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11.1 Do you have any tips for cutting and sealing fins used on HPR rockets?




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This article is from the Model Rockets FAQ, by Wolfram von Kiparski with numerous contributions by others.

11.1 Do you have any tips for cutting and sealing fins used on HPR rockets?

From: utidjian@remarque.berkeley.edu (David M.V. Utidjian):
To fill the grain in balsa fins and fill in the spirals in body tubes
use epoxy. I use HOBBYPOXY "Smooth 'n' Easy" Epoxy finishing resin.
For fins it does the trick in one coat... and sands easily... and
adds strength to the fins. I use those disposable brushes with the
metal handles and brush on a single coat after a preliminary sanding.
I then use auto body primer filler in gray and red-brown from spray
cans for the entire model. This gives very thin and even coats. I
alternate the colors of the coats to show where the low and high spots
are. My last sanding before paint is done with 400 grit wet/dry paper
and I do this wet... being careful not to get any inside the body tube.
[Another good coating-type epoxy is PIC 'Coating Poxy'...Buzz]
[NOTE: This is not for kids or the inexperienced!! This technique is
used in HPR where the added weight is not a penalty: Buzz]

From Bob Turner (NAR member, not on net):
Bob Turner (the DARS NAR section advisor) suggests using alcohol in
smoothing 'coating' type epoxies. The PIC 'Coating Poxy' instructions
suggest using your fingers to 'burnish' any surfaces (i.e., fins) filled
with the coating poxy. Bob suggests using a VERY soft cloth which has
been dipped in alcohol to rub the fins after about 30 minutes (or
whenever the epoxy starts to set and is just slightly sticky to the
touch). [I followed Bob's suggestion and got MUCH smoother fins over
the hand/finger burnishing method...Buzz]

From: jack@rml.com (Jack Hagerty):
When sanding fins, or any other balsa part that you want to be all
uniform, stack the parts together, even them up the best you can
(you'll be surprised at how uneven those die-cut pieces are!) on
the root edge and drive a couple of straight pins through them to
hold the stack in registration while sanding. For larger fins,
anything over about 2 sq in, use three pins. I find that the pins
that come in shirts are just about the right size. The small holes
that are left when you remove the pins are easily filled during the
sealing/filling step.

From: kaplowro@hccompare.com (Bob Kaplow)
I've found two handy tools for sanding big rockets. 3M makes these
sponge-like sanding pads. They are great for conforming to the
curves of tubes, nose cones, fillets, etc., and make quick work of
fillers. The second is a palm sander, just like Norm uses on TV. Big
rockets call for heavy duty solutions. Save the belt sander for
airfoiling the fins during construction.

Condensed thread on filleting fins; many contributors:
First, ALWAYS fillet high power fin joints, even fins mounted TTW to
the motor mount. This will add strength and improve the aerodynamics
of the model. The suggestions for filleting material include:
* 5 - 30 minute thick epoxies
* 30 minute (or longer) thin epoxy mixed with micro-balloons
until it has a thick, paste-like consistency; let it thicken
some prior to using it
* SIG Epoxilite (warning: this got very mixed reviews)

Always keep a bottle of rubbing alcohol handy when working with epoxy.
Dip your finger in the alcohol and run it along the fillet to smooth
out the bumps. It was mentioned that a pure epoxy 'topcoat' was
necessary on top of the epoxy/micro balloon mixture, although using
an alcohol-soaked finger to smooth the micro-balloons might eliminate
the topcoat requirement.

Use 30 minute epoxy with microballoons added. Let it sit for a few
minutes in the pot so it thickens, and then apply it. The microballoons
make it much less runny, so you don't have to keep watching the fillet
to make sure it's not dripping or running around the edges. Also do one
side of two fins at a time:

              \          /
               \        /     f = fillet, ^ = really bad version of body tube
                \f    f/      / and \ = fins
                 ^^^^^^

 

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previous page: 10.15 Just what is a 'hybrid' rocket motor? Who makes them?
  
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next page: 11.2 How do you keep in a high power motor in its mount, but still alow for the numerous lengths in which HPR motors are sold?