Imagine a propulsion system that could send spacecraft from low Earth orbit, or even from high within the Earth's atmosphere, to the Moon, Mars—or beyond—without using propellant. Such a system would eliminate the need to carry massive amounts of fuel from the ground into space just to get enough speed to make the journey and to slow down and stop at the destination. Then, once the mission is complete or the product is ready to return to the Earth, a variation of the same propulsion system would pick up the payload and send it home. All this is possible using a related, but different type of tether system called a Momentum Exchange Electrodynamic Reboost (MXER). Once implemented, the MXER may become the railroad of space.
The physics is simple. Recall your childhood yoyo and how you probably played with it, learning to do tricks until one day you got a little bored and decided you were going to play "David and Goliath''—using the yoyo as David's slingshot. Spinning it rapidly over your head you let go of the string and watched the yoyo become a projectile that hopefully didn't hit your kid brother or sister in the eye! This is the very same principle by which a MXER tether can be used to send a spacecraft payload on its way to anywhere in the solar system. A large, rotating tether system in space could be used to "throw" a spacecraft out of the Earth's gravity and on its way to the Moon, Mars, or virtually any other destination in the solar system—and then be used again for the next spacecraft.7
To explain how the overall "system" works requires first a careful explanation of each of its components. The key elements are: (1) the nonconducting portion of the rotating tether; (2) the electrodynamic portion ofthe tether; and (3) the power system. The first component is the bulk of the rotating tether system. A tether is nothing more than a cable connecting two or more bodies in space. In this case it must be relatively long (approximately 100 kilometers in length) and strong enough to not break during operation. Fortunately, modern materials are available that meet these requirements. To avoid being cut by micrometeors or debris in Earth orbit, the tether must either be braided into a rather substantial rope, larger in diameter than any of the particles or objects likely to hit it during operation, or designed such that any cut portion ofthe tether results in the loads being redistributed to other sections, keeping the system intact (see Figure 15.4). The latter approach, called a "Hoytether" is named after its inventor, Rob Hoyt. Rob's design looks like a ladder. If one piece is destroyed by a micrometeor, then the other parts of the ladder will carry the load and allow the overall tether to survive. Make no mistake, anything hit by a piece of debris traveling at 29,000 kilometers per hour will be damaged. The width of the braided tether will do nothing to help the portion that is hit to "survive" the impact, rather the width of the tether is large so that any hits will only "nick" the tether, leaving it mostly intact. This is because most of the micrometeors and debris are fairly small compared to the width of the more distributed Hoytether.
The MXER tether does not work by magic. Newton's laws apply, and once a payload is thrown it gains energy and the MXER tether station loses energy. The payload's energy increase is easily seen—it speeds up and is moving quickly away into interplanetary space. Where did the energy come from? The spacecraft's increase in kinetic energy came from a decrease in the MXER tether station's kinetic energy, hence its orbital altitude. In order for the station to throw another payload, it must regain energy and boost back to where it was before its last use. This is accomplished by the use of the electrodynamic, or conducting, tether embedded within the length of the overall MXER tether. This relatively
7 Sorensen, K., Conceptual Design and Analysis of an MXER Tether Boost Station. AIAA
small conducting section of tether, if power is provided, can push against the terrestrial magnetic field and reboost the MXER tether station to its operational altitude. Once the reboost maneuver is complete, MXER tether station will be ready and "recharged" to send another spacecraft on its way.
Where does the power required to reboost the system come from? In Earth orbit, solar arrays are capable of providing more than enough power to run the station and reboost it, as required. Ultimately, the MXER tether station will rendezvous with a spacecraft, capture it, spin it up, release it and then recover lost altitude and energy without the expenditure of propellant. The entire system operates on renewable resources, potentially allowing it to run autonomously for years. The cycle of operation of a MXER boost station is shown in Figure 15.5.
In addition to operating MXER tether stations in Earth orbit, one can be placed in low lunar orbit. The station orbiting the Moon cannot operate as efficiently as the one near Earth due to the Moon's lack ofboth an appreciable magnetic field and ionosphere. The lack of these resources makes it impossible for a lunar tether to reboost using electrodynamic tether forces. Replacing the conducting tether component of the system with a highly efficient electric propulsion system (for reboost) would still make it an attractive option for sending spacecraft and payloads on their
Rotating tether picks payload up from suborbital launch and tosses it into orbit
Tether s orbit drops as it transfers energy & momentum to the payload
Tether current pushes
Tether current pushes
FIGURE 15.5 The Orbit of a MXER tether boost station before and after operation. (Courtesy Dr Robert Hoyt, CEO, Tethers Unlimited).
return journey to Earth. This is especially true when you consider that the Moon has no atmosphere and therefore there is no reason that a long tether could not come very close to its surface. A tether dipping low into the Earth's atmosphere would experience incredible drag forces and be subjected to severe heating from its high-speed passage through it— probably resulting in the tether melting or otherwise breaking. This would not occur on the Moon. In fact, if the rotational velocity of the tether is controlled sufficiently, the tip of the tether could be made to dip down and touch the surface of the Moon—having for a moment near-zero relative motion compared to that point on the surface—thus potentially picking up any payload directly from the surface of the Moon and carrying it into space. Once there, it could be tossed toward its Earthly counterpart. Instead of relying on chemical rockets to get from the surface of the Moon to space, as was done in Apollo, a tether can simply come down from the sky and provide a relatively free ride home.
Consider how this might look to our lunar astronauts. After placing the cargo for return to Earth in a designated drop zone, they move back and watch as a rope descends from space, swinging across the horizon, appearing to slow down at the drop zone while the cargo is clamped to it. It then slowly accelerates along its arc back into space—carrying the cargo with it. The tether doesn't really slow down, of course, but it appears to do so because at that moment, both it and the Moon are rotating and have components of their velocities in the same direction.
There is another way that either or both of the MXER tether stations can gain energy and regain operational status following a "throw." If they "catch" an incoming payload, the kinetic energy of motion from the captured spacecraft would be conserved, resulting in the energy of the
MXER tether station increasing. The energy gained during a catch would reboost the station, minimizing or eliminating the need for the electrodynamic tether or electric propulsion system to do so.
MXER tether systems are not without their problems; fortunately few of them are really technical. For example, to maximize the utility of a MXER tether system, it would need to be placed in orbit above the equator. The United States has a very limited ability to launch spacecraft into equatorial orbit given that most of our launches occur in Florida, giving them an orbital inclination of 28.5 degrees. Either much time and energy would be wasted changing the orbital inclination of launched payloads in order for them to reach the MXER station, or a new launch facility would have to be constructed on the equator. Both of these options are expensive.
Nonetheless, one can easily imagine a network of such stations transferring payloads between the Earth and the Moon—efficiently and without the consumption of much propellant.8 Compare this with our current capabilities to transfer spaceships between the Earth and Moon and you will quickly see its advantages. The entire mass of the Saturn V rocket was required to carry enough propellant to send the tiny Apollo spacecraft containing three people to the Moon and back again.
Never again need we be so wasteful when Mother Nature is ready to provide us with all the energy we require—she just waits for us to be clever enough to use the resources she has provided. The next step would be to place a MXER tether station in Mars' orbit—emplacing the next stop on our space railroad.9
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