# Fin Size ### Help Support The Rocketry Forum: #### Fred22

##### Well-Known Member
Hi Guys Does anyone have a good simple for calculating the necessary fin area for a rocket?
Thanks
Fred

#### TheAviator

##### Well-Known Member
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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#### dave carver

##### ....what hump?
The low aspect isocilies(sp ) triangle type, like a StarLight Flash 24 have very small fins for their length. 22.25" long BT 50 based tube minium diameter 24 mm fin root is 2 7/8", leading edge is 2.5" and span is 1.25". With 3 fins drag is minimal and I dont know how much smaller you can go with a D motor in this airframe and maintain stability.

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..

#### Fred22

##### Well-Known Member
Thanks fellas that was helpful. I am dtermining fin size for my minuteman Cheers
fred

#### MattieShoes

##### Well-Known Member
The normal fin rules of thumb...

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

#### Fred22

##### Well-Known Member
The normal fin rules of thumb...

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
Thanks very much Cheers
Fred

#### Fred22

##### Well-Known Member
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

#### MattieShoes

##### Well-Known Member
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
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.

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#### luke strawwalker

##### Well-Known Member
Also remember that a span increase is more effective than a chord increase when considering the fin's overall effectiveness... OL JR #### Fred22

##### Well-Known Member
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.
The rocket should be about 28.75 inches long Man math makes my head hurt.
Cheers
Fred

#### Fred22

##### Well-Known Member
Also remember that a span increase is more effective than a chord increase when considering the fin's overall effectiveness... OL JR I'll keep it in mind cheers
fred

#### Fred22

##### Well-Known Member
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.
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

#### Pantherjon

##### Well-Known Member
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
I believe the formula can be used using either imperial or metric figures..metric would make it easier to measure the final results..

#### MattieShoes

##### Well-Known Member
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
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.

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.

#### Fred22

##### Well-Known Member
Since Im not doing an SLBM Ill skip the fathoms This may seem dense but how does one calculate the the fin chord? given I know how long it is at both ends aproximately and the total surface area. I very much apreciate your help with this by the way.
Thanks
Fred