Building a Doritos STAX rocket!

Photo Credit Peter Barrett

I'd never built an "odd rock"! Odd rocks are a category of rocketry reserved for those who create rockets from odd materials or make odd objects fly. I've seen loads of examples from flying cars to an office fan but it's never been an area I've delved into. At the milder end of odd rocks there has been a long history of people making rockets using confectionery packaging and in particular Pringles tubes. Pringles tubes are a very worthy rocket building material but they have been done a lot and I wanted to do something different. The Dorito's STAX tubes are a pleasing rounded triangular shape and I thought it might be a worthy candidate for a build. 

This build was very quick and dirty and everything was done in between work and other commitments and definitely took a back seat priority wise to other rocket projects. I started by drawing around the tube to get a rough sketch of the tube profile and took a picture of it on my phone. I dragged the image into inkscape and then hand traced to get a vector of the tube shape. The tubes aren't quite an equilateral triangle which adds a bit of faff! I laser cut some plywood centring rings into which I cut a hole to receive the motor mount tube. This was finished off with a PETG 3d printed motor retainer. 

The fins are 3mm balsa laser cut to a shape that kind of looked a bit dorito-esque! I originally planned to put the fins through the wall but ended up just gluing them on. I reinforced the oversize fins with a layer of tissue paper and glue. Due to the shape of the tube this rocket is impossible to simulate using a package such as openrocket. As such I decided to go very large with the fins to try and definitely pull the centre of pressure rearward in the airframe. The nosecone offered some challenges and I didn't really have time to create an accurate fitting cone. As a first attempt I decided to model a shoulder-less nosecone which would overlap the top of the tube somewhat rather than trying to accurately model to the tube. Having my rough nosecone model in FreeCAD to 3d print I then created a stack of lasercut plywood triangles to create a shoulder piece that could be sanded to a decent fit inside the tube. The recovery bridle was some thin kevlar and I flew this using a proven hexagonal annular parachute I'd made some time ago. Finished with a bit of satin black spraypaint and a quick vinyl cut graphic, DSTAX was finished! 

Photo credit Peter Barrett

Due to this not being sim'able, once together I decided to add around 18 grams of nose weight (some small flat lead panels epoxied into the nosecone bulkhead) and this seemed to make the airframe stable when performing the classis "swing test". The swing test is where you attach a string at the centre of gravity (with the airframe loaded with a motor etc) and then swing it in a circle, it should correct itself slightly into the wind and then remain stable, an old school approach that works well to give you some indication of the stability.  The other consideration was the maximum lift off weight limit of the target motor, an estes D12-5, which is 283 grams. All up weight of the rocket came in under the maximum at 224g. 

As you can see in the video the first light was successful albeit with a slightly late deploy of the recovery system. I think it's a little over stable by the way it turned into the breeze but it's certainly within acceptable limits. If I built another I would consider reducing the ridiculously large fin area somewhat!