Rockets and Missiles
Rockets that operate with solid and liquid chemicals are currently the only kinds of vehicles capable of lifting off Earth's surface for scientific research or military applications. But both types of chemical rockets suffer from one serious drawback for use in vehicles traveling through outer space. The fuels they use are much too heavy for long distance travel above the Earth's atmosphere. In other words, their specific impulse is too small to be of value in outer space travel.
Rocket engineers have long recognized that other types of rockets would be more useful in travel outside Earth's atmosphere. These rockets would operate with power systems that are very light in comparison to chemical rockets. As early as 1944, for example, engineers were exploring the possibility of using nuclear reactors to power rockets. The rocket would carry a small nuclear reactor, the heat from which would be used to vaporize hydrogen gas. The hydrogen gas would then be expelled from the rear of the rocket, providing its propulsive force. Calculations indicate that a nuclear rocket of this type would have a specific impulse of about 1,000 seconds, more than twice that of the traditional chemical rocket.
Other types of so-called low-thrust rockets have also been suggested. In some cases, the propulsive force comes from atoms and molecules that have been ionized within the rocket body and then accelerated by being placed within a magnetic or electrical field. In other cases, a gas such as hydrogen is first turned into a plasma, and then ionized and accelerated. As attractive as some of these ideas sound in theory, they have thus far found relatively few practical applications in the construction of rocket engines.
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