There are a lot of ways to hold fins on a rocket, most aren't very good or they are expensive and/or time consuming. A couple of nice features on any design is the ability to remove and replace a fin quickly and easily, as well as move the fins/retainers to another rocket. I've tried the usual routes, composite fin cans, bolted on fins, through the wall fins and surface mounted fins; they all worked, but I wanted something better, something that met those 2 criteria I mentioned earlier. A set of cast fin brackets seems like a good option. They won't be quick to make, but I can't imagine damaging them easily so they should last for some time. What really got me going on this again is that I now have a milling machine. I'll be able to cast the brackets a bit thick, then machine them down to the final size.
Here's the initial form to cast the brackets.
I started this project some months ago. I made a fiberglass lay-up on top of the Ganymede rocket between the fins. You can see this first lay-up of glass as the top layer in the picture above. It's about 1/4" thick and was trimmed to size after it cured. I tried packing green sand around the form, pulling it out and then casting the bracket vertically, at just 1/4" thick it was a tricky job to mold in the sand, then even trickier to cast the thin object. It worked to an extent, but I wasn't overly happy with the end result. Next I was going to try building the form a little thicker and sanding it down with a slight taper to make pulling it out of the sand easier. Now with the milling machine I decided to make a big, full thickness plug and try removing the excess on the milling machine.
So I cast polyester resin in the back side of the form, I added HDPE pellets just to minimize the amount of resin. Once the resin had cured I added some body putty to finish the sides, I then sanded the sides with a slight angle to make de-molding from the sand easier.
Here's the flask opened up, the drag still has the form in it.
Here's the form out of the sand.
Here's the cope back on top.
A couple of notes about my aluminum casting...
I'd been looking for bentonite clay for some time. The only places I could find it were from internet stores and it was terribly expensive. One day in casual conversation John was telling me about his day at work and mentioned shooting bentonite into a bore (he runs a horizontal boring machine for underground pipes/wires/fiber optic, etc.). I immediately stopped him and said, "What, you guys use bentonite clay?". "Oh yeah, we buy it by the truck load" he said. Needless to say I made him sell me a bag, and now I've got 50 pounds of bentonite to make green sand.
You can see the quick flask I made in the above picture, on top of it is a thin walled aluminum tube I use to cut the sprue hole. I polished it up a bit and it works great. You don't need an expensive tapered sprue cutter!
I was looking at what to use for parting dust. You can buy graphite powder to use as parting dust, which I have some and have successfully used it, but it's fairly expensive. The other form of commercial parting dust I looked at was calcium carbonate, wait, that's chalk! So I went to a home improvement center and bought a couple of bottles of white line marking chalk, you know, the stuff you snap lines with. It works great and costs about $2 a pound.
With the mold ready to pour, I'll see if I can get some aluminum melted tomorrow and do a pour. I'm not sure how well my scrap aluminum will work for this purpose. I'm thinking I should try to get a supply of used pistons to get a better quality of aluminum for casting. But my scrap 6061 may work, we'll find out soon enough.
30 November, 2009:
I did do an aluminum melt today, but not so much a pour... I had a nice almost full crucible full of molten aluminum, had just started the pour and I managed to drop the crucible off the tool and spill the aluminum everywhere. Some did manage to find its way into the sprue hole, more than I expected, but of course I didn't get a full pour.
I opened up the flask to find the mold about half full.
There was enough cast to get an idea of how the bracket might turn out, so I clamped it in my mill vise and made a few cuts to see how it would mill. It didn't go too bad, the first cut I made with a 1/2" standard end mill and made a right angle turn. The next cut I made 3 passes with a 3/8" round nose end mill. If it looks like this will work, I may go ahead and mill down the pattern to get it closer to the finished size, which would require less mill work on the finished part. But...
I can see I need to work our a few things with my casting set up before I get too far along. Obviously I need to
make a better tool to hold the crucible when making pours. After I made the big mess dumping the molten aluminum
everywhere, I decided to melt down more scrap aluminum and pour some ingots. After quite some time the aluminum
wasn't melting, so I stirred up the charcoal and found the ash was plugging the air intake. Once opened up the
melt progressed much quicker. But I used a lot of charcoal, and it looks like the time I can keep the forge hot
is limited because of the ash build up. So now I'm thinking I better make myself a new propane furnace for melting.
I'd like to do this inside, so that means I'd need a chimney, fire proofing, an insulated furnace, a high pressure
regulator and probably a couple of burners. More time and more money but I think I need to make these investments
if I want to pursue some serious casting.
This page may sit dormant for some time before I get things set up to do another attempt.