How to build a fiberglass rocket, part 7: motor retainer

As I’ve written extensively about before, it’s absolutely critical that your rocket has a good motor retention system. You don’t want the motor falling out of the bottom of the rocket. For one thing, it may land on someone’s head. More importantly, if the motor falls out, your rocket will not perform as intended.

There are different types of motor retention mechanisms, ranging from the extremely simple (e.g., wrap it with masking tape so it’s a tighter fit) to relatively simple (e.g., metal clips or hooks to keep it in place), to slightly more complicated but significantly more reliable hardware, such as a machined aluminum motor retainer. That’s what I went with for my previous rocket (54mm motor mount) and that’s what I’m using again here (75mm).

metal screws in wooden centering ring, on workbench with drill nearby
time to drill

The main question for me was: how am I going to attach it? The aluminum motor retainer has pre-drilled holes for screws, but generally you would attach it directly to the closest centering ring, which would be the “aft” one, or the one closest to the bottom of the rocket.

With this rocket, everything is fiberglass, and I wasn’t sure about drilling this many small holes in a fiberglass centering ring (“CR”). The ring is extremely thin, and the screw inserts would extend far beyond it instead of sitting comfortably inside it. Plus, I needed something to fill a gap between where I wanted my aft CR to sit, and where the motor retainer would be located.

So for multiple reasons, I decided to buy and use an additional wooden CR (two, actually, as I needed to fill a 1/2″ space and each wooden CR is exactly 1/4″ in width).

It worked out well. I used wood glue to attach the two CRs together, and then after it dried, I drilled holes and inserted each of the metal screw inserts. I then attached the aluminum motor retainer to the wooden CRs with screws, as pictured here.

black aluminum motor retainer attached to wooden centering ring
motor retainer

I was then able to attach the whole thing – additional CRs and metal retainer – to the motor mount tube using epoxy.

aluminum motor retainer attached to red fiberglass motor mount
business end of the motor mount

To be honest, this was one of the easier and more straightforward steps in the assembly of this rocket. This type of retainer is reliable, though, and now I have one less reason to worry when I launch.

There are still plenty of other things to worry about, plenty of things that can go wrong – sometimes catastrophically – and I’m sure at least one of them will.

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.

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.

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.

Catastrophic failure

Last weekend, I attended a launch event hosted by my local rocket club. It was a great experience, and I enjoyed watching others launch a ton of rockets, large and small. A launch report that the club sent out afterwards said that there were 111 total flights, ranging from motors sized 1/2 A up through G. (It was a low and mid power launch, so nothing higher than that.)

Rockets set up on range on grassy field
Rocket range

Something else made the day particularly memorable: my rocket self-destructed.

But first, a little background.

A rocket launch can go wrong in a lot of different ways. A LOT. One of the most common issues is a failure of the recovery system – for example, the parachute fails to deploy properly, and the rocket comes crashing down. But a rocket can be constructed perfectly and the launch can still end in disaster if the motor or engine malfunctions. Motors are designed by people with advanced degrees in chemistry and engineering, and constructed with expensive equipment in large facilities (with hefty insurance policies), but they’re effectively just small explosive devices and things can understandably go wrong at times. When a motor fails, it’s termed a “catastrophic failure,” commonly referred to in the rocket-launching business as a “cato.”

This launch day had an unusually large number of catos, including during one of my own launches.

A sad day - me holding the crumpled rocket
A sad day

I flew the “Mean Machine” twice, and the first time everything went well. It launched smoothly, flew high, the parachute deployed, and it drifted back to the ground. I recovered it and prepped it for a repeat performance.

But on the second launch, the motor suffered a catastrophic failure. It seems that the propellant inside the motor somehow exploded out of the casing, which meant a small fireball shot upwards through the rocket, superheating the body tube and blowing off the nose cone prematurely (and shearing off the parachute’s shock cord, severing it from the rest of the rocket). The body tube, which is normally very hard and difficult to bend, crumpled from the extreme heat. It then quickly cooled, and is now frozen and unbendable in its current sad and broken state.

Crumpled rocket on floor
Crumpled rocket

I’m reassured that this “cato” was solely due to a defective motor and not my shoddy construction of the rocket, and I’m further encouraged by the fact that something similar happened to several other people on the range that day. (Not that I’m happy it happened to anyone else, but at least we could commiserate.)

And to be honest, it’s not really a big deal – the fins are totally fine. With a little work, I can cut away the damaged part and re-join the two halves of the tube into a single straight rocket again. It’s a good learning experience, and it makes for a great video clip, too, which I’ll post shortly.

High power rocket construction: part 1 (motor mount)

I finally got my head out of my ass and started putting together this high power rocket. (My head is often firmly lodged in my ass, so extracting it is time-consuming and unusual.)

The basic parts are similar to those in smaller rockets. You build a motor mount (to hold the motor in place) by attaching three centering rings. These rings keep the motor mount tube centered – hence the name – within the larger body tube of the rocket. Then you attach the fins to the motor mount. All of these attachments should be made using a strong wood glue or epoxy.

Motor mount
Motor mount

Later in the process, the larger body tube will slide over the mount and will be flush against the edges of the fins, where they can be secured with glue again on the outside of the rocket. They’re held firmly in place, inside and out, which is important because of the high stresses that will be placed on them during launch.

Motor mount with fins attached
Motor mount with fins attached

Finally – you attach a steel eyebolt through one of the centering rings, using some washers and nuts and then a strong epoxy to hold it all in place. The purpose of this is so that you can attach it to a strong (and fireproof) cord inside the rocket body, where the other end of the cord is attached to the nose cone, along with a parachute inside for recovery. This allows the rocket’s nose cone to pop off just after the rocket hits apogee (its highest point in the air) and lets the parachute deploy, while ensuring that all the parts stay together on the way down.

Motor mount with fins, standing upright
The core of the rocket

As a side note, if you include an electronics bay (“e-bay”) in the rocket, which is optional, then you need two cords: one to attach the motor mount to the e-bay, and another to attach the other side of the e-bay to the nose cone, so again everything stays together. The e-bay also have steel eyebolts on both ends for attachment. Just FYI, I’m building and including an empty e-bay in this rocket; I’m not actually installing any electronics in it for the first launch. I want to keep things relatively simple for my level 1 certification flight and will start putting in some interesting electronics for the next launch after that.

If you’ve built and launched any rockets before, you’re probably rolling your eyes at how I’m oversimplifying much of this, and you also likely already identified several inaccurate statements I’ve made. On the other hand, if you’ve never done any of this before, I probably just confused you with a bunch of inadequate and lackluster descriptions.

In fact, I’m pretty sure I’ve failed to satisfy anyone at all with this post. But then, who cares?