How to install a rocket motor (without blowing yourself up)

Rocket motors are basically small explosives, so they are understandably treated as hazardous materials for purposes of transportation and shipping. Of course, you can be pretty confident they are safe: these motors are generally produced by large companies that have a tremendous amount of professional expertise, as well as hefty insurance policies.

Two companies primarily manufacture high power rocket motors: Aerotech and Cesaroni.

Shipping label on box: "Hazardous materials"
hazmat warning!

For my first high power rocket flight, I purchased an Aerotech I-140-14A “White Lightning” single-use motor with a 38mm diameter.

The “I-140” means that this is an “I” class motor (H or above in the alphabet is considered high power), and the 140 is the total thrust, measured in Newtons (N). In terms of high power rocket motors, this is not terribly powerful, but it’s still significantly more powerful than anything I’ve ever launched before.

The 14 is the number of seconds in the delay, after the motor propellant burns out, before the ejection charge fires to separate the rocket body and deploy the parachute.

Aerotech rocket motor
Aerotech motor package

“Single use,” as the term implies, means that this motor can be used once. The alternative is a reloadable motor. I plan to try these in the future, but single use is the most simple and straightforward type of motor.

The 38mm is the diameter of the motor; you would typically buy a rocket motor that fits into the rocket’s motor mount tube. The diameters of 29mm, 38mm, and 54mm are all fairly common in HPR, although there are even larger sizes too. You can also always buy a smaller diameter motor for a larger rocket, and secure it using a motor mount adapter, which is just something that fills the extra space between the smaller motor and larger rocket tube, centering it carefully.

Since I have a 38mm motor and a rocket with a 54mm motor mount tube, I have just such an adapter, and I’ll cover building and installing the adapter in another blog post. A key consideration is making sure the motor and the adapter are completely secured with some sort of a retainer (e.g. ideally not just masking tape).

Aerotech rocket motor and igniter
Aerotech motor + igniter

The motor here comes in a fancy yellow cardboard tube. Inside is also an igniter and a tiny vial of black gunpowder. The motor comes with instructions, but basically the gunpowder is inserted into one end of the motor and then covered with a plastic cap to seal it in. Later, when the rocket motor burns out, there will be a delay and then an ejection charge (shortly after apogee). The explosive force will be amplified by the black powder.

The motor is placed into the rocket like any smaller motor: inserted into the aft end of the rocket, after loading and securing the gunpowder on one end, and attaching the igniter on the other end. The only additional complication here is that I’m using the adapter, as mentioned above.

That’s it! The motors in HPR (and their installation) are really very similar to those in low or mid power rocketry, with small model rockets. The biggest difference is just the amount of propellant, and consequently, the amount of thrust.

Definitely looking forward to launching this thing, although I am expecting something less like the Falcon 9 and more like the Hindenburg.

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