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Coldfire is built in
what is commonly referred to as an antizipper configuration. That just means
that the top of the booster section has a parachute attachment point and
that the parachute itself is deployed from the aft end of the body tube
instead of the forward end of the booster section. This arrangement helps
avoid "zippering" the body tube with the recovery harness if the parachute
is ever deployed while the rocket is traveling at relatively high speed.
Such as when deployment is well before or well after the rocket reaches
apogee.
If the parachute is
deployed out the front of a body tube while the rocket is traveling at
high speed, then the recovery harness will end up pulling on the lip at
the front edge of the body tube to slow it down. At very high
speeds, that will result in the recovery harness tearing a slot right into
the body tube and down a short length until the body tube has come to a
stop. This tear is what is referred to as "zipper" damage. An
antizipper configuration generally eliminates this failure mode.
However, sometimes, even with an antizipper configuration the end of the
body tube can still get damaged. This can happen on high speed
deployments where the body tube tumbles before the shock cord comes tight.
An antizipper band was added to Coldfire in order to help prevent even
minor damage from that kind of scenario.
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The antizipper band is a
steel band that is embedded into the lip of the body tube to prevent the
recovery harness from slicing into the body tube. The band itself
is the 1/2" wide band from a steel hose clamp. I cut off the screwing
mechanism. The length of the band was cut to exactly fit the
circumference of the body tube. |
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The epoxy selected for this
step is Loctite Hysol E-120HP. It is a non-sagging, aerospace grade
with superior resistance to rapid temperature changes. It has high shear
strength and resistance to peeling. It can be purchased at
McMaster-Carr (part
number 6430A24) for about $12 for a 50ml cartridge. An applicator gun
(part number 74695A71) and a mixing nozzle (part number 74695A12) are also
needed. The gun uses a ratchet scheme to push a plunger into the
cartridge. This system is very handy
and a real pleasure to use! |
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Gloves are a necessity when
working with epoxy.
I prefer these nitrile gloves available from
Harbor Freight.
They are inexpensive and very tough. Much tougher than the vinyl gloves I
used to use. |
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Some
West System Epoxy was also used to wet out the end of the body tube
since it has a much lower viscosity than the Hysol E-120HP. After
wetting out the end of the tube, a ring of Hysol E-120HP was added to bond
with the hose clamp band itself.
The body tube is flexible
phenolic available from
Giant Leap
Rocketry. The outer glassine surface has been sanded away to allow the
tube to readily absorb the epoxy for a good bond. |
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The steel band is
positioned flush with the end of the body tube and pushed down into
the Hysol epoxy. |
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A layer of wax paper
is placed over the steel band and then another hose clamp is used to hold
eveything in place while the epoxy cures. |
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Antizipper band after the
epoxy has cured and the hose clamp and wax paper were removed. |
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The steel band was simply
added to the outside surface of the body tube. That will leave a
small step at the edge of the band between the surface of the body tube
and the sirface of the band. It would be nice to fill this step and
blend the edge of the band into the body tube. That was done by
applying some more epoxy. West System Epoxy
mixed with some #406 Colloidal Silica
filler was used. This filler makes a nice creamy mixture that will
not run or drip due to gravity. A generous about of the mixture was
smeared over the edge of the steel band and onto the tube. |
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Next the excess epoxy was
removed using a plastic scraper tool. The idea here is to leave a
small amount of epoxy that blends the edge of the steel band into the
surface of the body tube. That way there will not be a gap below the
fiberglass cloth that will be applied next. |
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After the epoxy on the
steel band had cured, the body tube was placed onto my setup for applying
fiberglass cloth. I use two saw horses with pipe holder fixtures on
them. A length of 2" PVC pipe is run through the middle of the body
tube and laid between the two saw horses. The body tube is centered on the
PVC pipe using two pieces of foam cut to roughly the right diameter and
placed just inside each end of the body tube. The foam pieces have a
hole in them to allow the pipe to pass through. |
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To fiberglass the body
tubes I use 6 oz plain weave glass. This is a roll of G7628 that I
purchased from CST.
They offer it in 50" widths which is perfect for 48" long sections of body
tube. |
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The fiberglass needs to be
cut to the appropriate length. I then sealed the end of it with a
spray adhesive. This photo shows a small 1/4" section of the
fiberglass exposed between two layers of newspaper. A very light
spray coat of adhesive keeps the ends from fraying. |
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I use 3M
Super 77 spray-on adhesive to seal one end of the fiberglass cloth.
This is certainly not required but it makes for a nice clean seam without
having to fiddle with any frayed strands of fiberglass. This stuff
dries in about 10-15 minutes. It can be found at most any hardware store
or paint store. I
only seal one end of the fiberglass. The end that is put
onto the tube last. The end that goes down first does not usually
suffer much stress that causes fraying. The end that goes down last
is typically handled a lot more and can tend to fray a lot. |
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These 3-inch foam rollers are
really handy for putting the epoxy onto the tube and for rolling the
fiberglass down into the epoxy. The foam roller can be tossed away
and replaced after use. |
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I use the
West System "Slow" mixture for fiberglassing
body tubes. (#206 Hardener.) It gives a reasonably long working time,
somewhere around 45 minutes to an hour. I find that by the one hour
mark, the epoxy has started to cure enough that it gets a little sticky
and is much harder to spread and apply cleanly. However, it will
take all night for it to completely cure.
The pumps that are available for West System
cans are well worth the minor cost. This epoxy needs to be mixed in
a 5:1 ratio, so the pumps make that easy. |
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I use the pumps to dispense
the epoxy into a 12 ounce plastic cup for mixing with a small wood stick.
However, don't be tempted to leave it in the cup! It will get very
hot and cure within about 10-15 minutes. Once it is mixed, pour it
into a plastic picnic plate. This increases the surface area of the
epoxy so that it will not self heat and greatly extends the working time.
The plate also makes it convenient to roll the epoxy onto the foam roller. |
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Now it is just a matter of
rolling a generous amount of epoxy onto the bare body tube and then
rolling on two layers of
fiberglass all in one go. The fiberglass should be
rolled down firmly in contact with the tube using the foam roller.
It works really well for eliminating any wrinkles or bubbles. The
fiberglass changes to a transparent state once it is saturated with epoxy. |
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And now the
end of the fiberglass that I sealed with the spray-on adhesive is the last
little bit to be rolled on to the tube. This
seam can be firmly laid down with the roller without any trouble from
fraying ends of glass. |
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The motor on my "body tube
rotisserie" setup can now be switched-on to slowly rotate the body tube
as it cures over night. Rotating the tube prevents any sags or high
spots in the epoxy. This photo shows the three pulley wheels that gear
down the motor RPM to something very slow.
For truly high performance tubing, some people will wrap the tube in a
constricting tape that squeezes out excess epoxy. They may even put
the tube into a curing oven to reach maximum strength on the epoxy.
I did not feel that was necessary for Coldfire. |
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The steel band is now
nicely captured between the tube and the two layers of fiberglass cloth.
The overhanging end of the cloth was then trimmed away with an X-acto
knife once the epoxy had cured. At that point the seam along the
length of the tube created by the edge of the fiberglass cloth was also
sanded down flush with the surface of the tube. |
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Finished tube with antizipper band in place. |
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This next section will
describe the process used to reinforce the booster coupler tube by applying
fiberglass cloth to the inside of it.
The coupler tube is
simply sitting on the table and held in place with two wood boards.
A wood dowel runs through the middle of the tube.
Fiberglass cloth is wrapped around the wood dowel. The fiberglass
can be slowly unwound and carefully applied to the inside surface of the
coupler tube. Although that is much easier said than done!
The wood dowel is free to
rotate but is held in place by some metal fittings that are normally used
to capture a broom handle and hang a broom on the wall. |
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Here is a side view of the
setup.
The wood dowel does not go
right through the center of the coupler tube. It is offset toward
one side to give more room to reach inside the tube to unroll the
fiberglass cloth and push it down against the inside surface. |
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This is a close up view of
the fiberglass cloth being applied to the inside surface of the tube.
Two layers were applied in one continuous process. |
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After the epoxy has cured
the coupler tube was removed from from this setup and the excess
fiberglass was trimmed off each end. |
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The coupler tube also
needed a large slot cut on one end to allow it to slide past the rail
guide retainer plate on the inside surface of the booster body tube. The
size of the slot was not very critical so it was cut with a coping saw.
More details on the rail
guide retainer plate are available here. |
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