ENGINE BASICS II

Most common model rocket engines used in schools, clubs and classrooms are of the 18mm (diameter) X 2 1/4 inch variety. The engine casing is cylindrical and is made of a hard, paper tube. Sealed inside the tube is a black powder propellant used for combustion. The phrase "black powder" often gives the impression of a loose, granular propellant but this is incorrect. The propellant in a model rocket engine is a solid mass created by exposing a black powder mixture to enormous compressive forces in the manufacturing cycle. The result is a hard, physically stable "slug" which - when enclosed in the engine casing - can be stored for years with no adverse effects on performance. At one end of the model rocket engine the propellant is sealed by a inert clay "cap". At the other end of the engine is a clay "nozzle" which is used to direct the hot gases produced by the engine at ignition. The clay nozzle and cap effectively prevent the model rocket propellant from being ignited by any type of open flame or source of heat.

Model rocket engines use an electrical system to initiate ignition. This system is comprised of a launch controller (which uses a standard 6v or 9v battery for power) and a model rocket igniter. The igniter is a simple construction consisting of two short wires topped with a small amount of black propellant. Within this "head", the large wires are connected by a very small "bridge wire" that - when supplied with electrical current - creates sufficient heat to ignite the surrounding propellant.

Ignition and Flight

Just before launch, the model rocket igniter is positioned within the opening of the engine nozzle so the propellant is in contact with the surface of the black powder slug. When electrical current is applied to the igniter (via the launch controller) the result is a small, intense burst of heat which is sufficient to ignite the end of the black powder slug. The hot gases produced by this combustion are compressed and escape through nozzle, pushing the remains of the igniter out creating the thrust required for powered flight.

The "burn time" of a model rocket motor is relatively short (generally less than one second) but - given the low weight of most model rockets and the high thrust-to-weight ratio of the engine - the model rocket is quickly propelled to considerable heights (from 400ft to over 2000ft depending on the rating of the motor). When the propellant is completely consumed thrust drops to zero and the model rocket begins the 'coasting phase" of flight.

During the coasting phase the model rocket engine emits a dense smoke that allows viewers on the ground to track the flight path of the model rocket. This smoke is the result of the combustion of the "delay grain". In addition to producing dense smoke, the delay grain acts as a physical timing mechanism, providing 2-8 seconds before the final ejection charge is ignited at the top end of the model rocket motor. This delay allows the speed of the rocket to decrease sufficiently enough to allow for the successful deployment of the recovery system.

Commonly, the ignition of the ejection charge coincides with the model rocket reaching maximum altitude (commonly referred to as the "point of apogee"). As the ejection charge ignites, the inside of the model rocket fuselage is quickly pressurized, ejecting the recovery system and nose cone from the front of the rocket. Still attached to the model rocket by a length of cable called a 'shock cord", the recovery system (usually a parachute or streamer) deploys and the rocket safely descends to the ground.