A Case Study: Manufacture of the HM Carbon Fibre Backup Structures for the
ALMA 12 meter Diameter Radio Telescopes.
By Johan G Braasch
August 2002
Synopsis
This paper presents an overview of the project to manufacture two backup structures and
sub components for 12-meter diameter HM CFRP composite dish structures. The project
was executed by Aerodyne Aviation Technology situated in the Strand, South Africa, and
presented unique challenges such as the required near-zero quasi-isotropic CTE for the
entire structure and geometrical accuracies in the range of 1 mm over 6 metres. Topics
for discussion include project deliverables, tooling strategy, quality assurance and
manufacturing challenges.
1. Overview of the ALMA Program
ALMA is a joint project between the National Radio Astronomy Observatory (NRAO) of
the USA, the EEC and Japan, to establish 64 mobile radio telescopes in the Atacama
desert in Chile with which to perform hereto impossible radio astronomy observations.
“The Atacama Large Millimeter Array (ALMA) project will bring to millimeter and sub
millimeter astronomy the aperture synthesis techniques of radio astronomy which enable
precision imaging to be done on sub-arcsecond angular scales.”
“The 12m-ALMA radio telescope shall be applicable to radio astronomical
observations at a frequency range from 30 GHz to 950 GHz. The primary reflector has a
parabolic shape and is 12 m in diameter. The Backup structure and primary reflector
panels are connected to each other by means of numerous normal-to-surface adjusters
and in-plane adjusters. The CFRP BUS is mounted on a pedestal, which can be rotated
around the vertical azimuth axis and the horizontal elevation axis. See Figure 2 of the
pedestal during assembly.”
Detailed FEA analysis, design and optimisation was performed by the designers, Vertex
Antennentechnik in Germany, to ensure that the antenna design complies with all
performance and survivability requirements stated in the NRAO 12m-ALMA Antenna
Specification.
The design was finalised as an assembly of 24 large “cake slices” or segments that form
the reflector backup structure. Each segment is a very large single autoclave part of
approximately 6 m x 2 m x 2 m. The construction is HM carbon fibre from Mitsubishi with
perforated aluminium honeycomb from Hexcel. All metal inserts are from invar. The
composite raw materials for one complete backup structure cost approximately 5 mR.
More information on the ALMA program can be found at www.alma.nrao.edu.
2. Project Elements
a)
b)
c)
d)
e)
f)
g)
h)
i)
Establish a suitable production facility.
Design the manufacturing process.
Design and manufacture various tools such as moulds, drill jigs, bonding jigs, etc.
Validate all main tools by using 3-D accurate laser measurement equipment e.g.
SMX laser tracker.
Create QA documentation and Process instructions for all the various production
tasks.
Select and train all personnel.
Manufacture and assembly of structures.
Design and manufacture of packaging.
Shipping logistics.
3. Schedule.
The contract was placed on Aerodyne on 15 August 2001. At that stage Aerodyne
selected a facility at Houwteq for the manufacture. The facility was an empty shell in
terms of composite manufacturing equipment and all facilities, from simple machine tools
to autoclaves had to be installed and commissioned. At the end of October 2001, after
only 2.5 months, the customer was shown the first successful segment moulding.
The first completed BUS structure, all 24 segments, was shipped at the beginning of
March 2002. It has since been erected on site in New Mexico in the USA (see Figure 3)
for trial evaluation. The second completed BUS structure was shipped to Europe at the
end of July for trial assembly tests, etc.
4. Tooling strategy.
a) As the Master Segment Plug (see Figure 4) was very sensitive to temperature variants,
the factory temperature had to be controlled to “tune” the pattern size. Two moulds (see
Figure 5) were drawn from the Master plug, using tooling resin and high modules UD
carbon fibre (same type as for the final component). This resulted in near zero CTE
moulds.
b) Only one drill jig per type was used to ensure exact similarity for all segments (see
Figure 6 for the top-hole drill jig). For the side drill jigs, metal laser cut master jigs were
used to make the final drill jig.
c) Flat machined steel tables were used to manufacture flat CFRP panels in the
autoclave.
d) A special moveable oven was designed and used for production (see Figure 7).
d) Ultimately about 30 different tool types were used to complete the project (see Figure 8
for the “nose” compacting tool).
5. Parts Manufactured.
a)
b)
c)
d)
BUS (CFRP backup structure) – Figure 9
CFRP Central HUB – Figure 10
CFRP Head Part – Figure 11
CFRP Telescope Mount Bracket - Figure 12
6. The CFRP BUS (CFRP Back-Up Structure).
A BUS structure consists of 24 segments, each with a 15° opening angle, and there are
eight unique segment types amongst them. See Figure 13 for a view on the production
line set-up. Each segment has five “rib” panels (see Figure 14 for a typical “rib”) and
about 240 invar inserts bonded to it. About sixty flat panels per BUS structure are needed
and each structure had almost 6000 invar inserts eventually bonded to it. In total about
300sqm Carbon prepreg per segment is used and the final mass of a complete segment
varies around 260 kg. See Figure 15 for a Vacuum bagged mould, ready to be processed
in the autoclave. As the components were to be shipped via sea freight, an adequate
packing method had to be decided on. With care four segments just managed to fit into a
40 ft container (see Figure 16).
7. Quality Assurance
A number of tests ware carried out for verification of the design - see Figure 17 for a
typical test panel. These focused on CTE and panel stiffness.
3D geometrical tool verification was done on all main tools to ensure dimensional
tolerances were reached (see Figure 18 – the Head Part tool 3D measurement). Each
manufactured part was measured to ensure geometrical compliance with the
specification, for example segments were checked after manufacture to ensure that the
reflector mounting points were all within 1 mm from the theoretical x-, y-, z-coordinates.
The project QA manager was responsible for all QA related matters and had three QA
inspectors assisting him. All manufacturing processes were documented and tracked as
required for build history purposes. Typical problems that were experienced were
machine-damaged prepreg (see Figure 19).
8. Conclusion.
The two prototype BUS structures will be evaluated by the end-user over the next few
months. If the design and manufacture proves itself then a further 63 structures are likely
to follow.
The Team:
Sias van der Westhuizen, Aerodyne Aviation MD and Project Coordinator.
Johan Braasch, General Manager of the Aerodyne Houwteq facility and Project Manager.
Colin Gaitskill, Quality Assurance Manager.
One engineer, three technicians and two toolmakers.
Three moulding teams of 10 people each.
One fitment team of 10 people.
With recognition of the intense and sustained effort of the production teams who worked in three 8-hour
shifts to meet the target completion dates.
Figure 1. Artists impression of an ALMA antenna
Figure 2. Pedestal being assembled
Figure 3. BUS assembly at New Mexico site
Figure 4. Plug tool verification
Figure 5. The two CFRP main moulds
Figure 6. Top-hole drill jig in operation
Figure 7. Post cure oven
Figure 8. Mechanical compacting tool
Figure 9. BUS structure
Figure 10. Central Hub
Figure 11. Head Part
Figure 12. Optical Telescope mount bracket
Figure 13 - BUS segments in production line
Figure 14. Typical “rib” section
Figure 15. Prepreg layed up in mould and Vacuum bagged
Figure 16. Four segments in a 40 ft container
Figure 17. Typical test panel
Figure 18. Head Part tool 3D measurement
Figure 19. Damaged prepreg