A Gift for Earth, Part 2
How It Works
The Skyhook Gateway system as described is not a science fiction dream. It could be built right now with today's materials and technology. In addition to the parts already described in the previous section, there would also be an ion propulsion module at the Midpoint Station that would be needed to control the orbital altitude of the Earth Orbiting Elevator.
The length of the lower cable used in this story, 1,088.6 miles, was selected due to it being long enough to allow the building of an economicaly viable sub-orbital spaceplane yet short enough that it would be affordable to build using existing graphite fibers. The length of the outward facing cable, 1,274.1 miles, was also selected for similar reasons plus the fact that the end of that cable is moving at just slightly less then escape velocity for that altitude. Escape velocity is the minimum necessary speed for a spacecraft to leave the gravitational field of the Earth.
In addition to making possible affordable aircraft type operations into Earth orbit, such a system would allow the launching of unmanned spacecraft, carrying cargo and scientific instruments, to virtually anywhere in the solar system with hardly any requirements for fuel or large expensive rocket motors. By using either a solar sail or an ion propulsion system for extra speed, course corrections, and deceleration to the desired planetary orbit upon arrival, these unmanned vehicles would make possible economical, detailed exploration of the solar system.
Manned vehicles launched from such a cable could then use their fuel to reduce their travel time between planets instead of using it for slow minimum energy transfer orbits as unmanned space probes do today. Such a situation would allow one-way travel times to Mars to be reduced from the nine or ten months of today down to five months or less.
Combining this high speed manned transportation with the slower cheaper cargo vessels would also make manned exploration of the planets both feasible and affordable. In addition to having their return fuel, planetary landing craft, and exploration supplies already waiting for them in orbit at their destination, fuel making equipment, housing modules, solar power arrays, and food growing equipment could also be sent in advance for the establishment of a permanent or semi-permanent base. Another advantage made possible by the Earth Orbiting Elevator is that the majority of these supplies, such as the rocket fuel, vehicle structural components, radiation shielding, food, water, and air, could all come from the Moon, thereby eliminating the need to carry these items up from the Earth.
Mining the Moon, moving asteroids into high Earth orbit and mining them for their elements, using solar sails or ion powered cargo ships for exploring the planets, using tethers to change a spacecraft's orbit without the use of rocket propellant, and Single Stage To Orbit space shuttles are not new ideas. All have been proposed and studied many times by some of the best minds this planet has produced. All of them have been found to work both technically and economically with the exception of the Single Stage To Orbit shuttle. Unfortunately, without an affordable way of getting into Earth orbit, all the other ideas become equaly unaffordable. This is why we need the Spaceplane and Earth Orbiting Elevator.
Single Stage To Orbit Launch Vehicles
For a single-stage rocket to accelerate to the speed necessary to reach low Earth orbit, it must start from the surface of the Earth with all the fuel it needs to get there. There are no gas stations half way up that it can stop at to refuel. So, besides needing to be powerful enough to lift all of its own weight, the weight of the payload, plus the weight of the fuel, the spacecraft's engines must also get good enough gas mileage to make the fuel last until it reaches the speed of orbit. The term used to describe and measure this fuel efficiency is called "Specific Impulse".
Specific Impulse is simply the number of seconds that a rocket engine will run when it takes off with exactly the same amount of fuel, in pounds, as the engine has pounds of thrust. Since a vertical take-off rocket won't take-off if its total weight is more then the thrust of its engines, specific impulse is a measure of the rocket engine's absolute best fuel efficiency, uncluttered by the weight of the payload or the spacecraft itself.
When the Space Shuttle flies into orbit, it increases its speed, on the average, by 20 meters per second every second that the engines are running. It must accelerate at this rate long enough to reach the equivalent speed of 9,000 meters per second to reach the low Earth orbit speed of 7,814 meters per second. The reason for the difference is that in addition to accelerating to orbital speed, a spacecraft must also overcome both the force of gravity that is pulling it back, and the resistance of the air as it flies through the atmosphere. So, for the Space Shuttle to reach the speed of LEO, its engines must run for a total of 450 seconds (450 seconds x 20 meters/second^2 = 9,000 meters/second). The specific impulse of the Space Shuttle's Main Engines is 452 seconds, enough to do the job as a single stage vehicle assuming the Space Shuttle carried no payload and had no weight of its own.
This is where the Earth Orbiting Elevator makes the difference.
As already stated, the speed for a low Earth orbit that is 150 kilometers high is 7,814 meters/second. If the bottom of the Earth Orbiting Elevator is moving at 70% of that speed then we have reduced the velocity required to low earth orbit by 2,344 meters/second. Subtracting that from the apparent velocity requirement of 9,000 meters/second to low Earth orbit equals 6,656 meters/second. Divide that by an average acceleration of 20 meters/second^2 and you get an engine burn time of 333 seconds. Since the Space Shutle Main Engine has a specific impulse of 452 seconds, we now have the equivalent of 119 seconds worth of fuel weight for the spacecraft and payload. With these types of number we could build an economically viable single stage sub-orbital shuttle using the engines and materials already developed for the Space Shuttle.
If the Earth Orbiting Elevator was placed in a higher orbit and the lower cable made longer so as to maintain its altitude at 150 kilometers above the surface of the Earth, the speed of the sub-orbital shuttle could be reduced even more at the expense of building a longer, heavier cable. The advantage of this is that the longer cable will make for an increased payload fraction in the launch vehicle thereby reducing its costs. A shorter cable will reduce the cost of building the cable but will reduce the payload fraction of the launch vehicle thereby increasing the launch vehicle's cost. The trade-off between the optimum speed required for the launch vehicle and the optimum length of the cable is determined by which combination will give the lowest freight bill.
Momentum Exchange
As some of you have probably noticed, there is a problem in that every time a payload is delievered to the bottom of the elevator or is released from the top of the elevator, there is a momentum exchange that drops the Earth Orbiting Elevator into a slightly lower orbit. Obviously such a situation, if left unchecked, would eventually lead to a large mess falling on the Earth. Actually, as serious as it might sound, it is not a significant problem. There are a number of methods available for managing the altitude of the Earth Orbiting Elevator. The most likely would be the use of a large array of very high specific impulse low-thrust ion engines located at the Midpoint Station to raise the the elevator's altitude back to its baseline altitude between delivery flights of the launch vehicle. The power and mass requirements of this propulsion system being determined by both the mass of the payloads and the amount of time between delivery flights. The propellant mass required for this re-boost operation is significantly less then that used by existing rocket powered upper stages due to the much higher performance of the ion propulsion system.
Complete reusability and this difference in performance between the Earth Orbiting Elevator's ion propulsion system and existing rocket powered upper stages is the heart and soul of the of what makes this concept work both technically and economically. The specific impulse of an ion propulsion system can be anywhere from 3,000 to 10,000 seconds depending on the amount of electric power that is available for powering the system. The best rocket powered upper stages have a specific impulse of around 450 seconds. Another way of thinking about it that wouldn't be to far off base, is that the entire Earth Orbiting Elevator system is nothing more then a very high performance reusable upper stage that replaces the expendable rocket powered upper stages on today's launch vehicles.