In the first article of this series, Return to Model Rocketry, I discussed some of the kits and kit manufacturers that exist today. However, a rocket without some way to propel it into the sky is just a cardboard, fiberglass, or carbon fiber tube. This article will talk about model rocketry propulsion in its various forms.
Model Rocketry Propulsion
This will not be a hardcore science lesson. The basics of a rocket motor are pressurizing a gas in a chamber. Consequently, this gas is allowed to escape the chamber through a small throat. Then, the escaping gas expands and produces a propelling force on an external nozzle.
The pressurized gas can be compressed air or compressed air with water which require no ignition or combustion of fuels. In contrast, liquid propellants, solid propellants, or a hybrid system all burn a chemical substance to produce heat and pressure. Liquid fueled rockets are complicated to build and service on the pad. Consequently, most model rocket engines are made with solid propellant.
Black Powder Motors
The original Estes motors are made from compressed black powder. These were the motors that I used a kid for rockets in Boy Scouts and just for fun.
One limitation to black powder motors is a size limitation. Once the motor gets over a certain size/volume, the propellant tends to crack. When the propellant cracks, the motor (and the rocket) tend to explode. So, that brings us to solid rocket motors made from composite materials.
The biggest change to model rocket motors has been the introduction of composite propellants. A composite propellant consists of a metal (often aluminum) powder, an oxidizer (usually ammonium perchlorate) and a binding plastic resin. This produces a “grain” propellant that has the consistency of a hard plastic or a hockey puck.
Many rocketeers are now making their own composite rocket propellants. I took John Wickman’s class in Wyoming to learn how to build your own composite rocket motors. The motor case and grains were built using PVC pipe. Here is the video of my class project.
A rocketeer can also buy these motors from vendors in the United States and Canada. They are listed later in this article.
Rocket Motor Impulse Classification
Rocket motors are classified by the total impulse produced then they are fired. The black powder motors I used as a kid were A, B, C, D and later E. Each letter up is double the impulse from the previous letter. So, a B motor generally has twice the impulse of an A motor. Composite model rocket motors are sold up to an O impulse. That is over 16,000 times more powerful than an A motor! These large motors can lift hundreds of pounds off the launch pad.
|Micro||0 - 0.3125|
|1/4A||0.3126 - 0.625|
|1/2A||0.626 - 1.25|
|A||1.26 - 2.5|
|B||2.51 - 5|
|C||5.01 - 10|
|D||10.01 - 20|
|E||20.01 - 40|
|F||40.01 - 80|
|G||80.01 - 160||Largest model rocket motor that can be used without certification|
|H||160.01 - 320||Level 1 Certification required|
|I||320.01 - 640|
|J||640.01 - 1280||Level 2 Certification required|
|K||1280.01 - 2560|
|L||2560.01 - 5120|
|M||5120.01 - 10240||Level 3 Certification required|
|N||10240.01 - 20560|
Rocket Motor Impulse Classification
Single Use or Reloadable Motors
Another development is that some of the composite motor companies sell motor hardware. Then, they sell propellant reloads that fit into this hardware. So, you buy the hardware once and then reuse it with purchased reloads. Here is an AeroTech rocket case with the reloads listed that fit it.
Hybrid motors a a combination of liquid and solid rocket motors. First, the fuel of the motor is a solid composite. Second, the oxidizer is a liquid or gas. For model rockets, the oxidizer is usually nitrous oxide. The oxidizer is loaded on the launch pad and the rocket fired immediately. Consequently, this dictates that the rocketeer have an oxidizer tank, regulator and line fittings at the launch pad.
There are four motor manufacturers that I have dealt with since I have been back in the hobby. To visit their website, click on their logo.
They built the first kit and motors I ever bought. And, they are still around and under new ownership. Their motors are inexpensive and readily available. All of their motors are made with black powder.
AeroTech builds composite rocket motors from A (Quest Aerospace) to N total impulse. I used their M650 for my Tripoli Level 3 certification flight. They are based out of Salt Lake City, Utah.
Cesaroni Technology (CTI)
This Canadian company is the primary rival of AeroTech. They sell motors in the same niche. Their smaller diameter reloads are pre-assembled which makes for easier prep at the launch site. I launched the Callahan’s Express on a CTI motor. My Friends4Michael rocket will be power by a Cesaroni N-impulse motor.
Loki Research is geared to the experimental rocket motor world. They sell rocket hardware (motor cases, nozzles, and closures) geared to homemade reloads. But, they also sell their own reloads for the hardware they produce.