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This page shows the
steps performed to create the fin fillets on Angelfire. Of course,
before adding the fin fillets the booster section had to be built.
Booster construction can be seen here. |
| Click on any photo below to see a larger image
of it. |
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The first step
for the fin fillets is to rough up the corner where the fin meets the body
tube. A small length of 1/4" square dowel was used with some 100
grit sandpaper wrapped around it. This helps give the epoxy something to
"bite into" on the otherwise smooth fiberglass surface. |
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After sanding I removed all
the dust by wiping the joint with a clean rag and isopropyl alcohol.
Next I
marked the body tube and fin with a straight line by putting a 3/8" wide wood strip into the
corner of the joint. This pencil mark was used to align the masking
tape shown below. |
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A wide masking tape was
aligned to the pencil marks on the body tube and fin. This blue 3M
masking tape is great stuff! It adheres well but will not leave any
residue when it is removed. It is also very easy to remove. It comes
off in one piece.
The tape was applied to the
body tube and fin in order to make clean-up a whole lot easier. The
excess epoxy will end up on the tape and can be lifted right off. |
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The technique I used to
create the concave fin fillets is one I first saw on the web, but I can't
remember where. The idea is to use a short section of PVC pipe.
The size of the pipe is selected to give the desired fillet size.
This photo shows the length of pipe I selected and I'm testing the fit to
see that it will shape the epoxy into the corner the way I want it and
leave excess epoxy on the blue tape. The piece of PVC pipe is
tangent to the body tube and to the fin, right at the edge of the masking
tape. |
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The short piece of PVC pipe
can even be rotated around the end of the fins and it will shape the
fillets on those areas very nicely too. |
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A little more care is
needed to rotate it around the forward end of the fin, but with a little
practice I developed the motion and technique that I wanted to use and
then was ready to add the epoxy and try it for real. |
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First step when adding the
epoxy was to wet the corner with plain
West System epoxy (no fillers.)
I used a small disposable brush to apply a generous amount to wet out the
surface. |
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Next step was to mix up a
batch of epoxy and thicken it with some
#406 Colloidal Silica filler.
It makes a nice creamy mixture. I made it thicker than mayonnaise but not quite as thick as peanut butter.
It was thick enough to hold a shape without running. It was then
applied into the fin joint and the PVC pipe was used to shape it and push
excess epoxy onto the blue tape. I found it best to rotate the pipe
around its long axis while I slid it along the fin joint. It also
helped to dunk the piece of PVC pipe in a small bowel of isopropyl alcohol
to wet the surface of it in order to help shape the epoxy without so much
smearing and pulling. All in all, it worked pretty well. |
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The photos above and below
show the results after several passes with the PVC pipe. The excess
epoxy was pushed out onto the blue tape. Once the fillet was formed
like this, the blue tape was then removed before the epoxy setup and
cured.
This method also ensures
that all three fin joints will have exactly the same amount of epoxy and
exactly the same shape. The results were much more consistent than
in the past when I shaped the fillets by free hand. |
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Before and after removing the
blue tape. |
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Once all three fins had
been completed, it was time to do some wet sanding to put the final finish
to the fillets. The photo below shows my setup for wet sanding.
The table is covered with a large sheet of 6-mil clear plastic like that
available at most any hardware store. (Home Depot for example.)
A clear plastic container has an inch or two of water in it to soak the
sandpaper before use. I use 320 grit "wet-or-dry" sandpaper. A
water spray bottle is used to wet down the airframe for sanding. Wet
sanding works really well for getting a nice smooth finish. The
sandpaper does not "load up" with debris and the epoxy mixture sands
reasonably easily. The edges of the fin fillets were blended nice
and smooth. |
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Wet sanding setup. |
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Working on a fin joint. |
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Applying some Icing. |
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After the sanding, I
noticed there were still some surface defects and undulations in the epoxy
joints. These were filled with Icing. In fact, I just filled
the whole joint and wet sanded it smooth again. Icing will sand very
easily. I buy Icing from my local PPG automotive paint store.
Squeeze out only enough that can be used in 5-10 minutes and mix in a very
small dab of hardener. It will cure in about 3-15 minutes depending
on how much hardener you add. |
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Applying Icing filler to
the fin joint with a gloved finger. |
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Setting up for more wet
sanding after the Icing had cured. |
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In the photo above you can
see how the Icing had filled in some low spots and was completely sanded
away at the high spots.
In the end, all this work
paid off. The fin fillets looked great! After the rocket was
painted the fillets look like they just "melt" into the body tube and fin.
I was very happy with the results. |
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