Scratch Dwarf King 4.0 Original Design / Scratch Built

Scratch - Dwarf King 4.0 {Scratch}

Contributed by Larry Brand

Construction Rating: starstarstarstarstar_border
Flight Rating: starstarstarstarstar_border
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Manufacturer: Scratch

This is a lighter, more affordable 4" version of of the 5.5" Dwarf King Mk.7 (7-tube tubefin) I presented earlier in EMRR. This design has the lowest drag coefficient of any tubefin I've built to date, Cd=0.68-0.70; it also differs from my big 5.5" Dwarf Kings in that this rocket is a much better performer than the 6-tube version, the Tea-Bird 4.0 I presented in EMRR. I try to explain why this is below. Dwarf King 4.0 replaces the "I-Pod 54" design I published in SPORT ROCKETRY last year - the latter had some high speed instability at near 500 mph I was uncomfortable with; the new model flies well at > 540 mph.

What gives Dwarf King 4.0 a big performance improvement over 6-tube Tea-Bird 4.0 (see graph and photo below) is the use of 7 tubefins and stubbier MadCow 4" plastic nose cone. Tubefin aerodynamics is still not 100% sorted out, so progress is still trial and error. But this is the best one yet. Construction is like the 5.5" version.

  • Body tube - 13 5/8" of 4" LOC tube
  • Tube fins - 7 each 3" lengths of 3" PML phenolic tube
  • Nose cone - 4" Mad Cow short (9") ogive black plastic
  • Motor tube - 11 1/4" PML phenolic 38mm
  • Centering rings 2 x LOC 4" w. 38mm cut-outs
  • Recovery - 36" and 15" nylon Top Flight chutes, 9' heavy para cord
  • launch lug - 2 1/2" of 7/16" ID thin wall aluminum tube
  • 5 minute epoxy used throughout.

By now you know the drill for fitting 7 smaller tube fins around a larger body tube: its inexact, so needs a custom trial fit and spacers every time. In this case, the spacers had to go between the tube fins and the body tube (i.e., 7 x 3" PML tubes were too big to fit around instead of too small). 1/16" ply spacers 1/4" wide and 3" long were glued to 6 of the 7 tube fins. One of these was glued to the body tube flush with the bottom with both positioned parallel on a flat surface for perfect alignment. Spacer strip against the body tube. Using the first tube fin for alignment, the succeeding 5 tube fins were glued in turn to its neighbor and the body tube. The last tube fin (the one with no spacer attached) was found to fit the remaining space perfectly, with no spacers needed. The centering rings were glued to the motor tube 1.5" from each end. In this case, I decided to anchor the recovery system with a 4" loop of stainless steel picture hanging wire passed through 2 holes drilled in the upper ring 1" apart, and secured with knots in the wire wrapped with duct tape. Use your own method if you like. A small EZ snap ring was attached to the wire loop at the level of the top of the body tube, giving some (?) anti-zipper protection, and the recovery system was attached to this. Motor tube assembly was glued into the body tube in the usual way with lots of epoxy. Note that the motor tube protrudes out 1.5" from the back of the rocket. The MadCow nose cone was modified by slicing off the hip, leaving 1.75" remaining; this gives more room for the parachute in a short rocket. Anchor was a a fitted 1/2" hunk of dowel rod wedged just above the cone hip and epoxied in place with lots of scrap and epoxy around it. The shock cord is tied off to this. I really like this set-up, but it takes a long time and a lot of sandpaper scoring to get epoxy to cure a tight bond onto the black stuff MadCow uses for nose cones (well, it wasn't mean't to be glued to stuff). In addition, I built a small "auxiliary" chute anchor from duct tape, and attached a second, 15" chute directly to the cone. The tube fins were reinforced on the outer surface by adding a strip of 2 1/2" fiberglass tape to the lower edge. Launch lug tube was epoxied on 1.5" above the tubefin can and in line with one of the triangular holes between tube fins. Finish was with purple and lime green Tamiya rattlecan, applied after the first few test flights, because I just couldn't wait!

Balance point is no further aft than middle of the launch lug (1.0 calibre). Test flight was on a CTI G69-5, using a medium Nomex® protector, perfect on the way up, but on chute deployment the glue joint fractured on my dowel rod in the nosecone, and the cone came down separately. I carefully re-epoxied the thing together, and added an "safety" 15" chute directly to the cone, and resumed flying. I was impressed that Cd was only 0.83 at just 196 mph; usually Cd is much higher at tubefin speeds below 200 mph. No more separation problems after this, but I decided I liked the 2nd chute being there, and it looks cool in the air. To pack for flight, the 15" chute is tucked between the 36" chute and the Nomex® (which I wrap around the shock cord, placing the 36" chute on top. Second flight was on one of the new (at the time) CTI G115 motors with the White Lightning propellant. Perfect flight, and 256 moh at Cd =0.80; Mike Jerauld of our DART clu caught a great shot of the launch with his new digital - note the Schlieren heat blurring of the La Jolla houses in the distance by the almost smokeless G115 motor burn. Later flights out in the desert were on the following motors, with altitude recorded (using a parasite pod) and speed/Cd calculations:

  • H143-6 1421' Cd=0.76 @ 327 mph
  • H153-8 1428' Cd=0.83 @ 339 mph
  • I205-8 2111' Cd=0.68 @ 466 mph
  • I212-7 1819' Cd=0.78 @ 437 mph
  • I255-9 2391' Cd=0.71 @ 543 mph
  • I287-8 2129' Cd=0.80 @ 521 mph

As the graph of Cd versus speed shows, there is no inverse speed dependence of Cd with this design - look at the comparison with the 6-fin Tea-Bird 4.0!! The combination of 7 tubes and the shorter MadCow cone makes a tremendous difference in performance. It will take an engineer to sort this out; I am a biochemist. The average Cd for Dwarf King 4.0 over the 196-543 mph speed range was 0.76 +/- 0.04, about what you get with a "normal"-finned rocket, and no steep slope to the curve this time. I honestly think I have finally gotten a tubefin design that performs like a 3FNC design - my goal. On the basis of this, I have started designing my Level 3 rocket - it will be a BIG, STUBBY tubefin, with 7 tubes, not 6.

PRO - Finally, a tubefin that flies with about the same performance as a "normal" rocket. Easy and quick to build from scratch. Durable and low-cost. The 5-grain Cesaroni motor case sticks out the back about 3", but no problem with balance; no nose weight needed with any motor.

CON - Still need to fiberglass a bit to prevent flutter and landing dings. MadCow cone takes extra care to get epoxy to stick to it (I will ask Paul for suggestions, since it wasn't meant to do this).

With determination, 7 years and more dollar investment in APCP that I care to admit, it is possible to get tubefins to perform as well as "thin-fin" rockets. I wonder what the Saturn 5 would have looked like if NASA had discovered this.

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