Robert's Rocket Project

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2022-01-17 17:30:11

Lots to talk about - been busy over the past few months and made a lot of progress.

Around the beginning of 2020, I finished up the structural attachments for the parachute and nosecone. I'm using a dual-deployment configuration with a 120 inch main (Cd 2.2) and a 60 inch drogue (Cd 0.97), both from Rocketman Parachutes. I used OSCALC to help with the sizing. I'm not a "rule of thumb" person so instead of using extreme lengths of oversized shock cord, I wanted to take more of an engineering approach, partially based on inspiration from this post. I ended up designing the cross-shaped bracket for the main to handle around 1000 lbf and the drogue to handle 200 lbf but I plan to beef up the drogue load capability (see below).

The nosecone is held in place during flight by two #4-40 nylon screws. I found various references on the shear capability of nylon screws but there was too much variability in the information I found so I set up my own shear testing rig with weights. My testing showed a #4-40 nylon screw fails between 40-45 lbf in single shear so two of them provides 90 lbf of retention. The design bay pressure for my pyroless recovery device is 7 psi resulting in 198 lbf of force when it pops the nosecone. Tests showed somewhat less than that due to leaks which I was able to partially address (including sealing connectors). Once I settled on a stable configuration, the nosecone has deployed reliably in 25+ ground tests. Instead of using a separate pressure transducer to monitor the pressure in the pyroless recovery device, I turned the entire pressure vessel into a transducer by mounting strain gages on the outside of it. This saves weight and has one less place for the compressed air to leak out.