Geometry lesson

A tiling system based on ancient dry stone walling techniques could provide a fail-safe way of fitting protective heat-resistant tiles to space shuttles, its developers claim.

Following the destruction of Columbia in February, space agencies have been urgently seeking a way of improving the design of the thermal shields used to protect spacecraft during their return to Earth.

The Accident Investigation Board’s report into the disaster, due to be published next month, is expected to find that a crack was made to the shuttle’s wing when it was hit by a 1.5kg piece of foam from the fuel tank’s covering. It has also been revealed that up to 100 dents in the shuttle’s body from debris were considered normal following a mission.

The tiling system was developed by Dr Yuri Estrin at the Technical University of Clausthal in Germany and visiting professor Dr Arcady Dyskin, associate professor at the School of Civil and Resource Engineering at the University of Western Australia. The technique involves the use of topological interlocking, where polyhedral-shaped tiles with curved surfaces are joined together using their geometry alone.

The concave parts of one block are interlocked with the convex parts of another and vice versa, creating a structure that is more resistant to heat and impacts than current systems. Each tile is held in place by six neighbouring tiles, meaning it is unable to move.

In tests, the system’s developers confirmed that even if some of the tiles are removed the rest of the blocks will remain in place. As there are no adhesives or direct mechanical connections between each tile, cracks are unable to spread through the structure.

Conventional tiles used on spacecraft such as the doomed Columbia are held together using chemical binders that act like glue.

However, if any gap appears between them as a result of trauma such as an impact or a flaw in the material, the binders can be destroyed by the high temperatures encountered during re-entry to the Earth’s atmosphere – withdisastrous results.

Tiles fitted together using mechanical joins are susceptible to concentrations of stress.

‘The idea of using the interlocking system for protective tiles came as a result of a larger project on interlocking. This was supported by the Australian Research Council (ARC); the German research council, DFG; the Humboldt Foundation and NATO,’ said Dyskin. ‘We do not have any project with any space agency as yet, but we plan to approach them.’

The interlocking blocks could also be used to construct more robust buildings in earthquake zones.

‘Flexible foundations made from interlocking elements would help prevent soil liquefaction (when seismic waves pass through loose, saturated soil, causing it to act like a liquid) – one of the reasons for collapse of structuresduring earthquakes,’ said Dyskin. ‘Also, structures built from interlocking elements should be more stable in an earthquake situation because they have an enhanced fracture resistance as cracks cannot propagate between elements. We joke that such structures would be difficult to break as they are already broken.’

Applications of the technology in civil engineering are being developed by the team as part of an ARC-supported project.

Dyskin said that using mortar-free construction could also resolve problems likely to be encountered by future space missions if research or other facilities were to be built on the Moon, asteroids or other planets.

‘Mortar-free masonry is the only method for construction where there is no atmosphere, such as the Moon or asteroids, or with thin atmosphere like Mars, since curing mortar in vacuum or at very low atmospheric pressure will lead to high internal porosity detrimental to mortar strength,’ said Dyskin. ‘Furthermore, mortar-free, extra-terrestrial construction is cheaper.

‘To produce traditional mortar, cement, sand and water are needed. This means that water must be made available and a cement plant would need to be built before any construction starts, because transporting these commodities in industrial quantities from Earth is expensive.’