Seveneves: A Novel (45 page)

Since velocity was measured in meters per second, so was delta vee. The delta vees bandied about in spaceflight discussions tended to be large by the standards of what Markus was now calling Old Earth. The speed of sound, for example—a.k.a. Mach 1—was three hundred and some meters per second, and most earthbound people would consider it awfully damned fast. But it hardly rose to the notice of most people who talked about space missions.

A common delta vee benchmark had been the amount needed to get something from an Old Earth launch pad to an orbit like Izzy’s. This was some 7,660 meters per second, or more than twenty-two times the speed of sound: an impossible figure for any object that had to fight its way through an atmosphere. Once a vehicle had reached the vacuum of space, though, things became simpler: rocket engines worked more efficiently, drag and aerodynamic buffeting were absent, and the consequences of failure weren’t invariably catastrophic. Getting
it from point A to point B was a matter of hitting it with the right delta vee at the right time.

Sean Probst’s delta vee history, from his departure from Earth until his departure from life, had gone something like this. The launch from terra firma to Izzy on Day 68 had required a delta vee of 7,660 m/s according to a naive calculation; but as any old space hand would know, losses due to atmospheric friction and the need to push back against gravity would have elevated the practical number to more like 8,500 or 9,000.

Once he had collected Larz and most of Dinah’s robots, Sean had needed to execute a plane-change maneuver to get from the Izzy orbit—which was angled at about fifty-six degrees to the equator—to the equatorial orbit in which
Ymir
was being assembled. This was one of those circumstances in which human intuition got it all wrong. The Izzy orbit and the
Ymir
orbit did not seem all that different in most respects. Both of them were a few hundred kilometers above the atmosphere. Both were essentially circular (as opposed to elliptical). And both went in the same direction around the Earth. The only real difference between them was that they were at different angles. And yet the delta vee required to get from one to the other was large enough that it had been necessary to launch a separate rocket, carrying nothing but extra propellant, just to refuel Sean’s vehicle in preparation for the plane-change burn.

Once
Ymir
had been assembled, a delta vee of some 3,200 m/s had been needed to place her in a very elongated elliptical orbit that had taken her out to L1. En route, the plane-change problem had once again reared its head. Essentially everything in the solar system, including Comet Grigg-Skjellerup, was confined to a flat disk centered on the sun. The imaginary plane through that disk was called the ecliptic. Conveniently for people who liked seasons, but not so good for interplanetary travelers, Earth’s axis and equator were angled with respect to the ecliptic by 23.5 degrees, and so
Ymir
’s initial orbit had been off-kilter by that amount. Fortunately, plane-change maneuvers
were much less “expensive” (meaning they required a lot less delta vee) when they were performed far away; and
Ymir
was, of course, going very far away. So, they had done the plane change out at L1 range, as part of the same burn, totaling some 2,000 m/s, that took her out through the L1 gate into heliocentric orbit.

That orbit, more than a year later, had intersected that of Comet Grigg-Skjellerup. As
Ymir
had drawn near to the comet core, she had used another 2,000 m/s of delta vee to sync her orbit with its.

All of these maneuvers, up to the arrival at Grigg-Skjellerup, had been achieved by using
Ymir
’s rocket engines, which were altogether conventional: they burned propellants (fuel and oxidizer) in a chamber, making hot gas, which was vented out of a nozzle to produce thrust. The final burn had emptied her propellant tanks, so this was a one-way journey unless the nuclear propulsion system could then be turned on.

No engine had ever been made that was capable of pushing a comet core around the solar system at any appreciable speed. For that, they had needed to embed the nuke-on-a-stick into the heart of the ice payload, construct an ice nozzle behind it, and then pull out the control blades, causing the reactor’s sixteen hundred fuel rods to become very hot. Ice turned to water, then steam, which shot out the nozzle and produced an amount of thrust actually capable of making a difference. So a few months had then been consumed disassembling
Ymir
and integrating its parts into a chunk of ice carved off the three-kilometer ball.

The question might have been asked: Why just a piece of it? Why not bring the
whole
comet core back, if water was so desirable? What was the point of sending a large nuclear reactor into space if you weren’t going to use it? And the answer lay in the fact that even a large nuclear reactor did not even come close to having enough power to move such a big piece of ice. The mission would have lasted more than a century, assuming the existence of some kind of a miracle
reactor that could operate at full power for that long. In order to get this done in any reasonable amount of time, they could only bring back the bare minimum of ice needed to rendezvous with Izzy and accomplish the Big Ride.

In any case, Sean and his surviving band had used the nuclear engine to impart a delta vee of about 1,000 m/s to the shard they had carved off Greg’s Skeleton, thereby placing it into a somewhat different orbit that had, a few months later, glided into L1. Sean had remained alive just long enough to yank out the control blades one last time and execute a delta vee that had basically reversed the maneuver they’d used to leave the L1 gate almost two years earlier. This had simultaneously brought
Ymir
into geocentric orbit while executing, as cheaply as possible, the plane change needed to enable a later rendezvous with Izzy. A couple of days later Sean had tapped out the “coming in hot, high, and heavy” message and dropped dead. Of what, they could only conjecture.

The retrieval team that was now being organized by Markus was going to use a MIV, or Modular Improvised Vehicle, assembled from a kit of parts: a sort of Lego set for the construction of spaceships, neatly sorted on a stack of modules, collectively known as the Shipyard, connected to the Caboose.

The Shipyard was a generally T-shaped contraption. One arm of the T’s crossbar, projecting from the port side of the Caboose, was studded with MIV parts. The opposite arm was a cluster of spherical tanks surrounding a collection of splitters. These used electrical power to split water molecules into hydrogen and oxygen, and piped them to chillers, which refrigerated the gases until they became cryogenic liquids that could be stored in the bulging tanks.

So much for the T’s crossbar. Its long vertical stroke was a truss terminated by a nuclear reactor: not a small RTG like the ones on the arklets, but a true reactor, originally designed to power a submarine, considerably souped up for this task.

Markus dubbed the Shipyard’s first product
New Caird,
after a small boat that had been used in Shackleton’s expedition to Antarctica. She was assembled and made ready for use in ten days: about one-third of the time they estimated it would take for
Ymir
to arc in from L1 and make her closest pass to Earth.

To design, assemble, and test such a vehicle so quickly would have been unthinkable two years ago. During the interval between Zero and the White Sky, however, the engineering staffs of several earthbound space agencies and private space companies had foreseen the future need to jury-rig space vehicles from standard parts such as arklet hulls and existing rocket engines, and had provided a kit of parts, lists of procedures, and some basic designs that could be adapted to serve particular needs. In effect,
New Caird
had been designed a year ago by a large team of engineers on the ground, all but three of whom were now dead. Those three had been sent up to join the General Population. Building on their predecessors’ work, they were able to produce a general design—enough to begin pulling the bits together, anyway—within a few hours of Markus’s decision. Details emerged from their CAD systems as they were needed over the following week and a half, and the necessary parts and modules were shuttled about the Shipyard until the new vehicle was ready.

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.