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.

Why I drove 7 hours through the mountains to a rest stop

I recently finished building my first high power rocket, and mentioned in my last post the difficulties involved in finding a launch site, especially at this time of year. I’d need to find a local rocketry organization that has a launch site (i.e. either owns the land or has permission to use it) and secures the FAA waiver for the appropriate date, and then – weather permitting – I could launch. Unfortunately, this is all easier said than done.

There are only a handful of organizations in the PNW, and most of them don’t hold any launch events in the winter. I did come across one group, Gorge Rocket Club (located in northwest Oregon) with a promising launch calendar even in winter months, and there was a recent weekend with a scheduled launch. I decided to go for it.

The FAA waiver allowed a launch window of just a few hours starting at 9:00am, and the location was in Goldendale, WA, which is a 4 hour drive from my home. This meant hitting the road at 5:00am when it was still dark outside for a nighttime drive east into the mountains. No problem so far. Just need to load up on coffee and a couple of podcasts.

Snow covered mountain road with pine trees on both sides
Snowy mountain roads in Snoqualmie Pass

The drive out there would have been a bit more scenic if it weren’t in the early hours before sunrise. Due to a combination of darkness and fog, I couldn’t really see much of anything. On the way back, I did see some of the scenery; Snoqualmie Pass in particular is beautiful.

But as I drove through the pass and dawn started to break east of the mountains, I noticed the roads getting increasingly icy, and snow was beginning to fall. That’s not a great sign for a scheduled rocket launch. The further I drove, the more heavily it snowed.

Finally, as I approached Yakima about three and a half hours into this drive, I got the not-totally-unexpected news that the launch was cancelled due to heavy snowfall and no visibility. It was total whiteout conditions.

Isolated rest stop with snow on ground
Not the most desirable location in Washington

I pulled over at a rest stop to make sure it was really cancelled, and took the opportunity to make use of the facilities while I was there. Then I began the three and a half hour drive back through the mountains to Seattle (this time with some better views).

I took a picture of a sign before I left the rest stop, though. The image basically sums up my day:

I didn’t literally fall, but I definitely slipped on some metaphorical level and landed on my ass. Also, my head physically separated from my shoulders when I heard the cancellation news.

But this is all part of the fun of launching rockets. I’ve read others’ stories about their first launches where something was cancelled, or the FAA waiver never went through, or the rocket launched but then suffered some catastrophic failure. C’est la vie. It was a pretty low key scenic drive.

I may get a chance to launch again in the same area in the next few weeks, though, and if so I’ll certainly take advantage of the opportunity. Fingers crossed!

High power rocket construction: part 7 (painting)

Time for the finishing touches.

Rocket fully painted white with red nose cone, disassembled with parachute and shock cord and e-bay
Anatomy of a rocket

After covering the rocket in white primer, I used a can of white spray paint to coat it again – everywhere except the nose cone, which I painted red. I considered making it white, too, for a uniform (if overly simple) finish, but a major issue with painting rockets is that certain colors can be really difficult to see against the sky.

White, silver, or blue blend in too well and it’s easy to totally lose track of the rocket once it gets high enough. For that reason, rockets are often really bold and vivid colors, and also more than one color.

I added the “Improbable Ventures” logo, too. First high power rocket, but definitely not the last.

Finished rocket standing vertically with pine trees in background
Finished rocket

The completed rocket stands about 6 feet high. Inside is a parachute, a shock cord securing it, and a small fire blanket to protect the parachute against the extremely hot gas from the motor when it burns out and fires an ejection charge, separating the rocket in midair. There’s an electronics bay, but right now it’s empty. Prior to launch, of course, I’ll insert the motor as well.

Having built a few smaller (low power) rockets definitely helped me better understand what I was doing when building this high power one. As I’ve mentioned before, most of the basic parts are the same, and it helps to understand why you’re doing what you are doing, and not just blindly following instructions, even if they are idiot-proof. (We will see.)

The rocket is done, so my next step is to wait patiently for an upcoming high power launch hosted by a local rocketry club. But I may be waiting for a while.

While I could theoretically launch this thing by myself at any time, it’s not really practical. First, you need a proper launch pad and rails to keep it vertical during liftoff (I don’t have the equipment, but clubs do). Second, you need to find a very large area of land – many acres – that meets a long list of conditions ensuring it’s safe for launching rockets, and you need to either own it or get permission from the landowner. The launch site needs to be far away from any buildings or major roads (you don’t want a rocket crashing down, or even landing relatively softly with a parachute, in the middle of an expressway). And finally, you need to get an FAA waiver for launching high power rockets. A club will regularly apply for these waivers, which are specific to a particular date and time window.

I’ve mentioned before that our local Seattle area organization (Washington Aerospace Club or WAC) doesn’t currently have a high power launch site, so, until it does, it cannot conduct or host high power launches. There are other clubs in Washington or Oregon if I’m willing to drive 4-6 hours each way (and I am), but almost none of them host any launch events in the winter months. Things usually pick up again in March.

I just might have a slim chance in early Jan or Feb to launch with an organization in southern WA or northern OR, weather permitting (i.e. no snow or whiteout conditions). It’s unlikely, but possible. In the meantime I’m going to dive into two related projects: (1) starting to learn about electronics and building out my e-bay for this rocket (for future launches), and (2) transforming our backyard garden shed into a small workshop for rocket construction.

High power rocket construction: part 6 (applying primer)

So if you’ve been following along with this step-by-step guide at home, the construction of your new high power rocket is now complete! All the major parts are assembled, and aside from putting a motor inside, you’re ready to launch. (Note: a motor is in fact required. More on this later.)

Some people do actually launch their rockets at this point without doing anything more, like applying primer or painting or adding any decals or other finishing touches. The idea behind flying your newly minted rocket “naked” is that it’s untested and may suffer some catastrophic failure during flight – so why invest a large amount of time making it look nice?

Rocket standing upright on flattened cardboard boxes on grass, sprayed with white primer
Trying not to turn the grass completely whit

But most people take some time to class the rocket up after building it, even before the first flight. You can spend as much or as little time as you want on this.

Here, I used a spray can of primer to cover the length of the rocket. The primer should be applied as evenly as possible, continuously moving the can and its nozzle so that it isn’t concentrated too much on any single area. Better to do many short, quick passes with the spray than to focus too long in one spot and build up an uneven surface. (Of course, it can always be sanded down to make the surface even again, but this just adds unnecessary extra work.)

Full rocket standing vertically with white primer applied
Rocket imitating the pine trees

Another tip is to cover anything that you don’t want to spray with masking tape before you get started, like rail buttons or the motor retainer.

Our yard and house are surrounded by massive pine trees, so this would not be an ideal place to launch a large rocket, for a variety of reasons. But that’s okay. Look at that rocket gleam in the morning light.

Next: painting and finishing touches!

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.

High power rocket construction: part 4 (rail buttons)

One issue that is becoming increasingly obvious to me is that I don’t have a proper workbench, or workshop, or anything remotely suitable for the gluing, cutting, and other madcap activities required for rocket construction or assembly. I’m just using a dining room table. Sometimes the line blurs between utensils and tools, and I end up spearing food with a screwdriver.

The point is, if I’m going to keep building rockets – especially bigger and more complex ones – I’ll need to find a better work space.

But anyway: RAIL BUTTONS.

Rail button close-up on rocket body
Rail button on rocket body

This is one of the more straightforward parts of the rocket build. Smaller rockets (low and mid power) generally have “launch lugs,” which are like straws. Paper straws, not plastic – we’re not barbarians here in Seattle.

The idea is simply that you set up a launch pad with a launch rod – just a long, thin metal rod – and the lug slides right over it. It keeps the rocket upright while launching.

But high power rockets are bigger and heavier and require a different solution. They typically use rail buttons, instead, which is just a variation on the same concept. The buttons are like guideposts that slide along a stronger, larger rail that, again, keeps the rocket straight during launch.

Rail button placement on rocket - top view
Rail button placement

The installation of these is pretty simple. They’re just metal screws with a plastic rail guide or button, and they can be attached to the exterior of the rocket body as pictured here. They need to be a certain distance apart, and one should be as close to the rear of the rocket as possible, but the exact measurements depend on the size of the rocket you’re building.

Rail buttons can be attached several different ways. What I did here was drill a smaller hole through the rocket body into the side of the wooden centering ring, and then drill the screw into that hole, so it connects directly and securely to the centering ring. This is true for both rail buttons. To help ensure they’re secured in place, I also added a drop of epoxy into the holes I drilled prior to inserting the screw.

Another technique would be to use a small bolt with a nut on the inside of the rocket body to secure it (and again, use epoxy on the nut to keep it in place). I had already put together the rocket body and motor mount by the time I attached these rail buttons, so there was no easy way for me to do anything on the inside. I probably should have attached the rail buttons earlier in this process, but this works fine too. I’m confident these rail buttons will hold.

[fast forward to both breaking off and me sobbing uncontrollably]

High power rocket construction: part 3 (rocket body)

The motor mount is built, and the fins are attached. (Note: sometimes this part of the rocket is also called a “fin can.”) What next, you ask?

Well, next, the motor mount or fin can goes inside the rocket body, and it’s glued in place. Because the fins go “through the wall” and are already solidly attached directly to the motor mount inside, this method requires cutting the rocket tube slightly in order to slide it over the fins. The cut tube can always be sealed up again later with wood glue.

Airframe: aft end and motor tube
Airframe: aft end

If you look closely at this first image, you can see where the rocket body tube was cut for each fin to slide past it. Fits like a glove!

As a side note, this end of the rocket is the “aft” end. Aft means rear. I confess that I did not initially know this. You may already be more acquainted with nautical terminology than I am.

You can’t see them here, but remember the motor mount tube has three wooden centering rings. Just before each centering ring slides inside the rocket body, flush against the body tube wall, you can add some wood glue to seal it. You can also add lots of additional glue to the final centering ring at the aft end, but that can easily be done anytime after this.

Fins attached to rocket body
Fins attached to rocket body

Finally, while the fins are already attached on the inside to the motor mount tube, they should also be glued again on the exterior to ensure an extremely secure bond.

You know what they say about a rocket that loses a fin.

Actually, I don’t know what they say, but without getting too deep into aerodynamics here, if your rocket loses a fin it will definitely be unstable during flight and will crash. And then you will feel bad.

High power rocket construction: part 2 (e-bay)

This post is largely meaningless because I’m not actually including any electronics into this rocket, at least for its first flight. I should probably have titled it “How to build an electronics bay without any electronics.”


Electronics bay wooden sled
Starting point for building e-bay: the sled

So first, what the hell is an e-bay?

An electronics bay (or “e-bay” for short) is where you attach any electronics that you want to fly in your rocket. It’s also sometimes called an avionics bay.

What kinds of electronics would you want to fly? Well, there are a lot, and it can get pretty interesting. A few examples of things are:

  • Altimeter. Measures the maximum height of the rocket (i.e. its apogee).
  • Explosive charge. If you put black powder on the outside of the e-bay and wire it up with some electronics on the inside, you can manually detonate the charge and cause the rocket body to separate on descent, for another parachute. This is called dual deployment, as you’re deploying two parachutes.
  • Camera. A GoPro camera can be installed on the outside of the rocket body, wired to electronics stored inside the e-bay.
E-bay mostly assembled: sled and coupler tube
E-bay mostly assembled: sled and coupler tube

The e-bay is actually pretty easy to assemble. It’s just a few pieces of wood, and some metal screws, washers, and nuts. You start by gluing together the wooden “sled,” and then sliding two very long metal screws through the slots, along one side. Each end has a circular piece of wood with an eyebolt (which can hook to other things like shock cords and parachutes) and it all stays together with some washers and nuts.

E-bay completed and assembled.
E-bay complete! Minus any electronics.

Now, if I actually had any electronics in this thing, it’d be more interesting. But I built it anyway for two very important reasons:

  1. It’s necessary to act as a coupler and keep the rocket body together so it can fly in one piece. Without this, there’s nothing to connect the top and bottom halves of the rocket.
  2. I plan to add electronics to this rocket for future flights.

So there you have it. Not your granddad’s e-bay.