(Contributed - by Larry Brand - 12/07/09)

Brief:
"The Walrus" is an extreme stubby version of my 7-tube tube fin "Dwarf King Mk. 7" presented earlier on EMRR. Being lighter and stubbier, it uses a LOC 5.5" cone and can be flown on G-motors at smaller non-HPR fields, or on 300+ Newton I motors.

I built it to test some more ideas on tube fin aerodynamics. Based on the results, I've selected The Walrus, scaled up by 35%, to be my L3 cert ship. BTW, "The Walrus", 105.7 FM, is the local 60's oldies rock station in San Diego. Inspiration for the name.

Construction:
Design is very similar to my Dwarf King Mk.7, only much stubbier, as stubby as it is possible to build. Ratio of length to width over the tube fin can is only 4.0, the lowest yet, and my equation for tube fin drag coefficient predicts minimum Cd = 0.65.

So I'll mainly lay out the changes, and refer to the earlier article for construction details, which are the same:

Parts for this scratch build:

• Body tube - 11 5/8" length of 5.5" LOC tube
• Nose Cone - 13" long 5.5" LOC cone (longer than rocket body tube!!)
• Tube fins - eleven x 3" lengths of 4" LOC tube (inc. 4 for half doublers)
• Motor tube - 8" piece of 38mm PML phenolic (yes, some motors will stick out, up to 3" or so aft of the end; CG is still OK)
• Centering rings - 1/4" ply, 1/2" from top and 1" from bottom of motor tube
• Launch lug - 3.75" piece of Garcia-Vega plastic cigar tube, 1.5" above tube fin can
• Chutes - 45" and 24" Top Flite nylon (cone has its own chute), large Nomex®
• Shock cord - 8' of heavy para cord

Assembly follows exactly the sequence used for my Dwarf King Mk. 7 described earlier in EMRR. A main difference is the shock cord anchor, which is by passing a knotted loop of para cord through a 1/8" hole drilled 3.5" from the top of the motor tube, with the external knot covered by a fairing made from the rounded end of the cigar tube (and a big gob of epoxy).

The major difference is in using the 5.5" LOC cone (the original short one). The bottom of the cone hip is cut off with a hack saw, leaving 2" to fit into the body tube. The recovery system attachment is to a piece of 3/4" hardwood dowel rod fitted to the inside of the cone and just above the cone hip indentation, anchored in place with lots of epoxy and bits of wood scrap (see photo). This lets you use the all wasted space of the cone bottom for the recovery system, since there is not much room for motors and chute! I use an auxiliary chute tied to the dowel rod for the chute, to insure against loss. Looks cool on descent! Painting was with rattle can Testor's flat light aircraft gray and flat white for the cone.

Flight:
The Walrus has been flown on as little as Aerotech single-use G-motors (G77R, G78 Mojave Green and G79W with 4 sec. delays and CTI G69 and G115 motors, giving exactly the 1500g max. all-up weight for our non-HPR Fiesta island site. Boost on these was what I call "Estes Big Daddy-like", 430 feet or so and 120-125 mph, very straight, and giving calculated Cd of 0.88 - 1.15, very low drag for this low speed. High power flights ranged from the AT H123-6 (834', giving Cd=0.8 at 170mph) to the CTI I345-8 (1295', giving Cd=0.80 at 360 mph). Top speed was 394 mph on a CTI I303-9 Blue Streak.

Best apogee was 1629' on a CTI I285-8 (Cd=0.73 at 389 mph). A plot (see graph) of the 10 motors tested shows that there is no speed dependence of Cd as seen in most tube fin designs, with constant (more or less) Cd values at all speeds from 170 to 394 mph - just like "normal" 4FNC rockets. Mean of all Cd values was 0.8u +/- 0.02; this number, instead of my speed-Cd plot, can be used in SIM predictions of performance. Not a bit of nose ballast required for any motor. This is important for scaling up the design - what I was looking for.

I selected a 7.5 x 43" version of The Walrus to be "M-Pod", my original tube fin Level 3 cert rocket. I have already built the first prototype (see photo) and it has so far passed all its flight stress tests on K630BS and K780BS motors to quality the basic design and construction for M650-power! See photo on M-Pod boosting on Cesaroni Blue Streak K-power, that blue flame is spectacular!!

Summary:
PRO - Very compact and easy to transport for a 5.5" rocket, at only 25" long! Versatile enough to fly G-power at smaller fields and when there is a breeze, or up to I power. With Cd = 0.80 and no speed dependence of Cd, The Walrus has the same flight characteristics as a "normal" finned rocket. Unique looks.

CON - Extreme stubby lay out means that motor selection is limited by case dimensions to about 12 inches. A J210 would be possible to stuff in with a 54mm mount, and sims out to about 2600' at only 345 mph, so the potential of The Walrus can't be realized - airframe is good to 500mph.

Other:
Remember with any 7-tube tube fin that the spacers needed between the tube fins for a perfect fit need to be test-fitted and fiddled with before gluing.For The Walrus, 3/16" square x 2.75" spacers were used. The first tube fin is always attached with no spacer added.