"So we built up another set just like those we have with us now. There was some extra disposable load in the Sirius which took the set to the orbit, and this allowed us to haul up the big, 40-square-meter reflector. That's the story of the station which we work."
McRae looked puzzled. "You imply that the Lunetta station is a Chinese copy of our own. Fve believed that it was the other way around and thought that our station was independent of the Mars vessels because such an independent station had proved its efficacy in the Lunetta orbit. But if the design was first worked out for the voyage, why wouldn't it have been simpler to have made the radio set integral with the ship?"
"There are mighty good reasons for the radio station being a self-contained unit separate from the ships. First of all, cargo vessels do not have comfortable living spaces like that yacht you're living in, the Polaris. They have a pilot's cabin, a tiny radio shack, and the crowded cargo bay of the landing boat, and that's all. Our transmitting and receiving spaces would have had to be built separate from the landing boat anyway.
"Then Peyton insisted that the attitudes of the ships be controlled, even while coasting, so as to avoid any propellant tank becoming too cold by being shaded for an excessive period. This would have prevented us radio people from setting the ship's attitude in accordance with the desired angularity for our reflectors.
"You'll understand, of course, that not only must our reflecting antenna be constantly directed at the Earth during the actual voyage and while circling in the Martian satellitic orbit, but the reflector of the generating plant must be directed at the Sun. It would be quite a trick to do that from a space ship whose attitude is determined by entirely different considerations. Right now, it's difficult enough to keep the ship's own power-producing reflector pointing at the Sun and still permit the skipper adequate freedom to position the vessel as he wishes. With our two reflectors, that would be a geometrical impossibility, for the angular relationship between Earth and Sun changes much during our voyage.
"The self-contained and separate characteristics of our station eliminate these difficulties entirely. After the maneuver of departure, we had two busy bees tow this station away from the Oberth and leave it some 1,000 feet from the vessel. We can stay here until the old man decides that we have to make another power maneuver. Here we can keep the transmitter compartment in whatever attitude will permit the reflectors to be appropriately directed."
"How do you control the reflectors?" asked McRae.
"With triple flywheels and in the same manner that the attitudes of the ships are controlled. There's a set of such flywheels here in the transmitter compartment. With them, we set the whole radio station, leaving the reflectors locked in position. Then we unlock the solar reflector which is swiveled at its center of gravity on an outrigger. The solar reflector has two flywheels with which we direct it at the Sun, after which a photoelectric cell automatically cuts the flywheels on and off to keep the direction. In the same manner we rivet the reflecting antenna to the Earth."
"Your current supply here is doubtless based on the same principle as the power plants in the ships?" remarked Billingsley.
"Yes, it's the same idea; solar reflector, mercury vapor boiler, turbine, and condenser, the latter being in the shadow of the solar reflector. The reflector is smaller than those on the ships, having an area of but 15 square meters. It collects solar energy to the amount of 8V2 kilowatts and feeds it to the boiler. This figure holds near Mars, but increases to 20 kilowatts near the Earth where solar rays are much stronger.
"Such kilowattage is not adequate for our transmitter, and besides, our turbogenerator is only about 30 % efficient. In the Martian orbit, we shall only extract some 2Vi kw of power from our 8V2 kw of solar energy, the remainder being reradiated to space by the condenser.
"Thus you see that our reflector only provides a quarter of the energy emitted by our transmitter. Moreover, to radiate ten kilowatts from the antenna, we must have an input into the transmitter of 30 kilowatts. For these reasons we must store energy in batteries.
We charge the accumulators with 2Vi kw for the full 24 hours, say 60 kilowatt-hours, and that allows us to operate the transmitting set about two hours daily. Our outgoing messages are handled during the ten minutes, occurring every two hours, when the station we are working is between us and the Earth. Of course, we can receive for much longer period because the receiver needs very little current."
"I suppose you just shift your antenna appropriately from transmitting to receiving when you're working the other station," said Billingsley. "Rather like talking over the telephone."
Lussigny smiled. "I'm afraid it's not quite as simple as that. Were you to try it, you'd find that all those millions of kilometers are rather more than just abstract figures. Radio waves move with the speed of light, 300,000 kilometers per second, and we are now about 20 million kilometers from Earth.
"Now, if I ask my friend Donald Flip in the Lunetta station, 'How's tricks where you are?' it'll take about a minute before my question reaches him, and I'll have to wait another minute before his, 'Just fine, Francis,' gets back to me. When we reach Mars, and when during the waiting time, Earth passes behind the Sun as seen from where we shall be, it will require almost 42 minutes for a radio wave to make a two-way trip between us and Earth.
So we'll have to "bunch" our questions and answers if we expect to be understood."
"How horrible!" groaned Billingsley. "It makes me homesick. How about it, boys, shouldn't we be getting back to Polaris?"
"I've another question for Dr. Lussigny," said McRae. "There's one thing about the power plant I've never quite understood. Every time I think of our flexible baby bottles, I cannot figure how a steam boiler can work in weightlessness. Liquid has to be evaporated in the boiler, and nothing but the steam is supposed to reach the turbine. It would seem to me that there might be frightful priming troubles. How is that avoided in weightlessness?"
Lussigny smiled again. "What you've just brought up has been one of the critical problems in the development not only of the radio power plant, but of the other plants in the ships. Here's the way it's done in principle. The condenser liquefies the mercury vapor along its walls, where it takes the shape of small globules, driven along by the still uncondensed vapor moving past. These globules are caught in a small chamber in which the condenser tubes terminate tangentially. The heavy liquid mercury whirls around the sides of this chamber from whence it can be returned to the boiler by the feed pump.
Residual mercury vapor, which keeps up the whirling motion, is recirculated to the admission side of the condenser by a small blower. Thus separation of mercury and vapor is done centrifugally, although in the boiler itself there is no sharp division between the liquid and the vapor phase. In principle the boiler is nothing but a tube set in the focal line of the gutter-like reflector. Mercury enters one side of the tube in liquid form and leaves the other side as vapor at 700 degrees Centigrade, being piped directly to the turbine.
"It all sounds a good deal simpler than it really is. We made preliminary experiments in the Lunetta orbit for months before the thing was working in principle. And it took more months to get the bugs out of the first trial power plants. Then we gave these a service test in the orbit. But finally we were successful, and I feel that we can consider this mercury vapor power plant as entirely reliable and functional."
Chapter 17 — A Nasty Little Aster
On the second of June, 1985, the voyagers to Mars had been under way for seventythree days. Their routine gave no indication to their senses that they were not floating idly in space, while they were moving actually towards their distant goal at a rate many times exceeding the muzzle velocity of a bullet. Today they would see the Earth pass across the brilliantly flaming surface of the Sun!