Aurora
The Aurora project was developed to extend the reach of our balloon payloads past the use of off-the-shelf latex balloons. By developing the means to fabricate zero-pressure balloons ourself, we can tailor make envelopes for any size payload. The martlet rocket project will require a balloon to be designed specifically for the task as will any large scientific experiments that we wish to launch.
Zero-pressure balloons have zero pressure difference between the gas in the envelope and that of the air outside, unlike latex balloons that have large skin tension. To ensure this pressure difference they are commonly left open at the bottom, or mechanically vented to allow excess gas to be released. Zero-pressure balloons are generally fabricated from polyethylene film of thickness 3-40um and can range in size up to several million cubic metres in volume. As they do not burst in the same fashion as a conventional latex balloon, long duration flights are also possible.
Whereas large (~2 tonnes) payloads have been carried by national space agencies, our payloads will be much smaller which allows new possibilities to produce simpler balloons. By experimenting with different manufacturing techiques and balloon shapes we aim to design balloons for small payloads that have a high volume to weight ratio, are economical to manufacture on a small scale and are reliable to use.
August 2008: Manufacture of small test balloons
Several balloons were maufactured using different shapes and folding techniques to investigate the possibility of only using straight welds to create 3D balloon shapes. The balloons were fabricated from standard 38um PE packing film, as a suitably cheap source of material for these non-flight tests. Three shape designs were tested and are shown pictured alongside "The Squid" that was built in December. Using thicker film with straight welds and no load tapes was found to significantly decrease manufacture time.
Rip seams were welded into the balloons to investigate the possibility of using payload cut-down to destroy the balloon envelope. The techniques tested were only partially successful due to the thickness of the film and further testing will be required. Three smaller balloons were also created and inflated at low pressure from a compressed air source to test burst seam configurations for sounding balloon applications.
December 2007: Building "The Squid"
In December we created our first zero-pressure balloon, nicknamed The Squid. The design is a natural shape balloon with 20 gores and was fabricated from ~8um film with ~40um UHMWPE load tapes at each seam. Impulse welding equipment was custom built for the purpose by the team over a period of several days and the balloon manufactured over the rest of the week. Problems such as residual thermal stress and handling errors were found when working with film of such thickness and welding complex shapes was found to be extremely time consuming without custom jigs.
The video opposite shows a test inflation of the 22m3 balloon using a standard hair dryer.