We need a better way to get to space(theconversation.com) |
We need a better way to get to space(theconversation.com) |
Also not one SABRE engine has even been built yet, whereas SpaceX has got very close to landing their rockets.
Just not ... in one piece.
Again, I don't know any of the mechanics or physics, but if energy was abundant and parts could be reused, would rockets really be all that bad?
Fuel is a small part of the cost of a rocket, most of the cost is in the engines and structures. Cheap production of hydrogen or oxygen is unlikely to make things much cheaper.
Also, any mass left over at engine cut-off requires a large quantity of fuel at lift-off and possibly necessitating a larger more expensive rocket. This would make re-purposing of boosters in orbit less likely.
The other option would be dropping the cable down, but you'd probably end up with a similar problem. I'm no physicist, but I'm pretty sure that dropping that amount of mass would shift the center of mass of the satellite-cable system and mess with the orbit.
Even if you manage to deploy the cable, you'd still have the problem of the counterweight. I've read that proposed solutions include a captured asteroid (we've just been able to land - or crash, depending on how you see it - in one), a space station/spaceport (that's definitely not cheap), an extended cable (which probably would require even more complex materials) or junk from the construction (still the same problem).
And we still haven't got to the point of security. How do you keep things (space junk, satellites, meteors) from hitting the cable? What would we do if several kilometers of ultra-strong cable fell down into the earth?
I think that space elevators are a nice fantasy, and just that. When we have mature enough materials and geopolitics, we'd probably be better off using them on other methods that seem to be far more viable.
Well, actually...
Package your cable up, launch it, move to geosynchronous orbit. Start deploying your cable. Once it gets long enough, say a few kilometres long, tides hold it rigidly pointing towards Earth. You solve the counterweight problem by also extending a cable outwards from your launch vehicle, so keeping the centre of gravity still. Eventually the bottom of the cable reaches ground level and you're done. On the plus side, you now also have a 36,000km long cable extending past geostationary orbit, ideal for interplanetary launches.
> What would we do if several kilometers of ultra-strong cable fell down into the earth?
Not much. If the point at which it's severed is low, the lower part will just fall down and you pick it up and weld it back on. If it's a bit higher, the lower part falls down and burns it. If it's a bit higher still, it goes into orbit.
People have been thinking about the engineering and geometry of space elevators for a long, long time...
http://www.ruf.rice.edu/~rau/phys600/p273.pdf
Many more here:
https://scholar.google.com/scholar?q=carbon+nanotubes+phonon...
Other problems - like an extremely capable cable material, ways to climb up and down etc - are getting gradually solved over last years, so I'm cautiously more optimistic than before.
Actually, much of the point is to build one that is significantly taller than that. The idea is that, if the center of mass sits at geostationary orbit, the elevator doesn't actually have to hold itself up. Centripetal forces do the heavy lifting. Try the wikipedia link posted by gp - the first thing you'll see is a very clear diagram.
You would have made the first 100km of the journey more fuel efficient, is that not a good enough reason?
[1] https://m.reddit.com/r/engineering/comments/2wstn6/i_am_very...
But, if they can solve such problems, great. I'm thinking that eventually we'll have some kind of electrical or hybrid mass driver (catapult) system for getting non-human cargo into low orbit[1], much cheaper (and quieter) and obviously could accomplish many launches a day for one-way missions.
You could get a large space station or interplanetary craft up there rather affordably using this approach. Specialized reentry vehicles as well. Launch the parts cheaply, robots assemble the parts in orbit, then launch the humans expensively.
[!] I am not a rocket scientist, so I haven't taken into account the orbital velocity you'd get from being rooted on the earth at takeoff, but it should be at most ~500m/s.
Still impressive if they can make it work, since of course Saturn V wasn't reusable.
I have not done the math, physics, etc on this, but it's an interesting idea worth exploring.
But that said, do we really belong in space when we cannot care for our own? Wouldn't that be akin to giving sugar to ants? only multiplying suffering and cruelty exponentially in space? Perhaps a refactoring of our culture and our methods of using/allocating/expending resources should be our first priority.
Both rail guns and light gas guns cap out at around 6km/s, and you really want 8km/s (more actually, for hypersonic drag loss) for this to be economical. Otherwise you need to launch a large enough rocket to make up the 2km/s difference, and you stuck solving the rocket equation again. Except you've subjected your rocket to 1000g, and the odds of it being reusable are really small.
For rail guns, the plasma discharge on the rails limits your speed. Around 6km/s.
For light gas guns, the working temperature of the gas cannot exceed the melting point of the barrel (or get close), otherwise tungsten/steel will get into the working gas and slow down the speed of sound. With hydrogen+tungsten, also around 6km/s.
There is a better solution, though :).
You'll also need some rockets to get into orbit. But I always wondered how much of the rocket one could replace with a rail.
Also, do you really think we can only fix one problem at a time? Did the birth of the transistor stop the civil rights movement?
I disagree but still upvoted this to get it to 0. I find the tought interesting but wrong. It seems more suffering is caused by various misguided attempts at refactoring culture etc and less by technology.
National socialism and communism are two widely known attempts at refactoring culture that has created more misery than all technological advances combined so far.
This would make an interesting topic to discuss how to design this system with our current tech, but that's another thread altogether.
They haven't mastered the ability to house, clothe, and feed the human family yet, despite having the technical ability to do so. As a result of this, you see the derivatives that exist today. Walk among Miami at night and you see homeless crackheads muttering to themselves while sipping on beer. The conditions that cause a person to do this would be probabilistically lower with a society that cares about one another more [designed-in] instead of fighting amongst each-other for paper slips.
If we absolutely need to go to space, we require research to determine the amount of materials needed to build this track length, and the renewability of those materials in reference to the earth. If this request for the materials meets a certain threshold, it will be dispensed, otherwise, the remaining materials can be reserved for further research to create synthetic alternatives. It helps to have a global database of known resources and their reserves, in fact, it might be a crucial requirement.
Imagine culling redis for this information, it sends a request to the global cybernetic hub, and it permits or denies the request, along with the terminal node you requested it to be sent to...
example: I request 13 Ounces of Gold (AU) at terminal #324-214-495-2341 to perform an experiment. (Since gold is currently in enough abundance for my request it is sent to my terminal as soon as technically possible, and any of the resource I do not use or can scrap back to the global hub, is done so)
I thought that the whole point of carbon nanotubes was that they are the only thing with the 'pull strength' (forgetting the term here) that could withstand the tension. If you weaved them, then you would rely on the Van Der Waal forces to keep the tubes together which is much weaker. Also you have to contend with the phonon interactions, anything tangential to that 'plane' has no effect on the strength, you have to be a part of the lattice to effect things.
Then again, I don't do research in this area.
Also, the whole point of leaving earth is leaving scarcity. There are meteorites with gold, we can and we should mine them as we see fit.
To complete your second statement, why do you believe we should mine them for gold when we cannot efficiently manage them here on Earth to begin with?
It would be a horrible expenditure of resources because when you look at a map of where humans designed and placed their cities, they are inefficient for travel and resource distribution.
Example: Manufacturing parts are created in Tennessee, shipped to Michigan, Manufactured in Michigan, and shipped to Florida to sell/distribute.
Instead, why not ship the raw materials to a equidistant manufacturing node/plant, create the material there, then distribute it. It's not done this way (most efficient) because money/corruption incentivizes backwards thinking of keeping "jobs" in sectors that are a waste of energy to complete and continue. Interesting times we live in.
Here is an example of an efficient and effective city design: http://i.imgur.com/shnIgHn.jpg
Regarding the second part... If the cable is severed, the lower part goes down and the upper part goews away as it does not have an anchor on Earth anymore (I think). Either way, you lose the cable. If it's a bit higher, maybe it doesn't burn up completely (for example, in Spain we've recently found space junk landing almost unharmed in a field [1]). And if it's several kilometres long (just a few hundreds) it could cause considerable damage.
1: http://elpais.com/elpais/2015/11/05/inenglish/1446724352_762...
I don't follow. If you didn't do this, wouldn't your centre of gravity still remain still? Just start at geostationary and make sure you have enough mass to act as a counterweight already on-ship (doesn't have to be in the shape of a cable). The center of gravity will remain in geostationary, the cable will reach the ground, and the ship/counterweight will end up at just the right height to balance it.
For the space elevator to work, the centre of gravity must be precisely in geostationary orbit (otherwise the entire elevator will drift relative to the ground).
You could achieve this by having a massy ship, and then carefully moving the ship outwards as you unreel the cable. But it's easier to have two cables. It also solves some other engineering problems, such as reaction effects as you control the speed at which the cable unreels.
What I'm saying is that without firing rockets, there's nothing you can do to change the trajectory of your centre of mass. When you "extend" the cable towards earth, presumably you are pushing on it in some way, and there will be an equal and opposite reaction. You don't have to "carefully move the ship outwards" - the ship (which is inherently "massy", as it has the rest of the cable as cargo) will move outwards by itself, exactly the right amount for the centre of gravity of the ship-cable system to remain in geostationary.
So I still don't see what two cables gets you apart from keeping the ship itself exactly at geostationary, which strikes me as unimportant.
You gain nothing from the height, because you still need to spend ~10km/s getting up to orbital velocity, but you don't have to push your way through the thick lower atmosphere any more. Rockets don't work very well in atmosphere, so you do get fuel savings due to more efficient engines.
I don't know if this has ever been tried for real.
In my defence I (a) am coming down with a horrible cold, and (b) read The Register...
Yes, but only for 2km/s, so there's that. Also this system can be used on other bodies such as Mars and the Moon.
But yeah, I'm more worried about the 1000g than anything.
The beamed power method gets about 20%, and works for humans (no g load). Escape Dynamics is trying it: http://escapedynamics.com/edispacelaunch/.
The difficulty here is more technical/scientific than engineering. Synchronizing microwave beams is hard, and if you don't you get destructive interference. I don't know if they have solved that problem yet (there have been a few proposed solutions, we'll see if any work).
Edit: Yes, rail would work great on the moon/mars. Actually there was a movie (Moon, with Sam Rockwell) where fuel is harvested on the moon and shot into orbit. Fun, crazy movie.
Edit: have you looked at these people's work? https://en.wikipedia.org/wiki/Quicklaunch
I think the oil derrick idea was a bit crazy (why make the problem harder than it has to be?), but the concept is sound provided that the rockets are fairly inexpensive.
I think the idea of launching to an arbitrary orbit appeals to engineers, but economically it would be more efficient to launch to a common orbit, and use cheap fuel to change orbits later if necessary. Of course, in-space refueling is only in the concept stage (but getting solved!).
To get above 6km/s, the only way I know of would be to use induction tracks (see Lawrence Livermore patent in doc). The original problem with the EM methods was getting high power switches to work repeatably. They used to cost a fortune, and last only for 10s of cycles. However this problem was solved by a completely different industry in the last couple of years (connecting wind power to the grid), and now you can get them cheap on Alibaba.
The major difficulty, actually, would be: 1) Sound pollution (not many places have high mountains, easy industrial access, and no local population). 2) Hypersonic drag (which is quadratic with velocity at that reynolds number). This is half as hard at 6km/s than 8km/s.
https://en.wikipedia.org/wiki/Treaty_on_the_Non-Proliferatio...
With a few exceptions, that's been working since the 70's.
This type of planning worked for Voyager (NASA/JPL).
fyi: cars suck. Dirty, inefficient, and it follows the same logic of giving every person in a 200 person hotel an individual elevator. It makes no sense, when you can instead design cities and transportation to be like the site I linked above (www.et3.com).
It's their choice though, if they want faster horses instead of cars, so be it. If they want faster cars instead of et3, so be it.
They prop and fall on their own sword I suppose.
