Tank Farm Dynamo

3

Imagine six very long parallel wires, hanging in space, always aimed toward the surface of the Earth 500 kilometers below.

At both ends the wires are anchored to flat rows of giant cylinders— forty in the upper layer, A Deck; and sixteen in the lower, B Deck. An elevator, consisting of two welded tanks, moves between the two ends, carrying people and supplies both ways.

I’ve lost count of the number of times I’ve explained the curious structure to visitors. I’ve compared it to a double-ended child’s swing, or a bolo turning exactly once always high. It’s been called a skyhook, and even a bean-stalk, though the idea’s nowhere near as ambitious as the ground-to-geosynchronous space-elevators of science fiction fame.

The main purpose of the design is simply to keep the tanks from falling. The two massive ends of the Farm act like a dipole in the gradient of the Earth’s gravitational field, so each deck winds up orbiting edge-forward, like a flat plate skimming. This reduces the drag caused by the upper fringes of the atmosphere, extending our orbital lifetime.

The scheme is simple, neat, and it works. Of course the arrangement doesn’t prevent all orbital decay. It takes a little thrust from our aluminum engines, from time to time, to make up the difference.

Since our center of mass is traveling in a circular orbit, the lower deck has to move much slower than it “should” to remain at its height. The tethers keep it suspended, as it were.

The upper deck, in turn, is dragged along faster than it would normally go, at its height. It would fly away into a high ellipse if the cables ever let go.

That’s why we feel a small artificial gravity at each end, directed away from the center of mass. It creates the ponds in my garden, and helps prevent the body decay of pure weightlessness.

When I entered the darkened control chamber, I moved quietly behind the chief flight controller and watched. The controller’s main screen showed the interdeck elevator stopped about three klicks above B Deck. The reason for its delay came into view in a few moments: a small delta-wing whose white tiles shone against the starfield. I stood in the shadows and listened as our operators conversed with the shuttle pilot.

“Pacifica, this is A for Arnold Deck control. You are cleared for orbit intersection. In a minute we’ll transfer you to B for Brown, for final approach. Extend your landing gear now.”

“Roger, Arnold Deck. Pacifica, ready for landing.”

The orbiter drifted toward B Deck. On the controller’s screen I could see Pacifica’s landing gear deploy in the deep black of space.

The inner face of B Deck was covered with a flat surface of aluminum plates, surrounded by a low fence of soft nylon mesh.

Pacifica was at the highest point of her elliptical orbit. Her velocity would, for a few minutes at apogee, be virtually the same as B Deck’s, allowing a gentle approach and contact. (A few purists still refused to call the docking a “landing.”) The shuttle gave off small puffs of reaction gas to align her approach.

It was a beautiful technique, and the unargued greatest asset of the Tank Farm. When Pacifica was secured to B Deck, she would be carried along in the Farm’s unconventional circular orbit until it was time for her to go. Then Pacifica would simply be pushed over the edge of B Deck, to fall toward the Earth again, finishing her original ellipse.

I looked at the screen showing the underbelly of B Deck. A great net of nylon hung below the plain of cylinders. Within, like a caterpillar trapped in a web, was Pacifica’s ET, the external tank that had powered her into orbit, sent ahead and snagged on a previous pass.

So the bad news boys had brought one of the magic eggs with them. I hoped it was a good omen, though it was probably just a coincidence of scheduling.

Until a year ago most of the orbiters visiting the Farm also delivered their external tanks, along with several tons of residual hydrogen and oxygen propellants in each. Then a new administration started reneging, stockpiling ETs at the Space Stations instead, and denying us our allotment. The Foundation took them to court, of course, and forced a delivery rate of at least ten ETs a year.

The new administration didn’t like losing face. Now they’d found a way to get even. Our contract said they had to sell us the tanks, but it said nothing about the water.

“Um, Dr. Rutter, could I speak with you for a minute?”

I turned to see an earnest-looking, black-haired young woman. She clutched a roll of strip charts. Emily Testa was a very promising new member of the Farm, sent up by the Italians, the junior partners in Colombo Station.

“This is really a bad time, Emily. Is it important?”

“Well, sir…” She caught my warning look. “I mean Ralph… Since I arrived I have been studying the problem of electrical currents in the tether cables, and I think I have learned something interesting.”

I nodded as I recalled the project I’d given the young newcomer to get her started. It was a nagging little problem that I’d wanted to have someone look into for some time.

The super-polymer tethers that held the Tank Farm together were sheathed in an aluminum skin to protect them from solar ultraviolet radiation. Unfortunately, this meant there was an electrical conducting path from B Deck to A Deck. As the Farm swept around the Earth in its unconventional orbit, the cables cut through a changing flux from the planet’s magnetic field. The resulting electrical potentials had caused some rather disconcerting side effects, especially as the Tank Farm grew larger.

“Go on, Emily,” I suggested. But I couldn’t help listening with only half my attention. Pacifica was coming in, gear extended like a fighter landing on an aircraft carrier. I could hear the controllers talking softly in their singsongy dialect.

“Well, sir,” Emily said, almost without a trace of accent, “I wasn’t able to find a way to prevent the potential buildup. I’m afraid the voltage is unavoidable as the conductive tethers pass through the Earth’s magnetic field.

“In fact, if the charge had anywhere to go, we could see some pretty awesome currents: One deck might act as a cathode, emitting electrons into the ionosphere, and the other could be an anode, absorbing electrons from the surrounding plasma. It all depends on whether…”

Pacifica touched down with barely a bump. Her landing gear flexed slightly as she rolled to a stop. The interdeck elevator resumed its descent as the orbiter was tied down by the B Deck crew. Her cargo was removed from the open cargo bay by giant manipulator arms.

Two spacesuited figures drifted down from Pacifica’s hatch and stood waiting for the elevator. It didn’t take a lot of imagination to guess who they were. Our bad news boys.

Emily went on single-mindedly, apparently unaware of my split attention. “…so we could, if we ever really wanted to, use this potential difference the tethers generate! We could shunt it through some transformers here on A Deck, and apply as much as twenty thousand volts! I calculate we might pull more power out of the Earth’s magnetic field, just by orbiting through it with these long wires, than we would ever need to run lights, heat, utilities, and communications, even if we grew to ten times our present size!”

The boys in the spacesuits got into the elevator. The crew loaded Pacifica’s cargo after them, encased in blue Department of Defense shrouding.

“Emily.” I turned to face the young woman. “You know there ain’t no such thing as a free lunch. Your idea certainly is interesting. I’ll grant you could probably draw current from the tethers, maybe even as much as you say. But we’d pay for it in ways we can’t afford.”

Emily stared for a moment, then she snapped her fingers. “Angular momentum! Of course! By drawing current we would couple with the Earth’s magnetic field. We would slow down, and add some of our momentum to the planet’s spin, microscopically. Our orbit would decay even faster than it already does!”

I nodded. “Right. Still, it’s a good idea. If we were getting all the water we used to receive, so we could run the aluminum engines as before, we might even decide to draw power your way.

“But our solar cells are really more than adequate. We could sell our excess to Earth, if they could only agree on a way to receive it.”

She looked a little crestfallen. “Keep at it, though,” I said for morale’s sake. “Maybe there’s a way to turn these electrical phenomena to our advantage. We ought to have a break coming about now.” I tried to sound as if I believed it. Emily brightened a bit.

The elevator started rising, on its way up here to A Deck. I had about an hour to get ready—to shave and shower away the aroma of my garden. It probably wouldn’t do any good, but I’d want to look presentable to the bad news boys.