ATM Unit Conversions and Trusses

[Aplanatic@aol.com]

Here's a nice page with unit conversions:

http://www.e-composites.com/commonFiles/conversion_table.asp

I needed to convert gigaPascals to PSI and felt lazy enough to go hunting on the web.  The reason I needed this conversion was to find the flexure in a carbon fiber tube truss structure for a large astrograph.  

The upper tube assembly (UTA) will weigh about 12 lbs.  The 8 truss tubes are 45 inches long and each of the four truss triangles will have an included angle of about 12 degrees.  I plan on using 9/16" ID carbon tubes with 1/32" wall thickness, see:

http://www.deltronix.net/cgi/acp_display.exe

The carbon fiber material in these tubes has a Modulus of Elasticity of approx. 30 million PSI.  The problem is to calculate the lateral deformation of the truss structure when the scope is slewed from zenith to horizon.  In other words, with all this weight and the skinny tubes, how much can I expect the collimation to change because of  deformation in the truss structure as it gets loaded by the weight of the UTA as the scope is pointed to the horizon?  You might be surprised by how small it is.  I could actually get by with smaller tubes.  Truss structures are remarkably strong.

Dave Rowe