Pre Covid I was mentoring a school team towards taking part in a UK rocketry competition and I'd bought a few bits and pieces on their behalf. Some estes BT80 body tubes, a matching nosecone and some F36 cti motors. Of course Covid scuppered the team and I had these bits sat here looking at me, so I decided to build something. I'd never really designed and built something for the upper end of mid power, I have models that I've designed up to D on 24mm diameter motors and then the bigger HPR stuff. I also realised that a simple reasonably wide airframe mid power rocket is an excellent test vehicle for testing DIY parachute designs and getting a bit of data.
"Chonk" Is quite a straightforward build in essence, I CNC'd some centering rings and CNC'd the through the wall fins. I decided to put a fine layer of fibreglass on the rings but I haven't glassed the fins. I find through wall fins make a very strong fin can and as such I felt with everything stuck to the 29mm phenolic motor mount tube that it was all going to hold together. Despite being a simple single deploy airframe I wanted to get flight data, where possible, to make it a decent test platform so I performed extensive butchery modification to the off the shelf NC80b nosecone.
I used the bandsaw to take off the base of the nosecone and then installed a 2mm threaded rod by pouring around 10mm depth of laminating epoxy into the nosecone tip. I left a nut on the threaded rod in the epoxy to act as a decent anchor. I also CNC'd a plywood ring to sit in the lip of the shoulder and receive the closing bulkhead and later I added a couple of M2 nuts and bolts to further hold the bulkhead and NC all together. I then made a bulkhead with a simple soft retainer and reinforced the bulkhead with fibreglass and a wooden centre washer. Inside the bulkhead I installed some uprights around the threaded bar area and installed 2 altimeters, a perfectflite Alt15 and an Eggtimer Quark the latter of which I had wired up to an openlog board for it to write flight data to an SD card.
To arm the altimeters I used the CNC screw switches which I designed and built quite some time ago, they worked flawlessly and maintained continuity throughout the flight. As the nosecone is quite heavy it's gets some decent loads/forces on ejection and deployment so it was a good test. A couple more test flights with this design and I might be tempted to make a few and stick them on tindie to sell.
Chonk flew last weekend in great conditions at Midland Rocketry Club and I'd made a 60cm 10 gore annular chute to fly in it. Chonk was probably slightly over chuted but a very steady and stable descent was made and I do like how people watching react to the annular chutes as they aren't a common sight at UK rocketry events yet! I reduced the delay to 7 seconds, but definitely it was a little late (even better test for the screwswitches!) and if I fly in this configuration again I'd take the ejection delay down to 6 seconds.
One of the best things that happened at the event was a fantastic rocketeer and accomplished modeller Pete turned up with a downscale micro version of "Chonk"! He'd seen me making Chonk on twitter and decided to make one using a micromaxx 6mm motor! Such good fun and his Micro Chonk flew very well!
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