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Space Elevator

neal.fun

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Re: Space Elevator

#401

Post by shagie »

andai wrote: post #390
shagie wrote: post #316

Earlier quoted context omitted.

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…

Well you just activated a neural pathway that's been dormant for several decades... you wouldn't happen to remember the result would you? ;)

I think the scale was on the hundreds of thousands of years. We're dealing with 700 ml of hydrogen and 70 ml of methane at standard pressures and scaling this up to 90,000 kg of hydrogen and 635,000 kg of kerosene (with the 1:1 methane).

Re: Space Elevator

#402

Post by matheusmoreira »

CaptainOfCoit wrote: post #385
matheusmoreira wrote: post #329

Earlier quoted context omitted.

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.

According to this comment:

https://news.ycombinator.com/item?id=45646741

Ascending the elevator will produce 0.002g of horizontal acceleration throughout the trip.

Re: Space Elevator

#403

Post by notahacker »

gizmo686 wrote: post #338
notahacker wrote: post #236

Earlier quoted context omitted.

Unless we're using it for humans the transit time isn't that big a deal; "last mile" orbital transfer times are often measured in days anyway. That "last mile" bit is going to entail independent propulsion anyway. Getting to the altitude if the ISS is a mere 10 hour trip at a sedately 40kph which isn't unpleasant even for humans, but the ISS orbits at nearly 29000kph (as will you if you let go of the space elevator a…

> as will you if you let go of the space elevator at that altitude) Doesn't the teather have a constant (24 hour) rotational period at every elevation? That is significantly slower than the ISS

fair point, you'd need to be orbiting at that speed to stay in that orbit, but you'd need propulsion to get the delta v to get there after letting go of the tether, but a lot less than to launch from ground level through the atmosphere. Or you could figure out the point higher up the tether to release where your orbital decay would intersect the IS orbits, but given the precision involved in that rendezvous you'd still want propulsion. You'd want propulsion for the last mile bit for pretty much anything other than building a station attached to the tether was kind of my whole point :)

Re: Space Elevator

#404

Post by jvanderbot »

LorenPechtel wrote: post #354
_carbyau_ wrote: post #349

Earlier quoted context omitted.

That whole "tyranny of the rocket equation" thing is why I am surprised the actual first stage for launching a rocket is NOT a ground based reusable "up-chucker". Basically, I would have thought that any momentum that can be imparted to the rocket before it has to rely on its self propulsion would be a huge help. Not talking about eliminating self propulsion, just an assist so the rocket could carry a larger payload…

Building your rocket to survive the upchucker costs more than the savings from being upchucked. Chuckers are the optimal large scale solution for airless bodies, but they're horizontal. You spread the acceleration out over a very long distance so you don't need a super beefy spacecraft and your humans won't turn to goo. Basically, a maglev train except it has track above as well as below and it doesn't have a maximum…

Except that you save maybe 30% of the cost to just launch from Earth. Once youre off planet you're over half way to anywhere, and you don't need to land on the Moon to go further

Re: Space Elevator

#405

Post by ekropotin »

Oreb wrote: post #104

How do we know how high pterodactyls were flying?

Scientists created a detailed virtual reconstruction of pterodactyl’s body and then feed it to computer simulation to estimate flight speed, endurance and altitude.

Re: Space Elevator

#406

Post by LorenPechtel »

ben_w wrote: post #384
LorenPechtel wrote: post #372

Earlier quoted context omitted.

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.

> 20kv/inch 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 i…

No, I wasn't eyeballing, but perhaps someone else was. I went looking for the dielectric strength of vacuum and I found a chart with values for a bunch of different things including vacuum.

And I don't understand the connection to the Van Allen belts--I'm talking about sunlight knocking electrons off your conductors.

Re: Space Elevator

#407

Post by kaonwarb »

hermitcrab wrote: post #215
raducu wrote: post #178

Earlier quoted context omitted.

> once we're up, its just a matter of force over time to create a nice orbit. It depends what you mean by "up there". ChatGpt tells me you'd free fall from 1000 km to 100km in about 8 minutes. It also did the math that you'd need 1.65G of sideways thrust to reach orbital speed. That's quite a bit of force for spacecraft sized objects. If you have an actual space elevator, sure, you can go to close to geosynchronous a…

>ChatGpt tells me you'd free fall from 1000 km to 100km in about 8 minutes You trusted an LLM to do the maths when it is just s = 5t^2?

Not quite; g drops materially between 100 km and 1000 km. 8 minutes I believe is quite close, whereas 5t^2 (or even 4.9t^2) will lead to underestimating time.

Re: Space Elevator

#408

Post by ben_w »

LorenPechtel wrote: post #406
ben_w wrote: post #384

Earlier quoted context omitted.

> 20kv/inch 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 i…

No, I wasn't eyeballing, but perhaps someone else was. I went looking for the dielectric strength of vacuum and I found a chart with values for a bunch of different things including vacuum. And I don't understand the connection to the Van Allen belts--I'm talking about sunlight knocking electrons off your conductors.

> No, I wasn't eyeballing, but perhaps someone else was.

I didn't say you did with that parenthesis, that was to indicate I was being very approximate with the pressures that correspond to your stated breakdown voltage: https://www.accuglassproducts.com/air-dielectric-strength-vs...

> I went looking for the dielectric strength of vacuum and I found a chart with values for a bunch of different things including vacuum.

That's even more wrong than looking up the value of acceleration due to gravity and applying "9.8m/s/s" to the full length of a structure several times Earth's radius (which was also being done in these comments).

Think critically: when you're reducing pressure, at what point does it become "a vacuum"? Answer: there is no hard cut-off point.

(Extra fun: https://en.wikipedia.org/wiki/Paschen%27s_law)

> And I don't understand the connection to the Van Allen belts

You mentioned free electrons. The thing Van Allen belts are, is fast-moving charged particles captured by Earth's magnetic field.

> I'm talking about sunlight knocking electrons off your conductors.

Very easy to defend against photoelectric emission.

Just to re-iterate, if you're lifting something up with a magnetic field, it's non-contact. You can hide the conductors behind any thin non-magnetic barrier you want and it still works.

Say, Selenium, with a work function of 5.9 eV. Tiny percentage of the solar flux is above that.

Even just shading them from the sunlight would work. Like, a sun-shade held off to one side.

Also, you could just have the return line inside the tether: If the supply is on the outside, return on the inside, you can even use the structure of the tether itself as shielding — coaxial voltage differential, so the voltage difference between supply and return lines due to load creates negligible external electrical field.

Honestly, this feels like you've just decided it won't work and are deliberately choosing the worst possible design to fit that conclusion. Extra weird as "but we can't actually build carbon nanotubes longer than 55 cm yet" is a great deal more important than all the stuff I've listed that we can do.

Re: Space Elevator

#409

Post by hermitcrab »

kaonwarb wrote: post #407
hermitcrab wrote: post #215

Earlier quoted context omitted.

>ChatGpt tells me you'd free fall from 1000 km to 100km in about 8 minutes You trusted an LLM to do the maths when it is just s = 5t^2?

Not quite; g drops materially between 100 km and 1000 km. 8 minutes I believe is quite close, whereas 5t^2 (or even 4.9t^2) will lead to underestimating time.

The radius of the earth is around 6300km. The difference in g at the start (between 6400km and 7400km) is 25%. But gets less as you fall. So it might make somewhere around a 10-15% difference overall? So s=5t^2 is fine unless you need a super accurate figure, in which case you need to do some calculus. I would trust an LLM with calculus even less.

Re: Space Elevator

#410

Post by BizarroLand »

_carbyau_ wrote: post #349
nroets wrote: post #251

Earlier quoted context omitted.

The rocket fuel needed to produce that 40 MJ weighs close to 1 kg, especially when you include the oxidiser. So the energy needed to accelerate 1kg of payload to LEO velocity is much more.

That whole "tyranny of the rocket equation" thing is why I am surprised the actual first stage for launching a rocket is NOT a ground based reusable "up-chucker". Basically, I would have thought that any momentum that can be imparted to the rocket before it has to rely on its self propulsion would be a huge help. Not talking about eliminating self propulsion, just an assist so the rocket could carry a larger payload…

I've also wondered about a balloon launch. Strap the rocket to an enormous blimp, it handles the initial 1-2m/s acceleration, saving an enormous first volley of rocket fuel needed to break objects at rest out of their state of rest.

After the rocket is clear, activate compressors inside of the blimp and return it to base for re-use.

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