Re: Tube/mount material, rigidity, damping, strength, cost. Optimize

Steve Strickland (lensnut@tpoint.net)
Tue, 1 Aug 1995 10:49:16 -0500

>"Tom Glinos, x4302" <tg@utstat.utstat.toronto.edu> wrote:
>
>> (2) Nothing replaces mass to encourage damping.
>
>More mass does not encourage damping. Alright, in practical situations
>you are probably right, since I assume you speak from experience. But
>adding mass by itself does not increase damping. You might make it
>stiffer by adding mass properly. You certainly lower the frequency at
>which it tends to oscillate. Other things being equal, a more massive
>structure oscillating at the same frequency with the same amplitude has
>more energy in it than a lighter structure, so it would oscillate
>longer. I suspect the reason a more massive telescope seems to dampen
>more is that it is harder to get it started. Tapping it the same way
>might transfer less energy to it. It would be better to somehow make
>it stiffer and more highly damped while keeping it light, but this is
>difficult.
>
>Clay Spence

This goes right along with the structural characteristics of a cylinder (tube). The inner and outer surfaces are held in compression/tension. The farther apart the two surfaces are, the stronger the cylinder will be. The intervening material does very little to increase strength, it only increases weight.

Massive material has low frequency sympathetic oscillations. Low frequency oscillations take longer to damp than higher frequency ones do. More mass also means more thermal inertia.

Simply adding mass will not increase strength very much and it will certainly worsen vibration damping times and take longer to reach thermal equilibrium.

Foam core composites have successfully addressed these concerns. The inner and outer fiber/resin skins handle compression/tension very nicely while the intervening foam does very little to add mass and thermal inertia. 

--
Steve Strickland
Lensnut@tpoint.net