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Coldfire was specifically
designed to fly on the
HyperTEK hybrid
motor system. This system uses ABS plastic for fuel and liquid
nitrous oxide (N2O) as the oxidizer.
The HyperTEK system
requires a special vent port that allows liquid nitrous to flow out when
the flight tank is full. In fact, when a white stream of liquid
nitrous is seen the LCO knows the tank has been filled and the rocket is
ready for launch.
Another complication with
the HyperTEK K240 motor is the diameter of the nitrous tank and the
diameter of the fuel grain. They are quite different. The fuel
grain is a standard 54 mm diameter that matches that of many other J-K
class motors. However, the tank is 81 mm in diameter. This
requires a special mounting system to hold everything in place in the
booster section yet still allow easy access for changing the fuel grain
and o-rings.
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The fuel grain is made from
injection molded ABS plastic. A phenolic nozzle is insert molded
into it. The inside of this grain also acts as the combustion
chamber. |
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The HyperTEK K240 motor
uses an 835cc flight tank as shown at the right and a fuel grain as shown
above. The two simply screw together.
This motor burns
for 5.6 seconds. It has a peak thrust of 96 lbs and an average thrust of
about 52 lbs. |
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The drawings above show how
the HyperTEK K240 motor system is mounted into Coldfire. The 54mm
fuel grain is first installed into a short motor mount tube. The fuel
grain extends out the top of this short motor tube so that the tank can
then be screwed on to it. Once the tank is screwed on, the whole
assembly is inserted into the booster section. The motor tube inside
the booster section is exactly the right size to accept the tank. The
centering rings on the short motor tube are also exactly the right size to
just fit inside the booster section motor tube with the tank. There is
also an aluminum thrust plate bonded to the short motor tube that contains
the fuel grain. The thrust plate has holes that line up with
threaded holes in the aft ring inside the booster section. Small bolts are
used to hold the short motor tube into the booster section. This
secures the entire HyperTEK motor system as well. The aft end of the short
motor tube also has a set of screw threads bonded to it that mate with the
tail cone. Once the motor is installed the tail cone is simply
screwed on and it covers up everything internal. There is also a
specially prepared nylon set screw that screws into the vent port between
the fuel grain and tank. This set screw is drilled out lengthwise so that
nitrous can flow through it.
To remove the motor, the
entire process is reversed. First the nylon vent screw is removed.
Then the tail cone is unscrewed. The small bolts are removed and the
entire motor system slides out the back of the booster. The tank can then
be unscrewed from the fuel grain. That allows the fuel grain to slid
out the aft end of the short motor tube. |
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The two piece tail cone
assembly is made from aluminum and sold by
Aero Pack Inc.
It is part number TRA5439PB.
The smaller threaded section of the tail cone assembly
gets bonded to the
short section of motor tube with JB
Weld epoxy. |
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This is the short motor
tube that is used to hold the plastic fuel grain. The right hand
drawing above shows the design for it. The photo at the left
shows a black threaded section that is bonded to the tube. These
threads mate with the tail cone. The yellow section is one wrap of
Kevlar cloth that adds strength to the phenolic tube. The Kevlar
cloth also overlaps the threaded section to help secure it to the tube. An
aluminum thrust plate is bonded to the tube at the other end of the Kevlar
section. |
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The photo above shows how
the entire motor system is put together. Normally the tail cone is
not screwed-on until the thrust plate is bolted to the aft centering ring
in the booster. It was added here just to show how everything fits
together. |
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The photo above is a
close-up view of the nitrous vent stem on the tank. A small drill
bit was temporarily inserted into the stem in order to get an accurate
reading of the distance from the aft end to the vent. This same distance
is then used to mark the motor tube as shown in the photo below so that an
access hole can be drilled in the right place. |
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Blue tape was added and
marked with the location of the vent hole that needs to be drilled. |
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A fixture holds the booster
section in position for drilling the vent hole using a drill press.
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Vent hole drilled!
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The next step was to
construct a wood spacer block to hold a threaded tee-nut in place over the
vent hole that was just drilled. The diagram below shows a cut-away
side view that explains how the wood spacer block, threaded tee-nut and
nylon set screw are configured. |
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The spacer block started
out as a piece of Poplar wood 1/2" thick. It was drilled with a 1/4"
diameter through hole and then drilled again with a 1/2" diameter partial
depth hole to countersink the tee-nut flush with the top of it.
The tee-nut has 1/4-20
threads that will mate with the nylon set screw. |
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The block was then
carefully sanded on a drum sander to shape it to match the radius of the
motor tube. The thickness was also taken down to the correct space between
the motor tube and the inside of the body tube.
The tee-nut was test fit at
this point and it was still flush with the top of the spacer block. |
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The spacer block and
tee-nut are now ready to be installed over the hole in the motor tube. |
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The tank was installed and
alignment with the fill stem was verified. |
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West
System epoxy was applied with a brush and then the spacer block was
taped in place to hold it until the epoxy cured. |
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Spacer block completed. |
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Now the trick is, how do we
drill a hole in the body tube at exactly the right place? To
accomplish this I used a pointed set screw from the local hardware store. |
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The pointed set screw was
installed with the point sticking out. |
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Fin can and body tube. |
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The body tube was slid over
the fin can and pressed down onto the pointed set screw. It was then
removed. The pointed set screw left a very nice little indentation
on the inside surface of the body tube. A very small hole was then
drilled through the body tube wall by turning a small bit by hand.
Once that bit penetrated the outside of the body tube, the body tube was
taken to the drill press and a full size 1/4" diameter hole was drilled.
This method allowed the
vent hole in the body tube to be accurately located. It would have been
very difficult to accurately locate the hole in both the longitudinal and
circumferential directions on the body tube by direct measurements.
This is because there are not any good reference points to measure "from".
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The next step was to
prepare some nylon set screws. They need to be drilled out
lengthwise to allow the nitrous to flow through them. Coldfire
uses 1/4-20 x 1.5" nylon socket head set screws. They are available at
McMaster-Carr as
part number 94564A100. Only one is needed (plus maybe a few spares)
but the minimum purchase size is for 100. Fortunately they are only $6.45
for 100. |
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To drill out the set screw
lengthwise, it needs to be held in a drill press. To do that I used
a long 1/4-20 threaded coupler nut that is typically used to join pieces
of threaded rod. |
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The nylon set screw was
threaded into the coupler nut. The coupler nut was then held in a
vise on the drill press. One end was drilled with a 1/8" diameter
hole to fit over the stem on the tank. That hole was only drilled as
deep as the stem on the tank is long. The remaining length of the
set screw was drilled with a 1/16" diameter bit. |
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This photo shows the hole
in the booster body tube that allows access to the threaded tee-nut for
the nitrous vent. |
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Just to check all the
alignments, the nylon set screw was installed with a standard hex headed
Allen wrench. |
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Everything fit perfectly.
The set screw was flash with the body tube and it mated perfectly with the
end of the vent stem on the nitrous tank.
The screw was then removed
as it would not be needed until Coldfire was ready to be put onto the
launch pad. |
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