381–390 of 414 posts
I love this guy. Re playing this gem https://neal.fun/stimulation-clicker/
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 clos…
> it's just hard to go up Going up is the comparatively easy part, it's not exactly rocket science. Going fast enough sideways so you stay up there is the tricky bit.
Some would argue is quite literally rocket science, even if suborbital rockets are much simpler, like they can just burn solid fuel.
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> No maglev train I ever heard of travels at 36,000 km/h. This is about two orders of magnitude faster. You think the problem is the speed itself, and not the fact that trains are close to sea level and at that speed would immediately explode from compressing the air in front of them so hard it can't get out of the way before superheating to plasma, i.e. what we see on rocket re-entry only much much worse because the…
Dielectric strength of vacuum is 20kv/inch. Thus your megavolt needs 50 inches of separation at an absolute minimum. And you're operating this in space where you have ionizing radiation. Free electrons with a big voltage differential? You're describing a vacuum tube.
Breakdown voltage is pressure dependent, not a constant.
Your figure is for (eyeballing a graph) approximately 2e-2 torr and 150 torr, less between, rapidly increasing with harder vacuum. The extreme limit even in a perfect vacuum is ~1.32e18 volts per meter due to pair production.
For a sense of "perfect" vacuum: if I used Wolfram Alpha right just now, the mean free path of particles at the Kármán line is about 15 cm, becomes hundreds of meters at 200km.
Though this assumes a free floating measurement, the practical results from https://en.wikipedia.org/wiki/Wake_Shield_Facility would also matter here.
> And you're operating this in space where you have ionizing radiation. Free electrons with a big voltage differential?
Mm.
Possibly. But see previous about mean free paths, not much actual stuff up there. From an (admittedly quick) perusal of the literature, the particle density of the Van Allen belts is order-of 1e4-1e5 per cubic meter, so the entire mass of the structure is only order-of a kilogram: https://www.wolframalpha.com/input?i=%284%2F3%29π%282%5E3-1%...
If this is an important constraint, this would actually be a good use for a some-mega-amps current, regardless of voltage drop between supply and return paths due to load. Or, same effect, coil the wires. And they'd already necessarily be coiled to do anything useful: Use the current itself to magnetically shield everything from the Van Allen belts.
Superconductors would only need a few square centimetres cross section to carry mega-amps, given their critical current limit at liquid nitrogen temperatures can be kilo-amps per mm^2.
But once you're talking about a 36,000 km long superconducting wire with a mega-amp current, you could also do a whole bunch of other fun stuff; lying them in concentric circular rings in the Sahara would give you a very silly, but effective, magnetic catapult. (This will upset a lot of people, and likely a lot of animals, so don't do that on Earth).
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> just clear the atmosphere But for what purpose? You just want to float around in a elevator carriage, aren't you going somewhere when you go with a space elevator?
Presumably the elevator would lift cargo, fuel, maybe even the actual rocket into orbit where maneuvering is easier. Once it's in space and in orbit, substantially less fuel is required maneuver.
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Presumably the elevator would lift cargo, fuel, maybe even the actual rocket into orbit where maneuvering is easier. Once it's in space and in orbit, substantially less fuel is required maneuver.
I think you might be talking about something else than a "space elevator"? A space elevator would just go up and down as far as I understand, not horizontally, and if you wanna end up in orbit, you need horizontal movement, not vertical.
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You can reach space using air breathing jets. You can’t stay in space using air breathing jets.
Didn’t Chuck Yeager burn his face off doing that?
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Thing is, you're going to invest massive amounts of R&D in something that might be impossible, when you could invest in actually building stuff in space so you only need to shoot humans out there.
Instead of massive amounts of R&D, why not small amounts for the next 100 years? Seems like this is something a tech bro could bequeath to humanity, Nobel style. A $250 million fund that pay out $10 million per year in grants. And maybe some larger chunks when the board of directors sees a good opportunity to do larger scale trials.
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> If you somehow manage to get magnetic fields involved, so you are not afraid of friction with the cable itself, at 1.3 max apparent acceleration […] This means that half-way after 58 minutes, the climber is traveling at 0.3 * 9.81 m/s² * 60 * 58 ~= 10.2 km/s ~= 36,720 km/h (!!!) relative to the cable. A tiny imperfection or wobble is going to make the climber crash into the cable, destroying both. A climber with a…
Oh, and 10 tonnes is bus-sized. For infrastructure at that scale, you want trains at the very least, and those are on the order of 1,000 tonnes. Multiply force, power, and current by 100 accordingly.
From what I've seen nobody currently directly launches more than 4.9 tons direct to GEO (Vulcan Centaur VC4). Starship is supposed do 27 to GTO (not GEO) when finished, but it's not finished.
If a space elevator lasts long enough to amortise the construction costs (nobody knows, what with them not being buildable yet), they would represent an improvement on launch costs relative to current methods, even if you were limited to 10 tons at a time and each GEO being a 2 hour trip.
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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
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 goog…