- Thread starter Fred22
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Hi Guys Does anyone have a good simple for calculating the necessary fin area for a rocket?

Thanks

Fred

If you don't mind a little math (or spreadsheet scripting) Google "Barrowman Equations." For many years, these were the staple of calculating rocket stability. RockSim and other simulation software packages have somewhat supplanted them, but that doesn't make them any less valid. Besides, they're free.

The equations won't give you a fin size, but you can start with the general rule of:

ROOT = BT_DIA * 2

TIP = BT_DIA

SEMISPAN = BT_DIA * 2

As long as you keep the sweep reasonable, these will produce a good flying rocket. Plug those numbers into the Barrowman Equations, and you can tweak them until you get a shape you like.

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Above example on a BT 50 tube gives you a fin approximatly 2" root, 2" span, with a 1" tip, centered you have a classic Nile Smoke profile fin, actually quite a large-ish fin of 3 sq. in.. Low aspect fin is 1.875 sq. in..

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3 fins

Fin Area (each) = 0.17 * [(rocket_diameter+0.5)*rocket_length]

4 fins

Fin Area (each) = 0.13 * [(rocket_diameter+0.5)*rocket_length]

Those are in inches and square inches

Root chord = 2 x rocket_diameter

Tip chord = 1x rocket_diameter

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Thanks very much

3 fins

Fin Area (each) = 0.17 * [(rocket_diameter+0.5)*rocket_length]

4 fins

Fin Area (each) = 0.13 * [(rocket_diameter+0.5)*rocket_length]

Those are in inches and square inches

Root chord = 2 x rocket_diameter

Tip chord = 1x rocket_diameter

Cheers

Fred

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Hmm, I get different numbers... how long is this rocket? Assuming a 10:1 ratio, the rocket would be 26 inches in length.So for a BT-80 Root cord is 5.2 inches.

Tip cord is 2.6 inches.

Semi span would be 5.2"

This is aproximate of course

Thanks

fred

Root = 5.2 in

Tip = 2.6 in

Area(3) = 13.7 in^2 (each)

area(4) = 10.48 in^2 (each)

The formula for area would be:

area = (root+tip) * span / 2

Therefore:

span = area*2/(root+tip)

For 3 fins, that'd be 3.51 in.

For 4 fins, that'd be 2.69 in.

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The rocket should be about 28.75 inches long Man math makes my head hurt.Hmm, I get different numbers... how long is this rocket? Assuming a 10:1 ratio, the rocket would be 26 inches in length.

Root = 5.2 in

Tip = 2.6 in

Area(3) = 13.7 in^2 (each)

area(4) = 10.48 in^2 (each)

The formula for area would be:

area = (root+tip) * span / 2

Therefore:

span = area*2/(root+tip)

For 3 fins, that'd be 3.51 in.

For 4 fins, that'd be 2.69 in.

Cheers

Fred

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I'll keep it in mind

cheers

fred

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How do these formulas change if at all when using metric measurement. I can use imperial but find it confusing. I mean inches no disrespect eitherHmm, I get different numbers... how long is this rocket? Assuming a 10:1 ratio, the rocket would be 26 inches in length.

Root = 5.2 in

Tip = 2.6 in

Area(3) = 13.7 in^2 (each)

area(4) = 10.48 in^2 (each)

The formula for area would be:

area = (root+tip) * span / 2

Therefore:

span = area*2/(root+tip)

For 3 fins, that'd be 3.51 in.

For 4 fins, that'd be 2.69 in.

Cheers

Bill

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I believe the formula can be used using either imperial or metric figures..metric would make it easier to measure the final results..How do these formulas change if at all when using metric measurement. I can use imperial but find it confusing. I mean inches no disrespect either

Cheers

Bill

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The only thing that would change for metric (other than the units) is how much you add to the diameter... The formula adds half an inch, which is 1.27 cm or 12.7 mm.How do these formulas change if at all when using metric measurement. I can use imperial but find it confusing. I mean inches no disrespect either

Cheers

Bill

So for centimeters:

3 fins

Fin Area (each) = 0.17 * [(rocket_diameter+1.27)*rocket_length]

4 fins

Fin Area (each) = 0.13 * [(rocket_diameter+1.27)*rocket_length]

And your answer will be in square centimeters instead of square inches. If you wanted to do it in millimeters, substitute 12.7 instead of 1.27.

Heck, if you wanted to do it in fathoms, substitute .007. Though I can't say I know the length or diameter of any of my rockets in fathoms...

Anyway, it's just a rough guide, but I think on standard looking rockets, it's a reasonable starting point.

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Thanks

Fred