"Martin Eastburn" wrote in message
...
Some tubes have sliding fit inserts of smaller tubes and making up
two or three
tubes inside makes for a strong tube looking device. Often times
layers are
stronger than solid.

Martin


https://www.aero-mag.com/read-all-about-it/
"Of particular interest is the unusual construction of the Spitfire's
main wing spar booms. Each spar boom is built up from five
square-section concentric tubes made of 11 SWG aluminium alloy and one
square section central plug. The tubes are made to fine tolerances and
fit tightly into each other. Using specially commissioned photographs
and original working drawings we show how at the wing root the spar
booms are made up of five thicknesses of tube, but as the loads on the
spar decrease progressively towards the wing tip, so the inner tubes
terminate one by one, until at the wing tip only the two outer
thicknesses of tube remain."

I have the book. It's a good effort but this is its weakness:
"Scope for an in-depth technical manual is necessarily restricted by
our self-imposed limit to 160 pages "