Print

Please login or register.
1 Hour 1 Day 1 Week 1 Month Forever
Login with username, password and session length

[Il Palazzo]

Welp. That's an eye opener. I didn't know it was even feasible for it to be this thin.
The only other risky thing about having such a space elevator that I can think of, is the danger the part staying in orbit(after separation) would pose.

[TheBronzePickle]

It's a big, easily trackable object presumably. It has to be heavy enough to act as a counterweight for whatever is being sent up the elevator so that it doesn't get pulled out of orbit.

[Virex]

Depends on the design. Some designs just use more cable as the bulk of the counterweight, which has the advantage of allowing the lifting of object well beyond geosynchronous orbit. That is very useful because at that distance, the cable is going faster than the local orbital velocity, so anything released from the cable at that altitude will experience a "centrifugal launch" into outer space. That makes launching things towards other space bodies a lot easier because you don't even need rockets for the bulk of the acceleration, but only to adjust your heading.

[TheBronzePickle]

The particular design I think of is the counterweight being extended past geosynchronous orbit but also being some kind of space station that can be used for space-based maintenance of the elevator.

[Il Palazzo]

It's a big, easily trackable object presumably. It has to be heavy enough to act as a counterweight for whatever is being sent up the elevator so that it doesn't get pulled out of orbit.

Even if, as has been said, it has a counterweight/station like that, there's still the thousands-kilometres long cable swinging somewhat unpredictably attached to it. Furthermore, it's thin enough to not be easily trackable on radar, and massive enough to cause potentially critical damage to any ship that happens to hit it.

[TheBronzePickle]

Who cares? We're presumably going to be building the thing so an accident doesn't happen, and if it does we just blame somebody for it and move on.

[Starver]

Coming a bit late to this conversation, (but it also helps me follow it, so it doesn't disappear off of my reading list again, until I actually do catch up), I also think we should diversify across space, as per several messages, including:

[...] There's also the fact that self-sustaining lunar and martian colonies (which are 100% possible with existing technology) would render humanity immune to any extinction-level event that doesn't grease the entire solar system. [...]

One counterpoint that could be said (hopefully I've not been ninjaed, though) is that when we are spread across the system, it's entirely possible that planetary-scale Extension Level Events could be considered a viable policy by those now with the power and will to deliberately send an asteroid into contact with their chosen target, helped by a remoteness from the target world.  Naturally there'd be others on the other side crazy enough to retaliate, and things could easily go MAD (in the cold-war sense).  So I don't think we'll be safe from ourselves.

Anything short of irradiation by 'nearby' supernova should at least be survivable if there are self-sustaining colonies on other worlds/in other voids (of which we should have enough that the inevitable accidents that do occur don't bring us back down to Earth, but serve as lessons to the others), but self-sustaining colonies bring with them a "new nation" mentality, bringing along with them the 'risk' (or promise) of declarations of independence from the home nation/planet and if anyone gets suitably aggrieved about the slowness (or speed!) of the process...  whoops.

I still think it's worthwhile to try, and have probably been watching/reading too much near-future Sci-Fi, whose premises involve (as background, if not as a plot mainstay[1]) such issues.

PS, I doubt I'll get a chance to get into space (too old, wrong career specialisation and not being American/Chinese doesn't help, for current space missions, and just plain too old to imagine that a more civilian space free-for-all can end up drawing me in while I'm still useful), but I'd seriously consider it if someone asked.  (And then probably flunk the zero-G tests, or something, and I'm probably not really pioneer material...)


Anyway, now to read on and find that I've been pre-empted (or even refuted) on everything I've just said.



[1] I'm thinking of things as diverse as some of the historic plot background to Babylon 5, in many ways the Red/Green/Blue Mars trilogy even (by extension) the Spacer Worlds of Asimov's pre-Empire/Foundation era time-line, but perhaps a little less so in this latter case.

[Starver]

The moon doesn't make a good staging point... its a gravity well that you need to escape whenever you want to go anywhere. (though i guess with some kind of rail gun it can make things stack up better)

In some ways I agree with you.  However, while it is indeed a gravity well which would need to be escaped again on jumping onwards, it is less of a gravity well.  Still awkward, but it could be that the lifting of resources from there would be 'cheaper', by cumulative measurement of all costs (financial, manning, resources needed in turn for the production, etc).

Not everything moon-based need be on the surface, of course.  Lunar orbit may not be a natural way-point for outward (and inward) travellers, and orbits have are largely stable points part way down gravity wells in themselves, but with a resupply station at that point it could optimise travel outwards.  (Also, with several such options available, buffer against unforeseen accidents, whereas moon-launched resupply modules could be dangerously conspicuous by their absence if something goes wrong.)

And if we adopt lunar-orbit as a proving ground for space-based systems (further from the various effects of the Earth, still close enough for minor emergencies to be dealt with) a lunar base could serve to either provide emergency supplies or become a refuge for 'lifeboats', much easier than could be done with only Earth as an equation.  (Noting that luna-capable lifeboat-technology would be different from the atmosphere re-entering types that we may also need to consider providing.)

The Moon's surface is also suitable for proving some extraplanetary habitation/working systems (living modules, mobile machinary etc), although obviously not against Martian sandstorms or the likes of those Titanous atmospheric pressures and weird atmospheres.  But long-term trials of hermetically-sealed environments are pretty difficult to authenticate on Earth, as proven with Biosphere 2, so (probably with over-engineered refuges as safeguards) finding the ways to construct and maintain practically providable base-structures on the Moon (having some different challenges from zero-G habitations, with circumstances actually easier to deal with in gravity) might be at least a side-project for the any lunar colony that was founded, albeit probably not a primary purpose.


We could of course skip out to wherever else we want purely from geostationary orbit, but that pretty much forces resource-gathering from asteroids (and perhaps careful shepherding of them, down to some practical Earth orbit... with 'oh dear' consequences if errors occur) if you're not going to be sticking to stringing everything out from the surface of Earth instead.


(My points are probably ninjaed, and/or admirably refuted, given how late I am in making this reply.)

[Starver]

Anything bigger [and more dangerous to a space-elevator] can be tracked with radar and appropriate precautions can be taken, such as movable shielding.

Or, indeed, using the natural oscillations of the elevator (and, as necessary, applying thrust to the geostationary[1]/counterweight elements in order to make change its oscillations slightly) to by-pass known threats.


Probably the best thing to do once we have a space elevator is to... make another space elevator.  There are (slight) chances that one that fails will directly hit another, but if something goes wrong[2] then at least you have a second (or third, or fourth) elevator with which to hoist to orbit the materials that (for the very first one) you had to send up by more traditional means.

(Assuming you didn't just capture a suitably-resourced asteroid and get most of your material from that... with the remainder counting towards the counterweight.)


[1] Technically, I suppose, geosynchronous.  There is a difference.  While the two words are often used interchangably, there is a subtle difference that most people don't appreciate.  (In fact, I'm almost tempted to check to make sure I've not got them mixed up.  Almost. )

[2] Those people slungshot out into the solar-system are the ones I'm most worried about...  Very much depending on where the break is.  There'll almost certainly be a launching station at the counterweight, which'll be a comparatively cushy job with at least some (upside-down) gravity to keep the body in shape, but when something goes wrong you're going to want to have independant means to get home housed up there, too!

[Starver]

Welp. That's an eye opener. I didn't know it was even feasible for it to be this thin.
The only other risky thing about having such a space elevator that I can think of, is the danger the part staying in orbit(after separation) would pose.

Unless it's "parts", I foresee the above-the-break segments heading outwards.  As per the immediately prior message.

Actually, let's save on posts, and see if there's any other replies I can add to this one.  (Should have done that earlier, but I'm nearly contemporaneous now, so it should flow better...)

The particular design I think of is the counterweight being extended past geosynchronous orbit but also being some kind of space station that can be used for space-based maintenance of the elevator.

I think docking would still be easiest at the geostationary station (or very close to).  The counterweight station (unless very massive and thus able to be quite close to the geostationary[1] part) would be travelling along an impossible orbit, were it not tethered to the Earth, and thus docking would give problems.  The GS-station could be more or less matched in orbit, and slowly approached without having to do any jolting bird-of-prey-claw-grab manoeuvre (by mechanisms or static hooks on ship or station) at exactly the right time.

Even if, as has been said, it has a counterweight/station like that, there's still the thousands-kilometres long cable swinging somewhat unpredictably attached to it. Furthermore, it's thin enough to not be easily trackable on radar, and massive enough to cause potentially critical damage to any ship that happens to hit it.

The big issue will be the natural vibrations (think 'taut piano wire', plucked and then vibrating for a loooong time, though probably artificially dampened[2])


I must admit that it was in the late '80s when I was most seriously involved in Space Elevator theory, while "we'll use some exotic carbon-based nanotubes, or similar" was far more of a pipe-dream than now (albeit that it still remains somewhat so), so while I don't think the physics has changed much, there may well be new thoughts that I remain unaware of about how to deal with the more awkward aspects of the physics we're still left lumbered with.


[1] Or, indeed, geosynchronous, as previously mentioned.

[2] Placing some form of thrusters along the length, if possible without impeding the carriage of the elevator cars/whatever, would actually aid in both dampening down the oscillations you didn't want and creating localised oscillations to deal with potential collisions.  Some people have had plans to put interceptor missiles in these various nodes, too, but that makes me think of Henry Jones Sr's tail-gunnery efforts.  It'd need so many safeguards, and could still prove to be its own in-built Achilles Heel.

[mainiac]

I don't think that the upper part of a broken elevator would fly off into space.  The center of gravity would be low enough that it would presumably settle into a stable orbit.  I haven't done the math but that's my guesstimate.

[Starver]

Some maths would have to be worked to see what cases did not just shoot off into space.

An elevator whose centre of mass is in GS-orbit would happily sit in that[1] if sheared off at the base, but if that was sheared off any distance up, the CoM would now be in higher orbit, and that (simplifying things) would be travelling faster than the natural orbital speed at that point.  (It's possible that it would have an elliptical orbit whose perigee was at that height, but swinging further out for the rest of the time.)

But most SE-concepts have the counterweight giving significant (FCVO 'significant') outwards pull, such that the SE is in constant tension.  Any cut of the cable (from ground level upwards) gives an outward force.  The maths says that there could be stable orbits, too, but the higher the cut, the more likely it is that the initial tangential velocity gives it escape velocity.

And in the specific case of a multiple-point failure (including deliberate use of demolition charges to create smaller fragments, when an initial severing is created), I'm pretty sure (from maths done by me a long time ago, so open to being wrong, here...) that a counterweight station disconnected from the GS one will fly off out of Earth's orbit.  That would be true unless it was a truly massive counterweight close to the 'mid-point' at GS, IIRC.  Actually, given that the use of the counterweight for launching interplanetary probes (I always envisage a floor hatch, like that of a bomber, and people pulling a lever at the appointed time to just 'drop' them into space) without any additional launching fuel, I think that we're always going to be pressurised (scientifically/economically) to deliberately put the counterweight beyond that critical point, even if we needn't.


BTW, I forgot to say to someone's previous half-question, that space elevators based at non-equatorial latitudes could be stable, only increasingly cumbersome and angular from the vertical at the point they touched the ground.

(Extreme case, to not intentionally refer to another poster's question: very near the pole[2] you might well be able to have a space-elevator travelling effectively horizontal from the anchor-point until it 'carries straight on', where the ground falls away, and heads up towards some sort of GS-like orbital point where a station is actually pulled into a "higher latitude" GS-orbit than a free object would be, and then onwards to the counterweight (again, slightly off of the ecliptic, but less so).  You'd need (roughly, give or take the difference due by Pythagoras, and of the catenary-like 'hang', towards the anchor end) cable of a length from Earth's centre to GS-orbit, however, rather than just cable of a length from Earth's equator to GS-orbit.  On the other hand, if you were happy with a polar-located anchor end being pulled in all directions, not just one, you could set your structure spinning at a speed other than Earth's spin rate, for longer or shorter structures.  Imagine one engineered (with tolerances to account for the perigee/apogee and off-elliptic directional differences, of course) to track the moon!  Good luck with engineering that one, though... )

Noting that while it would be potentially a stable equilibrium, in the perfect case, there'd be those undamped oscillations already mentioned.  For better or worse.  It might still be like working at the end of a very long, slow-period pendulum.  It wouldn't be like standing on top of a pile of milk-crates, where going too far over[3] is easy and leads to the inevitable fall.

I find the Space Fountain concept interesting, but the fact is that a loss of power results in the tower that you have no longer being 'held up' by the stream of whatever-it-is-you're-fountaining and only sitting there while it's unstable equilibrium point is still being stuck fairly close to.  The 100km-plus 'vacuum tube' (present in most ideas of this sort) is probably more destructive on its way down than those fragments of elevator-ribbon would be.  As far as I can see.  The advantage being that you've managed to place it more or less anywhere you want on Earth's surface, so can keep it 100+km away from anywhere that matters.


The speculation is interesting, though.  (And, I still admit, I'm a bit out of date on the whole technology.  Rusty on some of the bits I did once know about, too.)


[1] And, depending on other factors, continue to rotate in "absolute space" at the same speed as the Earth, if it had been perched up there enough to have settled down enough in that regard...  prevailing winds at the base might spoil that, however)

[2] Not at, both because the catenery of the cable would dive below the horizontal at that point, unless you could build a large enough tower to avoid that, and the fact that you're actually aiming at a point slightly below the tangential horizon.

[3] Actually, you could get it going too far over, but that'd take a large amount of effort, in bringing the counterweight past a certain angle.  By that time, you'll have had a lot of warning of what was happening and doubtless send out your Action Heroes Of The Hour to confront the Evil Overlord trying to slowly rain on everyone's parade.  It'd have been easier for him to just hack into the safeguard demolition charge controller/anti-ballistic-threat-missile-system and blow the thing up with its own defences.

[Starver]

Knew I'd have some time (off-line) last night, so grabbed a bunch of figures and tried to work the problem out.

Realised I'd completely forgotten how to make a definite integral, over the last two decades of being lazy and not needing to know, so abandoned my idea of working with lengths of cable (and, indeed, working out what counterweight length of cable (even assuming just plain, constant density cable) would exactly balance the GS-orbit to surface length), but I still ran some 'point' figures.

You don't want all the rambling calculations, I imagine (available on request, I suppose) but... looks like I might have been wrong.

On a hypothetical cable stretching from the equator out towards GS-orbit and beyond I have several 'r' radiuses from the centre of the Earth:
r_gs = 4.22x10⁷ metres for the geostationary point.
r_cw = 4.85x10⁷ metres (for a counterweight end equally distant outwards from r_gs as r_sl for a Sea Level groundstation is inwards).

(Note that a real r_cw would not be as far again out from the GS-orbit one as the GS-orbit is from seal-level, but I need to refresh myself with integration methods before I can make sure that the effective CoG (across the combined gradient consisting of both gravitational (receding, away from Earth) and centripetal (increasing, away from Earth) forces) is Geostationary.)


Anyway, by my calculations, the point r_x (the point, beyond which, rotational velocity is greater than escape velocity and cuts in the cable or just plain 'falling off' guarantee interplanetary flight[1]) is 5.31x10⁷ metres.  Beyond any of the above points.

Additionally, I looked at the perceived gravity (i.e. total force of acceleration away from the planet) at each of several points.

I had previously confirmed that r_gs was zero (and had the correct units) while checking I was using the right formulas and base figures when I started off.
For r_cw, it's 0.089-ish m/s²
For r_x, it is 0.14-ish m/s²

The theoretical length of cable at which gravity is roughly Earth Normal (albeit inverted) of 9.8-ish m/s², r_-E, is 1.84x10⁹ metres from the Earth's centre.  That's 4 to 5 times the distance the moon is at, if the figures I scribbled down earlier are correct.  (And the calculations, naturally.)


But I was doing this on paper, and I probably made some mistake, so feel free to correct me/ask for my workings, if you're at all bothered.  I just thought you'd appreciate less of a "wall of text" than usual.


[1] Missed a trick, there.  I could have worked out interstellar flight too, if I'd taken the appropriate figures home.  Indeed, even a interglactic launch, ever increasing engineering impossibility such a "passive launch system" they would have to be using...

[Il Palazzo]

While your calculations seem correct, I don't uderstand your choice of maximum elevator length(r_cw). Just adding another Earth's radius seems very arbitrary, not to mention that there needn't be any counterweigth - apart from just more cable - at all(and at that distance you'd need something extra than just the cable). The maximum legth will be only constrained by the material properties of carbon nanotubes, and even with today's estimates it would be enough to have the cable stretch way beyond 53 000 km.
Unless you wanted to have r_cw = r_gs*2 - R_E = ~7.8 *10⁷ m, that is the same legth of cable dangling outward from the gs point as connecting gs with the surface as a working estimate, and you simply made a mistake.

[Starver]

Spoiler: Original reply (click to show/hide)

While your calculations seem correct

...phew.

I don't uderstand your choice of maximum elevator length(r_cw).   Just adding another Earth's radius seems very arbitrary, not to mention that there needn't be any counterweigth - apart from just more cable - at all(and at that distance you'd need something extra than just the cable).

Outside of a gravitational/inertial gradient, a cable that is twice as long as "surface->geostationary" would mean that the mid-point would behave as the centre-of-gravity for all concerns.  Thus the arbitrary choice of the r_cw length.  (Exactly so that the 'ascending' cable itself was the counterweight to the descending one.)

But, with such a massive structure being perpendicular to the surface, increasing centripetal forces act upon the upper length of cable even while decreasing gravitational forces act in the other direction, and so a "cable only" structure from Seal Level to Geostationary Height to As Far Again Beyond That seems (to me) to have an effective mid-point (where an equivalent point-mass would orbit representatively of a vertically-stretched Elevator reaching, but not anchored to, the ground) at some point above GS-orbit.  I was thus originally going to calculate the length (less than the given r_cw, but still some way above r_gs) where the 'counterweight' length would reach.  But I've only half remembered the mathematical methods needed to do this, and have been too busy so far today to refresh my memory (I should still have some of my relevant A-Level books at home, somewhere, but probably buried under other junk, given how I didn't find them when I looked for them, briefly, last night).

With an actual counterweight mass on the far end of the cable, it could be even closer, but that could be a more complex calculation.  (Or simpler, if you assume effectively zero-density cable compared to the much more massive point masses of GS-orbit and CW-end and maximum strain capability of the SL-groundstation.)


And all of this is thinking hypothetically.  Indeed, you could have a cable leading out a tad under 93 million miles in length (so as to not quite touch the Sun, once a day, although a longer one would probably only actually need flexing out of the way on sets of days centred around the equinoxes, right?), in theory.  But if you can support a cable down from GS-orbit (at the bottom end, experiencing full Earth gravity) you can certainly support an 'outward' length of less distance (and under less outward acceleration).

They say the first 60 miles is the hardest bit in rocketry...  I suspect something similar can be said about the last 60 miles of your descending tether.

Erm...

Unless you wanted to have r_cw = r_gs*2 - R_E = ~7.8 *10⁷ m, that is the same legth of cable dangling outward from the gs point as connecting gs with the surface as a working estimate, and you simply made a mistake.

Did I copy something wrong, somewhere?  I only just read that end bit properly and...

Yep, I totally cannibalised my originally intended message (copied from paper into notepad... taking most care over the sub/sup BBCode being right, I must admit) before brought into work this morning, and so may have somehow used the incorrect r_cw value[1] when I decided to spare you all the gubbins.  Or perhaps copied the wrong bit off of the scrap of paper I did the original calculations on, earlier on.  Or copied the wrong bit onto the paper in the first place.  Oh well.

I'll go back to my piece of paper tonight (not long now) to see what else I might have gotten wrong.  But as far as bits of the tether being liable to fly off into oblivion?  Well, I thought I was wrong; but I was wrong, I was right! (Pending other big errors, naturally.)  Thus there may well be bits of tether (and, potentially, a counterweight station used for launching probes from) that would have !!FUN!! in the event of a break.  Another (conditional) Phew...   Cheers for that!

[1] Bearing in mind that I'd been working towards an arbitrary length for the counterweight length, before realising my integration was going nowhere and settling on the intended 'as far out again' measure.