After trying several ways to create the nosecone for the ASP (Atmospheric Sounding Projectile), I've decided I won't use any of those methods again. Here are some notes and photos of what I did, and a few suggestions for what I'd do differently. Any input would be welcome.
We (Luke, Ian and I) had earlier experimented with gluing up just the foam (styrofoam insulation), turning it to shape, splitting it and then gluing it to the wooden ribs that were to support it. Bad idea all around in my opinion. It was a pig to put (and keep) on the lathe and cutting the finished product into four equal pieces would be a pain. Maybe it'd be OK for a smallish cone but not for this one. By the way, gluing a piece of styrofoam to a wood disc and attaching that to a faceplate on the lathe works pretty good for pieces up to maybe a foot long and a foot in diameter.
The next experiment was to create a wooden support structure with a 1” dowel at the center and 1/4” plywood ribs for support. The dowel had four 1/8” deep dadoes cut into it to hold the plywood.
I then cut the ribs so that, when glued to the dowel, they would make a structure defining the size and shape of the finished nosecone. Stacking the rib blanks and taping them together made it fairly easy to cut them the same size and shape.
I drilled and cut foam circles just a little bigger than the wooden structure and quartered them.
A 3/4” center hole worked great and careful quartering produced a pretty good fit when I began gluing the pieces to the ribs.
The first attempt I made at this procedure can be seen on the lathe in the background of the picture above. I thought I might be able to do it cheaply by using liquid nails to hold the foam together. - -Wrong.- - The liquid nails ate the foam from the inside out and the whole thing blew apart under stress on the lathe.The assembly in the foreground has wood glue for the wood joints and Devcon epoxy for sticking the foam. I used 5-minute epoxy but I had extra hands there to put the cable ties on and help mix the epoxy. Working alone would definitely require slower epoxy.
This attempt went well. I planned on using a skew (angled knife) to turn the foam down until I hit wood and figured I'd get the right shape and a nice smooth finish that way. The first time I nicked the wood I could tell I'd have to change plans. Even with a razor sharp skew, the difference in density between the foam and the wood made the knife catch and pull. I ended up using a drywall sanding block and paper to get the finished shape while the lathe was running. One downside to sanding instead of cutting was that any amount of pressure applied to the sanding block wore the foam more than the wood resulting in a dip along one edge of the rib. Another downside to using the sanding block was that I couldn't get the nice crisp corners I wanted on the angles in the lower portion of the ASP's cone.
The Finished Product - Next time I'd make the wood ribs smaller than the finished cone size and fill in with 1/4” strips of foam. It would take some careful work with a skew and calipers to get the exact shape but I'd prefer that to sanding and I think I'd be happier with the results. Turning the foam on the lathe is easy and that's nice, but the stuff is so soft I ended up looking for a filler to take care of some nicks. Lightweight vinyl spackle seemed to do the trick.
Something else I'd like to try would be to turn a wooden form and create the cone out of fiberglass. Maybe glue a wooden support structure in later after the cone was removed from the form. That kind of glass work is beyond me right now but I'd love to learn. More to come, Evan
P.S. You can find out more about this project on Jim Yehle's pages at http://www.xmission.com/~jry/rocketry/projects/asp/asp-group-project.html.