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11.7 How can I strengthen my thick paper (i.e., LOC type) body tubes?




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

11.7 How can I strengthen my thick paper (i.e., LOC type) body tubes?

Various composite construction techniques may be employed to strengthen
paper body tubes. These same techniques may be used to build scratch body
tubes as well. An excellent article on composite construction techniques
appeared in the XXXXXXXXXX issue of High Power Rocketry magazine. Another
article dealing with strengthening HPR rockets appeared in the XXXXXXXXX
issue.

The two most practical methods for strengthening the paper body tubes
used by LOC, THOY, etc. are 1) reinforce the tube with couplers for most
of its length and 2) wrap the tube with some type of reinforcing layer.

The first option produces a strong tube, but has the drawbacks of high
cost (at $2-4 per coupler) and high weight.

The most common material used with the second option is fiberglass cloth.
Two ounce cloth is good for use on 2.5 to 4 inch diamter tubes. Five
ounce cloth might be used for larger tubes. R.m.r posters have recommended
several techniques for applying the fiberglass. Here are two of them:

From: bmcdermo@ix.netcom.com (Buzz McDermott)
1. Sand the tube with 320 grit sandpaper to slightly roughen its surface.
2. Mark a straight line down the length of the tube.
3. Lay out the fiberglass cloth on a flat, smooth surface. Use a square/
straight edge and a SINGLE EDGED RAZOR BLADE to cut the fabric to
a rectangle, allowing for at least 1" overlap around the diameter
and off each end of the tube to be covered.
4. Lay out and tape together enough wax paper on the floor of your
garage, basement,etc., to be larger than the fiberglass cloth in all
dimensions. Lay the cloth on the wax paper. Tape the wax paper to the
floor (but NOT to the glass cloth).
5. LIGHTLY spray one side of the cloth with 3M 77 adhesive. I mean
to put on a QUICK, VERY LIGHT coating of adheasive.
6. Lay the tube down on one edge of the fiberglass, using the line on the
tube as a guide to get the tube straight along the glass cloth.
7. SLOWLY roll the tube along the cloth, working out wrinkles with your
fingers. The 3M 77 should lightly tack the cloth to the body tube.
8. Once the cloth is on the tube, use thin *orderless* CA to seal the
overlap and edges along fin slots and ends of the tube. Using a plastic
bag over one hand gently rub the CA into the cloth. Also CA any
wrinkles that are left. When the CA dries you can use the single edge
razor to trim off excess cloth at the ends, feather sand the overlap
joint (with 320 grit), cut out fin slot openings, and sand down or
slice off any wrinkles in the cloth.
9. Brush on 20 minute 'finish cure' epoxy. Bob Smith 'Coating Poxy' and
Hobby Poxy 'Smooth N Easy' are good choices. Completely cover the
entire cloth surface. Be sure and gently work the epoxy into the
cloth. You want the cloth soaked and the epoxy soaking into the
body tube.
10. About an hour after you finish, the epoxy should be getting real
'tacky'. Soak some rubbing alcohol into a clean, lint free cloth and
use that to lightly 'buff' the epoxy. This will help smooth the
coating and get rid of air bubbles.
11. After 24 hours, sand with 240 grit wet-or-wry, WET, until smooth.
You are now ready to prime.

Two additional notes:

1. With lighter cloth (3/4 up to 2 oz), I sometimes soak cyano into the
entire cloth surface. I then sand with 320 grit VERY LIGHTLY. I find
I use much less epoxy and end up with a lighter rocket. This is a
good technique when weight is critical.
2. Always were latex gloves when working with epoxy. People do develop
nasty reactions to this stuff over time.

From wolf@netheaven.com (Wolfram v.Kiparski)

When using 3/4 oz. cloth, I find it easiest to first paint epoxy (thinned
with a little laquer thinnner) on the body tube and then lay the cloth
onto the tube. The cloth readily "wets out" when it touches the epoxy,
and adheres to the tube without curling up. The cloth can be gently
arranged and gently brushed to smooth out the wrinkles as you wrap it
around the tube. Extra epoxy can be dabbed on as needed.

For 3/4 oz. cloth:

1. Cut the cloth to size first. Cut the cloth slightly oversize so that
it is a little longer than the tube, and will overlap if wrapped
around the tube.

2. Mix your favorite epoxy and add about 5% laquer thinner. Paint
this onto your body tube with a china bristle brush. I use a 1.5
inch brush. Thinning the epoxy makes it spread easier, and will
help keep lightweight cloth from distorting and wrinkling. It will
also cause you to use less epoxy.

3. While the epoxy is still "wet," drape one end of the cloth onto the
body tube. Use your brush to smooth the cloth out. Brushing in only
one direction will help avoid wrinkles. Roll the tube slightly as
you smooth the cloth onto the epoxy-covered tube. The cloth will
pick up enough epoxy to wet-out. If it doesn't, add a dab of epoxy
to help it along. You can free both hands by placing the body tube
over a long wooden rod like the kind used for closet hanger rods.
Support the rod at both ends kind of like a giant toilet paper
dispenser.

4. 3/4 oz. cloth will stick to the body tube and tend not to lift up
before the epoxy has cured. Be careful not to brush too vigorously
when overlaping the cloth as you finish applying it. You might
wrinkle the bottom layer of the overlap, and experience a great deal
of frustration.

5. After the epoxy has cured, lightly wet sand with 220 grit sandpaper.
Fill in any low spots with spot putty and sand smooth.
A few coats of primer will fill in the weave of 3/4 oz. cloth,
especially if you lightly wet sand with 320 grit between coats.

With a little practice, this technique is easy to do, and adhesives
other than epoxy are not required.

From: dave@ddave.com ('Dangerous' Dave)
[Dave had the following comments about the above described technique. Dave
is an expert in the use of composites, fiberglass and laminating
techniques]
When the glass is fully cured, you can sand the lap joint till it
feathers into the ajoining surface. Any irregularities can then be
filled with a polyester filler (Bondo) and spot putty to blend the
surface so that it is unnoticable.

Don't use an adhesive to tack the glass in place. It will prevent the
resin from soaking into the fabric and will effect the physicals of
your epoxy. Cut your fabric to size allow and inch or so overlap that
you can trim off later. Wet your surface and then drape the fabric on
to it. Then stipple the resin into the fabric with a china bristle
brush. Don't use a paintbrush that is made from synthetics, i.e.:
nylon, polyester, ect.. The epoxy and/or your cleaning solvent will
disolve your brush and it may react with the resin.

Be sure and read my Safety Document on handling composite materials
before you do any of this.

You will get your best adhesion by completely removing the glassine.
Since resin can't penetrate it and will not bond well, you must remove
it in order to take advantage of any strength gains you get from
applying glass.

Visit my web and ftp sites for some more info on laying glass.
FibreGlast at: http://www.fibreglast.com has a very good section on
composite techniques.

[Editor's note: If you're going to work with fiberglass, epoxies, or
carbon fiber, check out DDave's web page, http://www.ddave.com/].

 

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