SAMPE Journal - November/December 2020 - 38

FEATURE / ONE-OFF COMPOSITE STRUCTURES
In order to verify the material properties more
than 700 mechanical samples were manufactured
and tested. Furthermore, a number of samples
were manufactured and tested in order to verify the
CTE properties of base laminates as well as sandwich structures.
Another important aspect of the material testing program was the processing of the prepregs,
especially in combination with Automated Tape
laying. Typically pitch fibres has an E-modulus of
>650GPa but only 0.3% - 0.4% strain to failure. This
makes the fibres very prone to breaking if not handled carefully. A number of test was performed to
verify that fibres and prepreg could be process by
ATL, without breaking during placement. These
tests showed that pitch fibre prepreg could be successfully placed using the ATL process, while retaining the same mechanical properties as those
achieved with hand layup. However, testing showed
although it was possible to place this material without damage to the fibres, the processing of this material was different to that of HS PAN fibres. Due to
the more than 3 times higher stiffness of the pitch
fibres compared to PAN fibres, the pitch prepregs
were prone to pre-releasing from the backing paper, thereby causing problems during tape laying.
In the end this problem was solved by changing the
backing paper used for the prepreg, selecting a paper with a slightly more "grippy" coating.
Top Plate
The top plate is a CFRP/honeycomb sandwich, with
a skin thickness >15 plies and a thick core. The 6x7m
top plate is produced in 4 segments, allowing for autoclave cure of the sandwich panels and also simplifies transportation of the backup structure. In the top
plate there are approx. 600 CFRP bushing mounted,
which holds the adjusters for the mirror surface.

Truss Structure
The top plate is supported by a CFRP truss structure, also providing the connection with the rest
of the telescope structure. The truss structure has
to support the top plate while rotating from 30° to
150°, keeping the total mirror deformation <10μm.
The truss structure also has to have a CTE similar to
the top plate, in order to not influence the shape of
the mirror during ambient temperature variations.
For the design of the truss a hollow box structure was selected consisting of full CFRP skins in
combination with standard CFRP profiles. During
the truss design all cross-sectional areas of the individual beams needed to be optimized to meet the
performance requirements resulting in an unique
design for all beams. This is needed to account for
the complex, asymmetric loads on the mirror backup structure.
MANUFACTURING
Manufacturing Concept
With lessons learned from previous telescope projects, such as the South Pole Telescope and ALMA,
it was a clear desire for the project to try and automate the composite manufacturing as much as
possible. While the primary goal of the design was
to meet the very stringent performance requirements, a secondary goal was to design a structure
that could be manufactured taking advantage of
automated manufacturing technologies. This resulted in a design based on flat or single curved
elements, which can very efficiently be manufactured using ATL. Furthermore, using a large, flat
base laminate as a starting point, it is possible to
cut blanks that can be formed into profiles. These
blanks can also be used as building blocks for a
larger, more complex structure, also using the robot to place these elements in the correct position.

Figure 5. Top plate and truss structure (courtesy
of Vertex Antennentechnik GmbH).

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SAMPE Journal - November/December 2020

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