Rocket ground testing for electronics bay

After a long self-taught (and frequently confused) journey involving assembling this rocket, and building the electronics bay, I just needed to track down some black powder, set up my ground station, and conduct a bit of testing. Basically, the idea here is that the e-bay (roughly in the middle of this rocket) has a flight computer on the inside and a black powder charge on the outside, and I can remotely detonate it, causing a small but controlled explosion that will separate the rocket and deploy the parachute.

The black powder was surprisingly difficult to find. The Seattle metro area certainly has no shortage of gun shops, but virtually none of them actually sell black powder. I called more than a dozen shops at increasingly far distances from the city limits in all directions, but it seemed like nobody sold it. Everyone pointed me to other shops.

Finally, I located a place about an hour’s drive away, and after they confirmed they had some in stock, I made the trip. The specific type of black powder I was looking for was FFFF.

Rocket on wooden stands in yard, ready for ground test
all systems go

Like Jason in ancient Greek mythology returning with the golden fleece, I completed the epic journey, explosives in hand.

When it comes to the rocket separation charges, you want to use an amount of black powder that is sufficient to separate the rocket with some real verve, but also not so much you damage or destroy the rocket. Based on an online calculator, I started with 0.5g or approximately 1/8 teaspoon of black powder. This was the lower end of the estimated range (roughly between 0.5g and 1.0g) but it’s generally a good idea to start with less, test, and work your way up.

Rocket on wooden stands in yard, ready for ground test (different angle)

How do you actually pack the black powder after measuring it out? It goes into the appropriate container attached to the outside of your e-bay (on mine, a small PVC pipe end cap. But the real explosive force happens when the powder is packed tightly and confined to a small, totally sealed area.

This requires adding some “wadding” in the PVC end cap, and then covering the opening completely with tape. Any kind of tape will do the job – masking, electrical, etc. Again, the idea is to ensure the black powder is tightly packed and stays that way, and remains sealed off.

Having prepared this, I readied the rocket by putting the e-bay inside and then activating or arming the flight computer. I also have a “ground station” that can communicate with (and control) the flight computer remotely, consisting of a small handheld Yagi Arrow antenna, a TeleBT (dongle that connects to the antenna), and laptop using Altus Metrum software.

Using the software on the laptop, I armed the flight computer’s pyro charge and the countdown began. 3… 2… 1… fire!

Rocket on the grass after separation charge
post separation charge

The ground test worked and the rocket separated perfectly. My conclusions? I’m going to do more testing and will increase the amount of powder – gradually – to the high end of the range to see what works best.

I also need a sturdier structure to hold the rocket in place. I’m just using some makeshift wood supports I quickly threw together.

But the exciting thing is that it worked – with the push of a button on a laptop, I remotely activated the separation charge. The rocket separated, and the parachute deployed.

Dual deployment, here I come!

How to build a rocket electronics bay

I was originally going to create a series of articles dedicated to this topic: building an electronics bay for a rocket. Having never done this before, and having no idea what I was doing when I began, it took me quite a while to figure everything out and to actually build this thing.

In the end I decided nobody cares how long it took me to do this, and everyone is better off with a summary, even if it’s a slightly lengthy one. Quick table of contents based on the section titles below:

  • Why am I here?
  • What you’ll need
  • More about the flight computer
  • Step 1: Decisions and planning
  • Step 2: Attaching the components
  • Step 3: Dual deployment capabilities (optional)
  • Final thoughts

Why am I here?

So to get started: I’ve covered in a few previous posts what an electronics bay (or “e-bay”) is, and why you might want to build one. Just to recap here, an e-bay is not strictly necessary to launch a rocket, but it lets you do a variety of cool things. For example, with the right electronics, you can measure and record exactly how high your rocket goes; fire charges to deploy one (or two!) parachutes with more precision during the flight; track its location after it disappears from sight and inexplicably lands far away; and much more.

Flight computer - circuit board
flight computer

But assuming you already know what an e-bay is and some of the cool things you can do with one, the next step is building it.

What you’ll need

There are a lot of different ways to go about doing this. A simple e-bay can have minimal components. For example:

  • an altimeter to measure the rocket’s maximum height (it actually measures barometric pressure and uses that info to deduce the height);
  • a battery; and
  • an on/off switch.

That’s it, for the main components. In addition, you’ll need:

  • copper wire to physically connect things together (if your switch doesn’t already come with wires); and
  • some way to secure everything in place during flight (e.g. screws, or glue, etc.).

This last bullet can include nylon screws and washers (which I used for the flight computer), or just a lot of glue, or rubber bands or zip ties… you can get creative.

This simple e-bay wouldn’t have any ability to communicate wirelessly with things outside of the rocket, but it doesn’t need to. As long as you can locate your rocket post-flight and remove the e-bay and altimeter, it will provide you with useful data.

You can also go toward the other end of the spectrum and make the electronics as complicated as you want. But the basic concept is the same. You have at least one circuit board or flight computer, powered by at least one battery and connected to an on/off switch.

More about the flight computer

I chose to start with the TeleMetrum, which is a flight computer from Altus Metrum. It combines the functions of an altimeter with a few other abilities, including firing two separate pyro charges (for dual parachute deployment), GPS tracking, and a radio transmitter – hence the long antenna.

Physically, the TeleMetrum is just 1 inch wide by 2 3/4 inches long. It’s amazing how much cool tech can be crammed into such a small board. The antenna is 7 inches long, and ideally for this particular board you’d want an e-bay with at least 10 inches of interior length to accommodate the board and antenna. My e-bay was less than 8 inches, though, so I needed to extend the antenna somewhat outside of the actual e-bay. The antenna is flexible wire, but it’s best to keep it as straight as possible.

E-bay partially built, mid-construction next to ruler
e-bay mid-construction

I’ve also previously posted about building the e-bay minus any of the electronics, so I’ll just skip ahead here, assuming you have already constructed an empty e-bay based on my spectacular instructions and are ready to add all of the fancy gadgets and components.

Step 1: Decisions and planning

As noted above, you’ll need to first decide exactly what you want in your e-bay. Do you just want a simple altimeter to measure height? Do you need to fire pyro charges to be able to do dual deployment? Do you want GPS tracking and radio communication with your rocket?

For my purposes, I wanted all of the above, which is why I selected the TeleMetrum after carefully reviewing the options.

I also got a rechargeable 900 mAh LiPo battery from Altus Metrum. It’s really small and lightweight. Finally, I got an unnecessarily large push-button on/off switch, and some 20 awg copper wire from Home Depot. As a side note, “awg” technically stands for American wire gauge, but this would typically be referred to as “twenty gauge wire.” Somewhat counter-intuitively, the larger the gauge number, the smaller or thinner the wire diameter. The one I bought, 20 awg, is sometimes called “bell wire” because it’s used for common household purposes that require small amounts of current, like doorbells or buzzers.

E-bay nearly complete
e-bay nearly complete

You can see from the pictures above how I placed the components in my e-bay. Simple, right? To be honest, I’d say at least 95 percent of this is just planning and understanding what you’re doing – making sure you have all the right parts, you understand how everything works together, and where exactly it will be placed. Once the planning is done, the rest is a piece of cake.

Step 2: Attaching the components

To attach the flight computer, which has pre-drilled holes right in the circuit board, I drilled 4 holes in the e-bay wooden “sled” and used 4 nylon screws. I also added a dab of epoxy to hold them in place, just in case. I’ve actually heard that nylon screws work really well for this purpose, because they will shear. If the rocket suffers a catastrophic failure or really rough landing, the impact may shear the nylon screws (which absorb most or all of the force), but preserve the flight computer intact. I don’t plan to test this out, but it can’t hurt.

The push-button switch I simply glued in place with epoxy. It’s important to note that you’ll also need to drill a small hole through the external wall of the e-bay, and that hole should line up with the switch. You should be able to push the button – and therefore arm or disarm your rocket’s flight computer and electronics – from outside the rocket by simply inserting a pencil, screwdriver, or other small thin object through the hole to push the button. Make sure the hole lines up with the switch!

Finally, to help secure both the switch and the battery in place, I cut a few very small pieces of wood and secured them using wood glue. This isn’t strictly necessary, but it helps give extra security to the switch, and keeps the battery from moving around. I also used a zip tie with the battery, which I can cut if I needed to remove or replace the battery – though that isn’t likely.

e-bay view of one wooden end cap, with steel eyebolt, white pvc end cap and barrier strip attached
e-bay view of one end cap

Depending on what kind of electronics you’ve chosen and/or what you want to do with your flight computer, you might be done at this point. If you just have a simple altimeter, or you aren’t interested in dual deployment (yet), you now have a finished e-bay. Congrats!

However, if you are interested in dual deployment, or you just enjoy exposing yourself to dangerous materials and explosions, read on.

Step 3: Dual deployment capabilities (optional)

For dual deployment, you’ll need – in addition to the above list – the following things:

  • two small (1 inch) PVC pipe end caps;
  • some small screws and matching washers;
  • black powder (recommend FFFF); and
  • electronic matches (“e-matches”) or electronic igniter, such as the MJG Firewire Initiator.

In addition, while not strictly necessary, you may find it helpful to also have:

  • two 2-way barrier strips; and
  • at least one 4-circuit male connector and one 4-circuit female connector.
underside of e-bay
the underside

You can see a white PVC pipe end cap and white two-way barrier strip in the photos above. One of each is attached on the outside of the e-bay, on each end, and they’re secured by drilling a hole, using a screw and washer, and also adding a few dabs of epoxy for good measure to hold everything in place.

The PVC pipe end cap will hold a small quantity of black powder, which will give you your explosive charge, separating the two parts of your rocket at the appropriate time and deploying your parachute. The black powder is ignited with the e-match, which is wired up to the flight computer (which tells it when to activate).

The reason for the barrier strip is to connect the e-match to the flight computer without having to disassemble everything every single time – it’s just more efficient to have permanent wires running from the flight computer to the barrier strip (which, again, is located on the outside of the e-bay for convenience), and to just be able to swap out the e-match more easily each time.

Similarly, the reason for the 4-circuit male and female connectors is just to more easily be able to pull your e-bay out and access things inside. With the connectors, you can simply disconnect the wire and pull things apart much more easily, and you can also use a shorter amount of total wire which takes up less space and doesn’t clutter up the inside as much.

finished e-bay, next to cardboard outer tube which doubles as a coupler in the rocket body
finished e-bay!

Final thoughts

In this last picture, you can see the overall build of my e-bay. It’s nothing special to look at, but hey – it’s my first one. I also thought it was important to leave as much additional space as possible in case I want to add more electronics later, either for redundancy or to provide new capabilities for the rocket (GoPro camera anyone?). But your layout is up to you.

If all goes well, this whole setup will allow me to do much more in high power rocketry and accomplish a variety of goals I’ve set for myself in 2020.

A few final thoughts and some helpful tips:

  • Plan. As mentioned above, most of the work here is just planning and understanding what you’re trying to do, and ensuring you have the right parts. Once you’ve solved for all of that, building is the easy part.
  • Glue. When in doubt, use more wood glue or epoxy, not less. You can secure the components many different ways and have a lot of options.
  • Layout. Leave room for additional future components (if you have the space for it). If not, no worries.
  • Get creative. My antenna didn’t quite fit within the e-bay so I drilled a small hole to let it poke outside. And since it’s kind of close to explosive black powder, I shielded it with part of a plastic straw.
  • Label everything. It’s good to sketch out what you’re trying to do ahead of time, and it’s also helpful to label parts as you go. You can see, for example, I wrote “TOP” with an arrow on the outside of the e-bay to make sure it’s inserted into the rocket body the right way. Once the e-bay is completely sealed up, it’s not always easy to remember which way is up!

Next up, I have some ground testing to do – before sending this thing thousands of feet into the air.

How to cause a fiery explosion

I’m getting all the parts together to build an electronics bay for my first high power rocket. Totally winging it here.

An electronics bay in a rocket, capable of dual deployment of parachutes, requires a couple of things: a flight computer or some similar electronic chip; a battery to power the chip; an electric match or igniter; and some explosive black powder. And, of course, some additional cords and pieces needed to wire everything together properly.

The basic idea is that the flight computer activates a “pyro charge” at the appropriate time. This electrically ignites the match, which in turn creates a spark inside some very tightly packed black powder. Which explodes – with some real verve.

It’s not as crazy as it sounds. The explosion and resulting expansion of hot gases causes the rocket body to separate at a pre-planned location, and a parachute deploys, assuming all goes well.

Firewire initiator, aka electric match
red end goes bang

This is a “firewire initiator” from MJG Technologies. In other words, it’s an electric match.

Apparently the federal Bureau of Alcohol, Tobacco, Firearms, and Explosives (“ATF“) generally regulates electric matches and igniters, but this is the only non-regulated version on the market. It’s made specifically for rocketry.

I previously uploaded a quick video of me testing one of these, but the real fun will be combining it with the compressed black powder, for one of your more vigorous explosions. Stay tuned for more ground testing.