Why metal beats cardboard: life lessons from playing rock, paper, scissors

The rocket construction is complete, but there’s one minor issue I still need to address. The rocket has a 54mm motor mount tube, meaning I need a 54mm diameter motor to fit inside. But I couldn’t find any H or I level motors (note: for the level 1 certification flight, the motor must be an H or I) that are 54mm. I could only find 38mm motors.

ENTER: THE MOTOR ADAPTER.

38 to 54mm adapter: cardboard
adequate adapter

The motor adapter is exactly what it sounds like. It allows you to adapt a motor of a given size to a differently sized rocket.

You can always use a smaller diameter motor in a larger rocket as long as you get an appropriate adapter; in my case, I just need a 38 to 54mm adapter. It’s like using a booster seat at a table if you’re too small for the seat. (Important corollary: if your motor’s diameter is too large for your rocket, you’re simply out of luck, and at that point you just need to build a bigger rocket.)

The rocket is made from durable and reinforced cardboard, so I figured a cardboard motor adapter would be sufficient. And it probably would be, but I wasn’t satisfied.

38 to 54mm adapter: aluminum
indestructible adapter

The cardboard adapter was extremely durable and fit perfectly. I had no doubt it would keep the motor properly centered. The only issue was retaining the motor – i.e., keeping it from falling out the bottom of the rocket. And not just falling, but forcibly ejecting out the bottom after the motor has burned through all its propellant and the explosive ejection charge happens at the other end.

I’ve explained this before but just to recap the serious danger: ideally the motor stays put, and the hot explosive gas at apogee forces the rocket sections apart (deploying a parachute). But if the motor isn’t properly secured, what can happen instead is the motor itself ejects and falls out the bottom. That’s bad. Even worse is the fact that the rocket didn’t separate as a result, and the parachute didn’t deploy, meaning now the entire rocket will come crashing down.

Retaining the motor is a big deal.

I did try attaching some small metal retaining clips, but I wasn’t confident they would hold under extreme conditions.

In light of this concern, I upgraded to a machined aluminum adapter. It’s more expensive, but the primary advantage here is that it looks fancier. Also, this adapter has its own retainer, so there’s no worrying about the motor ejecting out the bottom at apogee. Things will work as intended!

Rear (aft) view of a rocket, with aluminum adapter and motor retainer
The business end of a rocket

One other nice feature is that the 38mm adapter and its retainer fit perfectly inside the larger 54mm retainer. This allows both to be used at the same time for smaller motors, or alternatively, the adapter can easily be removed and the 54mm retainer can be used solo for larger motors.

I think I’ve exhausted this topic. In summary, metal > cardboard, and retainer > no retainer.

High power rocket construction: part 5 (motor retainer)

Motor retainer: helps prevent costly braces and unnecessary trips to the orthodontist during the rocket’s awkward teenage years.

Back of rocket with motor retainer
Motor retainer, end cap unscrewed

All kidding aside, the motor retainer is simple but important. Extremely important, actually. Anyone reading this who has flown rockets before – of any size – knows what I’m talking about.

If you haven’t, here’s the deal: a motor burns for a period of time (a couple of seconds, generally) and the explosive force shooting out of the bottom of the rocket propels it in the opposite direction. If things are going well, this direction is up, into the sky. But once the propellant burns out, after a brief delay, right around apogee, it triggers a smaller explosion at the opposite end of the motor. This is basically an ejection of very hot gas inside the rocket. That gas has nowhere to go, and cannot escape. The explosive force breaks the rocket apart, at a place where the rocket is designed to easily separate – and inside is a parachute, which gets pushed out. Science!

But the hot gas filling the inside of the rocket only has “nowhere to go” and breaks the rocket apart if the motor itself stays securely in place. If it’s not sufficiently secured, then this event will forcefully push the motor backwards, out the bottom of the rocket!

This is dangerous and is a big problem for at least two reasons. First, the motor will simply fall back to the ground, without any kind of parachute or recovery device, and it could injure someone. A high power rocket can have a pretty large and heavy motor.

Second, if the hot gas pushes the motor out of the rocket, then the rocket will not properly separate where it’s designed to, and the parachute will not have any chance to deploy. This means the entire rocket will come crashing down, which will almost certainly irreparably damage the rocket. The falling rocket – without anything to slow it down – could also seriously injure someone.

Back end of rocket with motor retainer attached
Motor retainer, attached

Enter: the motor retainer. This is a simple device, made of some durable metal (e.g. “precision machined aluminum”) and comes in two circular rings. One ring is permanently epoxied to the motor mount tube at the aft end of the rocket. The metal on both circular parts is threaded, and the other ring is basically an end cap that screws onto the first ring. The end cap prevents the motor inside from sliding (or violently ejecting) out the back. The reason it’s in two parts that can attach or detach is to easily allow you to insert a new motor, or remove an old one, after each flight.

Given what would happen if a motor fell out the bottom of the rocket, to both the rocket itself and any innocent bystanders below, having a high quality motor retainer in place to secure the motor can literally make the difference between a successful flight and total disaster.

Plus, it classes up the rocket.