Thursday, March 2, 2017

Structure: Carbon fiber cutting and bonding methods

Ever since I finally got the internal structure re-configured for optimal balance and aerodynamic stability using some serious 3D modeling, I have been working frantically like Doc Brown jacked on Red Bull to get the structural components built and assembled.

Soon I'll post a walk-through of the entire structure and the various components, but for now I thought I'd show you what the parts look like and discuss how I processed them. This picture shows the whole suite of structural components, which are all fabricated from carbon fiber.

Need more fiber in your diet? Carbon fiber, that is.

The closed sections (round and square tubes) were purchased from Rockwest Composites and cut to length. The round tubes are mortars for the parachutes, and in the photo you can see one of the 60-inch parachutes peeking out of its mortar tube - stay tuned for excellent videos of explosive parachute deployment! The square tube is the main strut that runs vertically down the center of the HAPP.

The flat sections, also from Rockwest, were a bit more tricky. These are, variously, the internal decks (round), three aerodynamic strakes (triangular), and holders for the parachute mortars (various small pieces). I purchased rectangular plate stock of various thickness from Rockwest and had to machine the parts using a CNC mill. Thickness ranges from 0.7 to 2.9 mm, depending on the part. The largest single plate was 24x24 inches and 2.2 mm thick (SKU # 403-22). Not cheap!

Although I've received my checkout to run the large CNC flatbed router at Maker Works in Ann Arbor, Michigan, in the interest of time I had the awesome Jason Davis at Pioneer Cuts do most of the work. This is what the process looks like:

To save weight and maximize flight altitude, I had to abandon the aluminum mounting system described in a previous post. Now I'm bonding the structure together with specialty adhesive - in particular, this 2K epoxy from 3M. This approach means the craft will not be easy to repair if any parts are damaged in flight testing, but such are the engineering tradeoffs in aerospace!

It also means the assembly process is a bit more complex. To hold the parts together while the epoxy does its bonding magic, I decided to 3D-print some assembly jigs using high-accuracy SLA. Here's one of the jigs. This one holds the end caps onto the parachute mortar tubes. I made a total of 5 different jigs. Thanks as always to Steve and the great 3D printing team at ThingSmiths in Ann Arbor.

3D printed assembly jig on
end of parachute mortar tube

All together, what you see laid out on the table in the first photo is the entire structure of the HAPP. It weighs in at 1671 grams - not bad for a craft that's 1 meter in diameter, almost as tall, and will support a lot of serious hardware, including twin pressurized gas tanks, pneumatic valves, cameras, a custom-molded carbon/kevlar aeroshell, and a few other important bits (like the pyrotechnic-deployed parachutes!). All of which will be exceeding Mach 1 on a plunging descent through the atmosphere after flying to the edge of space...

No comments:

Post a Comment