Composite Motors

When adults come back to the rocketry hobby you often hear the statement that “I used to fly little model rockets when I was a kid.” My, my, how things have changed. I like to say “You grew up, so did rocketry.” Today, flying an 8 foot tall rocket that weighs more than 15 pounds is well within the reach of an average hobby budget. This is due in large part to the availability of inexpensive, efficient, and reliable composite rocket motors.

Note: We refer to engines and motors interchangeably. I prefer motor because I associate the word engine with moving parts. A search of the literature will not clarify this issue. We use engine and motor, referring to the same thing, in various parts of this article. A conversation with a rocketeer will nearly always produce the same result.

So for a start let’s take a quick look at how common black powder motors (the ones you used as a kid) work. The common Estes or Quest rocket motors are made up of black powder mixed with a substrate that reduces chemical changes to the powder from heat/cold cycles and humidity. They feature a ceramic disk nozzle. There is a small cavity at the nozzle end to insert an electric igniter. When the motor is lit, it burns steadily from the nozzle end to the top end of the motor. Black powder motors are not efficient, but they are cheap.

Composite motors are made up of solid material consisting of Ammonium perchlorate (AP) oxidizer with a plastic binder for fuel and structural strength. They are often doped with metal or metal oxide powders in order to impart characteristics such as flame color. Ammonium perchlorate is an inorganic compound with the formula NH4ClO4. It is the salt of perchloric acid and ammonia. It is a powerful oxidizer, which is why its main use is in solid propellants.

Most Composite motors have a channel of some kind running up through the center of the fuel grains. Igniters for composite engines feature long leads, because the ignitor must light the top end of the fuel grain, rather than the base. These ignitors are also made up with a higher energy burning tip because they must generate a lot of heat to ignite the AP. Composite motor Geometry IllustratedWhen the engine ignites, the entire exposed internal surface of the fuel grains begin to burn. This engine burns from the inside out. As the fuel burns, the cavity inside the fuel grains grows and the increasing surface area produces higher thrust. There are several strategies to normalize or flatten that thrust curve so that the engine has enough impulse to get the rocket off the pad quickly in the first phase of the burn. In larger engines, the fuel grains may be separated by O-rings inside the case, so that the ends of each grain burn, in addition to the central cavity. Another strategy is to produce a star shaped cavity in order to expose more surface area for the initial phase of the burn.

The major drawback of solid composite rocket engines is that they may not be throttled or shut down. Did you know that many cruise missiles actually have a minimum range? Another artifact of composite motors is that they chuf (this is the technical term used in rocketry). That is, they do not appear to light right away. No smoke or flame will be visible until the internal pressure of the cavity comes up.On larger motors this can take several seconds.

In High Power Rocketry (HPR) we use very few single-use motors. There are some “D” through “G” class single A set of 29mm RMS motor cases and closures. use motors, but these are referred to as Medium Power (MPR). Several vendors sell machined aluminium cases (left) and closures for Reloadable Motor Systems (RMSTM). They come in standard diameters of 24, 29, 38, 54, and 78 millimeters. The length of the case depends on how many fuel grains are included in the motor you are assembling. Most rocketeers purchase a set of cases in order to support the range of motors they are interested in flying. I own a 38mm set and am saving up for a 54mm set.

Reload kits for RMS consist of fuel grains, fiber washers, O rings, spacers, and insulators designed specifically to adapt the motor to the recommended case. Motors are assembled in specific steps and each motor comes with illustrated assembly instructions specific to that motor. The motor reload is designed to fit together under compression so that no leakage of hot gas will burn through the side of the aluminium case. We have all heard about how important this A Reloadable Motor System Reload from the Space Shuttle Challenger incident. We use a liberal amount of grease (basically a petroleum jelly) to insure a good seal, and a bit more on the sleeve that slides into the case in order to make cleanup easier. Larger motors feature a phenolic plastic case to better insulate the motor casing from the heat of combustion.

Commercial HPR motors available to HPR rocketry range from “H” through “N”. Sale of motors H and up are restricted to rocketeers who pass a certification test with one of the national organizations. Level 1 certification may handle H through J motors. This test requires building and flying a rocket with one of these motors, witnessed by another Level 1 or above member. A certification form is turned in to the national organization, and the flyer is then issued an ID card indicating Level 1 certification. Level 2 includes a 50 question written test including technical and safety rule content and flight certification with a K - L class motor. Level 3 requires a design and flight proposal using redundant dual deployment systems to be reviewed by a Technical Advisory Board, followed by a successful execution of that flight. Since the applicant is not able to purchase a motor of the class they are testing for, a member with certification in that class must be able to purchase the motor for them. This member will also be the person who evaluates the tests.

While the National Association of Rocketry (NAR) uses only commercial motors, the Tripoli Rocketry Association (TRA) provides a framework and safety rules for research motors. Fabrication of J through M motors are commonplace. We make it a point to refer to this as research, and not experimental. Some Tripoli members have produced O and Q class motors and tested them. Static firing tests on high speed video are more common than actual flights. Commercial motors of this energy are only sold to academic research and commercial launch providers with the requisite engineering certifications. Ceseroni Technology is the leading provider of commercial high power motors in the U.S. Motors O and up are commonly delivered assembled, in a carbon fiber motor casing to be returned to the manufacturer following use.

Both TRA and NAR provide Junior HPR certification programs NAR members as young as 14 years old may fly Level 1 motors with adult supervision. The TRA program is not based on the age of the flyer, rather the candidate must pass a written safety test administered by an officer of the local TRA Prefect in order to use HPR motors with supervision. A parental consent form is also required.

Commercial composite motors have revolutionised the hobby of high power rocketry. They are safe and reliable. They do require specific skills and the careful application of safety rules. The hobby is self-regulating – which is a remarkable accomplishment.

Table of Total Impulse by Letter Designation of Rocket Motors