Space Cadet Thought Experiments

  1. There’s famous video of the 1970’s Skylab Astronauts running along the inside surface of a ring of lockers lining the wall. Because Skylab was basically a surplus moon-rocket tank, it was a bit bigger than any module of the current Internal Space Station - figures I’ve seen suggest that ring was a bit over 6 meters, or around 20-21 feet in inner diameter. (You can see the video on youtube, I won’t link it as most postings are from conspiracy theorists)

Question: could you unicycle it? If so, about how fast would you be going, and what size wheel would be best? Would gearing help? Would it be possible to use a giraffe sized such that your head remained stationary at the axis of the station?

  1. Imagine we have perfected skinsuit technology, such that a precisely engineered lycra-type garment is able to apply sufficient pressure and insulation to a human body for a spacewalk, without trapping any air around your joints and thus allowing a fair fraction of normal athletic performance.

Question: Could you unicycle on the moon? On mars? What size wheel would be best? What tire pressure, relative to what you’d use on earth?

Note: a little searching suggests these topics were touched on briefly back in 1999, but I’m curious on others thoughts before I start trying to figure out my own answers.

I gotta ask – what conspiracy are these theorists talking about?

Could you unicycle it? I’ll offer a 2-part answer. I remember seeing that video, way back when it was pretty new. The guys were going around those bins, but seemed to be short on traction so it was hard to get going and hard to maintain momentum. So I will theorize that, if we assume those doors are strong enough (probably not; weight is bad), and we assume the unicyclist can get some help building up some momentum to stick him/her to the sides, I believe they could probably continue to ride around those bins.

Or not, as being on a unicycle makes you taller and raises your center of mass, which might make it more difficult to accomplish within that amount of space. But maybe not, since riding a wheel might give you a smoother motion than running, so might help to maintain some traction as you went around.

Speed? It would have to be enough to keep you pressed into the riding surface with enough force to have traction. This is why it’s nearly impossible to ride a unicycle underwater; basically no traction because the water is so dense, and bouyancy makes you relatively lighter.

Wheel size? Small, to “lower” your center of mass. I’ll say 16" wheel based on my experience with 12" wheels, which is that it’s hard to pedal them fast without a lot of wobble.

The giraffe idea would make it way more difficult/less likely to be worked out. Your center of mass would be near the center of the space, making it super hard to generate any decent amount of traction under your tire.

Last but not least, the practicalities. I think those Skylab crewmembers could probably have gotten a lot better at running around that “track” had they had more time to practice. They probably had very limited time, and might have been under orders to be extremely nice to the bins as not to damage them. Plus bringing a unicycle up there would be very prohibitive in terms of weight! :slight_smile:

Yes and yes; I’d love to be the first! That is, assuming a spacesuit that allows sufficient freedom of movement like you said. Tire pressure? Minimal. Remember, both locations have little to no ambient pressure so you would barely need any. Or none at all if you use tires like what they had on the Lunar Rovers. Wheel size? Not important. It would be fun to have something like an Oregon, so you could do really high hops!

Engineer, my dad was an engineer (automotive) before he retired. From 1962-1967 he worked for NASA at Langley AFB, working on the Gemini and Apollo programs; mostly development and testing. He didn’t have any thoughts of unicycles back then, but I think I get some of my engineering genes from him. :sunglasses:

BTW, you can get an up-close look at those storage bins/lockers if you go to the Smithsonian Air and Space Museum in Washington DC. They have the mock-up of Skylab that was used for training. But it suffers from gravity.

These are all just silly questions. Everyone knows it’s impossible to ride a unicycle.

I could accomplish all of the above, but unfortunately I don’t wear lycra so I’ll have to pass. :stuck_out_tongue:

Ask Randall Munroe.

No, way, man, I know that unicycling is real because I saw it on TV!

He never responded last time I asked him a unicycle question.

Part 1:
I thought an engineer would just figure this out for himself. It’s actually a pretty simple highschool physics problem to calculate if I understand it correctly (but I never took highschool physics so hey, I could be wrong.) I’d guess the radius to be approximately 3 meters. Centripetal acceleration = V^2/R. V = sqrt(RA)
To ride fast enough that the acceleration is the same as that of earth V = sqrt(9.8
3) = 5.42m/s ~ 12mph.

You’d probably need a 29er to maintain that kind of speed but you shouldn’t actually have to go that fast since you really only need to go fast enough for friction. If you only went 8mph it seems like that would be enough since you would end up with about half as much force as you would on earth. Also the radius would actually be less than that since you should really be measuring from the center of the circle to your center of gravity. Really that calculation is just a ballpark to show that it’s definitely within the realm of possibility. I imagine that you would want to get up some speed by running around the circle while mounting. Once you’re on it’s anyone’s guess as to how long it would take you to learn to ride in such conditions but it’s certainly doable with some practice. And practicing new skills is the whole point of unicycling anyway.

A giraffe to hold your head in one spot seems like it would work assuming you can mount and get going. It might be a two person job, one guy could run around the circle and get the bottom spinning fast enough that the other guy sitting on the saddle could take over. Also it might be hard to keep from getting dizzy. But you’re an astronaut, you don’t get dizzy easily, right?

Part 2:
Of course you could unicycle on the moon and mars in that case. I don’t think that there’s an answer to asking which wheel size is best on the moon/mars, just like there isn’t one for when you’re on the earth. It all depends on whether you’re hopping off mountains or just going for the fastest possible speed.

The tire pressure would be different by the earths atmospheric pressure if I understand correctly so you would just want to subtract 14.7psi. But I don’t really know how gauges are calibrated so it might actually read the exact same. It’s even more confusing because there’s a fake zero pressure in there kind of like with the standard temperature scale. Someone who knows more then me could probably answer easily. If you didn’t feel like calculating just let the air out of the tire and use a CO2 refiller to get your desired hardness once there.

Speed required to ride a unicycle in Skylab? About 4.83 miles/sec! :slight_smile:

This video features three Skylab astronauts jogging - well, sort of …
Jogging begins around 3:30.


Making up a completely random number and posting a youtube video of what you think for no reason is fake will not prove anything. Get over it, there is no legitimate reason to think that it’s fake. And even if it was fake that doesn’t mean it’s impossible it just means that it hasn’t been done.

Also, as the video below shows it’s possible for humans to run loops on earth, in space it would actually be easier once you got going since there’s no spot at the top where there is a lot of gravity pulling you the wrong way.

Not a random number. 4.83 miles/second was Skylab’s orbital velocity. Aerospace Engineering humor - no offense intended. It was meant to give the OP, a fellow engineer, a chuckle. I posted the video link because John referenced it in his post. And for the record, I am positive the video is authentic.

Okay, that makes it better. :stuck_out_tongue:

It was great to see that video again. Those Skylab guys had a great luxury in the form of that big round space to play around in! Not that they had much time to play. But if that’s what they were able to do with minimal practice, imagine what can be done in such an environment.

It looked like their jogging speed was limited by traction, and what I imagine was air friction keeping them from generating more speed. So the hardest part for a unicyclist in there would be getting up to a speed where your tire would have some grip. With no traction, you wouldn’t be able to maintain speed, and you would keep drifting to a stop or something. Or spinning your wheel while going nowhere, which is kind of what happens when you try to ride on the bottom of a swimming pool.

I imagine a larger diameter “locker loop” would make things a little easier. Then it would be fun to get in there with a fat tire and try some of those flips the guys were doing!

Yes it would be possible to ride the walls of a virtical drum, but you’d need to get sufficiant velocity to combat gravity. I agree you wouldn’t need the full 12 mph but I think it would be much closer than 8, maybe 11 mph. This would depend on how grippy the surface and your tire are. Too much grip would increase rolling resistance, slowing you down.

I saw a guy ride one of those near virtical drums you sometimes see motorcyclists do in a circus. This guy attempted it many times, the farthest he got was around 3/4 a revolution before his speed and/or form faltered (It was clear sometimes it was one or the other) and he’d UPD. Looked to me like he was leaning up far too much, he should have been leaning towards the center, roughly in line w/ the uni.

There are several videos that have been posted in the video forum where street riders would hop up and ride a wall for 1/2-1 revolution.

JF has a point about hight of your center of mass from the surface being a problem, so I’d say the smallest uni that could easilly maintain ~ 11 or 12 mph, maybe a geared 20. Like JF mensioned small wheels woble more @ high pedal RPMs, also geared hubs put a bit of instability to the uni (I think I read that the geared hub is kind of like pedaling w/ a loose crank, but wobbling at less regular intervals). I remember reading about a geared uni w/ a similar drivetrain to a giraff, but the pedal axle being lower than the wheel axle (I think it was called the Unirex). That lowering of your CoM would help.

Yes you could ride on the moon or Mars. Size & type of uni wouldn’t really matter but the type of surface could make one better than the other just like on earth, like Shmolagin said. Air pressure would probably be a bit less since you’re hitting bumps w/ less force reducing the likelyhood of pinch flats. On Mars if coming from a 14.69 psi room to outside it would be 0.6% of what you might use one earth, so 30 psi on earth equates to 0.18 psi on Mars, before going outside, so I’d start w/ all the air pushed out of the tube before puting it in the wheel and add air when I got outside.

On the moon I’d do the same, but starting w/ as close as possible to getting ALL the air pushed out of the tube (I’m sure there vacuume pumps that can get you close enough). I wouldn’t be surprised if the air in a small wrickle you always see in a brand new tube (which is always more deflated than I’ve ever been able get a tube) might explode the wheel as soon as you open the door on the moon.

Yes the guy was effectively running in place because they were spinning the drum. But if they had in space and were spinning the drum to create artificial gravity, you could generate much more than one G (although all the times I’ve read of how scientists actually planning out how they’d do it, less than 1 G was always used ~ 1/2 - 2/3 G). This is what they were attempting to represent in the movie. They could have done it for real, even then, but going to space to get a true reprsntation prob would have cost MANY millions of dollars just for one short scene.