Ironing out the wrinkles

A researcher at the University of Arkansas believes that some inflatable structures used on space missions could be rendered impractical by small wrinkles on their surface.

Bob Reynolds, assistant professor of mechanical engineering at the University of Arkansas said that inflatable structures are ideal for many space applications, but very small wrinkles may make enormous mirrors impractical.

Reynolds has determined that wrinkles make the polymer membranes currently used for space-based inflatable structures, such as reflectors or communications antennae, unsuitable for use as mirrors.

Space-based mirrors, such as the Hubble telescope, have provided scientists a look at the far reaches of space and even time. But lifting huge mechanical devices into space is both expensive and difficult. As a potential solution to these problems, and a way to position even larger mirrors, some scientists have proposed using inflatable structures that would both increase the size and range of the mirrors and decrease the cost of deployment.

Because inflatable devices are made of membranes they can be folded to a very compact size, which makes lifting them into space less expensive. In addition, they can be used to make devices that are many miles wide.

However, the folding and compressing required to make the device small enough also produces wrinkles in the membrane. If these wrinkles are not removed, the final structure will not have the same dimensions or perform in the same manner as it was designed.

Reynolds conducted a series of experiments to determine if the wrinkles smooth out when the structure is inflated. He tested two thicknesses (13 micron and 76 micron) of a polyimide film membrane commonly used in inflatable space structures.

‘We only looked at small wrinkles with random orientation,’ said Reynolds. ‘Large, global creases were intentionally avoided, since their behaviour is likely to be somewhat different.’

Reynolds subjected the samples to tensile loading like that produced when the structure is inflated and measured the deformation caused by the wrinkles with an optical device that did not touch the surface of the membrane.

Although the wrinkle size decreased rapidly during the first two hours, the membranes did not flatten completely. Even after 24 hours, between 50 and 80 percent of the wrinkles remained.

‘This shows that it would be nearly impossible to remove all of the wrinkles with inflation pressure alone,’ Reynolds explained. ‘Because the collective impact of the wrinkles would be a serious deformation of the size and shape of the overall structure, this is an significant limitation in the use of these films in devices such as mirrors, where precision is critical.’