Manufacturer: | Scratch |
Brief:
5.5" Dwarf King is a simpler, cut-down version of the huge and fat K445-powered tubefin I presented in
ROCKETS Magazine. It has the option of being built with either 6 tube fins (Mk. 6) or seven tube fins (Mk. 7).
Both versions can fly on up to Cesaroni K445 or K500R power or on as little as an I212SS. The lighter Mk. 7 version can
also fly on H143SS or H153 motors, giving a very easy L1 shot to only 700-800'. This article is part of a series of
tubefin construction designs that support a tubefin aerodynamics article set to appear in Sport Rocketry in late
2009-early 2010.
Construction:
Dwarf King was named after Gimli, the short, fat, bearded guy with the battle axe in the Lord of the Rings
films, whose resemblance to me is purely coincidence. (However, he was actually played by a clean-shaven, skinny,
6'2" Brit, thanks to a fat suit and camera tricks.) Dwarf King is one of the fastest-building, lowest cost K-power
rockets anywhere, kit or scratch. The 6-tube Mk.6 is faster building, simpler and more resistant to landing dings. The
7-tube Mk. 7 looks better (I think) and is lighter by a pound. The rocket body construction is identical for both, so
I'll start with that:
Parts:
The build:
Finishing:
I painted my Dwarf Kings with a simple, freehand, two-color scheme, using Tamiya rattle-can. Note from the photos how
small the DK Mk. 6 is compared to my Ford Explorer! It is however, easy to fit this K-power rocket in your vehicle and
your work shop.
Finished weight of the Mk. 6 version was 2211 grams, owing to the weight of the heavier 5.5" tube fins. The 7-tube version can be built as light as 1660 grams. Dwarf Kings are flown on up to the 1400 gram 54mm K445 with not a bit of nose weight needed.
Flight and Recovery:
To insure perfect deployment with the 2-chute option, which I recommend, I pack the shock cord inside an XL-size
Nomex®,
stuff this into the rocket, then lay the folded main chute on top. The 30" X-chute attached to the cone is then
folded and stuffed between the Nomex®
and the main. Lastly, the nose cone is fitted into place. This way on ejection, the 30" chute will drag out the
54" main. When I began flying these, I used friction fitting of the motors, however, my fellow rocketeers told me
this was an unsafe practice with K motors and above so I bought an Aero Pack retainer and also fabricated a homemade
adjustable retainer, since with this design, the motor casing sticks out 3" or more aft of the airframe (see
"BEEMR" on this website).
Dwarf King is a beautiful flier on the Cesaroni K445 with 8 second delay, giving 3416' at measured Cd=0.82 at 500mph. The very similar K500 Red Lightning reload gave a 3256' boost with Cd=0.86 at 482mph. I measure altitude with a Perfectflite Alt15K in an external, taped on parasite pod to calculate these values.
Other Mk. 6 flight results were: 1693' on a J400SS-7 with Cd=0.87 at 379mph, 1366' on a CTI 285-6 with Cd=0.88 at 314mph, and 961' on a CTI I212SS-6 with Cd=0.87 at 223mph. With the Mk. 7 version, the J400SS-7 gave 1785' with Cd=0.81 at 465mph, 1857' on the CTI J330-7 with Cd=0.88 at 428mph, and 1096' on the I212SS-7 with Cd=0.93 at 286mph. The lighter 7-tube Mk. 7 also gave a straight,stable boost on a Cesaroni H143SS-6 to 747' with Cd=0.97 at 206mph.
What's clear here is that neither version displays the severe speed dependence
of Drag Coefficient (Cd). Together with the low Cd=0.8-0.9, one can see how these stubby tubefin designs fly almost as
cleanly as 3FNC rockets. With a 700-800' H-motor flight and J-power taking them up to only 1700' to 1900', the Dwarf
kings are ideal for flying L1 and L2 cert flights with the same rocket on the same day--a popular thing to do lately,
although the NAR office frowns on this.
Summary:
PRO: Easy and inexpensive to scratch build, compact to transport (for a K powered rocket!). Fly L1 and L2 with the
same rocket! No need for nose ballast with any motor.
CON: Mk. 7 tube fins are not as strong as Mk. 6 fins and can be dinged on hard landing. Long 54mm cases protrude 3-4" aft of rocket, and your $70 CTI casing protects your $2 tube fins from rocky impacts! Avoid flying these designs at speeds above 500 mph (based on your simulator) or you will be the test pilot!
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