Motor vs. Engine

You will find many sources that will refer to rocket "motors" as rocket "engines" and even the venerable Estes® refers to their motors as engines (the last time I checked).  To be technically correct, nearly all amateur rockets from the smallest to the largest use motors.  According to the American Heritage Dictionary, a motor is "something, such as a machine or an engine, that produces or imparts motion" and an engine is "A machine that converts energy into mechanical force or motion."  A machine is "A device consisting of fixed and moving parts that modifies mechanical energy and transmits it in a more useful form."  So the space shuttle engines are indeed engines because they have all kinds of mechanical moving parts such as pumps and valves.  A solid propellant rocket motor has no mechanical moving parts.  The only thing moving is the igniter as it is ejected out the nozzle and the gas and propellant particles resulting from combustion of the propellant.  There are no moving parts like there is in your car engine.  So unless I slip, I will refer to all propelling devices in this web site as motors.

Rocket Propellant

An internal combustion engine like your lawnmower, car, truck, small planes, etc. use a fuel like gasoline or diesel oil.  To burn, it must have two more ingredients, oxygen and heat.  The heat is supplied by a sparkplug or in the case of a diesel engine, the heat comes from compressing the fuel.  The oxygen is taken from the oxygen in the air.  In a rocket, the propellant is a mixture of fuel and an oxidizer and so oxygen is not taken from the air.  That is why a rocket can operate both underwater and in the vacuum of space.  I don't want to get too technical here.  I may add more technical aspects of the chemistry of propellants later or you can find any number of other web sites that go into great detail.  The oxygen can be pure oxygen such as in the space shuttle that uses liquid oxygen as the oxidizer and liquid hydrogen as the fuel (this is the most efficient propellant known), or the oxidizer can have the oxygen as a component of a chemical compound.  There are a great number of chemicals that are oxidizers.  Technically, an oxidizer does not even have to contain oxygen.  It only needs to be the chemical that gives up electrons in a redox reaction.

How a motor works

The most basic rocket motor consists of only three parts: case, nozzle, and propellant.

There is one more component actually in the rocket motor and that is the combustion chamber but in this simplest version, the combustion chamber is the void area in the center of the propellant.  To produce usable thrust, the pressure inside the motor must be higher than the outside atmospheric pressure (considerably higher).  The higher pressure inside forces the products of the burning propellant out through a small opening (the nozzle) at a high velocity. The thrust of the engine is dependant on how much mass is ejected out the nozzle in a given time and the velocity.  The velocity is dependant on the pressure inside the case and the diameter of the nozzle throat.  How much mass is ejected is dependant on the burning area  and the burn rate of the propellant.  The burn rate depends on the specifics of the propellant such as the chemicals in the mix, the ratios of the chemicals and the particle sizes used to make the propellant plus the pressure.  The burn rate of rocket propellants increases as pressure increases.  The burning area is controlled by the shape of the propellant.  As you can see, even in the simplest rocket motor,  there are a number of interacting specifics going on that a rocket motor designer has to keep in mind and design around.

The de Laval Nozzle

  A refinement to the basic motor is the de Laval nozzle.  There are three sections to this nozzle, the convergent section, the throat, and the divergent section.  High pressure, low velocity gas enters the convergent section and high velocity low pressure gas exits the divergent section.  To be efficient, the gases must be moving below the speed of sound before entering the nozzle, increase past the speed of sound in the throat and then the supersonic gas would increase velocity more as it expanded in the divergent section.  If the gas speed were to be supersonic before reaching the throat, the gas will slow down in the throat.  If the gas is not supersonic exiting the throat, it will slow down in the divergent section.  In an amateur motor, the nozzle can be approximated with a 45° convergent section and a 15° divergent section.

Propellant Configuration

One piece of propellant is called a grain.  This is left over from early pyrotechnics where the grain size of black powder was the size of each piece of powder.  So if the propellant is a single solid piece, then the motor has only one grain.  Single use small commercial motors such as those made by Estes are single grain motors.  Most high power amateur rocket motors have two to five grains in a core burning configuration (a cylinder with a hole down the center).  These are stacked up one on top of each other.  This configuration of two or more grains is called a "Bates" grain.  There are many other configurations that are used in commercial and military rockets and missiles and a few of these are being experimented with by a few amateurs.  Below are some of the most common experimental rocketry configurations and commercial configurations that have been used.  This is by no means an exhaustive list.  This shows an end view and generally a sectional cut although in many of the forms, that view is not very informative.

Inhibited Surfaces

Propellant grains are often cast as free standing grains and then inserted into the motor.  These grains can have one or more inhibited surfaces.  Most motors have the outside surface inhibited.  This can be done two ways.  The first way is that the grain can have a coating on the outside so that the propellant surface is not exposed.  This is often just paper of varying thickness that is stuck to the outside diameter or can be bonded using some type of adhesive.  It can also be a sprayed on coating such as aluminum paint, or can even be tape.  It can be more sophisticated such as a phenolic tube.  Another way to inhibit the outside surface is to case bond the grain to the motor casing wall.  This is usually done when the grain is actually cast directly into the motor casing.  This is another common method for smaller motors.  Conceivably, it could somehow be glued to the wall when the separately cast grain was inserted but I don't know of anyone that has done this.

In addition to the outside surface, one or both ends of the grain(s) could also be inhibited.  If no surface is inhibited, then the grain will burn from the outside in, from the inside out, and from both ends towards the middle of the grain.  The thrust vs. time can in this way be engineered to a variety of shapes by varying the number of grains, the surfaces that are inhibited, the core shape, and the diameter and length of the grains.

Ejection Charge

In commercial rocketry, there usually is no recovery mechanism because if it is a guided missile, the object is to blow up, and in the case of launch vehicles, the object is to get a payload into orbit and the launch vehicle is usually only for one use and falls into the ocean.  The shuttle solid boosters are recovered by parachute.  These are ejected by gas producing charges that are triggered electronically.  Amateur high power rocketry has evolved to a similar situation where often electronics (altimeters and/or timers) triggers a black powder charge that pressurizes the body tube and separates it from another body tube section or the nose cone and so parachute(s).  In small model rocket motors and the low end of some high power motors, an ejection charge is built into the motor.

The above diagram is what you would find in a paper casing such as Estes motors or homemade black powder motors.  It is also the same configuration for PVC motors.  In these two cases, the nozzle and header are normally clay or concrete.  The delay charge and propellant grain would be one solid piece in the case of black powder motors.  In PVC motors, they are usually made separately but are the same propellant composition.  The delay charge can also be a slower burning composition and could also include ingredients to produce more smoke for tracking purposes.  A portion of the delay charge equal in depth to the wall thickness of the propellant grain will be consumed at the same time the grain is burning and will be consumed during the burn.  The remaining amount will burn at near atmospheric pressure and so much slower.  When the Delay charge burns through to the header, it ignites the portion of ejection charge that is inside the header orifice and comunicates the burn up to the main ejection charge in negligible time.  The ejection charge is usually commercial black powder (4F) and is held loosely (not compacted) in place by a paper, cardboard, or other relatively fragile retainer cap.  The header orifice must be fairly small so that the majority of the ejection gases are used to pressurize the body tube and eject the parachute rather than be ejected back out the nozzle and out of the rocket.

There are other variations in what holds the ejection charges such as smaller tubes glued either inside the header or outside it but the operation is still the same.  Variations are most common in larger motors where a smaller diameter delay charge is desired.  The delay charge adds a small amount to the thrust of the motor, especially if it is the same formula as the propellant grain.

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