Rocket motors need to provide all the elements for combustion themselves (as compared to a jet engine, which uses atmospheric oxygen). So there are four key elements to a rocket motor:
- The oxidizer provides oxygen to drive the combustion chemical reaction. (Combustion, an oxidation reaction, is the combination of oxygen with some other substance.)
- The fuel is the substance with actually “burns” (is oxidized). This reaction between the oxidizer and fuel generates gas and heat, which provides the motive power for the rocket.
- The case keeps the motor together and under the correct pressure. Without pressure, the reaction will proceed too slowly and the fuel will burn without producing enough gas for efficient thrust. If the pressure gets too high, the motor will explode (CATO).
- The nozzle directs the gas formed by the reaction out the back of the rocket. The nozzle is formed such that the gasses are accelerated as they pass through the nozzle and provide efficient thrust.
Rocket motors fall into three broad categories: solid-, liquid- and hybrid-fuel motors. Solid fuel motors combine all materials into a mixture which is poured or packed into the motor liner and which hardens into a solid. These motors are easy to use and safe to transport because the components don’t require special containers and the motors are very unlikely to ignite accidentally. Liquid fuel motors keep the two materials in separate tanks and combine them only during actual motor firing. Hybrid motors use as gas and a solid, which are combined during firing.
Solid fuel motors are the simplest to use and the safest to transport and store and are by far the most commonly used in non-professional rocketry. Model rocket motors use black powder as both the oxidizer and fuel. Mid- and high-power rocket motors generally use ammonium perchlorate as the oxidizer and various binders as the fuel. Amateur solid-fuel motors generally use A.P. as well, but also use ammonium nitrate and other oxidizers. Metals and other additives are included in the motor to affect burn rate, alter the thrust curve and even make colorful flames. (Most substances which are good oxidizers are classified as explosives and regulated to some extent.)
Liquid fuel motors are more complex to set up and control, but provide a better thrust-to-weight ratio than solid fuel. Because liquid fuel motors generally use liquid oxygen as the oxidizer, the weight and complexity of these motors require very large rockets (they don’t scale down well). Various liquids are used as the fuel such as kerosene, alcohol or diesel fuel. Liquid fuel motors are rarely used because of the difficultly in building them and the difficulty and danger of dealing with liquid oxygen.
Hybrid motors attempt to blend the best of solid and liquid fuel rockets. The oxidizer is a gas (usually nitrous oxide) and the fuel can be almost anything which burns well (PVC is common). Hybrids are more complex than solids because the gaseous oxidizer needs to be pumped into the motor on the pad, requiring a complex launching system. However, the reloads are much cheaper than equivalent solid fuel reloads ($25 for a hybrid J grain as compared to $60 for a J solid fuel reload). Perhaps best of all, none of the components of hybrids are regulated by the B.A.T.F. Hybrid motors have even been introduced into high-power rocketry with systems from HyperTek and AeroTech which are certified for use at high-power club launches.
In hobby rocketry, motors are given a letter designation which specifies the total impulse (overall productive energy) of the motor with a range. Each letter indicates a doubling of the energy in the prior range so a “B” motor is twice as powerful as an “A” and a “C” is four times as powerful as an “A” (and twice as powerful as a “B”). That said, not every “C” motor has the same total impulse. Any motor whose total impulse is greater than 5 Newton-seconds (Ns) and less than or equal to 10Ns is a C motor, even though a 5.01Ns motor is only half as powerful as a 10Ns motor. By the way, commercial motors are made up to “O” size (the limit of high-power rocketry) and a full O is 40,960Ns, equivalent to 2,048 “mighty D” motors.
Submitted by: John Coker