Naked Science Forum
General Science => General Science => Topic started by: ngc1514 on 17/08/2011 17:59:44
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I am curious about the concept of using a floating platform as the lower terminus for a space elevator. Is the cable a static or dynamic system? Are there no tensile forces on the lower end of the cable from the counterweight out in space? If not, what the heck is holding the cable taut and straight?
Looked at several webpages on the elevator concept and didn't see anything that addressed this issue.
Thanks for any info.
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The geosynchronous orbit of Earth is at an elevation of 42164 km (26199 mi). I.E. The orbit where a satellite will stay in the same spot with respect to the Earth.
The Troposphere is about 17km thick at the equator.
Low Earth Orbit (LEO) begins at about 80km.
The orbits below the Geosynchronous orbit rotate faster than Earth's rotation. Those above the geosynchronous orbit rotate slower than the earth's rotation.
So, if your space elevator had a terminus at the geosynchronous orbit (or just beyond it with the counterweight). Then, stopping at the top of the troposphere would mean your cable would be 99.96% of the distance to the Earth. Any mass hanging at 0 rotational velocity with respect to the Earth, somewhere near the top of the troposphere would certainly cause pull on the space elevator cable.
But, it would also be very difficult to get to something hanging with 0 rotational velocity with respect to the Earth at the top of the troposphere.
What you would be more likely to do is to terminate the cable at the top of the troposphere with a rotational velocity somewhere between Mach 0.9 and Mach 4. Doing so, you could significantly shorten the cable from the more distant end, as well as somewhat lightening the weight of the platform. It might be possible to build a platform that was lighter than air (or close to it) at the surface of the Earth. It would not be lighter than air at altitude, but the weight would be minimal when accelerated to Mach 4. One would have to evaluate whether the drag caused by wings would be an issue. Winds and storms would be less of an issue at altitudes near the top of the troposphere.
Entering LEO would mean traveling up the cable beyond LEO, then dropping back down. Depending on where the counterweight is, you may still be able to get close to a geosynchronous orbit with minimal effort.
Sorry, I don't have the equations in front of me to calculate the length of cable necessary for an orbit equivalent to Mach 4 at the top of the troposphere.
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Space elevator dynamics is my area of expertise. The ribbon would be taught, although the tension at the Earth anchor point will be fairly low. A massive ship floating in the ocean would not be greatly affected by the connection to the ribbon.
The ribbon is static in equilibrium. Climbers run along it. That said, the ribbon will move (sway and oscillate) inadvertently due to a host of external forces.
For more info, check out my site: CHEEKY_ENGINEER.com, and click on the space elevator page.
FEEL FREE TO CONTRIBUTE HERE, BUT PLEASE DON'T USE TNS AS A REDIRECTION SITE.
MOD
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The tension in an elevator at the base would be designed to be a bit bigger than the maximum payload you want to lift, multiplied by a safety factor and then some more to deal with wind and other dynamic loadings.
If it was less than that, then the payload would never lift off; making it significantly more adds (enormous) mass and cost to the cable.