Last weekend, I decided to build a simple proof of concept that:

• Measured the altitude of a rocket at launch
• Send the peak altitude to a iOS device via Bluetooth
• Recorde altitude versus time (for later use in calculating velocity and acceleration)

The Build

Altitude Monitor

I decided to use an Adafruit nRF52 Bluefruit LE Feather as the BLE and MCU for the PoC. My rationale was that it expect to use a similar System on Chip device for the final product.

 All of the components used for the PoC test are Adafruit products (wishlist here->http://www.adafruit.com/wishlists/486595). 

• Adafruit nRF52 Bluefruit LE Feather (will use this same SoC for the final product)
• FeatherWing proto
• BMP280 Altitude Sensor
• MicroSD card breakout 
• Battery and Stacking Headers

Rocket

I used a 3D printed nose code and wings from the US Water Rockets website. I used a 73 mm diameter 1 liter bottle for the body of my rocket.

Launcher

Finally, I built a makeshift launcher using:

• Scrap wood
• three 3D printed rocket stabilizers
• Cork
• Bicycle pump and needle

Instructions to assemble the cork, needle, and pump can be found here: https://www.instructables.com/lesson/Pump-Rocket/

Note: This is not an optimal launching solution, as the rocket will launch before adequate pressure is built. For this flight, I was only testing recording altitude, so I didn't care about getting a lot of "air"

The Flights

Going in, I knew there were going to be a few problems:

• My rocket launcher was not optimized for pressure build-up, so I wasn't going to get a lot of altitude
• I chose not to build a parachute deployment mechanism for this PoC, so my rocket 3D printed parts wouldn't last long
• My SD card might eject on landing

Given all that, I was able to get 5 test runs in before I destroyed all my nose cones. This was enough to validate the proof of concept. Each flight was about 2 seconds, and I was able to record altitude every 300 ms. In addition, I was able to send the peak altitude to by phone via BLE.

Time (ms)	Altitude (m)
0	0
<300	1.99
600	8.33
900	10.91
1200	10.54
1500	7.56
1800	1.6

Results/Concerns

The current PoC can accurately indicate top altitude, but the data increments (300 ms) is wider that I would like. 

The SD card did eject on landing, so either we have to make the landing more stable or move to SPI flash (and sync data via BLE).

I also have a problem in that I am not able to start the test at launch (primarily because I start recording data, then I start pumping, and the rocket launches whenever the pressure imbalance forces the cork out).  To fix this, I need to hold the rocket in place, achieve the designated pressure, then start the measurements at the same time of the launch, with smaller data increments.

Next Steps

• Plot data into online graphics tools to determine acceleration and velocity
• Ideate on the correct location for the altitude sensor for the final product.
• Work on parachute deployment
• Determine if we should stay with SD card or move to SPI Flash
• Determine best way to synch launch with data capture