SAMPE Journal - November/December 2020 - 37

Figure 4. Mirror support structure (courtesy of
Vertex Antennentechnik GmbH).

tating from horizontal to a vertical position. When
translating these operational requirements into
structural requirements the following target values
was set:
* Mass target for mirror support structures:
M1 ≈ 1600kg, M2 ≈ 1400kg
* Thermal expansion (CTE): 0.0 ppm/°C
±0.2 ppm/°C (in all directions)
Meeting all three of these requirements posed
a major design challenge. In order to meet the CTE
requirement, also with regards to uniformity, it was
necessary to not only consider the in-plane properties but the full 3D behaviour of the structure.
Small, local CTE variations could result in global
deformation of the structure. In the end it was decided to use a hybrid structure, consisting of CFRP/
Honeycomb sandwich top plate, with a monolithic
laminate truss structure for support and attachment to the rest of the antenna construction.
In order to simplify the transport of the 7x6x1.5m composite structure it was decided to split
the structures, making it possible to move them by
trucks. However, this posed another challenge for
the design, having to create a structure that can be
assembled on-site while meeting both stiffness,
CTE and accuracy requirements.
Materials
One of the major challenges was to select material and design the laminates for the antenna
structures. Carbon fibres are known for their high
specific strength and stiffness, as well as their low
coefficient of thermal expansion (CTE). However,
normal high strength carbon fibres does not have

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the properties in order to meet the requirements
for the design of the mirror structures. In order to
meet the requirement it became clear that higher performance fibres were needed. One option
would be to use PAN based high modulus fibres,
which has a much higher stiffness as well as a lower
CTE. However, these HM fibres are also significantly more expensive, making it very difficult to meet
the cost targets. A third alternative would be to use
pitch base carbon fibres, which are known of their
very high stiffness as well as a low CTE, while at the
same time being more affordable than PAN based
HM fibres. One drawback with pitch based fibres
are their low elongation to break, making them
quite difficult to process, without damaging the
fibres.
After an extensive material testing program,
including both mechanical and thermal testing as
well as process testing, Airborne decide on using
on using prepregs based on Pitch fibres.
After the initial material selection the program
continued in order to detail and verify the specific
material properties. Because of the strict mechanical and thermal properties requirements extensive
work went into detailing material properties such
as fibre areal weight and resin content.

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SAMPE Journal - November/December 2020

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