15. Is it possible to keep building a faster rocket? (A K2S Question)
From The Space Library
Building faster rockets generally means that the specific energy and specific power must be increased. Improving specific energy can be approached in several ways. Exotic chemical propellants, such as metallic hydrogen, may provide very high specific energy. These materials may be very expensive to produce, if they can actually be produced, and they may be much more hazardous to use than existing propellants. Furthermore, the reaction temperature for these potential propellants is expected to be well above the melting point of current engine materials. Suggestions have been made that these propellants could be used in a pulsed detonation mode to reduce the average temperature requirements. Also suggested has been the use of magnetic fields to help contain the plasma exhaust to protect the engine walls. So, there are many interesting problems to be solved, but the benefits may be worth it. Specific energy can also be improved by the use of off-board energy sources. Examples that have been suggested include beaming energy to the vehicle with lasers or microwaves; the use of oxygen in the atmosphere early in the trajectory to offset on board oxygen requirements; and the use of ground based launch assist—a sled that gives the vehicle an initial boost. There are advantages and disadvantages with these concepts, but perhaps solutions can be found to overcome the disadvantages. Nuclear powered launch vehicles are often proposed. Certainly, nuclear reactors have far more specific energy than we need for launch. However, based on the designs of the nuclear rockets tested in the 1960s, the thrust-to-weight ratio is too low to be able to achieve orbit with a vertically launched single stage, limited primarily by shielding weight. Horizontally launched single stage nuclear rocket vehicles can, theoretically, achieve orbit. This is because the thrust requirements can be reduced since the wings provide lift and the reactor mass can be reduced appropriately. However, the radiation problems with such a concept are not considered environmentally friendly. Operational complexity, environmental, safety, political, and national security concerns will make this concept very expensive—perhaps not entirely impossible, but very difficult. Launch vehicles are occasionally proposed that are "all electric and magnetic." The concept suggests that when the reusable vehicle lands, it recharges its battery, fills its propellant tanks with water or liquid air from the atmosphere, and is ready to fly again. The engines consist of an arc jet, an electric arc that heats the propellant, and a magneto hydrodynamic (MHD) accelerator to further accelerate the propellant to very high velocity without a large increase in temperature. This concept is currently beyond our technical reach in several areas. Batteries currently can store about 0.1 mega-joules per kilogram. To be feasible the batteries must be improved to 200 megajoules per kilogram. Progress is being made that may one day achieve this goal. The MHD engines require large magnets, and magnet weight is also an issue with this concept, although there has been impressive progress in reducing the weight of magnets using new superconducting materials and carbon nanotube conductors. This concept may someday be achievable.
Answer provided by John W. Cole
Question and Answer extracted from the book Kids to Space - by Lonnie Schorer
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