21. Are there any alternatives to rockets for getting us to space? (A K2S Question)
From The Space Library
Alternative methods of getting to space, other than using rockets all the way, have been studied. a. The momentum exchange tether is a spacecraft on a string located in low Earth orbit. It unwinds a long tether, a cable, rotating in the orbit plane of the spacecraft. A launch vehicle carries a payload from the ground to high altitude but not to orbit. This requires much less energy than a launch to orbit. The mission is timed precisely so that the tether captures the payload from the launch vehicle. The tether then flings the payload in the desired direction, exchanging momentum from the tether spacecraft to the payload. The added payload energy comes at the expense of the spacecraft altitude. The tether spacecraft then re-boosts itself to its normal altitude using energy from the Sun and an electro-dynamic tether. Electric current flowing through the electro-dynamic tether creates a reactive force against the Earth's magnetic field, providing re-boost thrust. The physics for this concept is valid. The issues that exist are with the timing, and the fact that no simple demonstrator mission has been found. b. The space elevator is another rocket alternative, although it must initially be partially deployed with a rocket vehicle. The geosynchronous orbit is an orbit around the equator of the Earth at an altitude of about 19,300 miles where the orbit period is exactly equal to one day. Satellites such as communication satellites placed in this orbit appear to stay in one place in the sky. The space elevator concept deploys a tether, or cable, that reaches from beyond geosynchronous orbit all the way down to the surface of the Earth such that the center of mass is located at the geosynchronous orbit altitude. An elevator car would pinch the tether between a set of tires and travel up the tether to a station located somewhere high on the tether. This station would be a departure and arrival point to and from other points in space. There are two principle difficulties to be overcome to make this concept feasible. A material that has enough tensile strength that it will not break under its own weight is required. The best material found so far that has sufficient strength consists of single wall carbon nanotube fibers. These fibers are so thin that an electron microscope is required to see them; so, trillions of them are required to make the space elevator. The longest such fibers produced so far are about two centimeters long, but perhaps fibers long enough to reach beyond geo-synchronous orbit can someday be manufactured with the same strength as the shorter fibers. A second issue that must be solved is how to avoid collisions between the tether and satellites or space debris, and this may frequently occur several times per day. c. Solar sails are another alternative to rockets but are limited to regions of space far away from the Earth's atmosphere; usually in an orbit around the Sun. Photon pressure from the Sun impinging on the lightweight sail material provides a thrust to propel the sail. The thrust is perpendicular to the sail, so by tilting the sail in an appropriate direction one can increase orbit velocity moving the sail vehicle into a higher orbit, or decrease the orbit velocity allowing the vehicle to fall into a lower orbit. Beyond Mars there is little thrust since the intensity of the sunlight is weak. Close to the Sun, the solar pressure is much stronger and very high sail velocities can be achieved. Thus, sails can coast to the outer planets with this initial push. Breaking into orbit around another planetary body or providing delta V for a trip home must be accomplished some other way. Travel to and from Mercury and Venus may be ideal for sails. The effectiveness of sails depends on developing very thin light weight films that are not degraded by the space environment. d. The Star Trek transporter may have some basis for someday becoming reality. Quantum entanglement and quantum tunneling are being studied as possible ways for communications and perhaps for teleportation. These experiments so far deal with photons but research is just starting. This physics is not well understood and even if it is shown to be possible will be many decades away. Missions to the stars and warp drives also are not within our current technical capabilities, but are not entirely ruled out by our current laws of physics. Who knows what some clever scientist may think of? There are many, many space propulsion topics, concepts, and problems to be solved, and opportunities that will provide exciting and interesting work for future scientists and engineers, some of which will certainly enable missions that we cannot now even imagine. References: Hill, Philip G., and Petersen, Carl R.; "Mechanics and Thermodynamics of Propulsion", 2nd edition, 1992, Addison-Wesley Publishing Company, Inc. Reading, Mass. ISBN 0- 201-14659-2 Sutton, George P.; "Rocket Propulsion Elements, An Introduction to the Engineering of Rockets", 6th edition , 1992, John Wiley & Sons Inc., New York ISBN 0-471-52938-9
Answer provided by John W. Cole
Question and Answer extracted from the book Kids to Space - by Lonnie Schorer
 Buy This Book Click here |
