# Rocket tables: balsa density and tube ratios

The balsa thread led me to dig out 2 tables that I've made years back and
put them on my chad web site:

This is an extension to something I found in an old MIT journal, that I use
when buying balsa in the hobby store. By eye I select wood for lighter
color, which translates to lighter weight; and desired grain, often C-grain.
I can compare weight between sheets to pick the lighter ones. But I've never
been able to tell what the density actually is without weighing it. It's not
convenient to carry an Ohaus triple beam balance from store to store. So I
use something like
to get a
nearest gram estimate of what the sheet weighs. Then I look up the
dimensions of the sheet on this table, go down the weight column until I
find the weight of the sheet, and read the pounds per cubic foot off the
left.
Both the scale and a hard copy of this table fit in my wallet.
I've only posted the tables for 3" and 4" sheets. I made them for strips as
small as 1/16" and sheets as big as 12", but you can easilly adjut for
common sizes. If you have a sheet that is 1/8" x 2", treat it as a 1/16" x
4" sheet. A 1" x 1" block is the same as a 1/4" x 4" sheet. A 12x24x1/8"
piece of plywood is the same as 4x36x1/4", although the weight may be off
the scale. So divide the real weight by 2 (or 4 or whatever), then multiply
the density by the same factor.
If you carry a calculator then
density = grams / (width * thickness * 9.45)
for 36" sheets

I generated the first of these tube tables back when building my first real
scale model for NARAM-25. The model had 2 different diameters, so I needed
to find 2 tubes with a matching ratio. To use this table, calculate the ratio
for the real rocket (in my case 38:44 or .8636, or 1.157 if you flip the
fraction - either will work), then search the table for something close. In
my case, .853 is BT70:BT80, which is what I built.
The original version of this table, with maybe a dozen different tubes, was
published in The Leading Edge around 1983-4. I was surprised to see it
return in Peter Alway's "The Art of Scale Rocketry" a decade later. Alas,
this great book is out of print, but this greatly expanded table is still
handy.

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