THE ROCKET ENGINE
The home constructed rocket engine is very similar to commercial engines but also there are some important differences. Refer to Figure 1. The propellant is ignited through the nozzle and burns on the entire inside surface of the core. The burning produces hot gases under pressure which then are forced through the nozzle at a high velocity to produce thrust. These engines are called core burning as opposed to port or end burning engines. The large core burns evenly outward toward the case. The portion of propellant above the top of the core is thicker than the thickest portion of propellant around the core. When the propellant around the core is expended, the pressure drops and the time it takes to finish burning the top portion to the clay heading is equivalent to the delay charge in commercial model rocket engines.
(see Figure 2) As the last of the propellant burns, it ignites the ejection charge through the clay heading and ejects the rocket recovery system. The delay allows time for the rocket to coast to the maximum altitude.
The propellant used in the home constructed engine is not as powerful as commercial fuel and burns slower so the large burning area of the core burning engine is needed to obtain enough thrust. The reasons will be explained in the propellant section. Specific impulse is a measurement of propellant efficiency and is determined by dividing the total impulse of the engine by the weight of the propellant. Most commercial model rocket engines have specific impulses between 50 and 100 seconds. Large professional solid propellant engines commonly have specific impulses of 180 to 250 seconds. The home constructed engine normally has a specific impulse of 20 to 50 seconds.
In earlier engines the case was choked to restrict the exhaust gases and form a nozzle. (see Figure 3) This was done by tying string around the bottom while the case was still wet. These engines burned the paper in the nozzle and so enlarged it as the fuel was consumed and the pressure and thrust dropped off prematurely. The specific impulse of these engines were even smaller yet.
The clay heading is needed to contain the pressure as the method used to compress the propellant charge is not adequate in itself to contain the pressure of the exhaust gases. Also the commercial gun powder used for the ejection charge, to work properly, cannot be compressed and so cannot be placed between the clay heading and propellant. It is held loosely in place by the paper retainer.
The case is made from paper for the same reasons as commercial engines. It is strong, fire-resistant, does not conduct heat readily, and if it should burst, light weight harmless paper shreds are the only product.
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