Scientist Sees Space Elevator in 15 Years

http://www.mit.edu/people/gassend/spaceelevator/center-of-mass/index.html

Ok, here's another page, that I found fascinating and enlightening, and it proved my intuition correct! CG slightly beyond geo.

But if you don't get off at geo, then you need to be really sure that the cable won't snap while you are on it. Because beyond that point, if anything goes wrong with the elevator, then you are in serious trouble. At least don't put a space station at the anchor.

"Balance the weights and the ride is free."

"Well, not entirely free. I believe the difference would come out of the rotational energy of the Earth, wouldn't it? Like when you use a planet to slingshot a space-craft. Not that we would have to worry about moving to a 25-hour day anytime real soon..."

This change in rotational rate...in the length of a day (the precise time of one rotation) occurs when glaciers form (water mass is raised above sea level) or large reservoirs of water are dammed. Moving mass to a greater distance from the axis of rotation will slow the day by a small amount. But it can be calculated and measured. (millions of tons of weight in the examples above).

Sure. Pardon if I don't remember project names right off. But I'll describe the concepts.

1) Concept by ex-Air Force folks to build air breathing craft with large JP-4/LOX rocket in the tail. Concept drawing looked very much like an old British Vulcan bomber. Craft would launch from Edwards area and cruise on turbojets to an existing military area off the Washington State coast. It would hit a military tanker loading up on JP-4 (far more than it would normaly be able to get off the ground with). LOX is carried from takeoff. It would aim south, ignite the rocket, and go suborbital, re-entering military airspace off the California coast. At apogee of the flight at perhaps 300k feet and several mach, it would open large doors in the top not unlike the Shuttle, and kick out an upper stage that would continue into orbit. Performance of the vehicle is perhaps only a bit higher than the X-15 was. So it's very do-able. The advantage is in the ability to carry an upper stage internaly, and eject it during the 2-4 minutes it spends in "space".

2) Basically similar idea to No 1, but rather than carry air breathing engines for takoff, would be towed by 747/C5 class aircraft. Idea was tested by towing an F106 with a C141 a few years ago.

Those are two of the ideas that stick in my mind. There are others with exotic scram jet mid stages that should be considered too. I believe that the material science has been figured out, if you use Shuttle style tile thermal protection. These tiles have been bashed in the media, but I believe they would work fine as long as you don't put them next to a large external tank and accelerate the stack to hypersonic speeds exposing them to extreem shock waves in the space between shuttle and tank, and bashing them with FOD.

There are other ideas, but it's too late to think of them right now, and I think the above are the best.

Diameter of Earth ~ 7,659 miles
Diameter of elevator orbit ~ 65,829 miles

Circumference of earth ~ 24,000 miles
Circumference of elevator orbit ~ 206,809 miles

Speed of base (24 hour rotation) ~ 1,000 mph
Speed of platform (24 hour rotation) ~ 8,617 mph

The platform is 62,000 miles above the earth's surface. If the elevator cars travel at 100mph, they will still take almost 26 days to travel the ribbon. Delta-V will average out at just over 12 m/h². I think that will be manageable.

"The platform is 62,000 miles above the earth's surface. If the elevator cars travel at 100mph, they will still take almost 26 days to travel the ribbon. Delta-V will average out at just over 12 m/h2. I think that will be manageable."

The mass that balances the cable is at the top (65,829 miles), but the only point in the system that is in true orbital motion is the geosynchronous orbit point, 22,236 miles up.

If this is an elevator to orbit, then we will ride it to the geosynchronous point.

The higher portion is just for balance. (The center of mass should be at the geosynchronous point). So the upper part could be shortened by using more mass. (a captured asteroid).

See "Fountains of Paradise" by Arthur Clarke 1979, where this is laid out in some detail.

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