Thanks, Carl!

The way I did the Spear Tip was to build up the fin from multiple pieces, with the grain run optimally for each piece. I think these new designs would have to be built the same way. No one grain direction is going to cover all of the various stress directions. Typically, the grain for each piece runs parallel to the longest edges.

I'll work up a more accurate pattern for these and post them Friday.

__________________
Craig McGraw

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SAM 0044
AMA 352635

Plan #229...

Combination open-fin and ring-fin design.

One important note: Do not attempt to fly this design on anything less than a "C" motor, as the deployment V will make the model behave just as the name implies. Only with the "C" class or better could I get Dv rates in the safe region. Using "A" and "B" class, this model has Dv rates well over 30 FPS.

Length: 23.3"
Diameter: 0.908" (ST-8)
Fin Span: 3.80"
Ring Diameter: 2.22" (BT-70)
Weight: 1.25 oz

C6-3Q......498'......4 FPS
C6-3.......521'......5 FPS
C4-3.......562'......9 FPS
D10-3......747'.....11 FPS

All motors listed will achieve flight V on a standard rod.

Enjoy!

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Attached Files

Lunar Penetrator.rkt (59.9 KB, 104 views)

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Craig McGraw

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AMA 352635

One additional note: I tried substituting your 0.05" cardstock (paper) in place of the 3/32" balsa on one simulation, and the weight of the cardstock upset the stability margin back to a 0.7 when using the C6. It's heavier than the balsa.

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Craig McGraw

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AMA 352635

It may not be quite that bad in real life. We plug in closer densities in Rocksim. We use:

Mat Board 4-ply = 44.36 lb/ft3 (.050")
Mat Board 2-ply = 48.25 lb/ft3 (.025")
Spiral/Glassine = 53.2 lb/ft3
Balsa = 7.0 lb/ft3 (Nose Cones average, 6.0 through 8.0)
Balsa = 8.0 lb/ft3 (Fins average, 6.0 through 10.0)

These densities give us closer to actual results than the defaults in Rocksim.

__________________
Carl McLawhorn
NAR#4717 L2
semroc.com

In which case it would actually become a "*Terran* Penetrator."

It would be even messier if an actual Terran was immediately below said rocket.


(Which makes me wonder... Could you set up RockSim to simulate launching from the surface of Earth's moon? Come to think of it, RS has some Martian landscapes in its 3D view, too...)


Cheers,

--Jay

Possibly, if you fiddle with the various weights to make them 1/6 of their Earth weights. However, the moon has no atmosphere, so there's nothing for the fins to work against, and I'm not sure if RS allows for a vacuum.

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Craig McGraw

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SAM 0044
AMA 352635

As mentioned, here is a cutting pattern for the pieces needed to create the open fins of the Tradewind. This is perhaps the most complicated task in building this rocket. Some may feel this is an unnecessary step, but you have to remember the fin doesn't have all of the internal bulk to add strength. The grain of the balsa has to take all of the stresses of launch and landing. To give these fins as much strength as possible, you have to run the grain parallel to the leading, trailing, and tip edges. To achieve that, you have to use multiple pieces stitched together like a jigsaw puzzle. Or, maybe more correctly, think of the way a model airplane wing is built up. Same principle.

Addendum: I call out 3/32" thick balsa on this pattern, but I used 1/8" in the simulation (I just noticed this, BTW). Using 1/8" balsa gives a little more bulk in the structure of the fin, and tends to hold down the upper-end Dv using the "C" motors. 3/32" balsa will reduce the weight, and allows a very soft deployment using the "A" motor. It reduces the beefiness of the structure, however. It's a toss-up as to which thickness would be better.

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SAM 0044
AMA 352635

Actually, Carl, it confirms my observation in RS. Your 4-ply mat weighs in at 6 times the weight of the nose cone balsa, and your 2-ply weighs in at 6 times the weight of your fin balsa. The results come in the same -- even with so much material removed, the mat board fins weigh considerably more than their equivalent balsa counterparts. Up to six times as heavy.

If there is one advantage of using the mat board, it's the elimination of having to orient the board along a grain pattern. It's uniformly stable in all directions. I'll take a look and see if I can trim out some of the fin pattern and lighten up the final weight.

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SAM 0044
AMA 352635

Plan #231...

(I found a couple that I overlooked earlier...)

Another open-fin design. I'm intrigued by the way RockSim says these designs are as stable, if not more so by virtue of the reduced weight, than the traditional solid-fin designs...

One additional idea is to cover these open-frame fins with transparent MonoKote, but I don't know just how much weight that would add back. Still, it's an idea...

Length: 23.25"
Fin Span: 6.1"
Diameter: 1.04" (ST-10)
Weight: 1.46 oz

A8-3.......188'.......5 FPS
B4-4.......457'.......7 FPS
B6-4.......465'......12 FPS
C6-5......1000'......14 FPS

All motors reach flight V in the length of a standard rod.

Enjoy!

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Stimulator.rkt (70.6 KB, 106 views)

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Craig McGraw

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SAM 0044
AMA 352635

Craig, What about painting the hollow fins with CA? I know it will add strength and not too much more wieght. Yes, a messy and stinky job but it's a thought.


Here is a new design I call the Orion Cruiser. Also count me in for a couple of copies of that Barclone design catalog as well.

Enjoy and thanks Again, JP

James Pierson
NAR# 77907

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Orion Cruiser.rkt (92.6 KB, 88 views)