Friday, 27 January 2006

Self-Healing Spacecraft

Take an old idea - the self-sealing fuel tanks found in combat aircraft during WWII - and update it. That's what the European Space Agency did. Back in the late 30's, aircraft started to be made with fuel tanks that had an "inner" and an "outer layer. Between these was a latex rubber compound that solidified on contact with fuel. So poke a hole (say with a bullet or shell splinter) in the tank, the rubber would ooze out, and the tank would be sealed again.

From the European Space Agency :
"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, who oversaw the study.

He imagined such cuts as analogous to the 'wear-and-tear' suffered by spacecraft. Extremes of temperature can cause small cracks to open in the superstructure, as can impacts by micrometeroids - small dust grains travelling at remarkable speeds of several kilometres per second. Over the lifetime of a mission the cracks build up, weakening the spacecraft until a catastrophic failure becomes inevitable.

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.

In humans, the air chemically reacts with the blood, hardening it.
Really? Not As Such. I hope their materials science is better than the author's knowledge of biology.
In the airless environment of space, alternate mechanical veins have to be filled with liquid resin and a special hardener that leak out and mix when the fibres are broken. Both must be runny enough to fill the cracks quickly and harden before it evaporates.
Any vacuum-rated low viscosity 2-part epoxy would do. Such stuff exists, and is used in spacecraft at the moment, as well as part of s shuttle-crew's repair kit.
"We have taken the first step but there is at least a decade to go before this technology finds its way onto a spacecraft," says Semprimoschnig, who believes that larger scale tests are now needed.
The hollow glass tubes are only 30 micrometres in diameter. Filling alternate layers with expoxy would be tricky, but glass fibre is a great construction material when embedded in epoxy anyway.

This could be useful - but I'm not so sure. Most spacecraft structure is honeycomb aluminium, or titanium struts, not fibreglass. And for good reasons. As long as it retains structural strength, microcracks and micrometeorite holes make no significant difference.

Reading through the 174 page report (PDF) there's this :
A commercially available two-part epoxy resin system (Cytec Cycom 823) was successfully used for the repair of internal matrix cracking and delaminations through the thickness of a 16 ply glass fibre reinforced polymer (GFRP) laminate.
Aha! I was right!
It was found that once healing of the damage site was undertaken the laminate had a strength recovery of 87% compared to the ultimate failure strength of an undamaged baseline laminate (and 100% when compared to the ultimate failure strength of an undamaged laminate containing hollow fibres with no healing function).
So the self-healing comes at a cost : the laminate is only 87% as strong as one that can't self-repair, but the self-repair completely restores it when damaged.
The report continues:
A two-tier hollow filament selfhealing approach is recommended with UV-activated and vacuum-activated resin systems being employed for the repair of surface damage whilst two-part systems (resin and catalyst) are recommended for the repair of internal laminate damage. Existing commercial resins are proposed for both scenarios, and hence for further evaluation.
Neat. Makes sense.

Finally, the bit that allays my fears that this is a solution looking for a problem :
Advanced fibre reinforced composite materials offer considerable scope for stiffness-critical space structures, such as bus structures and solar array structures, and for dimensionally stable space structures, such as optical benches and antenna systems.
So although GFRP and similar materials aren't much in use today, they're suitable for the future. Good. I can easily imagine a situation where a 1/8 decrease in strength would be a good trade for self-repair. But just as often, an extra 1/8 increase in thickness might make repair un-necessary. Still a useful tool to have available. Duralumin and similar metal alloys are better in most cases, but often have really nasty coefficients of expansion, and tend to "sing" or vibrate.

You just need to be used to ESA's way of doing things, and be able to pick the important bits from the mass of ESA boilerplate. And that really is the hardest part of Rocket Science, trust me.

No comments: