Wednesday, 30 December 2015

Keep It Simple (Stupid) DIY Rocket Altimeter SD Datalogger project

Been working on this little project, a while back I prototyped a diy altimeter using an arduino pro mini (8mhz 3.3v) and a BMP180 barometric pressure and temperature sensor. I have now replicated the working breadboard prototype and made this tiny point to point/deadbug style version. 

The reason I have aimed to build it as small as possible is that I wanted the altimeter to fit inside the payload section of this small "Star Stryker" Estes Rocket kit I built a while back. This rocket payload section probably represents the smallest payload capable rocket on the market so the design will therefore theoretically fit into any payload section.

Weight wise it is 6.5g without the battery and currently 13.7g with a battery.. however the 200ma lipo could be replaced with something smaller (it was just what I had lying around!)

The device is activated by the switch that means that the payload section must have an access hole/port to enable the switch to be toggled with a tool.. but this is ok as the BMP180 sensor needs the payload section to be vented anyway to equalise the pressure and take an accurate reading. One slight addition I am going to make is to attach a larger LED to the pro mini. This LED will be lit on boot and will remain lit during a delayed setup part of the code (10 seconds or whatever is needed) to allow time for the operator to return to the launch controller. Then, when the LED is off, the main loop of the program is initiated writing data to the sd card at 20hz

The code currently is a bit shabby (I am no great coder!) and is just a mashup of  SD library and a BMP180 library sketches.. it's crude but it works! I'm going to work on the code with a good friend in the new year and when we have it polished (possibly dumping data to a .csv file for quick plotting) I'll opensource the project in case it's useful to others! 

Friday, 11 December 2015

OpenRocket and rocketry CNC workflow...

I've been playing with OpenRocket for a while now, essentially this little free java application allows you to design rockets and simulate their performance with an extensive database of rocket engines built in. It calculates both centre of gravity and centre of pressure and uses the Barrowman equations to calculate the stability (or lack of) in your design and has lots of aerodynamics code built in calculating drag coefficients etc. Being able to simulate launches and recovery is excellent and allows you to both optimise your design in terms of feasible altitude but also can allow you to fly within the limits of a particular altitude ceiling. Simulation also is important when you get up to qualifying certification flights for high power rocketry as your rocket needs to perform similarly in real life to what you have submitted in simulation data.

So all of this was exciting as I start to design my first scratch built rocket attempt, however an added excellent bonus is that Openrocket exports it's simulation data into a PDF document which I hadn't done until recently as I wanted to printout some data. The export function also will print out various other useful items from your design including fin templates and centring rings (rings that hold the inner engine tubes to the outer airframe tube) as PDF is a vector format it is readily accepted into most CAD/CAM environments and therefore it is a trivial matter to create tool paths to cut fins and rings etc on my CNC router. Slick!