5 reasons to put electronics into your rocket

I’m just beginning to learn about the different kinds of electronics that can go into a rocket. It seems like there are virtually limitless possibilities, but I’ll give a quick overview below.

TeleMetrum v3.0 flight computer chip sitting on stone backdrop
TeleMetrum v3.0 flight computer

A high power rocket often has an electronics bay (“e-bay”) or some payload area where you can put various types of payloads, generally electronics.

The options are really endless, but just to help provide some overall context, here are a couple of the major types or categories of electronics that can go into a rocket.

An empty electronics bay made of wood with metal screws and rods
An empty e-bay
  1. Altimeter. This is a simple device that measures altitude, or height. It uses changes in barometric pressure to determine height (starting by setting it to zero at the launch site, so that it has a starting point). It’s fun to launch a rocket, but it’s nice to know exactly how high it goes. I’ve heard great things about the RRC3 from MissileWorks, for example, as well as the StratoLoggerCF altimeter.
  2. Parachute release. If you wrap the parachute with a rubber band so that it’s closed tightly, it won’t automatically open when it’s released at peak height. By using a very small chip, such as the Chute Release from Jolly Logic, you can control when that parachute actually opens up and deploys.
  3. GPS/ radio beacon. It’s helpful to know exactly where your rocket goes, using something to record position data. It’s also helpful for finding your rocket after it inevitably disappears from sight and you have no idea where it landed. I’ve heard several people recommend the BeeLine GPS, for example, from Big Red Bee.
  4. Flight computer. This is a small chip (e.g. the one pictured at the top of this page) that integrates several useful functions into a single device. A flight computer generally contains an altimeter and GPS/ radio beacon, but also contains “pyro channels” which can control parachute deployment. A flight computer allows a rocket to “dual deploy,” meaning you can deploy two separate parachutes, and you have a greater degree of control over when the rocket parts separate and the parachutes actually deploy. The flight computer pictured above is the TeleMetrum, from Altus Metrum.
  5. Camera. Is there any limit to how creative you can get with putting electronics into a rocket? Not really! I’ve just begun to scratch the surface, but I know people put a GoPro or other camera on the outside of the rocket and record video during launch, so that you see the earth receding underneath. For some of the biggest rockets, you can even glimpse the horizon and the edge of the earth’s atmosphere.

My initial goal is to just figure out what I’m doing (I have no experience working with electronics) and put together the basic parts to create a functional e-bay with a flight computer. This is one of my 2020 goals – and specifically a January 2020 goal because I have a lot more to do this year.

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 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.”

Meh.

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.