Space Transport - Highways to the Heavens

It is so hard to get to space today that very few people realize how much activity is about to start there. The changes coming in the next decade will be due to rockets with reusable booster stages, and robotic equipment capable of executing complex behaviors through artificial intelligence, and of being operated remotely with agility.  The truth is though, that on the transport side, there are  well understood technologies well within our development capabilities that would transform our reach far beyond that. Moon Town uses three of them: nuclear thermal rockets, skyhooks, and mass drivers.

Nuclear Pod Ships

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Nuclear rockets can accelerate a ship twice as fast as the best chemical rockets of today. The reactor type used in the pod ships has a chamber filled with uranium fuel pellets. It spins at high speed so the pellets are held against the chamber wall and aren’t swept out of the chamber by the hydrogen spewing past. They are so hot, the hydrogen is superheated by them during its brief passage, and thus expands tremendously, creating thrust as it is pushed out the nozzle. This reactor type is called a rotating particle bed reactor. It was investigated by NASA in the early 90s in the Timberwind program.

The options for range, speed, payload capacity, and fuel efficiency provided by nuclear engines creates lots of new opportunities. Since the nuclear pod ships are only used in space, there is no need to think much about radiation. Nuclear fuel rods are benign until nuclear reactions have been started in them the first time. Before that, you don’t need to worry about radiation from them any more than you’d worry about being burned by a log that isn’t on fire. 

The pod ships build on the nuclear advantage with several additional architectures that make them even more versatile, efficient, and hardy.

A ship that never has to deal with more than a sixth of Earth’s gravity can afford to have radiation shielding around almost all of the reactor. Thus there is no need to isolate them from buildings and traffic. The nozzle opening just has to be plugged with an extra shield after it is docked or landed, then the shielding is complete. 

LANTR  is a technology that injects liquid oxygen into the nozzle of a nuclear rocket, where it combusts with the hot hydrogen jetting out there and greatly increases thrust. Stanley Borowski of the Lewis Research center designed it in the 90s. He calculated that by using this system, several times more oxygen could be used than hydrogen. Oxygen is a product that is produced in MoonTown all the time in reactors that reduce regolith minerals to metals. There is lots and lots of it. If an engine uses 7 times more oxygen than hydrogen by volume, only 3% of propellant mass is hydrogen. Hydrogen is scarce on the moon and far harder to store, so that is an improvement. The rocket would still be more fuel efficient than a chemical rocket and the reactor would last longer because it could operate at lower temperatures. Materials produced in MoonTown at much greater scale and quality than today also increase reactor life and sustainable operating temperatures – sapphire, quartz, and titanium carbide.

The ship can take one to four of the standard transport pods used in all space shipping. The front extension for the second to fourth pods slides back when the ship flies with only one pod, so that it can dock with an airlock if carrying a passenger pod. Flying in such a configuration would be done where a fast, direct flight between Earth orbit and the Moon is required. 

In ordinary flights, these ships make use of one or two skyhooks on their journey. When they take off from MoonTown, they need to reach an altitude of around a hundred kilometers and to have a speed of a few hundred meters per second in order to be gently grappled by the docks on the end of a skyhook arm. The fuel tanks are sized to have enough for that kind of flight plan. When needed, for heavier payloads, longer trips, or a tighter schedule, the oxygen tank can be replaced with a stretch version of greater diameter, and two or even three hydrogen tanks can be connected. 

Moon Town frequently launches special payloads that are attached to custom adapters that slide into the main spine of the ship where the front extension usually goes. Those payloads might be satellites or deep space probes, or components for space stations, solar power arrays, telescopes, or skyhooks.

Skyhooks

By Moon Town’s day, the moon is orbited by many rotating skyhooks of different kinds, in staggered equatorial and polar orbits. Some skyhooks are set up in pairs with compatible orbits that best allow them to pass spacecraft between them. In each set, one is for handling traffic back and forth from the lunar surface. It rotates more slowly and its arms reach closer to the surface. That makes it easier for a ship to reach the altitude and speed to be grappled by its hook. The other is for slinging ships into deep space, on their way to other planets, moons, or asteroids. It has shorter arms and rotates faster. 

For simple runs between the moon and Earth orbit, surface skyhooks are fine. The change in speed needed for a ship to break lunar orbit and start dropping towards Earth is only about 1 km/s. They are also used for transport between Moon Town and the outposts at the north and south pole.

For access to other points on the moon’s surface, there is a vertical skyhook in a polar orbit. It passes within a maximum of 100 km of every point on the lunar surface every 2 weeks. The farther a place is from the equator, the closer it is guaranteed to pass by. It doesn’t rotate. It is a cable that descends from a large mass in high orbit, such that the center of gravity of the skyhook orbits at an altitude of 5000 km. The result is that the platform on the foot of the skyhook moves above the lunar surface at a speed of only 222 m/s, and a ship can reach it if it has just 400 m/s of delta V. If it is willing to wait long enough, that ship can then get to within 100 km of any other point on the lunar surface, and land with another 400 m/s of delta V. A rocket can’t travel more than a couple of hundred kilometers for that little fuel, and land vehicles aren’t practical beyond about that distance. This skyhook has another far more intriguing benefit. The platform at its foot, orbiting some 10 km or so above the surface, is a destination in itself. Over the years, as the mass at the top of the cable has grown and the cable has gotten stronger, the mass of the foot platform has also been able to expand. It hosts an impressive array of telescopes and instruments that continuously map and monitor the lunar surface in exquisite detail. And it is a sort of cruise ship. More or less. It offers a stunning view of the surface and has a state-of-the-art shaped magnetic field generator on it that shields occupants from radiation. It can accommodate 200 people at a time in rather luxurious comfort, while they watch the earth rise and set twice a day. People from the many science and mining outposts are regulars on the platform. Other people go there for little vacations.

This skyhook is the one likely to collide with other skyhooks. Every once in a while it needs to do a dodge manoeuver in which the foot platform is set swinging so that one of the many skyhooks that orbit far below it will pass to the side instead of hitting it.  The powerful thrusters on the platform that get that going then progressively damp the swinging so it hangs plumb again. (This is not a popular time to be on the platform.) Other skyhook orbits are coordinated and maintained so that they never hit each other.

Low Earth orbit also has some skyhooks by this time. Most are small, but there is one great big new one that was mostly built by the moon. The chemicals for the giant cables that form its long arm came largely from Earth, but they were spun into flawless fibers thanks to the moon’s un-Earthly adroitness with chemical processes. The rest of its mass came almost entirely from the moon – the motors and engines that keep it spinning and in the correct orbit, the mass that keeps it stable, the panels that provide its energy, all the habitats and their equipment, all the maintenance robots constantly searching for and fixing any nicks or tears in the cable’s strands. 

When its long arm is at the bottom of its swing, and the skyhook is at the lowest point in its orbit, it is only 100 km above the ground and moving only 5 km/s relative to it. At that speed, specialized cargo vessels that come from the moon or processing facilities in its orbit can drop off and reach the ground without the need for the sophisticated heat shields of current reentry vehicles. They come in steep using only thrusters and grid fins for minimal guidance, and land in the oceans using parachutes, like Apollo command modules did.  They are completely reusable, and allow high-value products from lunar industry to begin to be sold on Earth. 

When the long arm is at the top of its swing and at the highest point in its orbit, it is 350 km above the ground and moving at 10 km/s relative to it.  That is enough speed to reach the moon’s orbit. A ship released from the arm at that point only has to fire its engines to do course corrections and braking to enter lunar orbit. If it then uses the moon’s skyhooks to do most of the braking needed to land on the moon, it can do the whole trip from Earth orbit to lunar landing with around a quarter of the fuel normally required. 

This skyhook also has a short arm much closer to the central hub of the skyhook complex, and podships actually usually dock on that arm. It requires a lot more fuel to come and go, but has space for several ships to be docked at once, and those ships can spend all the time they need there. 

Mass Drivers

A mass driver holds a magnetic vessel suspended in a precise place using hoops made of electromagnets. By building a very long series of such hoops along a straight track, a vessel placed at the beginning can be accelerated through them to high speed. Each time it passes through a hoop, the hoop switches polarity to repel the vessel, and the next hoop ahead of the ship switches on and attracts the vessel. This accelerates the vessel, and the hoops have to switch faster and faster as the vessel moves along.

There are a few options when designing such a device. If the mass driver accelerates the vessel to orbital speed this way, the track will either need to be very long (on the order of 100 km) or the payload will need to be pushed with higher acceleration forces. The higher the acceleration, the shorter the track can be. On the moon, vessels that withstand 10 gravities of acceleration can be launched on a track 13 km long. An alternate approach is to put larger engines and fuel tanks on the vessel. Unless the destination of the vessel is lined up nicely and has a large system to catch the vessels when they arrive, they need to be able to adjust their orbits and maneuver, at minimum. Beefing up those systems can allow them to handle the last few hundred meters per second of acceleration.

Moon Town’s mass driver is optimized to accelerate vessels at 3 gravities down a track 50 km long to orbital speed. It spends most of its time launching packages of solar cells and electrical and structural components for the giant solar power arrays in orbit at the Lagrange points in the Earth – Moon system. It also launches bulk materials made of titanium and its compounds, quartz and sapphire, and to a lesser degrees, other metals and metal compounds that are also commonly mined and processed from asteroids in orbit.