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.


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.