0697: "Tensile vs. Shear Strength"

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po2141
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Re: "Tensile vs. Shear Strength" Discussion

Postby po2141 » Fri Feb 05, 2010 9:06 pm UTC

Thats true, I hadn't thought about not having to carry fuel with you.

The only problem I have with the whole thing is that you'd need to apply a force to your cargo perpendicular to the cable, in the direction of the earths rotation. Because you arn't just hoisting the cargo up, we are standing on a revolving platform here people!

Its the Coriolis effect, I just remembered.

Its gonna be complicated to do that with solar panels and lasers.


@project2051:
Assuming you can afford to build a space elevator that is.

I guess the idea must be once its built you can start hauling stuff up 24/7 until you break even.
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Re: "Tensile vs. Shear Strength" Discussion

Postby OBloodyHell » Fri Feb 05, 2010 10:13 pm UTC

The energy to send your cargo flying into space still has to come from somewhere, the angular momentum of something in geosynchronous orbit is pretty spectacular. As you hoist something up, there will be a "draggy" effect as in order to keep station with the cable, your cargo has to be accelerated perpendicular to the cable, in the direction of the Earth's rotation.


You know, listening to people talk about orbital towers who don't actually know anything about the design proposals is amusing.

Most designs assume that there will be two elevators which raise/lower "in sync", one relinquishing potential energy while the other gains it, and the two meet/leave from the Geosync centerpoint. Energy use will thus be confined to overall transmission efficiencies/losses, i.e., entropy.

The issues with a space elevator which make actual creation unlikely tie far more to its potential danger as a terrorist-style target as well as the overall efficiency of use of risk capital.

Its attractiveness ties to its low costs to-orbit/from-orbit, which would be almost completely superseded should someone develop a reactionless drive system able to lift to orbit without reaction mass. (Energy being unlikely a problem with that tech available). While one can't predict such tech's development, the capital cost of an SE is rather prohibitive, and subject to "boondoggle" if it either failed financially in mid-build (30+ year capital reliability is hardly a foregone thing) or was superseded by some "supercool" tech, foreseen or unforeseen.

The danger of an SE lies not in someone "cutting the cable" close to ground but a catastrophic failure (likely caused by terrorist action) higher up. The potential energy represented by the cut-end of just 80km of earth-bound cable whipping down (which is what it would do) isn't pleasant. Now realize that there is 42 THOUSAND km of cable this side of the center point -- enough to wrap all the way around the earth. Not Pretty. That whip-crack you hear is the end of civilization.

Space Fountains are even more concerning in this regard, by my lights.

Don't get me wrong, I love the concept, it's the problems with it given the nature of humans and human social constructs that make it unviable.

I also think that a laser-launch system is a better mechanism for getting mass-to-orbit, esp. since you really, really want to get most of your mass from Out There and not ship it up out of a deep gravity well.

I think the Space Elevator was a favourite of Clarke's - I've only read The Last Theorem but the concept features in there quite heavily. Not a great book, incidentally, I was left distinctly underwhelmed given his reputation.


Between you and me, I haven't really been all that impressed with his later books. They are ok, but not up to the standards of his early-mid ones. This can be said for Asimov, too, and even, to a lesser extent, Heinlein (though RAH's style and focus varied at least three times in the course of his writing career, so it's apples-to-oranges in his case).

I think the problem with choosing this book as representative is the presence of Frederick Pohl as a co-author. I've read several of Pohl's books, and my response in each case was, "Meh." -- not bad, not particularly good, either. So it may wall be Pohl's influence which is at fault in your lack of impression of the book in question, as well as Clarke's advanced age -- this had to be one of his last.

I personally think Clarke is more notable for his important seminal concepts than his writing, and that makes him the least impressive of the "three Deans". I'd rate Niven AND Niven/Pournelle as better, more interesting writers to read, along with a host of others.

But The Fountains of Paradise was a great read just for the feel of the concept it promoted. It was a bridging ground between Very Large Structures (which we are unlikely to live long enough to see) and earthbound structures. Someone alive today could theoretically see an Orbital Tower. You won't see a Ringworld.

There is NO SUCH THING as CENTRIFUGAL FORCE


Which is
a) Categorically true
b) Categorically irrelevant to pretty much everyone except some purist physics types and some very special-scenario operators.

The distinction between centripetal force, inertial effects, and so-called "centrifugal force" isn't one most people give a rat's patootie about.

I see the term all the time, I might even use it occasionally myself, despite having about 50% of my college credits at calculus and above (another chunk in physics before I switched away). It's a geek issue. It doesn't matter to anyone in casual usage outside of geeks. Calling attention to its usage outside of an actual professional-level discussion (i.e., not this kind of informal BS session) marks one as a Sheldon-style geek. NTTAWWT. :P

Surely for a trip
to Mars
You'd need over
9000 jars.


Clearly you did not read the successor entry:

If a trip
to Mars you earn,
remember, friend,
there's no return

(Turns out Mars is in Germany somewhere.)


Yes, I think the cable would just dangle there


Ah, back on topic. Somewhat, yes. The cable would drift outward slightly based on the new center of mass/velocity in balance (i.e., it would seek a higher orbit based on the new "centermass speed".) More below.

punaccetable...


I have but one thing to say about pun humor:

"Pundits' A plaudit."

Seriously, how could you even -begin- to defend such a fragile thing from an opponent bent on destroying it ?


While I agree with the concern, it's nowhere near as fragile as it appears for the most part, depending a lot on the nature of the design, so any claims would be premature. I'm not saying it can't be broken, I'm just saying it's not QUITE as easy as Randall suggests. The most likely danger would be to use a nuke to break it apart well up along the way. As bad as a nuke might be thought, 10,000 km of cable whipping down would be much, much, much worse.

Im sure there are more efficient ways of causing havoc on earth that dont involve going into space first


Failure to grasp what the real nature of the danger mankind actually faces is.
The short form:
Tool=weapon -- hammer==club, screwdriver=knife(stabbing).
The better your tool, the greater your weapon. It is exceptionally difficult to design tool so that it cannot become weapon.
Central to tools is energy. The more energy in a tool, the more effective it is likely to be, but also the more capable it would be as a weapon.
So -- increasing the power of tools makes the power of the individual to perform greater and greater duties larger.
So, too, does this increase the power of the weapon in the hands of an aberrant individual or group operating under aberrant memes.
I've already noticed one prime example which someday I expect to see, which, AFAIK, no one has thought of yet (thankfully) -- and no, I'm not detailing it here.
Planes are wonderful tools. They're also, rather clearly, not particularly ineffective weapons.

Back to the original quote -- The total potential and kinetic energy involved in a space elevator (heck, involved in space travel) is literally phenomenal. When you say "more efficient ways" you don't grasp the sheer potential schrecklikeit you're at the heart of just by being Up There.

Hint: Orbital velocity is FIVE MILES PER SECOND. It's a simple calculation -- E= ½mv2

A large portion of the cable (dropped from geosync) is at a sizeable portion of orbital velocity, and with the cable being made out of carbon, alot of it will burn up.


I confess I haven't looked at the numbers, here, but it seems improbable. The cable is initially forced to be stationary, and, as it "falls" the coriolis effect will wrap it around, and even a short whip from that striking in the oceans would produce enormous tidal waves. I'd also suspect that the fall/wraparound would probably be slow enough that more than a trivial part of it would build up relative speed slowly and not wind up burning up. Another obviously relevant question would be the thermal conductivity of it. Not sure what the thermal conductivity of nanotubes is, offhand. Further, at least some designs have attached superconducting cables to do the lifting electromagnetically, and, since all electrical superconductors are also thermal superconductors, all heat becomes spread across the whole length of the cable... some part of which is likely to be underwater by the time another part of it hits the lower atmosphere enough to actually heat up.

It is stationary w.r.t the atmosphere while ...


Oh, dear. Look, the thing to realize is that it resembles a steel rod in orbit. The natural orientation of such is "vertically", with its centerline pointing through the center of the main orbital mass. Even if you "start" in a "horizontal" position (i.e., roughly in the direction of orbit) any perturbation (and there will be some) will cause it to drift "out of stability" and into its naturally stable orientation (stable balanced on a pin vs. stable at the bottom of a bowl).

The center of mass of the orbital tower is traveling in the correct orbit for the mass -- geosynch by design (not absolutely correct but close enough for this discussion). The "outer" end is traveling too fast for the orbit it is physically forced into, and wants to go "out". The "inner" end is travelling too slow for the orbit it is in, and so wants to go "inward". Hence it is an object under tension. It's been suggested that the best imagery is more like a suspension bridge than a "tower" acting against gravity. Now, if you lop off, say, the bottom fifth of it, the center of mass shifts upward. That center of mass is now going too fast for the orbit it is in (which is higher, of course) and so it drifts outward until it is going at the correct speed for the orbit, all the while retaining its "up/down" orientation. In short, it falls "up",not down, when cut off. (mind you, this is assuming it's cut on the planetbound side of its synchronous orbit)

It's the now unsupported earthbound end which is the dangerous part -- it still has all the potential energy which was tied up in it being supported there, AND is moving too slowly to stay there. It begins to fall, not straight down but along the equator, and the endpoint, when it strikes will still have orbital velocity, and a mass, even a small one, moving at orbital velocity is not an insignificant one. Consider Project Thor. Flying crowbars fully capable of taking out any armored platform on the planet.

So if the same energy would be required to get something into orbit with or without the space elevator then what's the real advantage of making it? Just the cool factor?

The article Mavrisa linked to pointed out you can reduce the weight of the spacecraft by 95% by not carrying any fuel with you.

The most obvious designs use mag lev to run two elevators in sync to and from the center. This allows the potential energy surrendered by one to provide energy for raising the other (much like a regular cable elevator uses a counterweight). The efficiencies are thus potentially massively greater than just the lack of need for carrying fuel, though that, too, is realized, and is hardly trivial.

I guess the idea must be once its built you can start hauling stuff up 24/7 until you break even.


Let's just say that, if you ever wanted to get an actually substantial percentage of humanity off-planet, it would likely require something in the elevator/fountain category. The potential carrying capacity of the thing is ginormous.

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Eikinkloster
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Re: "Tensile vs. Shear Strength" Discussion

Postby Eikinkloster » Fri Feb 05, 2010 10:43 pm UTC

po2141 wrote:@Eikinkloster

Nono, I meant a cable from the earths surface to an anchor at geosync - only the very top of the cable is in orbit - the higher you are the faster you need to be travelling to maintain orbit, until you escape altogether. The lower portions of the cable are not rotating sufficiently fast about the earth to maintain orbit at their lower height and so their weight hangs off the anchor.

This weight can be countered by spooling cable from the anchor outwards, if it is straight, it will be rotating around the earth faster than orbital velocity at its heigher altitude and so will exert a "centrifugal" (NB: I know, but you get me right?) force on the anchor, balancing the weight of the lower length.


Yes, *if* the lower portions are at the same angular velocity, the lower portions will be pulled down. If you keep the tangent velocity, your cable won't go down, simply, and you'll never have the cable moving in relation to the atmosphere. To make the cable go down you need to break it's tangent velocity. It will keep the angular velocity, and lose in radius, i.e., it will go down. Then, yes, it will break orbit, and keep going down because of the pull of gravity while keeping it's tangential velocity... I had actually ignored this part. But you'll just have to keep breaking the tangent velocity down to keep the angular velocity. Once the bottom anchor reaches the atmosphere friction will help breaking the speed down and keeping the whole system at the same angular velocity. To counter the weight of the cable and bottom anchor, you can simply thrust the whole top anchor up and ahead.
Angular momentum is the complicating factor here as it varies greatly from the bottom to the top of the lift. There is no "free" effort being provided by using a lift to get to orbit, you'd still need a comparable amount of kinetic energy to get there than you get from, say, a big ass-rocket. The anchor at geosync is moving at roughly 3km/s relative to the earths centre of mass, and so will any cargo have to be to reach it.


Yes, but that energy won't be supplied by the lift, since it is moving perpendicular to the tangent. It's the cable itself that will be accelerating the lift tangentially. Of course that kinetic energy will have to come from somewhere: the top anchor.
I don't think I understand the physics to try to prove it, but I believe the top anchor will then just rob the kinetic energy back from the planet itself. In this scenario the cable won't be in a 90º from the ground, but inclined against the rotation of the Earth.
The only energy that will have to be provided to the lift will be mgh.
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Re: "Tensile vs. Shear Strength" Discussion

Postby Eikinkloster » Fri Feb 05, 2010 10:46 pm UTC

po2141 wrote:Its the Coriolis effect, I just remembered.


No Coriolis when you're over the equator...
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Re: "Tensile vs. Shear Strength" Discussion

Postby Eikinkloster » Fri Feb 05, 2010 11:22 pm UTC

OBloodyHell wrote:You know, listening to people talk about orbital towers who don't actually know anything about the design proposals is amusing.


To talk about stuff without actually knowing anything about it is amusing too, and more than half of the point of discussing in xkcd and not in some expert level board.

So -- increasing the power of tools makes the power of the individual to perform greater and greater duties larger.
So, too, does this increase the power of the weapon in the hands of an aberrant individual or group operating under aberrant memes.
I've already noticed one prime example which someday I expect to see, which, AFAIK, no one has thought of yet (thankfully) -- and no, I'm not detailing it here.
Planes are wonderful tools. They're also, rather clearly, not particularly ineffective weapons.


Well said. This will be probably even more true of any super cool tech that supersedes a space elevator btw.
We are living on borrowed time. We're already very close from making commercial aviation inviable. How long before it will be really futile to try to prevent people with explosives from boarding airplanes? The Nigerian guy didn't blow up by sheer luck. How long before nukes become as accessible and portable as C4?

At that point, civilization will suffer a reversion or some sort. Either total barbarism, to a point in which nukes or even C4 will be very hard to manufacture, or some sort of totalitarism that will annihilate, Machiavelli style, most high profile aberrant memes.
Chaos Reigns!

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Re: "Tensile vs. Shear Strength" Discussion

Postby phillipsjk » Fri Feb 05, 2010 11:38 pm UTC

po2141 wrote:The only problem I have with the whole thing is that you'd need to apply a force to your cargo perpendicular to the cable, in the direction of the earths rotation. Because you arn't just hoisting the cargo up, we are standing on a revolving platform here people!


Power = Work/ time

Bradley Carl Edwards wrote:Ascending at 190 km per hour, the climbers would reach geostationary orbit in about eight days


We are talking days, not minutes or hours. You can presumably use the anchor platform to drag it back into position. The earth will lose a little angular momentum with every launch.

Edit:
Thus a “20-ton” Elevator is one that can support a single 20 ton climber at ground level. Typically, this means that the tether weighs 4000 – 6000 tons, and the climbers will actually weigh around 15 tons (since we have multiple climbers simultaneously on the ribbon).

- http://spaceward.org/elevator-feasibility

I wonder how traffic control would work? What if you want to climb back down? Don't current proposals grip both sides of the ribbon?
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Re: "Tensile vs. Shear Strength" Discussion

Postby po2141 » Sat Feb 06, 2010 12:52 am UTC

Eikinkloster wrote:It's the cable itself that will be accelerating the lift tangentially.

this is what concerns me, I just thought it might be problematic. Thats not to say not possible.

And I think someone said something about balancing mass going up and down at the same time? I didn't think of that, thats clever.
And something about it taking 8 days to get to geosync, that'd help too.


Eikinkloster wrote:
po2141 wrote:Its the Coriolis effect, I just remembered.


No Coriolis when you're over the equator...


Yes there is. The coriolis effect is caused by the difference in angular momentum dependant on your distance from the axis of rotation.

@OBloodyHell:
I admit, I only read your post through once, so forgive me if I misquote.

Hint: Orbital velocity is FIVE MILES PER SECOND.

that is closer to escape velocity

orbital velocity is roughly 3km/s (about 1.9miles/s) but yeah, a lot of energy.

The lower tens/hundreds of km of cable notwithstanding. if say, the bottom half of the cable ie:the lower 35000km-ish, were to come loose, dont you think that much mass, travelling at that speed would stay in orbit?

Ok, so lets do 100km falling, lets say 100kg/m of cable so 10000tons. the bottom 100km isnt rotating too fast so lets just treat it like a column, or better yet a point mass at 50km height, disregard air resistance and the PE=1e7*10*5e4=5e12J which happens to be just over a Gigaton of TNT equivalent.

But this isn't going to result in an explosion, it will be dissipated in myriad ways, possibly over the entire length of the cable depending on how it falls, and certainly over a period of time - the top of the fragment will take a fair few minutes to reach the ground, and there's air resistance. Bottom line, its not all going to flash into heat all at once. A nuke releases its total energy in a handful of microseconds and in a volume not much larger than the bomb itself. The cable will have a volume of, ooh lets say, 1e4 cubic metres? And it might take 10 minutes (straight-up guess) for it all to reach the ground? Thats all very abstract and probably terribly innaccurate. But do you think there will be a mushroom cloud?

Bits of rock with this much kinetic energy hit the earth all the time - ok so that count for the higher bits of cable more I suppose.

I agree it'd be bad - real bad, but "civilisation ending" I don't believe. And I guess you're right, this seems like more bang-per-unit-effort than building your own nuclear weapon and driving it to the middle of a city. That's if you don't need a nuke to break the cable I suppose.

and the endpoint, when it strikes will still have orbital velocity


No it won't, it was never going at orbital velocity in the first place but significantly slower, plus its got to get through the atmosphere too.

The project thor crowbars will be going at orbital velocity, and would be specifically designed to withstand the heat of re-entry, possibly with the aid of ablative shielding. Would the cable be clad in a heat-shield?

Erm..Im a bit nervous that this whole exchange might seem a little hostile, please take it as friendly discussion :)

A couple more friendly questions:
You dont think that the higher section would be subject to burning up in the atmosphere?
Anyone care to hazard an opinion on my suggestion that a large portion might stay in orbit? Albeit a much-lower-than-geosync one?


And this is about as long as a post I can make before I start forgetting what I wrote at the beginning!
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Re: "Tensile vs. Shear Strength" Discussion

Postby Eikinkloster » Sat Feb 06, 2010 6:07 am UTC

po2141 wrote:
Eikinkloster wrote:It's the cable itself that will be accelerating the lift tangentially.

this is what concerns me, I just thought it might be problematic. Thats not to say not possible.


Yes, I wasn't understanding this was your concern. I didn't get yet why it is a concern, though.

And I think someone said something about balancing mass going up and down at the same time? I didn't think of that, thats clever.
And something about it taking 8 days to get to geosync, that'd help too.


I also fail to see how this helps with the cargo. The balancing mass will balance the mass of the lift. not that of the cargo. Unless we have a continuous supply of cargo from space to bring down...

po2141 wrote:
Eikinkloster wrote:
po2141 wrote:Its the Coriolis effect, I just remembered.


No Coriolis when you're over the equator...


Yes there is. The coriolis effect is caused by the difference in angular momentum dependant on your distance from the axis of rotation.


Woops. Yes, I'm too lazy to try to really understand, but from what I could grasp browsing Wikipedia, that is Coriolis we're talking all along.
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Re: "Tensile vs. Shear Strength" Discussion

Postby accessory to oranges » Sat Feb 06, 2010 6:45 am UTC

project2051 wrote:
Pierrot wrote:For some reason I am not shocked that the elevator was cut. What I'm worried about is that who is going to clean up the 80km of cable that is connected to the counterweight moving at rotational speed. Which we all know it's going to inevitably cause tremendous damage.


Kim Stanley Robinson's Mar trilogy made good use of space elevators, including what happens when you let one fall on the planet.

that's my favorite sci fi book series ever, not only does it hold realistic elements in it, such as the space elevator, but it deals with the very real problems that we may face with the onset of "immortality"
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Re: "Tensile vs. Shear Strength" Discussion

Postby ZeroSkulleton » Tue Feb 09, 2010 3:14 am UTC

I can actually imaging that being sung to the tune of "How I hate the Night".

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Re: "Tensile vs. Shear Strength" Discussion

Postby Ephemeron » Wed Feb 10, 2010 9:18 pm UTC

I've thought of a material that could be used to make space elevators if carbon nanotubes aren't available. Where can you find that 'light inextensible string' usually seen in mechanics problems? You know the string I'm talking about! It would have the unique property of being able to take infinite tensional force, in addition to being weightless. In fact a long coil of it would simply float off into space. Just fly a big counterweight into geostationary orbit and... oh it would be blissfully easy to build a space elevator with this string, if only it existed...

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Re: "Tensile vs. Shear Strength" Discussion

Postby dennisw » Thu Feb 11, 2010 2:16 am UTC

Ephemeron wrote:I've thought of a material that could be used to make space elevators if carbon nanotubes aren't available. Where can you find that 'light inextensible string' usually seen in mechanics problems? You know the string I'm talking about! It would have the unique property of being able to take infinite tensional force, in addition to being weightless. In fact a long coil of it would simply float off into space. Just fly a big counterweight into geostationary orbit and... oh it would be blissfully easy to build a space elevator with this string, if only it existed...

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Re: "Tensile vs. Shear Strength" Discussion

Postby mszegedy » Thu Feb 11, 2010 5:37 am UTC

wisnij wrote:
fabiocbinbutter wrote:My theory is that a space elevator is plausible, but the material itself would not need to support the weight of the structure, it would merely be a channel to carry high-energy ions that do all the load bearing, both of the payload and the structure.

My theory is that you don't know what any of those words actually mean.


I think (s)he only doesn't know what ions mean.

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Re: "Tensile vs. Shear Strength" Discussion

Postby DaveMcW » Thu Feb 11, 2010 11:10 pm UTC

po2141 wrote:there's air resistance. Bottom line, its not all going to flash into heat all at once. A nuke releases its total energy in a handful of microseconds and in a volume not much larger than the bomb itself. The cable will have a volume of, ooh lets say, 1e4 cubic metres? And it might take 10 minutes (straight-up guess) for it all to reach the ground? Thats all very abstract and probably terribly innaccurate. But do you think there will be a mushroom cloud?


It's a ribbon!

Air resistance will slow it down to a few miles per hour, if it doesn't burn up in a cloud of carbon dioxide first.

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Re: "Tensile vs. Shear Strength" Discussion

Postby po2141 » Thu Feb 11, 2010 11:33 pm UTC

DaveMcW wrote:
po2141 wrote:there's air resistance. Bottom line, its not all going to flash into heat all at once. A nuke releases its total energy in a handful of microseconds and in a volume not much larger than the bomb itself. The cable will have a volume of, ooh lets say, 1e4 cubic metres? And it might take 10 minutes (straight-up guess) for it all to reach the ground? Thats all very abstract and probably terribly innaccurate. But do you think there will be a mushroom cloud?


It's a ribbon!

Air resistance will slow it down to a few miles per hour, if it doesn't burn up in a cloud of carbon dioxide first.



I tend to agree about the air resistance part.

I've seen it talked about as a ribbon, why a ribbon instead of a cable?
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Re: "Tensile vs. Shear Strength" Discussion

Postby WhiskerTips » Fri Feb 12, 2010 5:04 am UTC

cool

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Re: "Tensile vs. Shear Strength" Discussion

Postby hintss » Fri Feb 12, 2010 9:34 am UTC

FriedSushi87 wrote:I live near Disney and they have this balloon ride. You pay money to go 300 feet up in the air in a hot air balloon that is tethered to the ground similar to the comic. The entrance to the thing is 5 feet from the balloon. I'd imagine someone could easily conceal some bolt cutters or something and send 50-100 people flying in the wind across Disney World


here in orange county in CA, there is a giant helium baloon ride for free! no security, and one proabably 3" cable. and no perimeter fences. it'd be easy to sneak in at night and loosen some stuff...


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