It’s 2100 AD, and hackers and normals live together in mile-long habitats in the Earth-Moon system. The habitat is spun up so that the gravity inside is that of Earth, and for exercise, the normals cycle around on bike paths. But the hackers do their cycling outside, in the vacuum of space.
How so? With ion thrusters, rocketing out xenon gas as the propellant. And the source of power? Ultimately that’s the hackers’ legs, pedaling away at a drive system that turns two large Wimshurst machines.
Those Wimshurst machines then produce the high voltage needed for the thruster’s ionization as well as the charge flow. They’re also what gives the space bike it’s distinctly bicycle-like appearance. And based on the calculations below, this may someday work!
The concept of hackers in mile-long space habitats stems from the warped mind of yours truly, but the idea for the space bike comes from a short story called Grand Tour by Charles Sheffield, who was a mathematician, physicist and writer of hard science fiction.
The story is about an annual race called the Grand Tour du Système. The route starts out from low Earth orbit going to Lagrange point 4 (L4), a location along the Moon’s orbit where the Earth’s and Moon’s gravitational fields conspire to provide a stable position. From L4, the route continues halfway to the Moon and then returns to Earth orbit. Altogether it’s around 600,000 kilometers long.
The story doesn’t say how long the race takes but does give clues. The race is broken up into stages wherein the racers cycle non-stop, more-or-less, for around 36 hours. Each stage accelerates from 0 velocity relative to a docking station, then decelerates from a midway point so that the racer arrives at the next docking point also at, or close to, 0 relative velocity. It also says there are 30 variable length stages from Earth to L4, but we’re not told how many stages there are for the rest of the route. But guessing at 60 stages in total, that’s 2160 hours of stop-and-go pedalling (60 stages x 36 hours each).
The Tech: This Could Actually Work
Much of the story is about rivalry and camaraderie, but it’s really a vehicle for talking about the neat tech, and the evolution of it that supplies some plot twists.
For full details about Wimshurst machines, see our article that walks you through how they work. But basically, each Wimshurst machine consists of two counter rotating disks. Electrostatic induction charges sectors on the disks and sharp-pointed collectors collect that charge and normally carry it away to Leyden jar type capacitors to repeatedly produce exciting sparks. But in this bicycle, that charge is carried away to one of the two ion thrusters.
Given that these thrusters are powered by the high voltage and low current of Wimshurst machines, we can assume they don’t involve electromagnets. Instead they’re probably electrostatic ion thrusters, more akin to many of the ones currently used by satellites and long distance spacecraft, NASA’s Dawn spacecraft being one such example. Xenon is the fuel for many current day thrusters and the closest we find out about what fuel is being used in the story is when one character quips about another “drinking the heavy water again”. However, it is pointed out that for each stage they’re given exactly 50 kilograms of fuel.
Since the thruster details are missing from the story, we’ll talk about the Dawn spacecraft’s thruster instead. It uses xenon as the fuel. The xenon atoms are injected into the thruster where high energy electrons collide with them, knocking loose an electron and turning them into positively charged atoms, or ions.
The xenon ions then move to a pair of grids, the first one of which is slightly move negative than the positive ions. The two grids are spaced apart 1000 micrometers, or the thickness of ten human hairs. The second grid is more negative than the first and there are 1,280 volts across them. The result is an electrostatic pull on the xenon ions that rapidly accelerates them to 35,000 meters/second toward and through the second grid. It’s then that the rocket reaction takes place, the reaction to the thruster being an equal but opposite 1/50th of a pound (92 millinewton) thrust in the opposite direction. That’s what you feel when a sheet of printer paper is placed on your palm. Keep in mind that that’s from just one ion.
The xenon ions continue through the second grid but then there’s a problem. Since the grid is negatively charged with respect to the ions, the ions would be pulled back to the grid, negating the whole reaction. To counter this, a neutralizer is used which sprays negatively charged electrons into the xenon ions, neutralizing them back to uncharged xenon atoms.
Hacks And Issues
The first interesting issue that gets pointed out in the story is the problem of how to turn the bike around at each stage’s halfway point in order to begin deceleration. After all, the Wimshurst machine disks are rotating at high speed and act like flywheels — they resist having their orientation changed and so it’s hard to turn the bike around to get the ion thruster pointed in the opposite direction. Instead they had to slow down the Wimshurst machines to the point where they could turn around and then take the time to spin them back up again. That slowing down and speeding up again is not something you want to do when racing.
To get around this, it’s mentioned that someone once won a few stages by secretly adding a second ion thruster to the bike on the front. Decelerating then became only a matter of switching the Wimshurst machine’s output from the back thruster to the front one. Of course, all racers soon added front thrusters too, negating the advantage.
During the current race in the story, one racer comes up with another innovation to win a stage. He redirects his thruster exhaust in a direction that is out of alignment with where he wants to go, but in the direction of the nearest racer. To quote the story:
“We were throwing a couple of tenths of a gram of ion propellant out the back of the bike at better than ten kilometers a second, but we were being hit by the same amount, traveling at the same speed. Net result: no forward acceleration for us.”
A big topic in space today, especially with all the talk of going to Mars, is radiation. Radiation shielding is heavy and plays a big part in the races. Interestingly, the racers talk about the “weather” as we would on Earth. Of course they’re talking about the solar wind, one of the sources of radiation. We can just imagine future space dwellers speaking in that way: “How’s the weather outside?”, “Windy”.
According to the rules, they can attach as much radiation shielding as they think they’ll need for the forecasted weather. No mention is made as to what the shielding is made of. Naturally the racers want to minimize the mass by minimizing the amount of shielding used. But they can be penalized if they exceed the maximum radiation dosage during a stage.
However, one racer comes up with a radiation related trick to win the final stage and the overall grand tour. The final stage is forecast to have stormy weather, which would peak during the stage. His trick is to modify his bike so that after the storm peaks and declines, he jettisons unneeded shielding. That gives him a lighter bike and so he’s able to go faster and have an easier time decelerating.
Just how feasible is the use of electrostatic ion thrusters powered by Wimshurst machines for a race such as this? That’s a little hard to say since we’re talking science fiction, albeit hard science fiction.
Allowances have to be made to improvements in tech. Even these days there’s a lot of research into ion thrusters. For example, work has been ongoing into removing the need for injecting electrons into the exhaust in order to neutralize it — basically doing away with the bulk, and power requirements of a neutralizer.
We won’t pretend of be experts at orbital dynamics. We know for a fact that some of our readers are and we’d only embarrass ourselves. However, we can do some very basic, and rough acceleration and deceleration calculations. Given total race length of 600,000 km, and assuming it consists of 60 stages, each stage is therefore 10,000 km long and takes around 36 hours. And remember, each stage consists of accelerating from a 0 velocity relative to a docking station, to a midway point, and decelerating back to 0 for the next docking station. So we have 18 hours of acceleration and then 18 more of deceleration.
The formula for distance when accelerating is:
distance = [(vfinal + vinitial)/2]*time
Rearranging for the final velocity and plugging in numbers, we get:
vfinal = (2*5,000 km - 18 h * 0 km/h)/18 h = 555.56 km/h
The formula for the final velocity, if we know the acceleration is:
vfinal = vinitial + (accel * time)
Rearranging that and solving for the acceleration, we get:
accel = 555.56 km/h / 18h = 30.86 km/h^2
This is very rough as it doesn’t taking into account orbits and gravity.
But from the book, when describing the start-up procedure from a docking station we get:
“The starting signal came as an electronic beep in my headset. While it was still sounding I was pedaling like mad, using low gears to get initial torques on the Wimshursts. After a few seconds, I reached critical voltage, the ion drive triggered on, and I was moving. Agonizingly slow at first — a couple of thousandths of a g isn’t much and it takes a while to build up any noticeable speed — but I was off.”
A couple of thousandths of a g is 2/1000 of 9.8 m/s^2, which is 0.0196 m/s^2, or 254 km/h^2, and is more than eight times the above calculated 30.86 km/h^2. To give some context, a car doing 0 to 60 in 5 seconds accelerates at 43,200 km/hr^2 (accel = 60 km/h / (5 s * 1/3600 h/s) = 43,200 km/hr^2).
So assuming that couple of thousandths of a g acceleration is possible then we might just see hackers pedalling around outside of the habitats some time in the future. In the meantime we’ll have to use our bicycle-powered Wimshurst machines just for creating sparks in the night or hack them to work like strandbeests.