Once upon a time people thought we were on the verge of colonizing space. But then tight budgets and disasters cut the size of the STS fleet, and the cost of a pound sent into orbit remained in four figures. Visions of big O’Neill colonies and grand cities on the moon foundered without the bootstrap mass needed to build the dreams.
The lock passed me through. I stowed my hardsuit in a restorer locker whose nameplate simply read “Bossman.” While I racked my equipment, I recalled all the times I had explained the Tank Farm to audiences on Earth: to congressmen, housewives, investors—to anyone who would listen.
Back in the early eighties it was shown that the thirty-five-ton external tank can be carried all the way into orbit at zero cost to the orbiter’s thirty-ton cargo capacity. Thirty-five tons of aluminum and polymers, already shaped into vacuum-tight cylinders, delivered free!
And that wasn’t all. On arrival the tanks would contain another five to thirty-five tons of leftover liquid hydrogen and oxygen, usable in upper stage engines, or to run fuel cells, or to be converted to precious water.
At a time when the grand hopes for space seemed about to fall apart, the ET was like manna from Earth to Heaven. When the government didn’t seem eager to seize the opportunity—when they built their cramped, delicate, little “space stations” from expensive modules in the old-fashioned way—the Colombo-Carroll Foundation, a consortium of U.S. and Italian interests, offered to buy the tanks.
We would save them, until the world wised up, then sell them back. Meanwhile, the Tank Farm would provide orbit boosts via the tether-sling effect, saving customers fuel and time and paying our way until other investments matured.
For ten years the Farm had been on course, but it seems we’d omitted a few lines of fine print in our contract. The Feds had to let us buy the tanks at a fixed price, but nothing in the contract said they had to give us the residual hydrogen and oxygen, too.
It never occurred to us they’d not want to give us all the water we needed! Who in the world would have thought they’d ever want to take the Tank Farm away from us?
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.