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Space Elevator

(neal.fun)
1773 points kaonwarb | 1 comments | | HN request time: 0s | source
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jvanderbot ◴[] No.45643427[source]
Very cool. One thing I wish was better shown: space is close, it's just hard to go up. Our liveable breathable atmosphere is razor thin compared to the size of earth.

In most cases, 100km is less than the distance between sizeable metropolitan areas. It's a day long bike ride. Air runs out less than a bus ride across town. A 15k jog/hike would put you in the stratosphere. Those jet aircraft that seem so high are closer than that. Closer than your friends house or the local stadium probably.

Look at a map or globe with that in mind and everything feels so thin!

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amflare ◴[] No.45646338[source]
For a standard globe that you might see in a classroom, the Earth's atmosphere is about as thick as the paper glued to the outside that displays the map.
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mk_stjames ◴[] No.45647864[source]
That didn't sound right to me, and so I checked it as follows:

Estimate for a standard classroom globe at 13" in diameter (I'm seeing a rnage of 12-14 inches as typical). I'm reporting in inches because that is what came up first and most of the globes are for sale in the US. Mixing units here, but, it works out.

But, in meters, the diameter of the Earth is 12,742,000 m on average. if we use the 'Karman line' as defining the edge of what the atmosphere is, that is 100,000 meters. Solving for X ... (13" / 12742000 m)=(X / 100,000 m). gives us an atmosphere thickness of approximately 0.1". -----

Paper glued to the globe would have a thickness of maybe, 0.004" (thin paper) to 0.012" (like a card stock paper).... so that analogy is off by an order of magnitude or more.

Even if you use the mesosphere as the definition for the top of the atmosphere, that is still 85,000 meters and thus similar.

People can check the numbers I used.

* Perhaps the analogy should go more like: the thickness of the cardboard sphere the globe is made out of is about the thickness of the atmosphere. Because, having completely destroyed a globe once in my youth, I remember the cardboard shell being approximately a tenth of an inch thick. But, that's maybe not a great reference for the analogy because not everyone has cut apart a classroom globe....

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shagie ◴[] No.45648219[source]
https://en.wikipedia.org/wiki/Atmosphere_of_Earth#Pressure_a...

90% of the atmosphere is below 16 km.

16 km * (12" / Earth diameter) :: https://www.wolframalpha.com/input?i=16+km+*+%2812%22+%2F+Ea...

0.015 inches, 0.38 mm

... and tossing sheets of paper into that ( https://www.wolframalpha.com/input?i=thickness+of+paper ) ...

16 km * (12" / Earth diameter) / thickness of paper :: https://www.wolframalpha.com/input?i=16+km+*+%2812%22+%2F+Ea...

4

Note that that's copy paper rather than card stock...

Adjusting this to 5.6km (the 50% atmosphere amount) ...

5.6 km * (12" / Earth diameter) / thickness of paper :: https://www.wolframalpha.com/input?i=5.6+km+*+%2812%22+%2F+E...

1

So it's a matter of selecting the proper globe, proper paper, and proper threshold for the atmosphere.

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vee-kay ◴[] No.45648490[source]
I just love such nerdy debates on HN on a hypothetical scenario/example.

I think this thread would also be loved by the nerdy folks at https://Reddit.com/r/theydidthemath

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shagie ◴[] No.45649109[source]
If I recall correctly... my very first post on Reddit was doing calculations for a (practically immortal) person eating beans and storing the flatus for a trip to the moon (searching shows that this is a not-infrequent request). It was only concerned with quantity - not storage or the engine.

... and the source document for the numbers was based on a paper that is fairly easy to find given the proper keywords in google search... https://pubmed.ncbi.nlm.nih.gov/1648028/ (and I learned that methane more rare in flatus than not).

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LorenPechtel ◴[] No.45652344{6}[source]
Did you account for the weight of storage? Because I would think the tyranny of the rocket equation for such a low ISP would cause various levels of impossibility, not to mention the problem of getting enough thrust.
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1. shagie ◴[] No.45652418{7}[source]
It was based on “how long would it take to fill the tanks of a Saturn V?” … with the additional “let’s pretend we can 1:1 swap methane and kerosene.”