Tuesday, July 14, 2009

8" MLAS construction notes, part 1

I'm writing this to document how I'm building the nose section. If it works I'll use this for part of an EMRR review. You can follow the MLAS tag if you aren't familiar with what the real one looks like.

The ~2.5" body section and shoulder shown in the photo is 8.25" Sonotube. I really could have used a suitable power saw but, since I don't have one and wanted to get started, I cut the pieces by hand. The shoulder was made using the tried-and-true process of sectioning the tube and using the leftover strip to join the sections.

The nose is the top of a big plastic Easter Egg. Or is it the bottom? Philosophers, let the debate begin :) I'll try to explain how I cut it. There was a spot on the end-to-end seam that appeared to be the apex. I set the egg, with the 'top' section down, in the Sonotube. I measured the length of the exposed seam and subtracted that from the total seam length, divided by 2, and used this to mark points around the circumference. This provided a 'nose cone' that fit inside the tube. Next, I aligned the upside down egg in the tube using these marks and used used a ruler to measure the distance from the tube's rim to the point where the tube's outer diameter would intersect the cone. The marks were extended by this distance and I added a fudge-factor to account for the measurement being added along the surface rather than on a line parallel to the body tube. (Now, that should be clear as mud.) The egg was cut using a Dremel cut-off wheel. I probably should have done this more scientifically, but I lucked-out and the fit is good.

The resulting nose cone was really floppy and it didn't fit naturally on the tube. I slid the body tube section over the shoulder section and adjusted its position so that the shoulder supported the nose cone (i.e. the shoulder extends up into the cone by about 3/4 inch). I then glued the body section and shoulder together. To help with the fit, I filled the angled are between the shoulder lip and the body using Fill'n'Finish. When dry, the nose fit really nicely. I then screwed the cone to the shoulder in 4 places.

I cut four fins from 1/4" balsa. These are tapered in their leading and trailing edges and should look nice. I'm made the nose 'engine pods' from sectioned pieces of BT-5 whose ends are capped with cardboard. Holding anything onto the nose cone plastic will be difficult so I added some screws from the inside of the cone. I filled the pods with rods of pink foam and trimmed them with a small hot-wire cutter. I used a dab of epoxy where the screws penetrated the foam and tile caulking to hold the pods on. I'll post a photo a bit later.

Next up will be the inner structure in the cone. I plan to use a plywood ring centering a 3" tube. I had though this would hold a 'chute but now realize a G motor will extend halfway into the upper body. I may devise a piston scheme using telescoping 3" tubing.