Rocket Motor Technology

Introduction
There are several types of rocket motors used in the hobby. Historically speaking, the ancient Chinese's were among the first to use firework-type propellants. Later, much of this technology developed by the Chinese was borrowed by other groups.

Today the most common hobby rocket motors, or “engines” as they are some times called, are made of compressed black powder. In both mid-power and high-power rocketry most of the motors flown are composite-based typically made from ammonium perchlorate (AP). These AP motors typically provide three times the power of a typical black powder motor of the same size.

In addition to black powder and composite rocket motors, there are several manufacturers who are selling hybrid rocket motors. Hybrid motors use inert materials which when combined with nitrous oxide as an oxidizer create a very powerful rocket motor. These inert materials may include card board, paper and plastic.

History
The hobby of model rocketry began with what are now called ‘single use’ motors. Some companies call these rocket ‘engines.’ These motors were commonly manufactured using hydraulically compressed black powder which was contained in paper tubes.

Late in the 1960's, Irv Wait founded his Rocket Development Company and created the first composite model rocket motors called "EnerJets". EnerJets were later bought and sold by Centuri until 1974 when the company shut down the production line because of low profit margins.

In 1978 and 1979 two companies reintroduced composite model rocket motors. These companies were Small Systems Sounding Rockets or SSRS (later known as Crown Rocket Technology) and Composite Dynamics. SSRS chose 1.125" diameter paper phenolic motor casings with machined graphite nozzles. The other company, Composite Dynamics, used filament-wound fiberglass motor casings and cast ceramic nozzles for their motors. Both companies chose HTPB (hydroxyl-terminated polybutadiene) fuel binder which was a first in hobby rocket motors.

Composite Dynamics also developed its first motor to be called the E20. This motor had the same physical dimensions as an Estes D-12, however, the E20 produced almost three times as much power (40 newton-seconds).

Other manufacturers have continued to develop the composite rocket motor. New technologies include reloadable motor technologies which allowed the customer to use metal hardware cases over and over again by purchasing reloadable motor kits containing the propellant for each flight.

Single Use Motors
Like the name implies, a single use motor is designed to be flown once and disposed of. Typically these motors use a wound paper or plastic cases in smaller motors and are made of plastic, phenolic or even aluminum cased in larger motors.

Single use high power motors usually cost quite a bit more per flight than the a reloadable motor kit used in a reloadable motor. Most people in high power rocketry prefer to fly reloadable motors.

Reloadable Motor Technology
A reloadable motor is a motor designed to be used repeatedly by loading new motor parts into metal hardware casings. A reloadable motor can be broken down into three main individual components: an aluminum tube, forward closure and aft closure. The purpose of each closure is to hold the nozzle on one end and to seal the other end. In many motors the aft closure also holds a delay element and black powder charge used in recovery of the rocket. Motor cases come in different diameters and lengths to allow for various reload kits of different size and total energy.

Unlike smaller rocket motors which rely on an ‘engine hook’ in the rocket to keep the rocket motor in place, in reloadable rocket motors, the aft closure is usually slightly wider than the diameter of the case. When the motor is inserted into the motor mount tube the wider lip keeps the motor from sliding up through the motor mount tube while the rocket is under power.

A reloadable motor kit consists of one or more propellant grains, o-rings, a nozzle and various other items which are replaced after each flight. Some manufacturers like Kosdon TRM products use a graphite nozzle which can be used for up to 20 times before being replaced. In contrast, AeroTech motors use a different nozzle which is replaced after every flight.