In the period of 2006-2011 Airborne has manufactured 25 back up structures (BUS) for the ALMA antennas that are located at the Atacama desert in Chili at a heigth of 5000 m. The ALMA project is a very large astronomy project to help the astronomers to discover the cosmic origins.
The customer requirements
The ALMA antenna needs to function under high wind loads and over a larger temperature range. The BUS of the ALMA antenna needs to be very stiff and thermal stable in order for the antenna not to deform more than 25 μm under wind load, gravity and temperature changes. These kinds of requirements simply cannot be met with a metal back-up structure. Therefore the BUS is a carbon fiber composite (CFRP) sandwich structure consisting of 24 segments (pie pieces) with a very high stiffness to weight ratio and a very low thermal expansion coefficient. Also the sub reflector support structure is made of CFRP. Besides very stiff and thermally stable, the BUS should not degrade at the environmental conditions at 5000 m high, being a very cold, dry environment with high UV light levels.
The challenge of the ALMA project was to manufacture 25 x 24 = 600 BUS segments in a time frame of 5 years. Besides the planning it was important to deliver a high quality product for several reasons. The dimensional stability of the BUS is very important so the dimensional accuracy and the repeatability of the stiffness properties should be the same for all segments. As repair or maintenance at 5000 m is not easy and expensive, every segments was inspected thoroughly so that no hidden defects could be present in the segments.
A special manufacturing line was set-up and every process step of the BUS was optimized with the investment in special tooling, a large 5-axis milling machine, and a laser tracker for measuring in order to be able to guarantee an accurate segment. Most of the tooling was made from CFRP, designed and manufactured in house in order to have the same low thermal expansion coefficient as the BUS segments itself. This was needed to meet the stringent dimensional tolerances. Every segment was milled with a high accuracy 5-axis milling machine and measured with a laser tracker to ensure the right dimensions of the segments. Quality checks were implemented at every process step to ensure a defect free segment.
The total project was finished in time and the ALMA antennas are working well and will help to make a giant step in our understanding of the cosmic origins.