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The
Hindrocket
Fin
Design and Construction
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| Fin Design and Construction If I want to bring big rockets to fly at our
annual soirée in phoenix every year, I have to consider
how I can transport it 2000 miles cheaply and with out
risk of damage. The biggest cost comes from cubing-out
the shipping. Essentially, you'll pay much more to ship a
pound of air than you will a pound of dirt. Thus it means
that you will want your rocket to ship in the smallest
package possible. Usually this will mean that you will
need a convenient way to remove the fins from your
rocket, and then reinstall them.
In the case of the Hindrocket, I
realized that the for a rocket that was going to be in
excess of 15 lbs, that fins of this design would have to
be extremely rugged if they were to withstand landing,
even with a large parachute. And, if could make the fins
survive, I would have to further reinforce the body to
keep in from being thrashed by super strong fins. This
means still more weight and expensive materials. However,
I figured the only reason I would want to make the fins
so strong, is because of the difficulty in repairing
permanent fins. Thus if the fins were essentially
disposable, they only have be strong enough to handle
ascent (which is far less demanding). So instead of an
expensive composite lay-up, the fins are made from cheap
tempered hardboard. The method described here was perfect
for the Hindrocket application, it may not be the best
approach for a higher performance rocket, or a model that
gets a lot of use, but it worked just fine here.
To construct fins of this type, you
will at a minimum you'll need access a table saw and
above average wood shop skills.
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I designed fins and retaining mechanism using a
high-end cad system that I am fortunate to have access
to. However should be able to adapt this concept to your
designs by studying this page. |
As a cost and labor saving measure I made the fins from
low-cost 1/4" hardboard from Home Depot. I calculated that
the fins would easily handle the stress on this flight, and
because the fins would be removable, I wouldn't much care they
broke off on landing. In fact, I made a whole set of spare fins
at the same time I made the primary set!
| Here is he stack of fins, just after cutting on a
band saw. The paper pattern is still glued onto the top
fin. There are eight fins in the stack (an entire spare
set!). because they were cut at the same time, they are
all identical. Two 1/4" drill bits are set into the
stack to prevent shifting during cutting. the other holes
present are a phenomanon similar to crop circles. (Click image to enlarge)
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| Another image of the fins, after being
seperated from the stack. note the fron and back locking
tabs. the holes will not be visible outside the airframe. (Click image to enlarge)
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| Gluing up the motor mount. The motor mount has three
rings, with a center support ring. the upper most ring
will not be glued in place until after the fin slots are
cut. The forward ring will be placed behind the aft
ring while the slots are being cut.
(Click image to enlarge)
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| Aft view of the motor mount after being glued into
the airframe. note the two t-nuts for secureing the motor
clips Interior view of the motor mount before cutting
the fin slots.
(Click image to enlarge)
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| This is a CAD drawing showing how the table saw blade
cuts slots in the tubes and the centering rings |
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| Dave Erickson helped me build this
handy jig for cutting the fin slots. Note that he forward
centering has been temporary placed behind the aft
centering during cutting. the forward ring cannot be
glued in place while cutting the slots because the radius
of the blade prevents a complete cut. so we put it in the
back. note that the rings have been carefully indexed so
that we dont cut through a lightening hole, t-nut, or
harness mount. It takes two passes on the saw to get a
full slot.
(Click image to enlarge)
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| After the fin slot is cut we give it a trial fit.
Success! (Click image to enlarge)
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| This is the finished front ring. with the fin-stop
ring glue in place and the u-bolt for the harness mount
attached (Click image to enlarge)
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| Aft Retaining ring, top and bottom views.
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| This picture wasn't supposed to happen. the epoxy
didn't bond properly and the mount popped out. this
unlucky event did give me a chance to take some excellent
skeleton photos (Click image to enlarge)
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| Top veiw Note that top ring is upside down in these
last two pictures, I dont know why.
(Click image to enlarge)
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| Photo taken after painting. Fins are removed to
emphasize slots. (Click image to
enlarge)
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| One fin being inserted. It goes in tip pointed in to
the forward tab inserted into the forward slot (Click image to enlarge)
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| All the fins inserted (one fin at 2:00 is slightly
out) the retaining ring not yet installed (Click image to enlarge)
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| All fins in place and the retaining ring installed. I
use a washer-head self taping screw for holding the
retaining ring. Note the orange alignment marks. to get a
snug fit, the detents for each fin cut indvidually, so
the ring only fits one way. (Click
image to enlarge)
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| Finished interior: Note how the fins interlock into
the slots
(Click image to enlarge)
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| Removable Hardboard fins in Action! Heading to 2500
ft!
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| As predicted, two fins broke on impact (Click image to enlarge)
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| Business end of the hind rocket. |
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