Basic Rocket Technology
November 3, 2006 10:49pm CST
BASIC ROCKET TECHNOLOGY The basic principle of a rocket engine is that when fuel is burned in the engine, the reaction mass is expelled at high speed and pushes the engine in the opposite direction, in accordance with Isaac Newton's law of action and reaction. The energy to expel the reaction mass usually comes from some sort of exothermic (heat-producing) chemical reaction that causes the combustion products to expand violently and to stream out of a nozzle. In chemical reactions the actual reaction mass is usually the combustion products of the reaction. A number of other types of rocket engines are also possible, including ion engines using electrically charged ions. Thrust The thrust, or "push," of a rocket engine is measured either in units of weight (kilograms or pounds)Ñwhere one unit of thrust gives to the equivalent unit of weight an acceleration of one gravity, or g (9.8 m/sec/sec, or 32.2 ft/sec/sec)Ñor, more properly, in newtons. A newton is a unit of force that gives one kilogram an acceleration of one meter per second per second. For any rocket, thrust in kilograms can be converted to force in newtons by multiplying thrust by g. Large values are measured in kilonewtons. Efficiency The efficiency of a rocket engine is a much more crucial indicator of its performance. Efficiency is measured by a quantity called specific impulse, which is equivalent to the propellant's exhaust velocity divided by g. The resulting unit of measure is seconds. An equivalent concept is that the specific impulse value is the duration of time for which one kilogram of propellant can produce one kilogram of thrust. The higher the exhaust velocity and specific impulse of a rocket engine are, the more efficient it is. Solid-fuel rocket engines tend to have specific impulse values of up to about 200. Simple liquid-rocket systems, such as those using kerosene and liquid oxygen, have specific impuse values that measure in the mid-200s. Hypergolic systems such as those using hydrazine and nitrogen tetroxide, where the components ignite upon contact, have specific impulse values exceeding 300. Cryogenic hydrogen fuel can deliver values in the mid-400s. A simple nuclear engine such as the NERVA project of the National Aeronautics and Space Administration (NASA) in 1970 can deliver 800 to 900 seconds, but with significant complications in safety. Staging Rocket staging is also required in order to create vehicles of sufficient power for spaceflight. When most of a rocket's fuel has been exhausted, the rocket is carrying a great deal of empty-casing weight and is using a rocket engine whose thrust has become much too great for the remaining vehicle weight. Schemes were therefore developed to discard empty tanks and large engines during the course of a rocket's ascent. The simplest technique was to place an entire smaller rocket on top of a larger one; this is called tandem staging. Other approaches involve the use of side-mounted engines or even engine and tank assemblies than can be discarded in flight; this is known as parallel staging.