Bicycle Racing In Space Could Be A Thing

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 Story

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.

Route for the space bike race
Route for the space bike race

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.

How an electrostatic ion thruster works
How an electrostatic ion thruster works

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

Decelerating using front thruster
Decelerating using front thruster

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.”

Jettisoning unneeded shielding
Jettisoning unneeded shielding

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.

100 thoughts on “Bicycle Racing In Space Could Be A Thing

  1. To get the thruster to the front you do not necessarily have to overcome the gyroscopic forces of the wimshurst machines. There is no up and down in space, you could just turn back to front in a wheely-like motion, with the rotational axis parallel to the rotational axis of the wimshurst machines. I suppose the two machines have to run in opposite directions anyway, otherwise you would become spinning like crazy when you accelerate them, like a helicopter with missing tail rotor. So to turn upside down and back to front you need a variable speed gear between the front and back discs or just decouple them for a moment similar to doing a wheely on a normal rear driven bike. You can use them like momentum wheels in a satellite, when they are at the same speed you are still around this axis, when you want to rotate, they need to have slightly different speed.

    1. They absolutely would need to contra-rotate, but you don’t necessarily need to turn over end-over-end. The idea that gyroscopes resist a change in orientation is a frustrating myth. They turn just as easily as any object, but they redirect torque by 90°. To turn them you just have to apply torque to an appropriately rotated axis.

      Since there’s no real need to have the wheels in the same configuration as a bicycle, it would be pretty handy to have at least four of them in contra-rotating pairs with axes 90° to each other. If they were clutched you could then use them to pitch and yaw, and roll wouldn’t really be necessary in space.

      Should weigh a lot less than an extra ion engine, too.

    2. True. But since a wimshurst machine has two counter rotating disks it should have zero angular momentum and could be turned in any direction anyway.

      The gyroscopic effect of each of the two disks will cancel out.

  2. The problems of both eating and waste elimination in a space suit would have to be very thoroughly solved.

    Yes, I know, people are working on that already anyway. But… solving it well enough that astronauts can perform necessary maintenance on a spacecraft and solving it well enough that one would actually choose to spend 36 hours this way as part of a sport… That is a very different thing.

    Just imagine this article with the title “BICYCLE RACING IN DIAPERS COULD BE A THING”! That just brings a whole new meaning to it doesn’t it?

  3. And if you are drained of power while decelerating you will miss L4 and end up in some random orbit. Nice.
    Ever tried to hit your precious space station in L4 with a shuttle in KSP manually?

    1. Lagrange points don’t exist in KSP anyways because it uses simplified 1 body orbital dynamics based on spheres of influence. I think there are some multibody mods out there though.

    2. The race will probably be closely monitored, like bicycle races today – so, unless there will be loss of consciousness for the rider (and that can be checked too) – intercept would be relatively easy. Especially given that there are stops every 36 hours – with docking, etc – so there are ships with much better engines nearby.

  4. ‘A couple thousandths’ is 2/1000, not 1/2000, which changes the calculations like so:

    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).

      1. Many people use the word ‘couple’ to mean ‘exactly two’, but fourty years ago when i learned english, i took it to mean ‘about two’, in the given context i could live with a number between say .5 and 10.
        On the other hand, the thrust calculation depends strongly on the grid voltage. Increasing that will improve thrust. On the other hand, you can also do some energy calculations, based onsay 200W of max human output.

    1. They should, I think. Vacuum would help insulate against losses due to corona between the sectors on the disk. However, I’m not sure if it would be worse at the collectors where the corona is desired.

      1. “Vacuum would help insulate against losses due to corona between the sectors on the disk.”

        I don’t think this is true. Breakdown voltage decreases with decreasing pressure: it’s *easier* to arc in space.

          1. Yes, if you can maintain a significantly sub-Torr environment, vacuum will start insulating again. However it’s extremely hard to actually maintain that especially if you’re near a pressurized environment. (Plus the idea that a minor puncture in your living quarters would lead to electrical arcs is kindof the definition of making a bad situation worse.)

    2. Well, if you put them inside the pressurized cabin you have to deal with ozone production and possibly some minor nitric acid production. So given the choice, I would start on making charge collectors work in a vaccum.

      1. You people do realize there have been ion engines in use for decades right?

        And that space is full of electrical charge related events and plasma too I would add. It’s not like we have something new and never seen.

        1. I think you misunderstood, I meant the Whimshurst portion adn I believe the original post did as well. Since a mechanical transmission to drive it (Whimshurst machine) would be much harder to accomplish than a high voltage feedthrough to the ion engine. I know the ion engine requires vacuum to function. Technically savvy people tend to hang out here, not every post is immediately worthy of the mighty “you people”.

          1. Ah, good point re the mechanical transmission. The story doesn’t address either that or whether or not the Wimshurst machines are in vacuum. However, I sort of did and in the 3D model I made for the illustrations and animation, I used rotating cylindrical rods to transfer the rotation by the pedals, through seals of some sort, to the chambers where the Wimshurst machines are. I figured that using a rod would be the easiest to create seals for. I also put the Wimshurst machines in their own separate chambers so that they can either be in vacuum or not — at least the option is there and the cyclist wouldn’t be exposed to ozone.
            Not important, but for full disclosure, I figured the pedals would use bevel gears to transfer rotation to the rods
            You can best see this in the animation above. The actual rotating rod is inside a cylinder and the bevel gears are hidden inside a sphere between the pedals so that I didn’t have to draw them.

  5. I have here in my desk drawer an exposé for a story I meant to write when I find the time. Sigh, seems someone already has done that. But mine was about walking the space-dogs on an ion driven bicycle. Less suspension, more silliness. Now I don’t know if I should kill the project because someone has beat me to it or if I should finally give it a try.

    1. Science fiction stories are just as much about the implications/uses of the tech as they are about the tech itself — the “what if”. Sounds like you have a very different story in mind. I’m with Brian, write it up.

  6. Great, once again cyclists trying to butt in where they don’t belong (1 lane motorway anyone?) Just like those jerks in spandex blocking my car, that belongs on the road, I will cross my fingers, hope and pray, then laugh my ass off when they eat shit!

    1. Well I could just get rid of my bicycle and walk itto work at 9.5Miles a trip and 20-ish miles a day….
      Then I’ll get tired….
      then late…
      then slow to produce products….
      then fired…
      then the dole office won’t give me cash as it’ll be my fault….
      then I’ll become homeless…
      then I’ll fall into crime to keep myself alive:
      I hope your mom don’t mind if I knock her dead cold for the paper in her pocket??? I want to survive!

      If you answered no:
      Remember, all bikers are potential psychos in the making… do anything to their bike(s) and you’ll see no end of poblems… Doesn’t matter if it is Motor or pedals!!!!

      1. My mother is a biker, the motor (Real) kind. You can try to “knock her dead” but you’ll catch 2 .45 in the chest and 1 in the head like you deserve, god bless conceal carry. Maybe if you tried to keep your drivers license, drink a little less, or could afford a car your life wouldnt be so close to spiraling out of control and you wouldnt need to resort to crime?

        But I digress, you cant pedal 45mph and are therefore holding up traffic, and I can count on 1 hand the number of pedal pushers that actually obey traffic signals and signs, and all are under 12 (and half are my kids.) These 2 things are the reason for my hatred for bikes and the idiots that ride them on our motorways. The sad part is back when I first got my license I used to worry constantly about them, always scared they’d fall in front of me and Id kill them on accident or cause a crash trying to avoid one. Not anymore…

        Thanks to the first reply for the video, I laughed out loud : ) The dude in the truck even honks at them and the dipshit just keeps on going like he deserves to be there! If your taking up the entire road on a double yellow; i woulda done the same thing if I had my kids in the car, Im not gonna risk their lives to pass in the other lane just so you can laugh with your butt buddy about how much a prick you’re being. It does suck they caught the person in the car, next time back over the other one and take the GoPro!!

        And eff New York, I visited one and hate it there, its almost as bad as here in Chicago. Tho we do have an awesome township that, sensibly so, banned cyclists and anything without a motor from the road! Its a very nice place to drive now!

        1. Pretty much TLDR; I could care less if you cant makes ends meat cause your government blah blah blah… Take some personal responsibility for your life and quit being such a limey douche…

          I unavoidably saw something about my gun being taken from me, thats such a UK attitude and also why you pansies outlawed guns, knives, and if the Acid post on here was any indication, that as well. Cum Catapultae Proscriptae Erunt Tum Soli Proscripti Catapultas Habebunt…

          I could keep arguing with you, but I have a loving wife and children, they deserve my attention more than you : )

        2. Watched the video… OK,
          Race bike users think they own the roads… and the redways (Cycle routes) and the paths….
          The amount of times I’ve stood my ground and stuck to the UK legal side of the path/cycleway/road and those types almost have a head on collision.

          I don’t know what double lines in the road means leagally over in the big US of A, however no overtaking on such roads are legal at all in the UK.

          That racer-wannabe got what he deserved… They should of given way to traffic by threading themselves against the ditches (assuming you have ditches over there)
          One of the racers should of eased out to cling to the road edge in single file… They are Darwin-pool-of-failures waiting to happen.
          I HATE them “Cyclist” types as much as I HATE your attitude and as much as I HATE your presumptions about my lifestyle.

          1. Unferium: DID you watch the video? Yes, the bicyclists were side-by-side, but that is the only way for them to be reasonably safe on such a road. Note the width of the shoulder: if you were to try to ride on that, first you would have to be an exceptionally skilled rider, and second, the cars would pass you without slowing, and inevitably one out of twenty or so would misjudge and hit you with their passenger-side mirror. I’ve never been a hardcore biker, but I’ve been nearly killed more than once by trying to legally use a public highway and trying to be considerate by staying to the right. Nevermore. Now when I ride, I ride to stay alive, whatever that requires.

            Yes, here in the U.S., a double-yellow line definitely means “no passing”, but MOST motorists will ignore that when overtaking bicycles.

          2. I’ve had people brush their mirror closely to I and others, be it lone riding or group riding. It depends on a lot of things, mostly the driver. Always be aware of your surroundings. Give way where possible. Punch the drivers car if they are too close as legally they have to give roughly an arms distance thus making it impossible to punch their car. If they complain you punched their car: they’ll likely get the points on their license for not driving with due care and attention (Here in the UK, though it is disputable and appeal-able, thus not set in stone: a type of case law/bylaw)

            I assume UK helmets are designed by a masochist who like their hair to be ripped out and have rashes on their scalp whilst knotting up the hair into the sweat soaked plastics. In care I got banned from wearing a helmet on a pushbike as I was so discomforted by many with the same issue and on fidgeting around my last helmet I nearly fell off over the promenade: nearly falling a 15Ft drop into the sea on jagged rocks (Potentially more fatal than not wearing a helmet in the first place).
            I still don’t wear a helmet unless on a motorbike (Those helmets are comfortable IMHO) or unless I know I’m biking around for something stupid to do like random stunts.

            I’ve fallen off down a grass/mud hill at around 30+Mph because of a malfunction of the steering, survived ain’t the word…. It was literally nothing compared to falling off without moving due to a failing chain derailing, LOL

          3. @Unferium

            No one deserves to be hit by a car. Not even an asshole like you or the jerk you’re arguing with.

            Those cyclists were not breaking the law or riding in an unsafe manner, unlike the two assholes who unlawfully crossed into the left hand lane to overtake them.

          4. @wide eyed southern boy:

            I guess you wouldn’t mind if two or more cyclists had blocked you at the beginning of a 30-mile stretch of road and continued for the whole 3 hours of your usual 15-min journey. Especially when and if you had to be at work within an hour.
            I’m just guessing that as one such possible reason.

            IMHO, the cyclists were riding unsafe: too slow thus holding up traffic, not giving way (Yes I must have super cycling skills to pin to the shoulder, apparently).

            I’m sure it would have been impossible for the cyclist to have been hit at all, thus this video must be a lie and everyone must go wear tinfoil hats?

            Oh, I’m not an asshole… I just like trolling people who cant handle: Common sense, obviousness, reality, truth.
            Same as arguing with a flat-earther for the thrill and fun.

        3. आप एक कुत्ता चोद है! आप एक बेती चोद है!
          We can all use another language as cliche to try and look smart, I at least admit I am barely a beginner in Hindi.

          Oh and if you hadn’t skipped over, you’ll notice I pointed out what is said on advice sites about close combat involving guns, NOTHING ABOUT TAKING THE GUN AWAY FROM YOU AS YOU IMPLY “FROM A GOVT CONFISCATION”.


          Many American persons from America wrote how dangerous guns + close combat…. so is USA == UK now huh?

          If that is so then throwing a nuke your way would be a civil war!!!

        4. Quote:
          “Take some personal responsibility for your life and quit being such a limey douche…”

          That is a common cliche used by people who try to shift blame about:

          A. I already take responsibility for my life as I have improved my outcome and chances from the damage caused by the gov’t, most other victims have either: gone to prison, homeless, dying, dead, killed themselves…. I have a job, a roof, transport (Even if you want to ban it), friends, somewhat a life.

          B. Calling someone a limey douche because they have a valid opinion that you cannot full-force oppose, I could call you a “Biggot, redneck, Honkey….” but hey, that is just general.

          C. Telling someone to, “take some personal responsibility for your life” in regards to past events that the victim couldn’t be responsible for is like saying to a (usually female) rape victim, “Take responsibility for your life, it is your fault for being raped, it is your fault for being a woman”. That attitude only makes me want to troll you harder with such truthful and factual information like it is to troll a flat-earther with facts and readily performable experimentation.

        5. Yes, riding a bike at the right place can be a nice sport. But too many people mistake a bicycle for a means of transportation. Unfortunately in Vienna we have red-green city government and this leftist green party people even increase the amount of bike-ways and kill traffic lanes and parking spaces for that. It’s a shame, that a party with around 10-12% can do this.

          1. Come to the Netherlands (any city, town or village) or Denmark (most large cities and a lot of smaller towns) and dare tell me bicycles are not a means of transportation. They most certainly ARE. A lot of congestion would be solved if cities like New York were to truly accomodate bikes as a mode of transport and the general mindset of motorists changed accordingly.

        6. Hey, at least these two are using traditional units, rather than all that metric crap.

          I, for one, am cheered to see harmony regarding the truly contentious matters at play, and would even suggest that this exchange is much closer to amicable resolution than casual observers might presume …

  7. “Breaking News! The L5-Oz trace team is on a collision course with the Tycho City team.” Two days later. “The L5-Oz team is still struggling to avoid the collision with the Tycho City Team.”

    The rest of us in the the habitat are riding our bikes anti-spinward on the theory that if you can go fast enoug, you can take a jump that goes “vertical” and reach the axis.

  8. Based on the calculations, isn’t the 36hr a little long? I mean you could accelerate for 3 hours, take more than a day nap/break, then decelerate for 3 hours, and still be beating the time.

    What would an actual reasonable amount of acceleration be from one of these?

    1. I think you have to deviate a little from the traditional way of doing bicycle tours. The main difference is, you don’t “stop for the night” in space; each leg takes you to a slightly higher orbit, and there would need to be “rest satellites” at each of these orbits, where waste is collected and the next day’s food is distributed. So the whole time you’re going from low Earth orbit to L4, you would always be accelerating. Conversely, to get back down to Earth, you have to decelerate the whole way, as each stage would take you to a lower orbit, requiring a lower velocity.

      That’s about the limit to my expertise in orbital dynamics; I don’t know how it works when you compare accelerating at different rates during each stage. I.e., if you accelerate too quickly, will you overtake the next rest satellite in its orbit, and then have to pedal back to get your food for the next day? Seems like that could be a problem.

      1. If you accelerate later in your orbit, you end up somewhere completely differently. Most likely having to use a lot more time/fuel to compensate later on.
        Also your fuel is more efficient the close you are to the planet see:

        But my experience is limited here, only from KSP. Which doesn’t have the Lagrangian points, which could improve on the whole timing problem.

  9. I had thought about doing a quick dismissive rebuttal based on some under considered numbers such as:

    1. The actual fuel of this ‘rocket’ is the rider’s food. My most optimistic food source (a probably disgusting sugar/fat mix) would require about 4.5 kg per leg of the race. Oxygen requirement was about 10kg. Not a huge mass addition but noticeable.And the water would be heavy as in lots required from sweating – on the order of 1-2 litres per hour but lets assume a closed system based on about 5 litres (5kg) continuously recycled.
    2. As you move out from Earth you trade kinetic energy for potential energy in higher orbits at lower speed. Still, you need to tack the equivalent of about 80 m/s velocity on each leg of the outbound journey.
    3. I estimated that an imaginary super athlete could maintain 1000w output for 36 hours. This could drive a low end ion drive at 25 mN thrust. And unfortunately, thrust to weight ratio doesn’t scale down as well as one would like.
    4. I presumed (generously) that a liveable mobile mini-habitat + rider could weigh in at 200 kg (75 kg rider, 15 kg fuel, CO2 scrubbers, PET/kapton shell for radiation protection and reflectivity , cooling system, thruster, etc).
    5. Are they allowed solar panels for non-thrust activity like electronics, water purification, cooling? If not, the energy available for thrust would drop below the 1kW target.

    As it turns out, the numbers were close-ish – more like 72 hours per leg of the race (and not spending my entire day refining estimates) but far less improbable than I thought. And the guy who does this can mock every ultra-marathoner or iron man competitor in the solar system.

    Over time, technological advances could chew into the practical limitations. For example, I just read about an idea for thrusters for micro-satellites that self-assemble out of ferromagnetic fluid that may give a better thrust to mass ratio at the low end.

    I mean the race story is highly improbable but maybe in 100 years…

    1. “The actual fuel of this ‘rocket’ is the rider’s food. My most optimistic food source (a probably disgusting sugar/fat mix) would require about 4.5 kg per leg of the race.”

      The actual fuel of the rocket is the rider’s *body*. An ideal competitor here would balance out ability to output energy while minimizing actual food input. Fat storage in the body is pretty effective. So not quite so disgusting, although the in-flight meal might be.

      “I estimated that an imaginary super athlete could maintain 1000w output for 36 hours. This could drive a low end ion drive at 25 mN thrust.”

      That seems pretty bad for an ion drive, actually. You’d probably be looking closer to 100 mN thrust, especially assuming future technology. Ion drives have flown with 60 mN/kW thrust-to-power ratios, and 75 mN/kW drives have been demonstrated experimentally. Thrust-to-power is the better metric here compared to specific impulse, assuming that the thruster’s overall weight is negligible compared to the rest.

    2. I think your “super athlete” is going to take some serious genetic engineering. Riders in the Tour de France average about 250 W, and peak out at 400-500 W, and these are probably close to the best-conditioned athletes today. Note that these are actual race numbers, not the peak an athlete can produce on a sprint.

      Also, okay, the OP claims each leg can be done in 36 hours, and you say 72, and my own calculations (based on acceleration and the delta-vee from LEO to L4/L5) came to 56 hours. This is an extreme stretch; even at 36 hours the average power output would drop significantly.

      1. And now I see that I mis-read the original article. I don’t see where the 36 hour day comes from in the first place. Since there are many legs in the tour, there’s really no good reason for each leg to be that long. Since there’s no significant friction in space, there’s no penalty for making rest stops, so the race could be tailored for whatever the current crop of orbital-class athletes can handle. It’s not like you fall back to Earth as soon as you stop pedaling.

      2. BrightBlueJim The 36 hour day comes from the short story. It says that not all legs take that length of time, but since it was the only number given, I used that. Regarding their stopping despite, as you point out, there’s no significant friction in space and so they could just coast for a while, the docking stations in between stages are where they eat, drink, and recover for the next stage. I guess if they could carry all they needed or they could be resupplied without stopping, then the story could have been different. But I guess stopping in between stages also gives a chance for the story events that happen between them in person (there’s more than just the racing).

        1. Steven Dufresne: I understand why the waypoint stations would be required. But these stations would not be stationary; they would be in ORBIT, which means you would not “stop” at them, you would just catch up to them and stop accelerating while there. I’m not questioning your article, I’m questioning Charles Sheffield’s choice of the 36-hour leg time. Having not read his story, I can’t imagine why he would make such a choice. Is this supposed to show how humans have evolved by then? Because from a physics viewpoint it doesn’t seem justified.

          1. Yeah, it would be great to see his calculations. His being a mathematician and physicist and known for writing hard science fiction, I’m sure he did some. But then he also had to write a story, not just a technical paper, so I wouldn’t be surprised if he fudged some numbers to make a better story.

      3. How many watts am I using when I am thrashing my bike about fast, whereas follows:

        Within a week a bottom bracket with caged bearings is completely rotted no matter if the bike is fresh or second hand (sealed cassettes last a few years).

        I can snap a rear axle once every two months on average (Unless quick release types),

        The wheels warp over months unless double walled types then the spokes loosen and keep needing tightening,

        I can stretch my chain to the point gears malfunction (Skip, derail, etc) between 2 weeks to about 3 months depending on quality of the chain,

        I can chew the teeth of both low and decent quality sprockets within 4-months of such thrashing,

        I can crack some bottom bracket shells between 10 to 12 months (Probably low quality frames in these cases),

        I can get my heart to beat what feels like +2.5-ish beats per second (watched a second tick whilst counting beats) whilst/after pedaling up a 15* hill at about 15 to 20 MPH

        On hard-tail bikes suited for mountain touring, not racer bikes and not road bikes.

        By thrashing, Just riding hard… Not doing stupid stunts, i.e. I don’t bunny-hop, no jumping, etc.

        These days I take it a bit easier and thus the bikes last alot longer, except bottom brackets and non quick-release bolts. I am worried my heart maybe beating dangerously fast under such stresses.
        Also it is because of my heart over-working that made me wonder how much wattage I’ve wasted/used thrashing the bikes around…

        P.S. Decent bikes escaped being thrashed as they are for long term use and usually highly customized to suite comfort, especially as I am quite tall.

        1. Unferium, You might know all of this But i’ll toss it up for any & all who may not.
          How low is your seat?
          If you’re trashing crank bearings & frames, it
          sounds sort of like it’s low and your pedaling on a tangent to the pedal rotation.
          In other words knees pumping in a serious “V”.
          Back when i could still ride (knees are shot now, from manual labor & “ol Arthr”)
          I always set my seat height to have
          my toes pointed downwards when the crank arm was in line with the center/seat post.
          I can remember just about everyone who rode with the seat seat in a half squat height I.E. able to straddle the the seat with both feet on the ground, those people always had crank troubles & usually half trashed pedals too.
          Minor bicycle repair was a bit of a side hustle when I was teenager, so that’s why I was a little bit more tuned into the problems and some of their causes.

          For anyone who hasn’t set their seat at this height, give it a try. You’ll be surprised at how much easier, more efficient, your pedaling is!
          Oh and don’t forget the under-inflated tires.
          Those really drag you to a stop also.

          1. P.S. If you’re rough terrain riding then i get why you’d trade full seat height for non-squashed ‘nads.
            I guess it’s down to busted ball bearings or busted balls. :P

          2. Cheers for the tip as it’ll be useful for others who may scroll across the comments…

            I have my seat so I have to tip-toe to have both feet touch the ground, The stem length on my pedal/crank/sprocket set are usually tall and thus whilst one leg may V, the other has to reach.

            I go for a minimum 20″ frame, however I have to have handle bars like those of the tall BMX style or “dutch” style otherwise I land up with my ass in the air and my head dragging near the front wheel:
            Ok exaggerated, but gives the idea of the pain and discomfort riding without the height.

            Bottom brackets that are not of the OK to decent sealed cassette types and are just caged bearings usually are full of lint/mud/etc when stripping them down to put in a cassette. It is the non-sealed mud-filled types that go from zero to twisted in a single cycle in my experience.

            Tires are usually inflated to or close to their rated maximum: I usually have wide (2.25) tires pumped at their rated 65PSI. I have the wide tires for comfort and for going around bendy lanes. I can’t seem to not slip on anything smaller than a 1.5″-ish thick tire.

            I’m thinking of building a racer/hybrid using ceramic sealed bearings bottom brackets on a 2nd hand light alloy frame with slightly loosened and moderately greesed bearings on the front and back wheels to reduce friction, rim brakes, 58T or 60T on the crank and the smallest possible set of gears (By teeth) on the freewheel with the minimum number of gears to save weight.
            Then I’d fit a speedometer and go to my favourite hill outside traffic hours (When I’m not getting in peoples way) to see how fast and hard I can thrash that contraption down said hill.

          3. Quote:
            “P.S. If you’re rough terrain riding then i get why you’d trade full seat height for non-squashed ‘nads.
            I guess it’s down to busted ball bearings or busted balls. :P”

            LOL, By the time I ate food and typed up then I see your post…. Makes sense also :)

        2. Unferium: Can’t compute the power without knowing the mass. All of those other factors are usless.

          You said you are tall, and if you ride bike instead of driving, you’re probably pretty fit, so I’ll WAG at 90 kg for you and 15 for the bike, for total mass of 105 kg, but I’ll call it 100 kg to make it easier to scale to your actual combined mass.

          15-20 mph = 6.7-9 m/s, so we’ll call it 8 m/s.

          Tangent of 15 degree slope is .268, * 9.8 m/s^2 = 2.6 m/s^2 vertical acceleration, which we need for force. That same .268 * 8 m/s = 2.14 m/s vertical velocity, which we need for power.

          f = m * a = 100 kg * 2.6 m/s^2 = 260 kg*m/s^2 = 260 N.

          P = f * s / t (where s is displacement in m) = 260 N * 2.14 m / s = 556 Watts

          But that’s how much power is actually doing work. Since a mountain bike is probably no more than 80% efficient due to losses in tires and chain, we divide by .8 to get applied power of 695 Watts. Which if you’re being honest with your numbers is pretty damn impressive. I would question whether you’re REALLY maintaining 15-20 mph on a 15 degree grade, but only you know for sure.

          Ride safe.

          1. Ok, I’m impressed as well… I’d expected about 350W to 400W because I’d not consider myself that healthy.

            I’m 6ft2 last I measured, and I stopped growing about 2-3 years after, so possibly 6ft3 at most… I’m of average build (Bordering slightly skinny) so probably correctly guessed weight I suppose.

            Though maintaining speed on the 15* hill depends on if I feel healthier than usual that day or, rarely, if some numpty is swerving behind me beeping their horn whilst ignoring my indication that I’m pulling over soon to let them past anyway.

            I certainly haven’t got the stamina to keep a full 15+ MPH over more than 2Miles flat-ish struggling to keep 15 after the first two whilst overheating then struggling to keep 10MPH by the end of the third mile and settling at a cruise speed of 8MPH average (Takes an hour these days to get to work).

            The 15*-ish hill isn’t that long a ride 200 to 300 yards I’d guess (Trying to visualize it)… Usually when I get to the top and pedaling becomes too much free-spinning then I just roll the last bit.

            Which gets me to the following: I always run out of gears to change into in the first two miles, so I pedal fast and waste energy as heat instead of using gravity and pulling on the bars for more pressure as an aid (The gravity and pulling the bars up bit is probably why/how I chew through bottom brackets, lol)

          2. Ok, I’m impressed as well… I’d expected about 350W to 400W because I’d not consider myself that healthy.

            I’m 6ft2 last I measured, and I stopped growing about 2-3 years after, so possibly 6ft3 at most… I’m of average build (Bordering slightly skinny) so probably correctly guessed weight I suppose.

            Though maintaining speed on the 15* hill depends on if I feel healthier than usual that day or, rarely, if some numpty is swerving behind me beeping their horn whilst ignoring my indication that I’m pulling over soon to let them past anyway.

            I certainly haven’t got the stamina to keep a full 15+ MPH over more than 2Miles flat-ish struggling to keep 15 after the first two whilst overheating then struggling to keep 10MPH by the end of the third mile and settling at a cruise speed of 8MPH average (Takes an hour these days to get to work).

            The 15*-ish hill isn’t that long a ride 200 to 300 yards I’d guess (Trying to visualize it)… Usually when I get to the top and pedaling becomes too much free-spinning then I just roll the last bit.

            Which gets me to the following: I always run out of gears to change into in the first two miles, so I pedal fast and waste energy as heat instead of using gravity and pulling on the bars for more pressure as an aid (The gravity and pulling the bars up bit is probably why/how I chew through bottom brackets, lol)

            (Repost, Not spelled my handle correctly, thus got put on hold)

      4. “Riders in the Tour de France average about 250 W, and peak out at 400-500 W, and these are probably close to the best-conditioned athletes today. ”

        Tour de France probably isn’t the best comparison here, because that’s only a specific torque/rpm point, and you’re primarily managing air resistance anyway. And there are other concerns that wouldn’t apply in a controlled environment. But I agree with you that I don’t think you’d realistically be able to get a rider to output more than ~500 W anyway.

        That being said, I was surprised at the route proposed – until I saw the date of publication (1987). Why was I surprised? Because the obvious route choice isn’t L4 to a low Earth orbit and back. That route sucks – for one thing, you’d have to be concerned about satellites and the Van Allen belts, and especially near Earth, it’d be really difficult to actually have waypoints you can dock with. Plus, honestly, the route is boring: near Earth, gravity is simple. This is why the date made sense to me: back in 1987, the ludicrously complicated N-body math for farther out in the Earth-Moon system hadn’t been studied much.

        The obvious route choice is *between Lagrange points* – something like L1->L4->L2, or to get even more insane, the full loop through L1->L4->L2->L5->L1. Orbital mechanics for the Lagrange point orbits are *way* more complicated. From lunar L2 you could end up at the Earth-Sun L1 for extremely little energy. So there, you get a situation much more like real cycling, where it’s much more about route planning than anything else. The simplistic 3-body treatments here are from the late 70s (“Traveling Between the Lagrange Points and the Moon”, by Broucke) but the delta-V required between lunar Lagrange points and the Earth-Sun L1 point is extremely small (like well below 1 m/s), and the delta-V required between Earth-Sun Lagrange points and almost *any other* similar Lagrange point in the solar system is tiny. So orbits out there are super-complicated.

    3. “(a probably disgusting sugar/fat mix)” – add some cocoa and call it chocolate :-) In fact I once read an article about a bicycle courier and he told that his lunch usually consists of a good chunk of chocolate (and one of bacon) – concentrated energy and in the meat of the bacon also some proteins.

  10. If the discs are counter-rotating at the exact same speed (I assume they would be geared that way anyway), there would be no net angular momentum, and would turn as easily as if they were stopped. That’s a big “oops” in the original story, I presume.

    1. *This assumes that the structure is strong enough to handle the forces required when the wheels are at speed. Having not read the original story, perhaps that is the reason that the slowdown was needed. Martin’s idea would work, but it might be easier to have an independent wheel that you could spin up or down to apply the required torque to turn the bike.

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    1. Spammers contact info should be used to reply to the assclowns that use a 500 word, “dictionary dump” of (usually non-related) keywords in Craigslist ads.

  12. Xenon is about $1.20 a gram. So $60k to fill your bike.

    I dont think wimhursts will do it either, too high of voltage and too little current. You would be better off running a generator to power the drive electronics. If you really wanted to you could make the generators the wheels to keep it in your bike form factor.

    1. You are correct about Wimhursts. There’s a REASON we use electromagnetic generators and motors, and that reason is the energy density that can be stored in magnetic circuits compared to electrostatics. Sorry I don’t have the numbers at hand, but from what I recall from a university power electronics course, the difference was several orders of magnitude.

    2. And that $60k is for each leg of the tour, of which there are approximately 60, for $3.6 million. But hey, aluminum used to cost more than gold, so maybe in the future there’s cheap xenon, or a cheaper alternative.

      1. There are a lot of non-Xe ion thruster designs out there: the Xe thrusters are just the ones that have been actually used on spacecraft. But spacecraft ion thrusters are optimized for specific impulse, whereas here you’d actually want to optimize for thrust-to-power ratio, making non-Xe thrusters reasonable. Iodine, for instance, has been used in Hall effect thrusters with good thrust-to-power ratios, and iodine is an order of magnitude cheaper.

        1. There are also some designs using mercury. Of course that is not considered very “environmentally friendly”, but outside of the earth’s atmosphere that’s a non-issue. Hg would offer a very good density, a small tank.

      2. You could use krypton, it is cheaper but has a lower mass so less thrust. Xenon is just rare, I dont think there is anything you can do about it. After looking at xenon for a project I came up with a way to extract it from old xenon arc lamps. Most arc lamps are pressurized to about 10-15bar so if you are doing low pressure plasma stuff where you dont need much they can be a cheap supply of xenon.

  13. Having a fixed allocation for fuel is unfair, since it favors smaller riders. I have to assume that this sort of event only becomes practical once the mass of the major components is brought down to the same order of magnitude as the mass of the rider, so the rider’s mass is significant. I know that in some motor sports, ballast has to be added to normalize the driver’s mass, and I don’t see this being a practical thing to do where every kg counts. Also, it’s pointless, because all competitors will out of necessity carry the minimum fuel required for them to complete the course, since adding fuel mass decreases acceleration. This is not something you have to mandate since f=ma takes care of it for you. The primary limiting factor is how much power a human body can put out, and I don’t think it’s unfair to favor the strongest competitor. This is a sport, after all.

  14. Hate to be a party-pooper, but I think you’ve got some problems with the Physics. First, yes it’s true, a Wimshurst machine can generate a lot of voltage, but unfortunately almost zero current. What you need is POWER, and lots of it. Better to use a good ol’ rare-earth-magnet generator.

    Also, I think you vastly underestimate the amount of uumph you need to go from one orbit to another. Though I’m not clear what orbit you start with, if you’re starting from a near-earth orbit you’ll need something like 7 km/s delta v to get to L4, and you’ll still be going 200 m/s (450 mi/hr) when you get there. Hope the docking station has a very stout bungee cord.

    Finally, there’s the small matter of carrying along enough food, water, _AND_ breathable air for the trip. Even if you use a rebreather, you’ll still need a LOT of CO2 scrubbers.

    My guess is, by the time the bike is finished, it’ll be much bigger and heavier than the Apollo LEM.

    I like the general idea, though, and certain ion engines are the way to go. I just think you need to start with a less demanding race course.


    1. jackcrenshaw: I agree with you about Wimshurst machines, but that’s really a minor point because permanent magnet generators can be made in the same bicycle configuration. The more significant thing is how self-sufficient the bikes have to be. Since the races are conducted as a series of “legs”, my assumption is that consumables can be replenished at the end of each leg, so they only need to be self-sufficient for the duration of the longest leg of the course.

      Your assertion that you would have to adjust your velocity once you “got there” sent me on a Wikipedia crawl that led to things I THOUGHT I knew, but didn’t. Which is the most I can expect from a Hackaday article, so thank you for your post.


    2. A much saner option would be to basically have the waypoints be in fixed orbits, and have the “legs” just be transfers between them. Then you could also have the race be insanely longer, because when they stop at the waypoints they coast until the next orbital transfer point. With proper orbit choices (like Lagrange transfer orbits) you’d be talking about much more reasonable delta-V costs, well below 1 km/s. The downside would be that you’d need a *very* capable vehicle for rescue issues, although again with the low delta-V’s required it might not be that bad.

  15. Jack’s own alternative plot:

    Years ago, I was working with a group that had a large building with a zero-friction air table as a floor. We used it to simulate space docking, etc., in a 2-d environment.

    I got the bright idea, what if we gave folks something like a chair and a fire extinguisher. They could race a course by using the fire extinguisher as a rocket engine, directing the thrust as you needed to control both your path and your attitude (in retrospect, a big rudder might be nice).

    Then I got the BEST idea. Instead of racing a 2-d course on an air table, how about a 3-d course in some zero-g habitat? The track could have twists and turns in all directions like some gigantic bowl of spaghetti.

    Over time, I’d guess the “fire extinguishers” would evolve into sophisticated air-powered thrusters, or even hydregen-peroxide tgrusters like spacecraft use. In fact, the “chair” would evolve into a full-up, one-man rocketship.

    If that’s the zero-g equivalent of a go-kart, could a Formula 1 racer be far behind?

    1. My buddies and I had access to a similar facility in college and decided to try tobogganing using a couple of big plastic trays from the cafeteria only to find we were going too fast to stop when we reached the far end.

      It hurt. A lot. And then we got suspended for a week and threatened with expulsion if there were any further ‘incidents’.

      The English have a way with words you gotta admire. Specially when you’re being slow roasted in a calm, professional manner, without a single swear word or raised voice.

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