Monday, 13 August 2018

Making Drogue and Main parachutes... aka SEWING IS HARD!

I'm trying to get the ODR rocket build finished and am finalising the electronics bay for dual deployment where it will blow a charge and release a tiny drogue parachute at apogee and then deploy another charge to push out the main chute at a predetermined altitude on the way down.

I wanted to use a crossform parachute after all the work I put in working through the maths of them (paper and calculator spreadsheet here) and as ever I wanted to make them myself...although 5 hours in on the sewing machine I began to question why!

For my Panta Rhei flight I used a reasonably heavy ripstop nylon and I cut it with a soldering iron which gave an excellent edge that required no hemming, however this wasn't suitable for the job for the ODR rocket. I needed to consider packed sizes of the chutes as ODR is a slim rocket with not masses of room in either section for the recovery system. As such I wanted to use some very lightweight ripstop nylon I have as it would pack much smaller, however I also needed it to be strong as with dual deployment the main chute is deployed when the broken air-frame is falling at some speed. I designed the drogue to bring the falling air-frame down at about 20 meters per second as I had seen similar fall rates on online that others had used. The main is designed to further drop the descent rate to around 5 meters per second which is quite a large change in velocity so it needs to be pretty strong.

I decided that I would cut the nylon traditionally and then stitch a double hem to add strength to all the edges. I first used baking paper to cut an over size pattern and then cut the rectangle sections and pinned the hems. This was essentially the same process for the tiny drogue and for the main. I made 2 rectangles and then pinned and stitched them together to form the cross. The sewing machine I borrowed of my daughter is an excellent albeit small machine and it didn't have the clout to stitch through the 2mm nylon cord I was using for shroud lines so they are hand stitched onto the drogue. For the main chute I wanted to reinforce the area that the shroud lines attached to on the chute and attached some small squares of ripstop into each corner, this took ages to do as I hemmed the squares first and then attached them but it should hopefully increase the chances of the chute surviving deployment!

If I had access to a more capable machine and also had more room in the air-frame and could overbuild the chutes slightly I would definitely consider making the lines continue all the way through the chute to make a very strong version.

I've not quite finished yet and have to fit the shroud lines to the main but I've finished the shroud lines on the drogue chute and am pleased with the way I have worked out to terminate the attachment end. I first measure out the desired length (1.6 times the length of the rectangle side from the calculations which equalled 56cm) and then stitched the lines together. I then individually stitched a matching length loop into the end of each of the 8 shrouds and then I stitched the 8 shrouds together and finally with the aid of a bit of stitching and a few drops of superglue I wrapped all the shroud ends in some thick polyester thread. Very pleased and I will use the same technique with the shrouds on the main.

Finally .. whilst I had my daughters small sewing machine set up I stitched some small Nomex fireproof blankets to protect the parachutes from the deployment charges. I definitely rushed these through and they are rough but functional!..

So there we go, learnt loads and got to grips with the small sewing machine... I could definitely do with continuing a bit of sewing to increase my skill at it!