This week’s post is all about integration to see how the payload fits into the rocket. Let’s first review the content of the rocket’s electronics bay (or e-bay). At least, most of the components, because few items are still missing and others may evolve. In particular, I may add few extra sensors such as a high-g accelerometer (in the 200g range) and will add a camera to capture the view. There are two main computers on-board. The Joule, described in my last few posts, will be the test and development platform. This board will host the software to run experimental avionics applications, and whatever else one may want to do at high speed and under high acceleration. Although the Joule could handle the flight computer functionality, I’ve decided to not put all my eggs in the same basket, and go with a dedicated flight computer. I’ve picked the Altus Metrum TeleMetrum v2.0. It is a dual-deploy controller board associated with a GPS and telemetry module. This computer will control the explosive charges to perform the dual deployment. In the short, this simply means that the computer will monitor in real-time the altitude – with a precise barometric altimeter (and not relying on the GPSes) – and ignite two sets of charges at two different preset altitudes.
The first explosion, at the flight apogee, will eject the drogue parachute. This chute will allow the rocket to fall back to earth at a controlled but high speed. This is done, so the rocket cannot drift too far from the launch site because of the altitude winds. The second exposition deploys the main chute, which allows – in theory – a soft and controlled landing. In other words, this board is the most important computer in the rocket. For the same security reasons, it will draw its power from a dedicated battery. Note that I’ve presented in a previous post (titled Telemetry), my plan to use an XBee-based solution to a provide secondary telemetry source. Currently, I expect the Joule to collect the GPS data (from the Adafruit GPS board) and use the XBee to send the telemetry – position and experimental data – to the ground station. There is another option though – as mentioned in that post. I could collect the GPS data directly from a programmable XBee module. If you are interested to see the difference between a programmable and a non-programmable XBee Pro, feel free to scroll to the end of the pictures where I’ve added a shot on the topic. Last but not least, let me write a few words about the e-bay (electronics bay) itself. The cardboard cylinder is my mock-up. The two eyebolts are the attach points of the two chutes. The electronics are installed on the sled that slides on the two bars. The e-bay itself slides into the two tubes forming the frame of the rocket and is maintained by shear rivets. Although it is fully functional, my rocket is made out of fiberglass. The green-ish cylinder shown in the photos is the actual e-bay I will use. The good news is that it is slightly longer and that I will use a longer sled. Ideally, I would like to 3d print it, but I don’t have a 3d printer… Even though the actual e-bay will have more room, it will be seriously jam-packed! Have a great weekend.