The majority of the primary structure is complete (some skins not shown). The skins are rolled 2024-T3, 0.016 thick, with dimpled holes for #6-32 countersunk fasteners. I made a jig to drill the holes for the stringers and it generally worked with just a few of the holes needing to be tweaked afterwards. With such a thin skin, dimpling makes a stronger joint and is more tolerant to misalignment than a regular hole would be. The biggest pain was making the frame couplers that are used at the fuselage frames to tie the skins together. The fins are trapezoidal, made out of 6061-T6, 0.090 thick, with a 10 inch root, a 4 inch tip, and a height of 6 inches. A flutter analysis shows a 1.5x margin over the expected flight profile. The fin mounts are adjustable in 1 inch increments in case the CG ends up in a different location than predicted. I had to add some reinforcing rings to stiffen up the fin mounts as the stringers alone (even with the skin attached) were not stiff enough. The access panels didn't turn out too bad but the thin skin made them a bit tricky to fabricate. The pressurization bay has one for the helium fill and the avionics bay has one for the power panel.

The next steps are to integrate the parachute, nosecone, and the pryoless recovery device. Then all the existing components can be re-installed into the airframe.


I performed component testing today of the pyroless recovery device. The device is based on Troy Prideaux's Pyroless Deployment Device with some changes. Similar to Troy's design, it uses two different sized pistons to generate a small differential force that can be handled by the latch mechanism. It also uses a double-flat gear motor and bearings to retain the piston assembly. The main difference is instead of using gears to transfer the motion from the gear motor to the release shaft, it uses the motor shaft directly since it is designed for a 6 inch airframe and there is more room. The design pressure is 304 psi which should generate about 7 psi and 198 lbf on the nosecone with an 18 inch long recovery bay. The gas will probably be N2 since I will have that on hand at the launch site for bay purges prior to launch. To fill the chamber, I'm using a Dill Air Controls VS-743 aircraft tire strut valve which will be located in the pressurization bay with a 1/8 tube connecting it to the pyroless release device. This eliminates the check valve inside the device and allows remote fill. The plan is to use strain gages directly on the chamber instead of a separate pressure transducer to monitor the pressure in the cylinder but I haven't added them yet. The total weight of the device is 0.394 lbm, a bit heavier than I wanted. The chamber OD is 2.0 inches with a wall thickness of 0.034 inches and an internal volume of 11.7 in^3.

Prior to pressurized air testing, I hydrostatically tested it to a proof pressure of 600 psi. There were some minor leaks in the upstream plumbing but none in the device itself or its fittings. All I had on hand today was compressed shop air so I used 120 psi to check out the operation. For the first run, I ran the gear motor at 3 V (the rated voltage) but the shaft turned too fast for the mechanism to release. For the 2nd and 3rd run, I ran it at 1.5 V and it released as expected. I had planned for up to a 0.1 inch gap between the top of the tube and piston but when it released, the actual gap was much less. I was expecting the momentum of the piston assembly to continue to move it but it never hit the stop. That is good because I was worried about repeated hits against the stop wearing out the assembly. Since it is filled remotely, I used 1/16 inch 3003 aluminum tubing to provide a flexible connection between the fixed fitting and lower piston. I originally planned to use stainless steel tubing but it was too stiff for a flexible connection. After each run, the o-ring at the top piston departed the assembly so other than the gas, that is the only consumable. The main advantage of this device is that it can be tested multiple times prior to flight and is completely inert prior to the launchpad where it will be pressurized.

The main work the past few months has been to complete the structural design of the skin and integration of the fins and launch lugs. It turned out to be significantly more design work than I expected. However, the four major stringers are drilled so airframe assembly should start soon.