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This page shows some of the
construction processes that were used to create the Starfire booster
section. The Starfire booster section has three large fins and
allows for a central 75mm motor and six 38mm outboard motors. The booster
section also contains an electronics bay at the top. This bay will
hold the timer electronics and battery for igniting the outboard motors.
It will also hold a mini-DV camcorder for capturing on-board flight video.
The top of the booster also has a parachute attachment point to support
the weight of the booster during the recovery phase. |
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Design drawings for the
booster section.
Click on each drawing at
the left to see a larger image of it.
Click here to access the complete set of design drawings as a single
PDF document. |
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Here is the full set of
parts for the booster section of Starfire. There are three G-10 fins
that have been cut and sanded. There is a section of body tube with three
fin slots cut into it. Along side that is the 75mm diameter motor
tube. The coupler tubing is shown here with six 38mm motor tubes in
it. The various plywood centering rings can also be seen.
They have all been drilled and are ready for final assembly. Three
5/16" stainless steel threaded rods will be used hold everything together
and to provide the structural hard point mounts for attaching the recovery
harness. There are also three pieces of 1/4" clear tubing that will
be used to provide wiring conduits for the out-board motor igniter wires.
More details about all these parts are in the drawings above.
Now all we have to do is
put it all together! |
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The left most photo shows
the booster "fin can" after some of the pieces have been epoxied together.
There is still more work to be done on it before the body tube can
installed over it and epoxied into place. |
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Part of the process of
gluing the body tube in place was to clamp the three sections between the
fin slots to the rear bulk head. This photo shows the clamps holding
everything in place while the epoxy cures. |
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The booster section was
hung upright to allow the epoxy on the internal centering rings to spread
out and make even connection to the inside of the body tube. This
avoids having it all puddle to one side if the booster was laying
horizontal.
My workshop is in an
unfinished room in the basement, so it makes it very easy to hang things
from the trusses above! This often comes in handy for holding large
pieces while working on them. |
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Here is the back end of the
booster section after all of the epoxy steps have been completed. You can
see the white epoxy fillet that runs around the inside of the tubing that
extends past the end of the rear bulkhead. This ring of epoxy really
helps prevent landing damage to the tubing. You can also see the
holes and slots for the out-board motor igniter wires. You can also
see the three embedded T-nuts for holding on the motor plate. This
is the nicest it will ever look! Once it has flown, the back
end will be coated with black soot from the motor. |
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This is an aluminum plate
that is used to hold the motors into the recessed ends of the motor tubes
at the rear bulkhead. The plate is 1/8" thick and gets bolted on
using some 10-32 machine screws into the T-nuts in the rear bulkhead.
The holes in the rear
bulkhead are sized to allow the motor aft closures to be recessed.
The holes in this plate are smaller than the aft closures and will hold
the motors in place. |
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We are now ready to create
the fin fillets. This shows how strips of blue masking tape are
positioned along the fin and body tube. The spacing is set by laying
a long piece of 1/2" square stock into the joint and marking the edges
with a pencil. Then add the tape along the pencil lines. The
3M blue making tape works especially well for all rocketry applications
because it adheres well but will not leave a residue when it is removed.
Plus it is much easier to remove than regular "old style" masking tape.
This photo also shows the cradle fixture that
was built to hold the body tube. The cradle has a slot in it that
allows one fin to be positioned in a vertically downward orientation.
This puts the other two fins in the perfect position for applying the fin
fillets. |
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The fin fillets are
West System Epoxy mixed with #406 Colloidal
Silica filler. Enough filler is added to make a paste just a little
thicker than mayonnaise. Thickening the epoxy is necessary to
prevent it from running down inside the joint between the fin and the body
tube. Prior to
scoping the mixture into the joint, I brush on an application of unfilled
epoxy just to thoroughly wet the surfaces. |
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After the joint is filled,
I use the back side of a plastic spoon to smooth the joint and give it a
nice concave shape. It helps to wet the back side of the spoon with
isopropyl alcohol. This takes a little bit of practice to make a
perfect joint, but works pretty well. |
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Once the joint has been
shaped, the blue tape is removed before the epoxy in the joint has cured.
This prevents gluing the tape to the fins! It also leaves a very
nice straight line edge on the fillet as the tape is pulled off. |
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Both sides are now done and
we leave it to sit overnight for the epoxy to completely cure. Then
of course, we rotate the rocket and do it all again on the next pair of
fin fillet joints. |
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After all the fin fillets
are done, we end up with some very ugly looking "goobers" at the forward
and aft ends of each fin. These just need to be large enough that we
can carve them down and create a nice streamlined shape. |
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It is very easy to shape
the ends of the fillets using a small sanding drum on the end of a Dremel
tool. Don't forget your dust mask. You don't really want
to be breathing this stuff! |
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A little bit of final touch
up with some sandpaper and it is done. I find that wet sanding with
water and some 220 or 320 grit Wet-or-Dry sandpaper works really well for
this. The Colloidal Silica filler in the epoxy is especially nice
for sanding. |
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Once all the ends of the
fillets have been shaped, it is time to wet sand the entire length of the
fillet as well as the rest of the body tube and fins. This puts a
nice smooth finish on all of the surfaces and helps to blend the fillets
into the body tube and fins. I use 320 grit wet-or-dry paper for
this.
The booster section is
sitting in a cradle fixture and both the cradle and table are covered with
a sheet of 6 mil plastic. |
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I use a small plastic tub
with water in it to wet the sandpaper and a hand sprayer to wet the
surface to be sanded. |
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Icing can be used to repair
surface defects and to fill any edges that need to be blended. I
bought this tube from my local 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.
This stuff is very easy to sand once it has cured. |
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Here is a photo of the
fillet with the Icing applied. It is now ready for final wet
sanding. |
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Another photo of some Icing
repairs to some defects near the front end of the fin joint. These
have been sanded and are now ready for paint. |
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