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The rocket engine is a relatively simple device in which propellants are burned and the resulting high pressure gases are expanded through a specially shaped nozzle to produce thrust. Gas pressurized propellant tanks and simple propellant flow controls make operation of a small liquid-fuel rocket engine about as simple as operating an automobile engine. Why then do so many amateur rocket engines fail or cause injury? The reason, usually and simply, is that the amateur is not accustomed to high pressure devices operating near material temperature limits. His normal everyday life is, instead, filled with devices and gadgets operating at low pressures and at low thermal energy levels. With proper design, careful workmanship, and good test equipment, operated in a safe manner, the amateur can build small, liquid-fuel rocket engines which will have hours of safe operating life.

The puropse of this publication is to provide the serious amateur builder with design information, fabrication procedures, test equipement requirements, and safe oeprating procedures for small liquid-fuel rocket engines.


A liquid rocket engine employs liquid propellants which are fed under pressure from tanks in to a combustion chamber. The propellants usually consist of a liquid oxidizer and a liquid fuel. In the combustion chamber the propellants chemically react (burn) to form hot gases which are then accelerated and ejected at high velocity through a nozzle, thereby imparting momentum to the engine. Momentum is the product of mass and velocity. The thrust force of a rocket motor is the reaction experienced by the motor structure due to the ejection of the high velocity matter. This is the same phenomenon which pushes a garden hose backward as water squirts from the nozzle or makes a gun recoil when fired.

(figure 1)
Figure 1 Typical Rocket Motor

A typical rocket motor consists of the combustion chamber, the nozzle, and the injector, as shown in Figure 1. The combustion chamber is where the burning of propellants takes place at high pressure. The chamber must be strong enough to contain the high pressure generated by, and the high temperature resulting from, the combustion process. Because of the high temperature and heat transfer, the chamber and nozzle are usually cooled. The chamber must also be of sufficient length to ensure complete combustion before the gases enter the nozzle.

(figure 2)
Figure 2 DeLaval Nozzle

The function of the nozzle is to convert the chemical-thermal energy generated in the combustion chamber into kinetic energy, The nozzle converts the slow moving, high pressure, high temperature gas in the combustion chamber into high velocity gas of lower pressure and temperature. Since thrust is the product of mass (the amount of gas flowing through the nozzle) and velocity, a very high gas velocity is desirable. Gas velocities from one to two miles per second (5000 to 12000 feet per second) can be obtained in rocket nozzles. Nozzles which perform this seemingly amazing feat are called DeLaval nozzles (after their inventor) and consist of a convergent and divergent section, as shown in Figure 2. The minimum flow area between the convergent and divergent section is called the nozzle throat. The flow area at the end of the divergent section is called the nozzle exit area. The nozzle is usually made long enough (or the exit area is great enough) such that the pressure in the combustion chamber is reduced at the nozzle exit to the pressure existing outside the nozzle. If the rocket engine is being fired at sea level this pressure is about 14.7 pounds per square inch (psi). If the engine is designed for operation at high altitude, the exit pressure is less than 14.7 psi. The drop in temperature of the combustion gases flowing through the nozzle is high and can be as much as 2000-3000 F. Since the gases in the combustion chamber nay be at 5000-6000 F, the gas temperature at the nozzle exit is still about 3000 F.