As you probably guess, a spacecraft life is a tough one. It has to survive in space where it is exposed to extreme temperatures and cosmic radiation without any help. This is why engineers working for the European Space Agency (ESA) have developed new processes that mimic human reactions. When we cut ourselves for example, our skin starts an autonomous healing process and is able to repair itself. In a similar way, by using a polymer matrix like an epoxy with micro/nano-encapsulated healing agents, they've designed self-healing spacecrafts that will fix their own cracks in space. This could double the lifetime of spacecrafts -- or reduce their cost by a factor of two -- in about a decade.
Here is how one of the scientists describes the concept.
"When we cut ourselves we don't have to glue ourselves back together, instead we have a self-healing mechanism. Our blood hardens to form a protective seal for new skin to form underneath," says Dr Christopher Semprimoschnig, a materials scientist at ESA's European Space Technology Research Centre (ESTEC) in the Netherlands.
And here is what he did with his colleagues.
The challenge for Semprimoschnig was to replicate the human process of healing small cracks before they can open up into anything more serious. He and the team at Bristol did it by replacing a few percent of the fibres running through a resinous composite material, similar to that used to make spacecraft components, with hollow fibres containing adhesive materials. Ironically, to make the material self-repairable, the hollow fibres had to be made of an easily breakable substance: glass. "When damage occurs, the fibres must break easily otherwise they cannot release the liquids to fill the cracks and perform the repair," says Semprimoschnig.
The illustration below shows different pictures of this resin filled hollow fibre self-healing composite -- clockwise from top left: 30μm diameter hollow fibres, time-lapse sequence of healing process, infusion into damage site.(Credit: ESA)
And this second one describes the basic method of the microcapsule approach, tried since 2002. "When the microcapsules are ruptured by a crack, the monomer is drawn along the fissure, where it comes into contact with exposed catalyst, which causes polymerization, thereby filling and repairing the cracks with a structural material." (Credit: ESA)
For more information about this self-healing process, you can read this long ESA report published in December 2005, "Enabling self-healing capabilities - a small step to bio-mimetic materials" (PDF format, 174 pages, 3.22 MB).
Here are some few more technical details about the materials used in this study.
Polymeric structural joints or polymeric composites are susceptible to cracks that may either form on the surface or deep within the material (fibre/matrix interface) where inspection/detection is often impossible. Once these cracks have formed they will weaken the material further and may lead to major damage.
Imagine now for instance a polymer matrix like an epoxy where micro/nano-encapsulated healing agents and a catalytic chemical agent are incorporated. When a crack spreads now through the matrix it will sooner or later encounter a microcapsule and break it. Due to capillary forces the healing agent will flow along the open crack front and will inevitably come into contact with the catalyst. This will trigger the self- healing reaction (polymerisation) that will close the crack. Thus (nearly) no evidence will be noticeable after the healing process.
Let's switch back to the ESA news release to learn about the benefits of such new materials, which could be integrated in spacecrafts ten years from now.
The promise of self-healing spacecraft opens up the possibility of longer duration missions. The benefits are two-fold. Firstly, doubling the lifetime of a spacecraft in orbit around Earth would roughly halve the cost of the mission. Secondly, doubling spacecraft lifetimes means that mission planners could contemplate missions to far-away destinations in the Solar System that are currently too risky.
Sources: European Space Agency, January 20, 2006; and various web sites
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