Manufacturer: | Scratch |
Brief:
I designed this odd, little 5-tube tubefin with one purpose in mind: as an ultra low-cost, quick-building L1
certification for 14 year old rocketeers under the new NAR junior cert program. After spending so much effort figuring
out how to make tubefin rockets more efficient, I "retro-engineered" this one (I think that's the right term)
to have very high drag, the better to use H-power in a small, inexpensive rocket. I call it "ElCheapoCerto",
but it is basically a double size version (2.33x scale, actually) of the neat new 5-tube tubefin "TOOBOH" now
avaiiable from Discount Rocketry.
Construction:
ElCheapoCerto ("ECC") is constructed from 38mm motor tube with 2" postal mailing tubing for the tube
fins. Parts count is seven (body tube, tube fins, nose cone). There is no internal structure and it flies on 38mm
motors or 29mm motors using a standard adapter. It can be scratch built in an hour for about 20 bucks including cutting
out parts and painting. The parts list:
Assembly sequence:
Flight:
To date, I have rarely found it necessary to add nose ballast with any tube fin design. But to insure safety with the
310 gram H73 motor and casing on a rocket weighing only 425 grams, I did put 50 grams of copper BBs into the nose cone.
Plus, it lowers the maximum speed by about 10% or so, and I try not to fly non-glassed tube fins at more than 500 mph.
This is not needed with F and G motors. First flight was on an AeroTech F42-4 using a 29-38 adapter and was perfect and
vertical despite a 10-12 mph breeze. It went to about 600-700'. For all later flights, I taped an 18mm parasite pod
with PerfectFlite altimeter to the side so I could calculate Cd from the altitude achieved (this increases the
effective diameter from 42mm to 46mm but otherwise doesn't affect flight dynamics. Next flight on an H73-10 went to
1981', giving calculated Cd=1.90 at 295 mph, which was about what I expected based on 6-tube tube fins. Next flight on
an elderly G38-7 I had in my box was a disaster--the EconoJet CATO'd like a cherry bomb, shattering the LOC adapter
into pieces and splitting open the aft 12" of the rocket between two of the tube fins. I decided I could salvage
the thing by just permanently gluing a new 29-38mm adapter into ECC. The idea worked great but meant that all future
flights would have to be 29mm motors. Not a problem. Next came two perfect flights on H97J-10 motors with average
height of 1992' and Cd=1.74 at 444mph. I think this is probably the preferred motor for L1 in this design, along with
the 38mm H73J-10 as a second choice (slower climb and less wind resistance). No other H-motors are recommended although
I did (on a dare) make one successful flight on an H180-10, which the Perfectflite recorded as 2136' and Cd=1.76 at 509
mph (faster than is prudent unless the tube fins are glassed). That said, I blew a tube fin off ECC both times I tried
flying it on H128-10 motors. Not good. So H power in ECC should be limited to the H73 and H97 Blackjack motors. The
minimum Cd value obtained in flight testing of ECC is about 1.75, which compares reasonably with the calculated value
of minimum Cd = 1.63, based on its dimensions above the tube fin can and the regression equation for 6-tube tube fins.
So 5-tube tube fins and the usual 6-tube tube fins perform similarly. Huge Drag Coefficients are needed here to avoid
overspeeding the airframe of this small, light rocket. Flight testing of ECC with additional F-G motors gave even
higher Cd values at speeds of 200 mph and lower as expected--about 2.30 to 2.80. Details of these test flights:
Summary:
I would say the goal of a unique, simple, and cheap L1 certification rocket for 14 and above rocketeers seeking to
certify under the new NAR rules has been achieved. For fun flying, it will go to 1300' or more on G-power, which is as
high as you want to fly at our island launch site anyway.
CONs: Hard to see at 2000'. Limited H power choices (H73J-10 or H97-10). Tube fins can be dented by handing on hard surface.
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