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!

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