HELP! Marking guide for LOC 3.9" tube?

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Well-Known Member
Jan 25, 2009
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I need to mark a 3.9" LOC tube for three fins. Template Widget, which I normally use, says that the paper I use is too small. So I'm in a bind. I had a thought though. I did a little math:

Circumfrence of 4" tube: 4" * 3.14 = 12.56"

Since there are three fins on said rocket, I divided this number by three:

12.56 / 3 = 4.186

I think that 4.186 is just about equal to 4 and 3/16". So, I'm thinking of taking a strip of paper that I'll length to about 12.56", then marking it every 4 3/16ths inches will give me a marking guide.

Opinions? Thoughts?

Print this out. The red line is exactly 4". Once its printed, set your tube on top of the red line and mark the 4 fin lines. It should match your LOC tube perfectly. Then take an angle bracket or wall jam and extend the lines. Enjoy.
Here's what I do for all my rockets, with any odd size tube, including glassed tubes:

Take your sheet of paper.

Wrap it around the tube.

Mark its EXACT overlap length.

Take it off the tube again.

Fold this EXACT length into x number of equal parts, where x is your number of fins. Remember how to fold a letter to fit an envelope? Same idea :p

Unfold it and mark the creases.

Take this and wrap it around the tube and mark your locations (at the creases).

Check the distances between the marks with another piece of paper marked to this distance (i.e. measure the distance between two marks and check to make sure it's consistent all the way around).

Ta da, marked tube! Hope that helps :)
I also forgot to mention that in the "Handbook of Model Rocketry" there is a similar diagram to what I posted above as an attachment. (Rock2p actually pointed this out.) You can take the circular marking guide diagram and blow it up on a copier for larger diameter tubes. I still use the one Russell gave me. ;)