h a l f b a k e r yA riddle wrapped in a mystery inside a rich, flaky crust
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Having spent a few very fun hours yesterday abseiling and
seeing a half dozen hot air balloons whilst driving to work this
morning, I naturally put the two activities together.
Therefore, a mass transit system involving an airship and a
lot of zip-line ropes, so one can hook up to a passing
airship
and reel in as it moves on then zipline (fast, head-down
abseil) as you pass over your desired location.
Release the carabiner, roll to a stop, get to your feet, dust
off and jauntily head into the office, whistling tunelessly to
mask your many minor injuries.
That's a great idea! This should be easy!
Land_20luge_20commuter_20lanes
(vaguely related) [normzone, Jul 02 2012]
Alternative means of adding peril to your commute
Chased_20By_20A_20Leopard_20Commute
[hippo, Jul 02 2012]
An even more dangerous way of getting to work
Catapult_20to_20Work
[hippo, Jul 02 2012]
[link]
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Now hiring: Air traffic controllers with experience with balloons and ziplines. (link) |
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I'm sure there would be some who wouldn't enjoy it. |
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The rest of the world seems to know it as Australian
rappelling or "rap jumping". It's a scary thing to do,
but very good fun once you get over your fear of
instant death. |
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That's true. It takes a bit of faith in yourself to walk
face down, down a cliff. |
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It never occurred to me to grow my hair long enough
to get it caught in anything... nor to allow my face
close enough to the descender to catch it on the
rope. |
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Just one small detail ... |
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As each commuter lands and detaches from the rope, the load on the balloon will be reduced by their mass. |
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The balloon will therefore no longer be in equilibrium, and will rise. When the second passenger unclips, it will rise even faster. |
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Now, a helicopter can compensate almost instantly, by dropping the collective pitch ... but a balloon has to vent gas. |
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Allow 100Kg per passenger, including harness, briefcase, bowler hat, tightly furled umbrella and tame porcupine. Drop off two passengers and that's a 2000N upward force on the balloon. For a remaining mass of basket, pilot, 2 passengers and 2 porcupines, estimate 500Kg. That gives a vertical acceleration of 4 m/s-2, nearly half a gravity. |
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In 5 seconds the balloon will ascend 50m, neglecting air resistance (which would be significant, but this is the 1/2B); those ropes better be plenty long ... |
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I didn't say it was a balloon... I said it was an airship.
Entirely different beastie, [8th]. |
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My problem is kind of the inverse. Take that same 100kg passenger. Let him fall at, say 1/2g for, say, 50m. Let him then try to slow himself to a stop in 5m. What's the resultant downward acceleration of the airship (and resultant slower deceleration of the passenger), and how fast does the passenger slam face first into the pavement as a result? |
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// Let him fall at, say 1/2g for, say, 50m. Let him then try to slow himself to a stop in 5m. // |
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That's bungee-jumping, not abseiling. In abseiling, the initial acceleration is modest, and then the abseilee descends thereafter at a constant velocity of no more than 2 m/s. There's no "shock-load". |
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An airship will suffer, proportionately, the same lift as the balloon. Being that much larger, capable of carrying 12 passengers, 11 porcupines and an echidna (fashion victim !), unloading 1.2 tonnes in a matter of a few seconds is going to have an effect, even if the airship is partially reliant on vectored thrust for lift. |
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Depends on the abseiller. Having played counterweight to a much lighter person for a theatrical production, I can tell you that many (free, not cliff face) allow themselves to go much faster. |
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The upward thrust of the balloon will be balanced by launching N-Prize entries from the balloon as its occupants descend to Earth. |
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Can we do with bank executives, but without the zip wire? |
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<notes confusion on abseilee or abseiler> |
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[8th] is right... abseilers don't descend particularly
quickly. Even rappellers would rarely exceed 3-4m/s. |
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// abseilers don't descend particularly quickly. // |
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Or at least they're not supposed to, if they're doing it right. If they get it wrong, then yes, apply Newton's Second Law and substitute the value 9.81 ms-1 for "f". |
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You may neglect air resistance. |
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I don't think venting the lifting gas can be considered elegant. |
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However, as previously mentioned, vectored thrust can also be used, so only an approximation of trim is required.
I can think of several alternatives: |
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1) A specially constructed airship could reduce its boyancy by reducing the size of its floats. Either by constricting the floats, or by pumping lifting gas out of the floats and into compression tanks. |
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2) The commuters could clip on weights when they uncouple from the line. Great if there's stuff which needs to be transported along the route. The stuff would of course need to be winched up reasonably promptly. |
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3) The airship could partially run on hydrogen held in floats, and use that preferentially when underweight. |
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4) The commuters could each have their own small balloon - accounting for a significant fraction of their weight. While this would unfortunately reduce the maximum abseiling speed available, it might mean that they could just jump out and float down. It would also mean that climbing a boarding tower would be less strenuous. |
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I think (3) is my favourite option. |
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[Loris], I agree. You could create an airship covered in blisters, with each blister containing sufficient hydrogen to offset the weight of a particular passenger; upon offloading of that passenger, his/her assigned gas cell can be vented to maintain neutral bouyancy. |
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If done rapidly, it would greatly resemble popping large-cell bubble-wrap all over the airship. |
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//I don't think venting the lifting gas can be
considered elegant. // That is precisely what I told
Sturton. |
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