Science Forums 2

[CraigD]

Originally Posted by Moontanman

A space elevator has huge potential but lots of bugs to work out.

Even including the words huge and lots, this is likely an understatement.

Originally Posted by Moontanman

I like the idea of a space elevator as well but i wonder about it's flaws. Like it's fragility, an errant aircraft or even more likely a purposeful aircraft could knock it out easily and then all the man years of construction and mega tons of materials would be lost.

I suspect that if and when the challenges of making a space elevator are met, aircraft strikes will not be counted as among the hard ones. Given the strength of the materials required to make a space elevator, aircraft – which are already pretty fragile compared to most manmade structures – would be like tissue paper against steel in a collision. Also, note that only a small fraction of the total structure of a space elevator would be at altitudes aircraft can reach.

It’s also worth noting that, although common in popular science fictional and speculative descriptions of them, it’s not a strict requirement that space elevators reach all the way to the ground. There’re numerous advantages to not reaching the ground or anchoring them, but rather having their terminals at high altitudes (10 km or more), to which passengers and cargo must be transported via fairly conventional aircraft. (Imagine what it’d be like to land a 747 on a “runway in the sky” at 12,000 m altitude! )

Originally Posted by Moontanman

I've also read that a trip up the elevator would take weeks or even months, I've looked but I can't find where I read that.

You don’t really need a reference to figure this out.

geostationary orbit is 35,786 km above the Earth’s surface. Divide by your best guess at the speed some sort of climbing elevator car could safely achieve. The fastest conventional elevators have speeds of about 10 m/s, giving a travel time of about 41 days. A fast train can go 50 m/s or so, for a travel time of about a week. Since most of the trip would be in a near vacuum, much greater speeds might be possible, but so much depends on the detailed engineering of the system that it’s hard to guess if high speeds are practical, or worth their cost and risk.

Originally Posted by Gardamorg

Space elevators would be great if achieved, no, they would be monumental.

I think nearly everyone agrees on this

Originally Posted by Gardamorg

This is because a self sustaining fusion reactor is THE greatest propulsion method, and a craft can be built large enough to fit a large fusion reactor on board if it doesn't have to escape earth.

Being able to put huge masses into orbit at costs potentially as low as surface shipping would almost certainly revolutionize spacecraft construction, but we should be wary of calling any energy source THE greatest.

As the ieee.org article Gardamorg links emphasizes, despite decades of research, nobody’s really confident that self-sustaining fusion power will be possible anytime in the near future, or even if it can be accomplished, how much net power it will produce.

Also, as the wikipedia article “energy density” describes, the theoretical maximum energy/fuel mass ratio for fusion is only from 3 to 20 time that of fission, and about 1/140 to 1/300 times that to the absolutely greatest fuel, matter-antimatter.

OK, I take it back, it seem pretty safe to name the greatest potential energy source, so long as it’s antimatter.

It’s also important to note that even having a tremendous source of energy doesn’t solve all your propulsion problems. Rockets need reaction mass, even if only in the form of the relativistic mass of photons, and the less reaction mass available, the more energy is required to produce the same thrust. The wikipedia article “nuclear photonic rocket”, while not primarily about such a system, has some applicable discussion.