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LBlankenship
01-30-2011, 05:54 PM
I need second/third/fourth opinions...

I have a spinning habitat. My characters are going to be working on the outside surface of the habitat and what's spraining my brain is how you would get in and out of an airlock hatch that's flat on the surface of the habitat. Because when you're in the airlock, the opening into space is going to be in the floor and "down" as far as you're concerned. But once you're outside it should be free fall so long as you're moving at the same speed as the surface.

Sounds like a tricky transition, even with safety lines. Any thoughts appreciated.

More specifics, if you want them: it's an accelerated asteroid, about 25km diameter, providing about one-third gee inside. I figure about 4.6 rpm. Please correct me if I'm wrong!

Astronomer
01-30-2011, 07:05 PM
But once you're outside it should be free fall so long as you're moving at the same speed as the surface.Could you clarify this one point? I'm not sure what you mean by this. It sounds like you're saying that once outside, a person could float geosynchronously above the spinning asteroid without any means of attachment or acceleration, which I disagree with.

An "airlock elevator" that lowers the people/cargo out to the surface -- where a receiving scaffolding structure awaits -- seems like a reasonable way to do it. Once outside, the receiving area would provide a safe staging area where safety cables can be secured and equipment can be organized.

Is there a reason the airlock can't be on the spin axis at one end of the habitat?

Have you selected an asteroid? There are two in particular that would make a great candidate.

Lhun
01-30-2011, 07:14 PM
I have a spinning habitat. My characters are going to be working on the outside surface of the habitat and what's spraining my brain is how you would get in and out of an airlock hatch that's flat on the surface of the habitat. Because when you're in the airlock, the opening into space is going to be in the floor and "down" as far as you're concerned. But once you're outside it should be free fall so long as you're moving at the same speed as the surface.You can't move at the same speed as the surface, since the surface isn't moving in a straight line. Freefall will carry you away from the surface on a tangent, so the airlock would actually be in the "roof", if you're attached to the surface.
The options there are pretty much magnetized or otherwise sticky boots, or just rope.

Sounds like a tricky transition, even with safety lines. Any thoughts appreciated.Lines and handholds are essential. Trying to get inside a rotating object with a jetpack is extremely difficult. (a computer controlled jetpack can do it of course) Usually, you'd place the airlocks at the rotational axis, but for a really big habitat, that's too far away from the middle. Though people might still use only the hub airlocks unless there's some time critical maintenance to be done.

More specifics, if you want them: it's an accelerated asteroid, about 25km diameter, providing about one-third gee inside. I figure about 4.6 rpm. Please correct me if I'm wrong!http://www.artificial-gravity.com/sw/SpinCalc/SpinCalc.htm
Putting your figures into it i get about 300 g, or 0,15 rpm, depending on which variable to alter. Maybe you mixed up rotations per minute and minutes per rotation?
The formula is a = 0.011 * rē * d (a=centrifugal force in m/sē, r=rotation rate in rpm, d=distance to center in m)

Astronomer
01-30-2011, 07:58 PM
Putting your figures into it i get about 300 g
Whoa. I didn't look at the numbers, but yeah, with a 25Km diameter, it doesn't take much spin to get a G. At 4.6 rpm, you'd be covering 361,300 meters every minute just standing in one spot. That's a tangential speed of over 21 million meters per hour.

Slow that puppy down!

LBlankenship
01-31-2011, 12:50 AM
Have you selected an asteroid? There are two in particular that would make a great candidate.

It doesn't have to be any specific asteroid, but it does have to be close to the orbit of Mars. But not in a Lagrange point. I gather that a group called the Hungaria asteroids are good candidates, but if you know one in particular I'd be glad to hear about it.

Thank you for the math corrections. Bookmarked the calculator page. 300g sounds like a decimal's in the wrong place...

I was thinking that there would be a sensation of falling/being thrown through the open door on the "floor" on the way out of the station. Ropes and sticking to the surface are the primary safety measures. Getting around on the surface sounds like it's going to feel like crawling around on a ceiling. Except there's no floor. Actually, if there's one-third gee being felt on the inside, won't it also feel like one-third gee on the outside?

FTR, it's a relatively small settlement dug into the asteroid. Probably covers a few square kilometers and less than a kilometer deep. No axial airlocks, docking facilities, etc. The surface isn't someplace people go (and the characters aren't supposed to be out there, actually) so no elevators or any amenities like that. If there was maintenance to be done, they'd send a robot.

Dommo
01-31-2011, 05:18 AM
Basically with a habitat of that size, and the slow rate of revolution of roughly .15 rpm, handholds and safety lines are all that are needed.

Watch the Anime Planetes. What you will notice is that there are handholds EVERYWHERE, and that they often use "toe bars", which are bars that sit about 4 inches or so off the grounds so that you can "latch" your feet in place while standing around. The show is fairly accurate, but it might give you some ideas.

Astronomer
01-31-2011, 05:33 AM
Less than a kilometer deep? That sounds fun! So it will be a circular ribbon of civilization about 1Km x 78Km. That will present some challenges for getting around inside the thing, which could make for some very interesting plot points. If they criss-cross from one point on the rim to another on the circular flat end on tracks, make the tracks arc segments and one-way to account for Coriolis forces.

Okay, so you're keeping it simple -- have to work within a budget, limited resources and all that. I get it. Still, I would be reluctant to have them actually stand on the outside surface with sticky/magnetic/harness boots, since that would be exceedingly uncomfortable and unnatural, even at 0.3G.

How about something along the lines of mountaineering equipment? A grid of T-rails on the outside that takes them generally where they want to go (their safety lanyards connect with a ball in a track, allowing the tracks to intersect), and then rock-climbing gear (essentially) to "blaze new trails" and lay more track. Have the harnesses designed to hold them feet-out when sliding along the rails, and back-out when working, which would have the same feel as a mechanic working on his back underneath a vehicle.

Actually, if there's one-third gee being felt on the inside, won't it also feel like one-third gee on the outside?Pretty much. With a diameter that large, another few meters won't be noticeably more accelerated.

I hope you post some of it in the SYW section. I'm dying to see what you do with this. :)

Thank you for the math corrections. Bookmarked the calculator page.Yeah, thanks, Lhun. That was thoughtful. I've bookmarked it as well. Don't tell anyone, but web pages (and Android apps!) like that (time dilation calculator, heat transfer, energy conversion, etc.) are the "Secret Sauce" in all my writing. ;)

LBlankenship
01-31-2011, 07:54 PM
Okay, so you're keeping it simple -- have to work within a budget, limited resources and all that. I get it.Yeah, I read some of the threads in this group and I feel like I'm making my characters work with sticks and stones... :)


Still, I would be reluctant to have them actually stand on the outside surface with sticky/magnetic/harness boots, since that would be exceedingly uncomfortable and unnatural, even at 0.3G.If you grew up in just 1G, sure. But I figure if you grew up in mixed gees, it's just the way it is.


How about something along the lines of mountaineering equipment?Funny you should mention that... I did suddenly find myself wiki-ing climbing gear when I got to the middle of the first scene.

Some kind of rail/track system for getting around in emergencies sounds right. Fits better than the kinda kloogy thing I'd come up with. It'll still be inconvenient, which is fine.

Maybe I will post these parts in SYW -- it's kinda deep in the book and surrounded by girl cooties (http://sff.net/paradise/girlcooties.htm). :D There's a cargo delivery sequence that'll need fresh eyeballs on it, too.


Watch the Anime Planetes.Dang, that's been on my to-watch list for years now. Where does the time go?

Pthom
01-31-2011, 09:59 PM
Read Greg Bear's "Eon."

The access to the interior should be not on the perimeter, but at the axis of rotation (i.e. the "hub"). At this location there is very little angular momentum and therefore the only "gravity" is that provided by the asteroid itself.

Also, I believe that once on the exterior surface of a rotating body, the reference frame changes and a person could walk around without being "flung off" as might be the intuitive case. The moon rotates quite rapidly--but Neil Armstrong (and pretty much everything else there without a rocket motor) remained mostly "stuck" to the surface.

However, on an asteroid that's only 25 km in diameter, even if it were made of solid lead, it wouldn't contribute much in the way of gravity to hold an astronaut down. Use hand holds where there are constructions--climbing-type gear elsewhere. On such a place, it wouldn't take much to reach escape velocity.

movieman
01-31-2011, 10:39 PM
Also, I believe that once on the exterior surface of a rotating body, the reference frame changes and a person could walk around without being "flung off" as might be the intuitive case.

Nope. The only thing keeping you going around in circles on the inside is the skin of the habitat... go outside that skin and you'll fly off at a tangent, which will appear to you as though you're being pulled away from the curved surface below.

If the gravity inside is 0.3g then once you go outside you'll feel as though a 0.3g force is pulling you away from the surface.


The moon rotates quite rapidly--but Neil Armstrong (and pretty much everything else there without a rocket motor) remained mostly "stuck" to the surface.

Rotation reduces the effective gravity, but the moon rotates so slowly that its gravity easily provides enough force to keep you on the ground. As you say later, an asteroid won't have enough gravity to do that unless it rotates extremely slowly.

Astronomer
02-01-2011, 12:35 AM
The moon makes one rotation every 29 days or so. Its mass so overwhelms its centripetal acceleration that it makes a spinning 25Km asteroid look like an amusement park ride.

Lhun
02-01-2011, 04:16 AM
Also, I believe that once on the exterior surface of a rotating body, the reference frame changes and a person could walk around without being "flung off" as might be the intuitive case.Reference frames are just arbitrary constructs to do physics calculations, you change them at will, not by moving around.
If no force acts on you, you behave according to inertia. On the surface of such a habitat, the only force acting on you is gravity (negligible at that size). So you move in a straight line according to your velocity. Which will move you away from the surface at a tangent, since that keeps moving in a circle, because it experiences centrifugal force (caused ultimately by the molecular forces holding the rock together)

Nope. The only thing keeping you going around in circles on the inside is the skin of the habitat... go outside that skin and you'll fly off at a tangent, which will appear to you as though you're being pulled away from the curved surface below.

If the gravity inside is 0.3g then once you go outside you'll feel as though a 0.3g force is pulling you away from the surface.You only feel the centrifugal force as long as you're attached to the surface. As soon as you're in freefall, you move away from the surface but don't experience any force. You also don't accelerate any longer, you simply keep moving away at a tangent, at the speed of the rotation.

movieman
02-01-2011, 09:47 PM
You only feel the centrifugal force as long as you're attached to the surface. As soon as you're in freefall, you move away from the surface but don't experience any force. You also don't accelerate any longer, you simply keep moving away at a tangent, at the speed of the rotation.

I was talking about the surface: you'd need to provide some force to keep yourself hanging on as though gravitiy was pulling you 'upwards' relative to the surface at 0.3g.

However, if you flew off at a tangent and were still regarding yourself as hanging onto the surface of a planet and being pulled away by a force, you'd see yourself accelerating away with decreasing acceleration until you reached a distance on the order of the radius of the habitat; even though you were traveling at constant velocity tthe habitat surface 'above' you would be accelerating away as a consequence of its rotation.

For height h and initial velocity v from a habitat radius R, since you're leaving with a tangential velocity, (R + h)^2 = R^2 + (vt)^2

hence h^2 + 2Rh = (vt)^2.

For h >> R, that becomes h = vt, which is what you'd expect from constant velocity.

For h << R, that becomes h = 1/2 (V^2/R) t^2, which is 1/2 at^2. So initially you'd still see yourself as accelerating at 0.3g away from the surface even though your velocity is constant relative to the center of the habitat.

You'd presumably also feel that you were rotating as the closest surface vertical would be rotating away from you... at least until you looked at the stars and realised you stable and the surface was rotating.

Astronomer
02-03-2011, 09:53 PM
Living in an O'Neill habitat has its challenges and quirks. If you lived in an O'Neill habitat and you jumped straight up, you would land (select one):


A - Spinward from where you began.
B - Antispinward from where you began.
C - At exactly the same spot from where you began.

Which way would you orient a tennis court so as not to give any player an unfair advantage? Should the center line (the one perpendicular to the net) be in the center?

Do ballistic projectiles still follow parabolic paths through the air? Regardless of direction?

Would a cable suspended from two ends (as for a suspension bridge) still follow a centenary curve? Regardless of direction?

You have the opportunity to have a lot of fun with this. It's an unusual setting that would be lots of fun if properly brought to life.

LBlankenship
04-10-2011, 07:06 AM
Hey, everybody. I'm wrapping up the first draft of the novel this all related to, and I was wondering if anybody is still interested in seeing the approximately four thousand words that contain the most physics-oriented parts of the story. It's one-and-a-half contiguous chapters. I'll post it in SYW if people want, or I can email text files.

Though heck, if anybody wants to beta the whole thing (75-80K) for scientific plausibility I'm not going to say no. :)

Astronomer, I was tempted to settle on an answer of D - your head explodes from thinking about it too much - but in the end there wasn't a whole lot of jumping and no time for sports. The O'Neill habitat is a correctional facility -- somewhere in between a prison and a mental hospital. Warden said no tennis, and golf was right out too.

Though I suppose a 3-point throw in basketball would still be interesting. Or would it be an incentive to invent new sports? I kinda threw in "Martian football" in the background without thinking about it, but playing (American) football on a 1/3-gee planet has got to have complications of its own.

Astronomer
04-10-2011, 08:47 AM
I'd be very interested to see this passage in SYW. Thanks for updating us on your progress.

Wojciehowicz
05-14-2011, 06:23 AM
In the case of an O'Neill cylinder, if the airlock were along the outer skin, the artificial gravity would have the airlock beneath your feet, and exiting that airlock would be like going down into a manhole on a street, except that there's no ground beneath your feet. EXCEPT... that it would be fine IF you treated the outer surface as being the ceiling and a floor as what you're looking for. If you go down to the cellar, the first floor is now your ceiling. Same thing. Where do you put your feet? You would need something like a window washer rig, or an interior construction/renovation scaffold, attached to the outer skin somehow.

I would think that if the outer surface were ferrous, good powerful electromagnetic bearings could be used to roll across the surface, and then attachment points used to fix it in place for the duration for safety. If it were aluminum, it might be better to use a system of tracks and rails over the outside. In that case, a sort of upside down manipulator system like a multi-armed mutation of an earth mover might help. Your butt is on a seat pointed away from the center of rotation, the work to do is over head, and you can see it in front of your cab as well as watch the monitors for the cameras mounted on the manipulators for a work's-eye-view. Space is below the work and under you.