h a l f b a k e r yI CAN HAZ CROISSANTZ?
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The conventional image of a future space
station is a huge metallic ring doughnut
with artificial gravity generated by
rotation. Why not build the station as two
rings, one inside the other. You could
then spin them using electricity
generated from solar panels rather than
ejecting your
(valuable) fuel into space.
[a few notes six years on:
• The comment about ejecting fuel into
space applies mostly to getting the
station started.
• I have seen a counter-rotating space
station on screen -- Babylon 4 out of the
TV series Babylon 5 has a single collar
that rotates in the opposite direction to
the rest of the station. [aside: if you
watch the exterior shots in Babylon 5
closely there are
some nice little technological touches
that are never mentioned in the
script.]
• In 2003, I wrote an annotation that
refined the idea. This is posted
below:
"In fact the more I think about it, the best
solution (admittedly for a truly massive
space station) seems to be to keep the
outside still relative to local space and
rotate [possibly internal] drums attached
to the
stationary superstructure. Think of a
twin-tub washing machine and you'd be
about there." ]
(?) Ion Thrusters
http://www.hughespa...eets/xips/xips.html
Works on electricity, uses a nearly-negligible amount of "valuable fuel" [rmutt, Jul 16 2001, last modified Oct 21 2004]
(?) The Use of Nanofibers in Space Construction
http://www.distant-...2001_nanofibers.htm
Lots of space colony stuff [FloridaManatee, Oct 04 2004, last modified Oct 21 2004]
From 2001
http://www.daviddar...Space_Station_V.jpg
[ldischler, Aug 10 2007]
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Would there be any way to get from one space station to
the other without going outside? |
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PotatoStew: Not easily. The direct route sounds a little 'slice and dice' for me. |
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The only answer I can think of (short of donning a space suit) is to construct external tubes that meet at the hub (station 3 with zero g) where they'd be joined by rotational valves (keep your fingers well clear). |
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To move from one station to another you'd have travel to the hub and 'rotationally acclimatise' (hold onto a bar which slowly spins you up to speed so that you are matched with the appropriate exit tube). |
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Put it this way - I would not recommend living and working in different modules. You would not want to do this journey regularly. |
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"Look kids... Parliament... Big
Ben..." |
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The problem is centrifugal force only works in large vehicles. At something the size of the shuttle, it'd have to be spinning around it's long axis so quickly it'd tear itself apart. |
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Rods: Parasite drag? Fleas in dresses? Serously, though, I assusme that you're talking about very small forces here? Aren't you? |
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Starchaser: The forces woule be quite small - depending how fast you brought the object up to rotational speed. The reason why no-one does it is the large amounts of fuel needed to rotate a large body. |
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Here's a few figures got by plugging away at F=mrw^2. |
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A 10cm diameter mousewheel would need to be spun at 135 rpm to get one gravity at its edge. |
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A 100m radius space station would need to be spun at about 3 rpm. |
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To get a centrifugal force to balance the earth's gravity a the surface eg (ie to generate a zero g environment on the earth's surface) you would need to travel at 550 000 mph. You would circumnavigate the earth once every 3 minutes.
[oops, sorry - quick slip of the calculator finger - that should read 17 500 mph at a little under one rotation an hour. Whewww -that did seem a little fast] |
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[Starchaser]: by definition the forces around the edge of the shuttle hull would be 1G because that what you are aiming for. They'd be a little larger (2-3G?) at the wing tips but still waaay less than takeoff/re-entry stresses. |
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I think the *real* reason the shuttle isn't spun for artifical gravity is that it is such a narrow tube. If you stand on the floor, your body crosses the central axis so you would experience 1G at your feet, progressively fading to zero somewhere around your heart and then negative G for your upper body. This would be extremely uncomfortable, would play havoc with your circulation system, would make eating very tricky and if you jumped up a couple of feet, your centre of mass would cross the central axis and you would fall onto your head on the ceiling. |
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Since once the space stations were rotating they would basically keep on rotating (no drag) you could then decouple the two stations and separate them. Then there would be no eddy currents introduced by interaction. Although wouldn't at least one of them have to be magnetic for this to be a problem? |
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They might still spin down slowly due to interactions with the earths magnetic field I suppose. Could you prevent this by making all the joggers go round counter-spinwise? |
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Heh - you could put the two wheels edge to edge. Where they nearly touch, they've have zero relative velocity. So create a "transfer capsule" - a small chamber with a couch on a pivot. Get on the couch, strap in, punch a button. The capsule is released by one wheel at the same time the other grabs it (don't mess this up!). "Gravity" flips directions, the couch flips over. Unstrap and open the door on the other side, and you're there. Fun! |
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Or, put the wheels side by side, but put capsules on arms between the two. The arm alternately spins to match the wheels. When it is matched with one of the wheels, it locks to a door, the door opens, and people get in. The transition through zero-G and back is more gentle, but also lasts longer - less convenient. |
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I'm not 100% sure where I saw this, I was definitely a kid at the time; it may even have been in Arthur C. Clarke's original space flight proposal non-fiction from the 1940s. (where he invented, but did not patent, communication satellites). |
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To get around the small vessel / high rotational velocity / fall on your head problem, you build the space craft in two sections - one of which is the living quarter, the other contains lots of heavy, but not-often accessed stuff for balance. |
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The two halves separate after launch, and reel themselves out to the far ends of a long cable, and then rotate one about the other like a bolo. This gives you the necessary radius for a slow 1-G equivalent rotation, without having to build and maintain a gigantic titanium torus. |
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This idea with the dead weight and the living capsule
rotating around each other I believe to be the most
feasible. Although, to save on cost per useable space, it
may be more affective to have two separate capsules
rotating about each other instead of the dead weight.
Either way, the gravity gradient within the capsule(s) must
be considered. Depending on the radius of rotation, you
may have 1g at your feet but 0.6g at your head. This
would be extremely disorienting and impossible to live in.
In order for this gravity differential to be tolerable (less
than a 0.05g difference between a persons feet and
head), the rotation arm (radius) would have to be at least
90 meters long. At this length, the angular velocity would
be just over 3.1 rpm for 1g at the middle of the body (still
a rather high spin rate... there could be no windows or
else vertigo might result while stargazing). This poses one
last problem: how could a spacecraft possibly dock to
such a space station that is constantly in circular motion?
For this, a ring station with a hub in the middle might be
the only answer. I guess this is a good reason why non of
this has happened yet (not to mention the extreme costs
required for any of these venues into space exploration). |
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The 'gravity field gradient' you describe is actually a centripital acceleration gradient, but that's nit-picking; you have a point. |
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[st3f] is describing a standard O'Neil space colony (ref. Gerard O'Neil, NASA}, who proposed such a system in 1969. I believe O'Neil's colony was intended to be built on a massive scale, so the gradient you describe would not be an issue. The problem with massive space structures is the cost of putting them into space. |
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NASA has been evaluating ways of avoiding microgravitic hypertrophy of muscles and bone structure by implementing the alternative system that [BunsenHoneydew] suggested. The capsule, separates from the booster stage, extended on a long cable. The cable attaches to the roof of the capsule and a rocket impulse is used to spin up the rotation. The cable could be very long, reducing your gradient problem. |
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Nice to have you on board; you seem like a like mind. |
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Florida: Can you find a good link for an O'Neil Space Colony. I've googled and can find no evidence that he intended spinning elements in opposite directions to reduce the amount of fuel used. |
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That said, I'll admit that I wasn't the first to get here. Steve Degroof's link casually mentions counter-rotation as if it's commonplace and, in Babylon 5, the exterior shots of Babylon 4 show one of its sections spinning opposite to the others. |
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In fact the more I think about it, the best solution (admittedly for a truly massive space station) seems to be to keep the outside still relative to local space and rotate drums attached to the stationary superstructure. Think of a twin-tub washing machine and you'd be about there. |
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Am I missing something here? Why are we worried about "wasting all that fuel"? You spin the thing once, and you're done with it. |
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Worldgineer..Awww, u've spoiled it.
I've been wondering the same thing.....but was afraid to ask.....lest i be revealed as an ignoramus.
chorus line>>>>>>>>>>>We still may be missing something of course......
ive been hoping there would be some intrinsic vaue in the notion APART from saving the minute amount of fuel needed........repeat chorus |
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Y'know [st3f], the O'Neil references I've found on the internet refer to cylinders, so maybe I'm wrong. I have had many discussions with so-called experts that described these as I did above, so maybe I misinterpreted their description. |
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I've just thought of a problem: |
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If the two rings are counter rotating, then to transit from one to the other there'd have to be a transition from spinning one way to the other. |
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FloridaManatee: See above... :o) Transit may be simplified if you added a significant external superstructure, but you'd still need to 'spin down' and 'spin up' when moving from one module to another. |
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Worlgineer, peter2: Nope, thats all it is. A way of using electricity to spin up a space station rather than ejecting fuel into space. Bear in mind that electrical energy can be obtained by sticking out solar panels, wheras chemical fuel must be brought from somewhere else. rmutt's mention of ion thrusters does weaken this argument. |
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The only other advantage (that I can think of at the moment) comes if you add a stationary external structure which increases the docking area and allows docking without having to match the spin of the space station. |
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Yes, but you are aware of Newton's First Law |
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I. Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it. |
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....meaning that, in the instance u speak of, that thing is gonna keep on spinning so where's all this chemical fuel supposed to be used?? |
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There's no friction, no retardant force to slow the sucker down from spinning as fast as u desire.. |
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You have called out some large ornate solutions to a non-problem........while generating many pointless new prblems........ |
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but, as i said before, maybe something else will come out of this. |
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// ...so where's all this chemical fuel supposed to be used?? // |
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I think he's talking about getting the thing spinning initially. |
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exactly. not a big amount i would have thought. |
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I don't think he meant it to replace a 'big' amount of fuel. I think he's suggesting this as an alternative way to do it, using no fuel at all. Again, I can't speak for 3f. |
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The 'what's wrong with the way we do it now" argument does tend to negate the purpose of the halfbakery. |
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i retell a story form eddie tho' "The American's spent many hours and many dollars to get a biro to work upside down in zero G the Russians just used the pencil " |
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The thrust need be delivered once only. |
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peter2: "You have called out some large ornate solutions to a non-problem..." |
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Does this cause you discomfort? |
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i am discomforted,
but not seriously, dont worry |
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think for a minute that all the extra mass of ur craft now needs assistance to enter orbit in the first place. |
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//worldengineer help// Hey, don't look at me - my question's been answered. If st wants to create the most complex device off earth to spin this thing, it's fine with me. Who knows, if elegant enough it might weigh less then the fuel mass it's replacing. Think of a well-geared little clock motor hooked up to a small solar panel and two huge, tangentially touching space stations. Yeah, it might take years to get the things up to speed, but it can happen. |
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24 Hours later......>>sound of me pulling my head in..... |
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[Starchaser] Though something the size of the shuttle would have to rotate extremely quickly in order to achieve 1G acceleration at its extremeties, it would not tear itself apart. |
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My evidence for this comes in the form of an ordinary shuttle, in its hangar... doing nothing. In one G. In addition, one might picture the shuttle on re-entry, experiencing 4+Gs, yet still not tearing apart (barring wing damage.. er..). Moreso, I present myself as a possible vehicle to be a centrifuge, since my feet do not rip off whenever I do a chin-up. |
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re: below surface zero g monorails: Since the thickness of the earth's crust is small compared to its radius, it's unlikely that you'd need new figures. They're only approximations anyway. If you want to think about theoretical tunnels in the plastic regions of the earth, remember that, as the radius of the tunnel tightens, so the gravity due to the earth will be less. |
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I'm not sure about the wasting of fuel, but I remember reading somewhere that the idea of creating counter-rotating paired space stations was to avoid a gyroscopic effect. Please correct me if I'm wrong. How much fuel could be wasted in anycase once you get the thing going (unless you are trying to stop the thing)? |
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Excuse me if I'm being naive; but does the 'gravity gradient' actually exist? Even for a small radius of rotation; the only force acting on the body will be the centripetal force provided by the 'floor' of the spacecraft/station in order to constrain the person to a circular path (Otherwise the person would fly off in a straight line). The only force acting on the person would be the force through the persons body, from their feet, or whatever part of them was touching the spacecraft. Thus there would not be any 'gravity gradient' or 'centripetal acceleration gradient' or any thing of the sort... the force acting through their feet would compress them slightly causing the rest of their body to accelerate perpendicular to the outer surface of the circle. So their head would experience the same force as their feet, and would accelerate at the same rate. |
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A difficult one, bitter_chicken, but I'd say that the simulated gravity gradient would affect a human body. Simulating gravity by rotation results in a difference in momentum between you and the surface you think of as the ground. If you set up a spinning spacecraft small enough that you were taller than the centre point then you would feel as if your upper body was upside down. |
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If this feels counter-intuitive think about being in this situation and holding a ball at head height. OK, now let it go. What happens to the ball. Does it stay still? No. It is already travelling with the linear speed of the body-part that was holding it; a linear speed resulting from the rotation you are using to simulate the gravity. |
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So what happens? Well, when you were holding the ball it was travelling in a small circle. The force needed to keep it in that circle you perceived as the weight of the ball. When you let it go, the ball carries on moving in a straight line relative to the outside world. Because the rotating world is spinning in the same direction as the ball is moving it gets closer to the edge and appears to fall toward the floor. |
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Split the body into bits (mental exercise only) and you realise that to stand up still relative to the rotating floor each body part has a different momentum depending on its height from the floor. The higher the body part, the lower the momentum. The lower the momentum, the lower the force. |
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If you get your head around this, you get a few strange tricks. On a rotating space station:
- Even though the floor looks as if it curves up away from you (that's because it does), to your simulated gravity it is a flat plane. Roll a marble and the marble will tell you the floor is flat.
- If you can throw a tennis ball against the direction of spin at the right speed it will orbit (effectively the ball is stationary and the space station just spins around it). This will only happen in one direction; throw the ball in the other diretcion and it will appear to drop faster. This could make tennis interesting.
- Drop a ball and it won't quite go straight down. (I need to check my maths (in reality *do* my maths) on this one so don't take it as gospel) |
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yes - its all clear to me now :-) |
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by the way, I did the maths for the dropping ball - and you were right on that: |
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The difference in time between when the ball falls, and when you think it should fall, is given by |
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abs( ( T/(2*Pi) )*( tan(theta) - (theta) ) )
where (theta) is angle swept in radians,
and T is period of revolution |
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If you have one "wheel" inside the other, the speeds would need to be different for both to have the same gravity. Why not have both the same size & next to each other, like the station in 2001: SO In both cases it will be hard to tranfer anything from one ring to the other. Since there's no air there would be no friction so there's no need for rockets & fuel, execpt for correcting any unwanted movement. with 2 rings where will be some friction, so then there WILL be a need for more fuel! I can see how 2 wheels spinng opposite directions appear more stable than one, but there may not be a need for such a thing. |
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You don't need any fuel to keep a space station rotating. Once it's rotating, it just keeps rotating, with no air resistance to slow it down. |
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[StarChaser] No, the Space Shuttle could generate artificial gravity by rotating, no problem. In fact, tearing themselves apart is more of a problem for large vehicles than for small ones - essentially the size of a space station with 1g artificial gravity is limited by very much the same mechanical considerations as apply to single span bridges on the Earth's surface. |
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However, it's true that small vehicles would have to rotate relatively fast, which has other disadvantages - notably that you'd actually be conscious of the Coriolis effect when standing up from sitting or vice versa, or when lifting or lowering anything, and you'd be conscious of differences in the apparent weight of things according to how far from the axis of rotation you were. |
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Oh, gawd in space . . .. This one is a classic. |
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The original idea wasn't bad, although unclear on whether it meant initial spin-up, or (un-needed) perpetual power. |
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But it creates a very complex system for the wrong reason. If you want lots of windows and outside walls, fine, but building two stations is more expensive than one big one, and the counter-rotating hub/junction is always going to be a pain. |
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The ideal for a no-rocket spin-up of a space station has been mentioned in science fiction many times: Build two counter-rotating wheels, yes, but make one of them very small. In other words, make a flywheel within a single station, and spin the flywheel the hell of a lot faster than the station will be going, in the opposite direction. |
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That's two counter-rotating wheels, still, but it's a lot easier to make and maintain and live in than the posted idea. |
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One advantage of an opposing-flywheel spin-up over a rocket-powered one is that you can put the brakes on, and stop the station, if you ever want. |
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If you'll never want to stop, take the flywheel out, carefully, and get rid of it somehow. The station will never stop spinning--it's in space. |
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I think the space station in Ender's Game was like this. |
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Anyway, I was going to mention that the 2 halves don't have to be identical. Also they don't have to be rings. You could have a hub with all the heavy stuff rotating one way and 2 arms with living quarter capsules rotating the other way. |
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Microsatellites have been launched to measure the Earth's magnetic field using large coils.
However, since the satellies were launched on the cheap, piggy-backed on other launches, the ejection mechanism caused the satellites to tumble, so before measurements could take place, they had to be stabilised.
Because the devices were so small and cheap, the fuel load and complexity of conventional thrusters were out of the question, so the coils themselves were used with electrical shunts to slow the rate of tumble - the coils acted as electric motors directly coupled to the body of the satellite.
Why not extend this principal to provide fuel-free spin? |
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Um, because that system was only useful for STOPPING the tumbling. It was the electrical equivalent of putting on the brakes--the tumbling motion of the coils through the earth's magnetic field generated small amounts of electricity, which was wasted as heat. There were no moving parts. (The word "motors" should have been "generators", as the two are the same thing, in simplest electrical construction.) |
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You COULD spin up a space station by pushing against a coil in the Earth's magnetic field (instead of a flywheel), but the coil would slip like hell, and you'd still need to put forth all the energy needed to get the station moving, PLUS whatever the coil wasted. |
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Emphasizing once again: spinning the station up to speed takes energy, keeping it up to speed takes nothing. |
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//keeping it up to speed takes nothing.// |
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Except when it's joined to a counter-rotating structure by a vacuum-hardened bearing gasket thingy. Friction, people... |
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In a single station, what's the effect of having all the inhabitants take their daily jog in the same direction around the radius, ending up back where they began after a few laps? |
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I believe your ship will stop spinning when you stop jogging. Why do I feel another goldfish ball discussion rolling at me? |
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Wow. This one runs and runs. Most
annotators don't seem willing to read the
entire thread so it runs in near little circles
rather than straight lines. How
appropriate. |
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[thinks: Maybe I should add something to
the idea. :) ] |
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Ah, this idea has low annotation friction. Once you get these things spinning... |
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Yeah, the friction involved in counter-rotating stations is one of the bad things in this idea. I should have been clearer that I meant that a single, non-counter-rotating wheel will spin forever. |
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The activities of people inside a single wheel will all cancel out at the end of the, um, "day", but I'd not want to be conducting precise measurements whilst the gym is holding classes. A troupe of joggers will affect the whole station, yes, but only until they stop. (And there are going to be some kids who'll try to run down the corridors fast enough to stop rotating and float, but they'll hit a wall sooner or later.) |
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Only if everybody who leaves the single-wheel station jumps one way, and everyone who arrives thumps in from another, would you get some losses. |
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Personal pet peeve on this topic: Every science fiction movie ever made. |
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"Personal pet peeve on this topic: Every
science fiction movie ever made."
...hmmm strange. The only place I've seen
a counter-rotating space station is in
Babylon 5 and that was after I wrote the
idea. |
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If I run fast enough in the opposite direction, can I don my Superman cape? |
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I'd say you can fly/float if you can cancel out the rotational speed. |
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I can't say I've ever seen a counter-rotating space station in a movie, but I didn't mean that. What I have seen is a LOT of craft where part rotates and part doesn't, which causes almost as many problems (as evidenced by the heavy rumbling noise on the soundtrack). Poorly-faked weightlessness and assumptions of gravity are . . . <incoherent frothing noises> |
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