Rocket Flight

Abstract

Rockets are necessary to enable space flight, because any other form of propulsion cannot impart the necessary amount of velocity to put a spacecraft into orbit. Another important reason that rockets are indispensable for space flight is that they can provide thrust without requiring the presence of an atmosphere. Chemical rockets are typically employed to produce both a large thrust and a reasonable specific impulse by using a chemical reaction in a combustion chamber that generates an enormous amount of heat. This heat is then used to accelerate the resulting gases in a specially designed nozzle, which converts the thermal energy into kinetic energy of the exhaust. By Newton’s third law of motion, the exhaust gases apply an equal and opposite reaction on the rocket (the thrust) which causes it to accelerate in a direction opposite to that of the nozzle exhaust. A chemical reaction requires at least two substances, called the fuel and the oxidizer. The combined mixture of the fuel and the oxidizer is called the propellant, which results in a combustion. The propellant can either be a solid mixture which is ignited by a heat source, or two liquids which must be stored separately in tanks before being pumped into the combustion chamber and ignited. For example, a crude firecracker rocket has a mixture of powdered charcoal and sulfur as the fuel, and potassium nitrate as the oxidizer. A more sophisticated solid propellant mixture is fine aluminum powder (fuel), and ammonium perchlorate (oxidizer). Examples of liquid propellants are kerosene (fuel) plus liquid oxygen (oxidizer), hydrazine (fuel) plus nitrogen tetra-oxide (oxidizer), and liquid hydrogen (fuel) plus liquid oxygen (oxidizer). Solid rocket propellants are easier to store than liquid propellants, and do not require a sophisticated pumping mechanism. However, once ignited, it is difficult to stop the solid rocket combustion, while the combustion of liquid propellants can be controlled by manipulating the volume of the propellant pumped into the engine (called throttling). Thus liquid propellants are favored over solid propellants in space flight missions which require intermittent (or impulsive) firings. The chemical rocket principle is depicted in Fig. 7.1. Variations of the rocket principle are the acceleration of charged gases in an electromagnetic nozzle (plasma rocket), and nuclear propulsion wherein the heat is produced by a nuclear reaction rather than by a chemical combustion. However, these alternative forms of propulsion can currently produce only a very small thrust and hence cannot be used to launch a spacecraft into orbit.