Before I even came close to starting to build I played around with the design in RockSim. Here is the picture RockSim made:
If you have a copy of RockSim and would like to play around with my design....
|The design has a few interesting features. The first is that I
am using the space inside the nose cone. This usually is just dead
space. I cut off the base of the shoulder so I could get the main
parachute into the nose cone. I then had the problem of attaching
the shock cord and adding nose weight but I had figured out a
solution to both of these problems while building my late 5.5"
ATACMS. Cut off just enough of the tip of the nose so that you can
drill a small hole in it and push a piece of tubular kevlar through
it. Then spread the kevlar and apply some epoxy. Use thickened
epoxy to reshape the tip of the nose. Then to add weight just drop
the required weights into the nose tip and add some more epoxy. The
kevlar is not going to pull through the nose tip and the weight is
firmly attached to the kevlar cord.
Here you can see the nose hanging by the kevlar cord as the epoxy cures.
|I applied a layer of 6 oz. and then 2 oz. fiberglass cloth to PML 4" tubes using West Systems epoxy. This was my first try at vacuum bagging tubes and it turned out very well. I didn't take any pictures of the process but if you are curious check out the process at InfoCentral. The only variation I made to the process was to double the length of bag and double it back into the tube. This eliminated the need to support the tube with couplers, did away with bulkheads at the end, and cut down on the volume of air that had to be pumped out.|
|After cutting the tubes to length and cutting the fin slots, I
was ready to start assembly. I had already been working on the fins
for quite a while. I started by cutting the fins out of .062" G10
and then adding a skin of 1/8" balsa on both sides. I then sanded
airfoils into the balsa. This is much easier than sanding a thicker
piece of G10 (and lighter!).
At right you can see the pieces for the motor section laid out ready to be put together. I realized after (Doh!) nearly finishing the fins that the forward (fixed) fins were really 2" too short. The G10 didn't stretch very well so I made 2" extensions.
|Here you can see the assembly just before a section of body
tube went on. The shock cord attach cords are in place (tubular
kevlar), one centering ring, and the fixed (forward) fins. I
drilled 1/8" holes in the root area of the fins and then tied
kevlar line through them. That should help to keep them from taking
a side trip during the flight.
The fins were tacked on with 5 minute epoxy and then filleted with 30 minute epoxy.
|After epoxying the body tube in place over the fixed fins, I
then filled the cavity between the motor mount and body tube with
two part expanding foam (from PML). This was my first use of this
product and it works pretty well.
The cavity is completely filled for and aft and then I cut out some of the foam to make space for centering rings.
|Next up were the aft manuevering fins. On the real thing these are controlled with servo motors to provide guidance control. That is a bit more than I can handle so I just stuck 'em on The same as the forward fins. There is another centering ring just in front of the fins and they have a slight groove cut in them for the fins to fit into.|
|While filleting the aft fins I also put on the external fillets on the forward fins.|
|Before proceeding I just couldn't resist putting the pieces together. And while I was at it I put a 38/600 (I284) and 75/6400 (M1315) motor case in the picture as well. After this I epoxyed the aft body tube in place, foamed the cavity, and epoxyed in the aft centering ring. The aft centering ring is recessed so that the Aerocon motor retainer was flush with the body tube.|
|Just about the last step in the assembly of the motor section
was one more layer of fiberglass. This went from fin-tip to fin-tip
all the way around. I had previously put a band of 2 oz. glass in
the gap between the fins. I needed to fill in the fin slots so the
two part foam wouldn't some shooting out and that seemed a good
thing to use.
I also filled the gaps between the forward fins and the extensions with thickened epoxy. After all of this epoxy cured the fins were remarkably stiff.
All that is left now is: fill, sand, fill, sand.... prime, sand, prime....