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Space Elevator

neal.fun

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Re: Space Elevator

#321

Post by marcosdumay »

estimator7292 wrote: post #269
CobrastanJorji wrote: post #261

Earlier quoted context omitted.

I don't think there are any physics reasons why it'd be impossible, but certainly we can't do it with existing technology. You'd need an air breathing jet that could get a vehicle to go about five or six times faster than any current such engine has ever achieved (i.e. around mach 20-30), which is perhaps ridiculous, but I don't think it's necessarily impossible, just something we don't know how to do. There have bee…

Can an air breathing jet actually attain those velocities? I thought most supersonic aircraft use rockets after a certain point

> Can an air breathing jet actually attain those velocities?

There's no theoretical limitation on how fast an air breathing jet can move. You just have to redesign everything every few mach numbers, and deal with the atmospheric drag.

Re: Space Elevator

#322

Post by arkmm »

rich_sasha wrote: post #264
jvanderbot wrote: post #143

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…

Likewise, it is crazy to me when I realised how thin our oceans are. I used to think of them as super deep (I mean, they are) but even the Mariana trench is a mere 0.2% of the Earth's radius. Average ocean depth is more like 0.05%.

As a follow-up to this, even though water makes up 70% of the Earth's surface, it's only 0.02% of the Earth's mass.

Re: Space Elevator

#323

Post by floxy »

mr_toad wrote: post #90
icetank wrote: post #77

Earlier quoted context omitted.

The issue of the line falling back to earth is solved by putting the base of the elevator on water. If the top part of the elevator was cut of you could even detonate charges along the line to make sure all pieces fall into water.

A space elevator on Earth would be over 35,000 kilometres long. Depending where it broke it could wrap halfway around the Earth.

Let's say the cable gets detached really close to the geosynchronous tether point (that's the worst case, right?). How much of the cable will burn up in the atmosphere? What is the density of the carbon nano-tube / graphene ribbons? And what is their terminal velocity? Has anyone proposed dimension of a graphene ribbon tether? Like it is 300 mm wide by 0.01 mm thick?

Re: Space Elevator

#324

Post by Retric »

ianburrell wrote: post #315
Retric wrote: post #246

Earlier quoted context omitted.

You can reach space using air breathing jets. You can’t stay in space using air breathing jets.

There isn't enough air at high altitudes for jets to reach space even if you count 100km as space. The highest jet record is 37km in MiG-25. The scramjet record is 33km. I found source that says the limit is 40km at Mach 15.

The MiG didn’t reach 37km in level flight, which is probably where Mach 15 @40km comes from. Instead the MiG was doing a nearly parabolic trajectory starting with Mach ~3 worth of kinetic energy.

The NASA X-43A hit Mach 9.6 as an air breathing engine (test used a rocket at lower speeds for cost reasons) which in theory should be capable of ~65 km assuming it could survive a similar maneuver. Actual limits are heavily influenced by how much thrust you can generate while slowing down etc not just max velocity.

So yea no actually built air breathing aircraft can hit space, but it’s within the realm of possibility.

Re: Space Elevator

#325

Post by ben_w »

adwn wrote: post #312
ben_w wrote: post #308

Earlier quoted context omitted.

> A tiny imperfection or wobble is going to make the climber crash into the cable, destroying both. A maglev train is several centimeters from the rail; if someone made the carbon nanostructures (the only known material strong enough are atomically precise carbon nanotubes or graphene, but the entire length has to be atomically precise you can't splice together the shorter tubes we can build today) this badly wrong,…

> A maglev train is several centimeters from the rail […] No maglev train I ever heard of travels at 36,000 km/h. This is about two orders of magnitude faster. > We already have megavolt powerlines. That's transmission over long distances , but you need to handle and transform all that power in a relatively small enclosure. Have you seen the length of isolators on high-voltage powerlines? What do you think is going t…

> 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 air at the altitude of peak re-entry heating is 0.00004% the density at sea level?

> What do you think is going to happen to your circuit if you have an electrical potential difference of 1 MV over a few centimeters?

1) In space? Very little. Pylons that you see around the countryside aren't running in a vacuum, their isolators are irrelevant.

2) Why "a few centimetres"? You've pulled the 10 tons mass out of thin air, likewise that it's supposed to use "one kilovolt" potential differences, and now also that the electromagnets have to be "a few centimetres" in size? Were you taking that number from what I said about the gap between the train and the rails? Obviously you scale the size of your EM source to whatever works for your other constraints. And, for that matter, the peak velocity of the cargo container, peak acceleration, mass, dimensions, everything.

> For comparison, light rail typically uses around 1 kV, while mainline trains use something like 15 kV.

Hang on a minute. I was already wondering this on your previous comment, but now it matters: do you think the climber itself needs to internally route any of this power at all?

What you need for this is switches and coils on one side, a Halbach array on the other. Coils aren't that heavy, especially if they're superconducting. Halbach array on the cargo pod, all the rest on the tether.

Right now, the hardest part is — by a huge margin — making the tether. Like, "nobody could do it today for any money" hard. But if we could make the tether, then actually making things go up it is really not a big deal, it's of a complexity that overlaps with a science faire project.

(Also, I grew up with 25kV, but British train engineering is hardly worth taking inspiration from for other rail systems, let alone a space elevator).

Re: Space Elevator

#327

Post by hinkley »

blueflow wrote: post #313
rozab wrote: post #303

Earlier quoted context omitted.

The most important feature of a launch site is having no populated areas downrange. Kilimanjaro would have Mombasa downrange. I don't know of any launch sites significantly above sea-level, the marginal performance increase wouldn't be worth the logistical nightmare. It's easier to fly up a 747 than build a launch facility on top of a mountain.

> The most important feature of a launch site is having no populated areas downrange. Kilimanjaro would have Mombasa downrange. This was part of the Plot of Halo: ODST, where fragments of the space elevator collapsed onto New Mombasa.

The description of a space elevator falling is rendered in horrific detail in Kim Stanley Robinson’s Mars trilogy.

Twice.

Because the only known material strong enough at the time was diamond, and diamond doesn’t ablate much when falling through atmosphere.

One of the early space elevator research companies specifically designed a cable that was made by stacking successive layers of material both to slowly increase carrying capacity by building the tether in iterative layers, and hoping it would ablate or at least reach a quick terminal velocity in the case of catastrophic failure.

It is undoubtedly the case that the senior staff on that project were familiar with the Mars trilogy.

Re: Space Elevator

#328

Post by hinkley »

rozab wrote: post #303
hinkley wrote: post #299

Earlier quoted context omitted.

In a parallel universe where Africa is covered by world powers, Mount Kilimanjaro would make a pretty good launch facility. Reduced rocket equation needs for being nearly 3 miles high. If you start in thinner atmosphere you need less fuel to punch through it. You’re also higher when you hit Max Q. This is essentially what Scaled Composites and Virgin Galactic were trying to do with their cargo plane system, only you…

The most important feature of a launch site is having no populated areas downrange. Kilimanjaro would have Mombasa downrange. I don't know of any launch sites significantly above sea-level, the marginal performance increase wouldn't be worth the logistical nightmare. It's easier to fly up a 747 than build a launch facility on top of a mountain.

Mombasa is the shortest path to the ocean from the top of the mountain, that’s true. However that is not a good angle for an equatorial orbit.

What’s the downrange safety cone look like for space launch sites around the world? A little S curve in your insertion orbit would certainly waste a bit of delta V. But not all orbits are equatorial anyway.

Re: Space Elevator

#329

Post by matheusmoreira »

CaptainOfCoit wrote: post #100
matheusmoreira wrote: post #99

Earlier quoted context omitted.

I think it could but does not necessarily have to reach a station. The elevator just has to clear the atmosphere, right? As they say: once you're in orbit, you're halfway to anywhere.

> 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.

Re: Space Elevator

#330

Post by hermitcrab »

cauliflower2718 wrote: post #307
hermitcrab wrote: post #190

>Above this altitude is known as the "death zone", because there isn't enough oxygen for human life. Being pedantic, this should be "there isn't enough oxygen for sustained human life". An acclimatised climber can survive tens of hours.

I was wondering about this, because the link shows a bunch of birds in the "death zone".

Different species will have different 'death zones'. I'm not sure any bird would stay at that height for 10s of hours though.
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