Seveneves - Страница 71

New Caird would have to execute one burn to reach an orbit that would intersect Ymir’s and another to match her velocity, so that the crew could board the ghost ship and take the helm. The total “mission delta vee” for that journey, from its departure from the docking port on Izzy to its arrival at a similar docking port on Ymir, was some 8,000 meters per second.

The conversation turned now to mass ratio: a figure second only to delta vee in its importance to space mission planning. It simply meant how much propellant the vehicle needed at the start of the journey in order to effect all the required delta vees.

Laypersons tended to substitute “fuel” or “gas” for “propellant,” making the obvious analogy to the stuff that had been burned by the engines of cars and airplanes. It wasn’t a bad analogy, but it was incomplete. In addition to fuel, most rocket engines needed some kind of oxygen-rich chemical (ideally, just pure oxygen) with which to burn it. Cars and planes had simply used air. Rockets stored the oxidizer in a separate tank from the fuel until the moment of use. The two chemicals were collectively referred to as “propellant,” and their combined weight and volume tended to dominate space vehicle design in a way that hadn’t been true of, say, automobiles, whose gas tanks had been small compared to their overall size.

A convenient figure for characterizing that was the mass ratio, which was how much the vehicle weighed at the beginning (including the propellant) divided by how much it weighed at the end, when all the tanks had been emptied. If you knew how good the engine was, and how much delta vee you needed, then the mass ratio could be calculated using a simple formula named after the Russian scientist Tsiolkovskii, who was credited with having worked it out. It was an exponential: a fact that explained almost everything about the economics and technology of spaceflight. For if you found yourself on the wrong side of that exponential equation, you were completely screwed.

When the relevant numbers for the Ymir retrieval mission were jacked into the Tsiolkovskii equation, the result was a mass ratio of about seven, meaning that for every kilogram of stuff — Markus, Dinah, other personnel, miscellaneous robots, etc. — that they wanted to arrive safely at the docking port of Ymir, they needed to allow for six kilograms of propellant at the moment of departure from Izzy. This wasn’t all that difficult to achieve, especially for a vehicle that would never be exposed to the rigors of passage through the atmosphere.

The payload in this case was a single arklet hull that had been augmented with a “side” door: an airlock that could accommodate one person in a space suit. Other than that, it had been stripped to the minimum complement of equipment needed to keep a crew of four alive for a few days. To its mass, of course, needed to be added that of the actual humans and their food and other essentials. The lightness of a bare arklet hull was startling; the newer hulls, made of overwrapped composites, weighed in at eighty kilograms. Stripped of everything that made it comfortable and inhabitable over the long term, and including the “side door,” the maneuvering thrusters, and a reasonable supply of thruster propellant, the mass of New Caird was about ten times that. The humans weighed three hundred kilograms. The rocket motor that would be doing all the important burns weighed another two thousand. So, in round numbers, the payload mass — the stuff that actually had to get delivered to the docking port of Ymir—was some thirty-five hundred kilograms. The mass ratio of seven meant that its propellant load, at the beginning, was going to be some twenty-one thousand kilograms of liquid hydrogen and liquid oxygen.

The Shipyard had been stocked with several cryogenic propellant tanks of various sizes, some designed to hold LH2 (liquid hydrogen) and others built to the somewhat different specifications needed in the case of LOX (liquid oxygen). The chosen tanks were bolted together in a stack with the rocket engine mounted “below” and thermal protection wrapped all about. New Caird proper — the arklet with the humans in it — projected forward on a scrap of scaffolding just long enough that her maneuvering thrusters wouldn’t damage any of the other parts when they came on.

While the MIV was being constructed, twenty-one thousand kilograms of water had to be split into hydrogen and oxygen, chilled to cryogenic temperatures, and stored. The Shipyard’s port side already had some LH2 and LOX premade. In general, though, they tended not to keep a lot of them on hand, because they were tiresome substances to work with. The demand was supplied by the naval reactor on the Shipyard’s long arm, which was brought up to full power for the first time since it had been launched, piece by heavy piece, from Cape Canaveral on a series of heavy-lift rockets. Pumping juice down heavy cables to the splitters, it was able to turn twenty-one tons of water into gases and chill the gases to cryogenic temperatures while the other preparations were being made.

This was a lot of water — roughly fourteen liters of it for every surviving human. The Cloud Ark recycled water, of course, and was far from running out of the stuff. Nonetheless, the idea of taking that much of it and spewing it into outer space, never to be recovered, gave many people pause: especially the Dump and Run partisans.

There was a strong counterargument, which was that New Caird’s objective was to take possession and control over a piece of frozen water that weighed as much as Izzy herself, including the giant piece of iron to which Izzy was attached (and would continue to be, if the Big Ride advocates had their way).

Once New Caird had reached her, Ymir could presumably be slowed down, and brought to a rendezvous with Izzy, by firing her engine. And that was a primitive beast, but it had a basically infinite supply of energy in the nuclear reactor, and a vast stock of propellant in the form of ice. The “steampunk” propulsion system had much lower efficiency, however, than a properly engineered rocket motor. Consequently, the mass ratio that would be needed to slow Ymir down from the high-speed elliptical orbit with which it was falling into Earth’s gravity well, to match the much slower, circular orbit of Izzy, was about thirty-four, which meant that 97 percent of the ice currently attached to Ymir was going to be melted, turned into steam, and jetted out its makeshift nozzle just to slow it down. The remaining 3 percent, however, would still weigh as much as Izzy and Amalthea put together. Split into hydrogen and oxygen, it would supply the rocket fuel needed to power the Big Ride, all the way up to Cleft.

“I DIDN’T EXPECT IT TO BE BLACK,” DINAH SAID. SHE WAS HEARING HER own voice as if down a mile-long sewer pipe. She was pretty sure she had lost consciousness a minute ago. Maybe she wasn’t all the way back yet.

Markus was slow in responding. Maybe he had blacked out too. Maybe he was just distracted. “Comet cores are covered in—”

“Stinky black stuff, yeah, I know that, Markus. Remember who I am?”

“Sorry. Not enough blood in brain.”

“But this is just a shard that Sean broke off of Grigg-Skjellerup. Why’s it all covered?”

“I don’t know,” Markus said.

They were looking at Ymir from a distance of ten kilometers and closing. They were viewing her on their tablets, through a zoomed-in video camera. Vyacheslav Dubsky, floating closest to New Caird’s forward end, put his face to the vessel’s tiny window and searched the black sky for the black ship, but the squint on his face suggested it was still too far away for naked eyes to be of much use.

“Maybe he was doing us a favor,” Dinah said. “The black stuff has all kinds of goodies on it. Carbon, obviously. But also nitrogen, potassium—”

“Micronutrients,” Markus said, “that the Cloud Ark will be needing.”

“So maybe he used the robots to scrape some of it off Greg’s Skeleton, and loaded up on the gunk,” Dinah speculated.

“We will know soon,” Vyacheslav said. “Presumably he left a document.”

“Which we will not be alive to read, unless we stick the landing,” Markus pointed out, “so no more chatter from now on, please. Slava—” and he broke into a string of bad Russian meaning something like I trade places with you now. Vyacheslav responded in equally bad German. Both men were perfectly fluent in English. But they made a private joke of butchering each other’s languages, ostensibly as part of a project to preserve Old Earth’s linguistic heritage. Markus then added, “The rest of you, buckle up.”

With the deft movements of one who had been in space for two years, Vyacheslav glided aft. He was one of the veteran Russian spacewalkers who had come up to Izzy way back on A+0.17, in the first launch after the moon had blown up. He had been a mainstay of the Scout and Pioneer eras, racking up more time in a suit than anyone, wearing out three Orlans. He was a little worn out too, being sallow and gaunt compared to the strapping hero who had emerged two years ago from the same Soyuz that had carried Rhys and Bolor-Erdene. Markus replaced him at the forward window and buckled himself into the pilot’s seat.

Behind that was a row of three acceleration couches, mounted on a frame that spanned a diameter of the arklet’s hull. Dinah was loosely belted in on the port side. A few minutes ago she had been tightly belted in. She had not adjusted the straps. The entire couch, and its supporting truss, had been deformed by the same burst of gee forces that had left her so woozy. To starboard was Jiro Suzuki, a nuclear engineer who had been involved with the design of Ymir’s nuclear reactor core. It wasn’t clear whether he was conscious; but then it never was with Jiro. Vyacheslav, the fourth member of New Caird’s complement, settled into the middle position and pulled the top straps of the five-point harness over his shoulders.