him and be in his umbra during one third of the time required for each circle. Then the problem will become much like Lunetta's. The mean output of our 16 kW generators will be reduced to 11 kW per vessel, and that's getting close to the tenuous current requirements of the air and water regeneration systems. It is, in a sense, fortunate that the crews of the ships awaiting in their satellite orbits the return of the landing craft will be reduced by the number of the landing party. This will diminish our current requirements during this critical period.
"Exhaust steam condensers present a particular problem in the Mars vessels. Those of you who have visited Lunetta will remember the spokes connecting her rim with the central station. These serve as condenser tubes reducing the aqueous vapor to liquid which the feed pumps can return to the boiler. It was possible to use this expedient on Lunetta because the spokes are shaded by the rim, due to Lunetta's rotating in the plane of the ecliptic. But the Mars vessels are not wheel-shaped, and their attitude in space with respect to the Sun may change continually. Hence we propose to locate the condensers behind the reflectors, because, since the reflectors must always face sunwards when the turbines are running, there will invariably be shadow behind them.