Tuesday, February 26, 2013

A True Serrurier Truss?

What I have been considering building is a half-Serrurier truss, in the style that has become popular because of Kriege and Berry's work that led to the Obsession telesocpe.  This article here, as well as the Wikipedia article on the Serrurier truss, point out that a true Serrurier truss has many advantages over the half-Serrurier truss.

One of the advantages is balance point for the telescope is simplified; you can put the place where the telescope attaches to the mount in a more central location (which is not so important for my equatorial mount).  Another advantage is the two halves of a true Serrurier truss work together, allowing much better stiffness for the size of the tubes relative to a half-Serrurier truss.  A third advantage is that the length of the tubes is substantially shorter, which both simplifies getting this into a automobile, if that becomes necessary, and simplifies finding off-the-shelf carbon fiber tubes.  The downside is that it increases the number of connectors and adds another piece of tube at the pivot point of the telescope -- and this will, I am afraid, increase weight.

Or will it?  I don't need as long as a base to reach from the mirror section to the mount saddle -- I can construct a hexagonal center ring out of a piece of aluminum (or maybe even carbon fiber plate) that is 2" wide and mount that to the equatorial mount saddle plate.  The carbon fiber tubes can be much smaller to achieve the required stiffness.  The big advantage of this approach is that the tubes can still sag, but because the two ends are sagging the same amount, the optical path of the primary and secondary mirrors remains in parallel.

UPDATE: Perhaps I am making this too complicated.  I had dismissed a solid carbon fiber composite tube because it would cost me $3200.  (That's a bit rich for this.)  But I did not even consider the possibility of making a carbon fiber hexagon or octagonal tube form carbon fiber plate.  Perhaps I should have.  Simpler, less portable, but it would only weigh several pounds, and would drop the total weight of the telescope down to about 45 pounds.


Rorschach said...

I think you are going about this all wrong. Carbon fiber is excellent for light weight, but it is also very springy. If the intent is lightweight and rigidity, then choosing a springy material for your construction material is probably the wrong way to go about it. There is a reason why beryllium and it's alloys are used for spy satellite frames.... they are extremely stiff for their weight. AlBeMet metal matrix composite would be ideal for this. it's modulus to density ratio is 3.8 times that of Aluminum or Steel. it's thermal conductivity is 125% of Aluminum 6061 Alloy, it has the CTE of a ceramic and it is lighter than aluminum as well. Downside is that it must be machined under environmental controls and the machine carefully decontaminated to prevent beryllosis It is the material of choice for optical satellite frames.

Clayton Cramer said...

I'm confused. Carbon fiber composites seem like they are very stiff for their weight. At least, the examples that I have found seem remarkably stiff.

Are there suppliers of beryllium alloy tubing out there? Or do I have to buy it from someone who also sells plutonium?

Clayton Cramer said...

Using this calculator http://easycalculation.com/mechanical/deflection-round-tube-beams.php with a 35 pound force, 12 inches long, 20.1 diameter, wall thickness .03", it claims a deflection for carbon fiber and steel are quite similar: .0000078 for carbon fiber, and .0000070 inches for steel. That's sound pretty rigid!

Windy Wilson said...

Compared to AlBeMet, Carbon Fiber apparently is very springy. . . .0000078 sounds pretty rigid to me. As a non-engineer I was unaware of the calculations you are talking about, and I am very interested in seeing the results of your work.

My father was a carpenter and blissfully unaware of any of this sort of calculation. Consequently the stuff he made was strong (important with three boys at home) and heavy (also useful, as it kept us from picking stuff up too casually). The disadvantage was the 8 foot boat he made was heavy enough to go into combat, and it took two men and a boy to move the @#$%^& thing.

Rorschach said...

Brush Wellman sells it, along with Beryllium copper alloys which are probably cheaper and easier to work with (calling it beryllium copper is a bit of a misnomer, it is mostly copper with just a little beryllium) as long as you are machining it and not grinding it, then it is pretty safe to work with. carbon fiber is very strong for it's weight but strength and rigidity are not the same thing.