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Having acquired
some paper roll
cores, I decided I'd scratch build an upscale Big Bertha. This
one
is called QL Bertha - QL for Quite Large. (That should still save
plenty of cute adjectives in case I want to build an even bigger one
someday.)
With a diameter of 3.2", it turns out to be very close
to a 2x scale
up of the Estes Big Bertha. It uses 1/4" birch ply fins and has a
29mm motor mount. The nosecone is made of cedar. I turned it
myself.
I have made two high power flights with it so far using
an H70 and an
H125. Both were level one attempts, and both resulted in
zippers.
The first one I was worried about, but the second one was not
expected.
Numerous simulations were run, and all indications were that ejection
would
be very, very close to apogee.
I suspect that all factors were just too close to the
edge. Although
the tube is kraft paper, it is still not as strongly made as standard
rocket
tubes, so it is less likely to withstand a hard jerk from a
less-than-ideal
deployment.
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birch fins, thick-walled
tube (~1/8") and cedar nosecone, the rocket is relatively heavy, and
hence,
has a fairly high ballistic coefficient, so it will quickly gain speed
if ejection does not occur right at apogee.
It has a 10' long, 1" wide nylon strap for a shock
cord. It was
folded accordion style with each loop taped with 1/4" masking tape to
help
absorb any jerk at deployment. Nevertheless, it was probably not
elastic enough.
The first zipper was repaired with fiber glass. I
sort of wish
now that I had peeled the forward 6" of the airframe and applied a
complete
wrap of glass instead of just glassing over the first zipper.
That
probably would have made the rocket much more resistant to
zippering.
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Epilog: All in all, after two failed attempts, I felt this rocket was
a little jinxed, so I cooked up a LOC HiTech H45.
I got my L1 with it at
LDRS in Amarillo in 2002.
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After it languished in
limbo for a couple years, I was finally inspired to repair it again,
and this time I used a catalog paint scheme along with some flberglass
around the opening to harden it against zippering. Here's the
finished product. I like the way it turned out.
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Here's a blurry liftoff
shot. I can't recall the motor, but it was a 3-grain Aerotech
reload (29mm).
It flew successfully that day with no damage. Ever since, I've
been reluctant to put it back up, but I'm not ready to retire it just
yet. I may want to give it one more flight on one of the new
motors available.
In building the rocket, I compiled the catalog artwork from over the
years. This was useful in the shop to make sure I got all the
color details just right. catalog-art2.pdf
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12-11-2008
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