Space elevators for earth are probably unworkable, though they could be pretty good for Mars (from Phobos) and maybe for the moon.
A space-elevator must climb about 40000km, even at 200km/hr that will take more than a week and requires a huge amount of power to be supplied to the climber. That makes it a very slow and expensive method for getting mass into space with huge maintenance and capital costs dominating any potential savings from energy efficiency. Additionally the materials issues are likely beyond our abilities to solve, we have had nanotubes for 20 years, yet our inability to find a suitable matrix to join them means that we can still not even equal the strength of carbon fibre composites. There has been very little progress made on this problem. The near earth space environment is really nasty, with atomic oxygen that will eat away the carbon, micrometeoroids that will very frequently impact and damage the cable (even <1mm can be catastrophic at >1km/s, and we can't track stuff that small). Space junk is a huge issue, that is only getting worse and is expected to get into catastrophic run-away territory in the next few decades as satellites start to collide with each other and create ever increasing clouds of shrapnel. Staying in Low earth orbit is useful because junk and debris naturally re-enters over relatively short timescales due to increased drag. It is far from certain that a space elevator could be made to work due to environmental conditions and micrometeoroids even if the materials were good enough. But there is probably a better option: A partial elevator that bridges a few hundred to a few thousand km between a higher space station and a very low space station that skims along at 120-150km altitude. This low space station/dock travels at much lower speed compared to a orbital speed as it is partially held up by the upper counterweight station a few hundreds to perhaps 1-2000km above. This cuts down on the velocity required for a launch vehicle greatly, making reusable rockets much easier to build, with greater factors of safety and reduced re-entry heating as well as increasing payloads for existing rockets by large multiples. The shortened cable length reduces the cable mass, micrometeoroid problems and allows use of existing materials. You also gain a platform that can be used to suck up the much denser gas molecules in low orbit in a big vacuum pump to very cheaply provide nitrogen, oxygen and some water for the stations. The cable itself also serves as propulsion in what is called an electrodynamic tether, simply by using the earth's magnetic field and firing electrons off into space from a gun in the space station. Rotovators (cables spinning end over end) are another version of this, but are a much trickier problem for launch vehicles to rendezvous with. On 11 April 2012 20:37, Jed Rothwell <[email protected]> wrote: > The very first or "pilot" ribbon would be only 2 cm wide in the atmosphere > to reduce wind impact, 5 cm in space, and 10 cm in danger zone from space > junk. 1 micron thick throughout. So a 1 cm hole would be a serious problem. > The first 230 climbers would carry only additional ribbon, which is attached > to the first ribbon, gradually making it broader, thicker and heavier. > > The first climbers would weigh 900 kg. The very first one is 380 kg of > machinery and structure, and 519 kg of payload (ribbon). As more layers are > added and the ribbon becomes stronger, the climbers and payload increase, in > steps or around 12 kg per climber. > > The ribbon will be first widened to 30 cm, then increased in thickness. > > I have no idea how this can be done at 200 km/h. > > The first 230 climbers would be left at the far end of the ribbon, 100,000 > km from earth, as counterweights. Until you make a second ribbon I guess all > the climbers would be one-way. As I said, it would be far cheaper and > easier to lift a second ribbon with the first one already in place, and a > few people at the other end to fix glitches. > > The pilot ribbon has to be dropped, not lifted, obviously. That's the hard > part. The other hard part is developing the ribbon material in the first > place. Drop the pilot, run up 230 climbers, and Bob's your uncle -- the > whole solar system opens up. > > - Jed >
