MK I
This rocket started out as a stock Estes Fatboy
that my son Christian got for his birthday.
Seeing that he was only six, he would need lots
of "help" building it. Our local club
MASA, was staging a Fatboy competition so I
desired to create something a little different. I
wanted to modify it to make it capable of mid
power flight. I also wanted to experiment with
rear ejection. The model has gone through several
involuntary modifications, and I am currently
working on Frankenboy MKIV
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The first modification (MKI) to
this Fatboy was to discard the balsa fins and to
replace them with aircraft plywood. Second, the
motor mount was increase to 24mm to allow for
black powder D, and Single use composite E motors.
The third modification was the rear ejection
system.
Aestetically, I never liked the look of
decapitated rockets coming down on a parachute. I
wanted this rocket to come down looking as smooth
and sleek as it went up. For that reason, I did
not want an ejecting engine pod such as is used
on the Estes Sizzler. I wanted to see Rocket,
chute, and nothing else on decent. My solution
was to mount a long BT-20 Tube along side of the
engine mount. The BT80 tube has just enough room
for the BT20 when a BT50 is mounted in the center.
The 18" parachute would be wrapped into a
thin-rod shape and inserted in to the tube. To
make sure the parachute tube would not
excessively imbalance the rocket to one side. I
added a mass of putty opposite it to balance it
out. This, the plywood fins and the epoxy I added
for strength resulted in my having to add
considerable nose weight. |
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| I finished off the
parachute tube with a neat little cap made from
an engine block and the punch out from a
centering ring. It was held onto the shock cord
with length of Kevlar thread. The shock cord was
made from multi-strands of Kevlar thread braided
together by my long-suffering wife, with a length
of elastic cord for shock absorption. The Kevlar
cord was epoxied into the nose cone and would be
exposed to ejection blast. The elastic would be
kept on the cool side of the ejection. |
Concerned about the reliability
of this contraption, I rigged an ejection
simulator out of my son's "Stomp Rocket"
Launcher (This is a toy in which small missiles
are propelled by displaced air coming out a
bladder which is "stomped" with your
foot). In the simulation this ejection method
proved very reliable. With only moderate pressure
the chute ejected cleanly, and virtually unfurled
due to shear momentum.
When finished the rocket looked pretty good, with
no visible seam between the tube and the nose
cone, other than that, indistinguishable from a
Stock Fatboy
The first launch of the Frankenboy MKI was on a D12-3
at the MASA Fat boy flyoff. The Frankenboy got 2
feet off the end the launch rail, did a chicken
dance, then flopped onto the sod. Seconds later
the parachute deployed flawlessly onto the lawn. |
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I was very surprised, as RocSim
predicted an altitude of about 500-ft, and the
swing test demonstrated I had positive stability.
What did not consider was the exit speed on the
rocket leaving the rod was too low, resulting in
an unstable flight. Post crash inspection
revealed not a scratch of damage on the paint job
(we launch at a nice, spongy sod farm), but when
I went to re-pack the chute, it would not go in.
What had happened was that the parachute tube had
collapsed from the pressure of the ejection
charge. I had suspected this as a potential
hazard, but since it did not occur during the
simulated ejections, I thought it was safe.
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MK
IIIf I were going to try
and fly this again, I would have to redesign the
parachute compartment. One thing that is tr ue of
rear ejection models is that they are difficult
to repair internally, having only one end to work
with. In this case I had to extract the remains
of the BT-20 parachute tube through .75 diameter
hole. I felt like a rocket proctologist.
Reassessing the design I realized that more
weight would be needed to stabilize the rocket. I
would also have to redesign the parachute
compartment to eliminate the external over-pressure.
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After some consideration I
eliminated the parachute tube altogether and
created a kidney shaped parachute compartment
between the fin tenons. Additional nose weight
needed to be added. This necessitated extracting
the remains of the parachute tube that was still
rattling about the upper cavity of the rocket.
This problem lead me to discover a useful tool
for the range box; the fishhook removal pliers.
This tool has a small jaw on the end of a long
arm and operated by a pistol grip. It is very
useful for manipulating inside body tubes to a
depth of about 8 inches. The nose weight was
added by holding the rocket upside down and
dropping the appropriate amount of split shot
through the parachute cavity, where it settled in
the nose cone. I then thinned epoxy with mineral
spirits and poured it through a paper cone,
through the parachute cavity into the nose cone.
It took a couple shots to completely secure the
weight. I reused the recovery laundry from the MK
I version (Stock plastic Parachute and kevlar,
and elastic Cord.
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. The result was a heavy, but
stable projectile. A D12 motor was now out of the
question. The motor of choice would be the
Aerotech E15-7 or E30-7 motors. Design of the
parachute compartment precluded RMS Motors. The
Franken Boy MKII was Ready for the next MASA
launch. This times both the flight and the
recovery were perfect. The E15-7 motor lifted it
straight up to about 1300 ft. where the chute
unfurled flawlessly. The next time I launched
it was during a visit to Colorado the next month.
This time on an E30, it shot off the pad like a
mortar. Everyt thing went fine until the ejection
of the chute. The parachute shrouds ripped
through the plastic chute. (this was rather
dramatic to watch) As it fell, it began to move
so fast we couldn' t follow it to the ground. I
spent well over an hour looking for it. The high
velocity impact destroyed the forward two thirds
of the body (as well as a yucca plant),
Fortunately, the fin unit was intact, so
rebuilding it was a worthwhile effort.
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| MKIII For
the Frankenboy MKIII, I decided to go for broke.
I considered abandoning rear-ejection, but I
liked the look so much, I had to give it another
try. I also decided to go for largest motor that
would fit in this configuration. The 24mm G55-10
For parts I bought a second fat boy kit, since
this was the lowest cost way to get the nose
cone, tube and decal (Michael's Crafts and a
fortuitous 20% discount). Now that I had access
to the front of the rocket, I extended the
parachute compartment to make room for more shock
cord and a nylon chute. The engine tube was
extended all the way to the front of the rocket.
No engine block was installed except in the cap
on the end of this tube. Ejection Gas exhaust
holes are drilled into the sides of the tube, and
balsa supports are added to compensate for the
loss in stre ngth caused by the holes. More
weight is required for stabilization when a G55
is used over an E30. Provision was made for an
insertable counterweight that would go in before
the motor. This way the weight could be optimized
based on the motor used.
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Different length counter weight tubes would be used
with different length motors I added considerable
internal bracing to make sure the engine tube, with the
heavy counterweight and motor would stay fixed to
airframe. The Shock cord was attached like a noose around
motor mount to ensure in would stay connected to the
rocket The plastic chute was replaced with a nylon chute,
but I retained the length of heavy-duty elastic between
the kevlar and the Chute, This proved to be fatal.
The next launch I attended was the SSS G. Harry Stine
Memorial Launch, at Rainbow Valley Arizona (Yr. 2000).
This was the perfect spot to launch a small rocket with a
big motor.
The LCO gave the Frankenboy a huge build up,
complementing me on fitting both rear-ejection and a G
motor into a dime-store rocket. The launch was very
impressive (See Below). It shot way out of sight. The
crowd hooted and cheered when the chute came out, a tiny
dot of orange against the deep blue desert sky.
Unfortunately, It was soon clear that the parachute was
not attached to the rocket, as it was drifting down way
too slow. Someone soon found the airframe, and it was as
badly damaged as the last time.
Sure enough, the elastic cord had snapped. This was the
last time I would use elastic in a rocket. I feel
confident that the nylon chute would not have been
damaged by the shock cord.
MKIV
The MKIV project is sitting on my workbench, waiting for
other projects to be completed. I have so many other
things I want to build and try that I am going to let
this sit for a while. When I do complete it, It will
probably be an improved version of the MKIII
Lessons
Learned.
Paper tubes cannot bear external pressure: Important
to consider in a rear ejection design like this or if you
have a novel baffle tube arrangement. I was mislead
because the chute came out a such a low pressure, I
thought that the pressure would be relieved before the
pressure got high enough to collapse the tube, but I was
wrong. Glassing the tube may have solved the problem, but
it would add a lot of weight. To do this right your need
to design your internal structure so that the parachute
tube does not experience external pressure. This is what
I did with versions MKII-III and it works fine.
Use Nylon Chutes: These are marginal enough in
low power rockets. Don't use them with rockets heavier
than a few ounces. The investment in a nylon chute will
pay for itself. Remember that you can share your chutes
between rockets, so the investment is minimal
Don't use Elastic Cord. Use Kevlar whenever
possible in mid-power rockets. Again, it's worth the
investment. For shock absorbsion, use the accordion tape
method.
Get a second opinion: Had I shown the shock cord
arrangement to someone knowledgeable, I would have been
easily talked out of it, Poorly considered decisions will
eventually fail you.
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