Posted on 10/03/2007 5:33:50 PM PDT by KevinDavis
FARMINGTON - Light-powered vehicles zip along a paper-thin tether that stretches thousands of miles into space.
"Is this like, 'Beam me up, Scotty?'" asked Davis County Commissioner Bret Millburn.
Sort of. The California-based Spaceward Foundation is testing ideas for a space elevator, and Utah will play host to the competition this month.
(Excerpt) Read more at sltrib.com ...
Is 62,000 miles a bit of an overstatement? There is only 62 miles between the earth's surface and the end of the atmosphere. Perhaps it needs to be that long to work as a counterbalance?
The anchor weight needs to be in geosyncronous orbit in order for the space elevator concept to work. That is 62,300 miles up.
Thanks for the info, never realized the tether would need to be so long!
Now just wondering why they need light propulsion, the platform should be able to just winch up the tether.
There’s one question the Space Elevator FAQ doesn’t seem to answer - how does the payload get its “sideways” velocity?
Does the coriolis (I think that’s the right concept) force cause the ribbon to “lag” the launch point, with the sideways component of the force exerted by the distorted ribbon then provide the sideways acceleration?
It doesn’t need any sideways velocity, because the elevator’s center of mass is located at the geosynchronous altitude, where the orbital speed relative to the surface of the Earth at the Equator is zero.
The elevator cars do “winch up” the elevator cable... but the power to run the “winch” has to come from somewhere. Eventually there will be a solar power station at the top of the beanstalk that will beam the juice down to the cars, but for now it’ll have to be sent up via a laser beam.
I don't think that's right.
An object on the surface of the earth has a "sideways" velocity relative to a fixed frame of reference of slightly in excess of 1000 mph.
When it gets to the top of the 100,000 km ribbon, on its way to Mars for example, it's got a speed relative to that same fixed frame of reference of roughly 17,000 mph.
Where'd that other 16,000 mph come from?
Using a conventionally-launched orbiter, the launch rocket climbs vertically for a short period of time, but quickly rotates off the vertical to start developing the 18,000 mph necessary for low-earth orbit.
Yes, but as altitude increases, orbital period decreases; in other words, the farther “out” an orbit is, the slower its motion relative to the Earth’s surface. At an altitude of approximately 35,786 km (22,240 statute miles), the orbital period of the space elevator’s center of mass is equal to the Earth’s period of rotation, 23.934 hours. The elevator is moving “sideways” at the same speed the Earth’s surface is moving “sideways”, and so the elevator appears to hover over a point on the Earth’s surface.
Yes, but as altitude increases, orbital period decreases; in other words, the farther “out” an orbit is, the slower its motion relative to the Earth’s surface. At an altitude of approximately 35,786 km (22,240 statute miles), the orbital period of the space elevator’s center of mass is equal to the Earth’s period of rotation, 23.934 hours. The elevator is moving “sideways” at the same speed the Earth’s surface is moving “sideways”, and so the elevator appears to hover over a point on the Earth’s surface.
Not true.
The angular velocity is the same; 360 degrees per 24 hours, but in 24 hours an object on the surface of the earth moves 25,000 miles (the earth's circumference), but an object in geostationary orbit moves over 150,000 miles.
P=2p*sqrt(a3/m)
where P = the orbital period, a = the semi-major axis of the orbit (same as the radius of the circle for a circular orbit), and m) = the gravitational parameter (~398601 km3 / Sec2 for Earth).
Where P=24 hours, a=35,786 kilometers.
NASA has a handy online calculator that allows one to calculate the period of any circular orbit with an altitude >185 km. Try it!
Tether the payload to a counterweight in orbit at the platform. As the payload rises the counterweight and tether are paid out to a higher orbit, then winched down, not up, from the platform.
The problem is that for a satellite that's above the geosynchronous altitude, as would be the top of the space elevator ribbon, it's natural period is more than a day.
The period of the moon, for example, is 28 days (but you knew that ;-) )
The anchor at the top of this 100,000 km ribbon would have to be orbiting the earth at a rate of once a day, faster than a free satellite at the same altitude, which is what gives it the ability to "lift" the ribbon. If the ribbon were to disconnect, the satellite would immediately go to a higher altitude orbit, in the manner of a rock being swung on a string if you let go of the string.
Even ignoring the tether, going back to the geosynchronous satellite, it travels about 150,000 miles in 24 hours to keep up with the point on the surface of the earth it's above, whereas that point on the earth only travels 25,000 miles in the same 24 hours.
Actually, the space elevator IS in free orbit! The mass of the counterweight above the GSO station is equal to the mass of the ribbon below, so the system as a whole is in balance — a very long, thin structure with its axis pointed at the center of the Earth. Think of the elevator as a satellite 100,000 miles “tall” and you’ve got it.
I suppose that is probably true also, but still doesn’t answer the question of “How does something traveling at 25000 miles (the circumference of the earth) per day at the surface of the earth accelerate to 150000 miles (the circumference of a geosynchronous orbit) per day under the “Space Elevator” concept.
The only thing I can think of is that the elevator doesn’t go straight up as shown in the drawings, but rather is slightly angled to the west.
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