"The murky views which some scientists advocate as to the inevitable end of every living thing on Earth . . . should not be regarded as axiomatic. The finer part of mankind will, in all likelihood, never perish - they will migrate from sun to sun as they go out. And so there is no end to life, to intellect and the perfection of humanity. Its progress is everlasting." Konstantin E. Tsiolkovski
How the Earth Orbiting Elevator Works
The Earth Orbiting Elevator, called Skyhook Gateway, is an intermediate form of the elevator-into-space concept described by Arthur C. Clark in his book "Fountains of Paradise" that can be built with today's materials. It works by placing a space station, called Midpoint Station, in an intermediate altitude Earth orbit and hanging a cable down to 150 kilometers altitude above the Earth's surface. A second slightly longer upward pointing cable will be reeled out at the same time to serve as a counter balance to the Earth pointing cable and as a jumping off point for the rest of the Solar System. There will be docking terminals at the endpoints of both cables. The lower terminal is called Skyhook, the upper one is called Gateway. The upward and downward pointing cables, the upper and lower docking terminals, the Midpoint Station, plus the two elevators for traveling between the endpoint terminals and the Midpoint Station, are the components that make up the Earth Orbiting Elevator.
Once in place, the Earth Orbiting Elevator will become a permanent piece of transportation infrastructure like a highway, a bridge, or a railroad track. It will remain in place as long as it is maintained. Also, in the same way that the Moon always shows the same face to the Earth, the Earth Orbiting Elevator will automatically and naturally maintain its vertical orientation as it orbits around the Earth. The Skyhook Station will always be at the bottom. The Gateway Station will always be at the top. This process as it regards the Moon, is often referred to as being "tide locked". The technical name for this process is called "gravity gradient stabilization".
What causes gravity gradient stabilization is the fact that the force of gravity gets weaker as you get farther away from the source of the gravity. Just like a light bulb in a dark room, the farther away from the light you get, the dimmer the light becomes. In the same way you will feel lighter in weight as you get farther away from the Earth's surface. If you think of the Earth Orbiting Elevator as a really long stick, it then becomes apparent that the bottom end of the stick, which is closer to the source of gravity, is going to weigh more than the upper end of the stick which is farther away. Once placed in the vertical orientation, it then becomes natural for the stick to want to maintain that orientation since the bottom end weighs more than the top.
Another process that occurs as a result of gravity getting weaker with increasing distance is that the velocity for orbit also gets less with increasing distance. Since it is the center of gravity of the Earth Orbiting Elevator system (located at the Midpoint Station) that determines the system's orbital velocity, and since the Midpoint Station is farther from the Earth then the Skyhook Station (greater distance = lower orbital velocity), the Skyhook Station will therefore be moving at less than orbital velocity for its altitude. The advantage of this is that a Spaceplane flying to the Skyhook Station will not have to accelerate all the way to orbital velocity in order to deliver a payload to the station. This makes for a substantially smaller Spaceplane which in turn means lower launch costs. Another interesting aspect of this process is that the Gateway Station, which is farther from Earth then the Midpoint Station, will be moving much faster than orbital velocity for its altitude. This means that a spacecraft or satellite that is released from Gateway Station will get a free velocity boost from the Earth Orbiting Elevator.
This difference in cable velocity versus orbital velocity as both change with altitude is shown on the chart by the blue and yellow arrows to the left of the drawing of the Earth Orbiting Elevator. The yellow arrows showing how the velocity for orbit decreases with increasing altitude while the blue arrows show how the velocity of the cable increases with increasing altitude. The place where the two velocities are the same, where the line formed by the blue arrows crosses the line formed by the yellow arrows, is where the center of gravity of the Earth Orbiting Elevator is located. It is this point that also determines the orbital altitude, orbital velocity and amount of time it takes the overall system to orbit the Earth.
What determines the specific amount of velocity savings a Spaceplane receives when flying to the Skyhook Station, or the amount of velocity boost that a spacecraft receives when released from Gateway Station, is the lengths of the upper and lower cables that are part of the Earth Orbiting Elevator system. Slower Spaceplanes require a longer lower cable while faster Spaceplanes can get by with a shorter one. For spacecraft being launched from the Gateway Station, a longer upper cable means a greater velocity boost while a shorter upper cable means less velocity boost.
What follows are some examples of cable lengths as a function of Skyhook Station or Spaceplane velocity measured as a percent of orbital velocity for that altitude. Please note that for all examples, the Skyhook Station is at an altitude of 150 kilometers above the surface of the Earth. Increases in the lower cable length result in an increase in the orbital altitude of the Midpoint Station. The lengths for the upper cable are followed by the Gateway Station velocity measured as a percent of escape velocity for that altitude.
For a Skyhook Station or Spaceplane velocity equal to 85% of orbital velocity a lower cable length of 464.2 miles is required. The upper cable length necessary to balance the weight of the lower cable is 497.7 miles. This gives a Gateway velocity of 88.11% of escape velocity for that altitude. Total cable length is 961.9 miles.
For a Skyhook Station or Spaceplane velocity equal to 80% of orbital velocity a lower cable length of 650.6 miles is required. The upper cable length necessary to balance the weight of the lower cable is 709.0 miles. This gives a Gateway velocity of 91.33% of escape velocity for that altitude. Total cable length is 1,359.6 miles.
For a Skyhook Station or Spaceplane velocity equal to 75% of orbital velocity a lower cable length of 857.5 miles is required. The upper cable length necessary to balance the weight of the lower cable is 965.3 miles. This gives a Gateway velocity of 94.96% of escape velocity for that altitude. Total cable length is 1,822.8 miles.
For a Skyhook Station or Spaceplane velocity equal to 70% of orbital velocity a lower cable length of 1,088.6 miles is required. The upper cable length necessary to balance the weight of the lower cable is 1,274.1 miles. This gives a Gateway velocity of 99.03% of escape velocity for that altitude. Total cable length is 2,362.8 miles.
Once the Earth Orbiting Elevator is in place, the Spaceplane will be able to carry payloads directly to the Skyhook Station without the need for an expendable upper stage. This increases the useful payload of the Spaceplane from 17,500 pounds to over 40,000 pounds per flight. In other words, our stage and a half to orbit Spaceplane just became a Single Stage To Orbit Spaceplane with over twice the payload capacity it originally had.
The last major component of the Earth Orbiting Elevator is the propulsion system. Every time a payload is delivered to the Skyhook Station, it pulls the entire system down into a slightly lower orbit. Left unchecked, this process would eventually pull the entire Elevator system down into the atmosphere where it would burn up. The solution to this problem is the installation of either an ion propulsion system or an electrodynamic propulsion system at the Midpoint Station. The propulsion system will be used to raise the Earth Orbiting Elevator's orbital altitude back to its original height between flights of the Spaceplane.
For more information on the Earth Orbiting Elevator contact SAE at 400 Commonwealth Drive, Warrendale, PA 15096-0001 U.S.A. Tel:(412)776-4841 Fax:(412)776-5760 and ask for a copy of SAE Paper 942120 "How an Earth Orbiting Tether Makes Possible an Affordable Earth-Moon Space Transportation System" by Eagle Sarmont.