Self-Healing Spacecraft? Tiny Tubes Ooze Epoxy

Ben Harder
for National Geographic News

January 27, 2006

Spacecraft, heal thyself.

Aerospace engineers are developing self-repairing materials that "heal" themselves when damaged.

Shuttles, probes, and satellites sheathed in the stuff could safeguard craft from the wear and tear of space, researchers say.

Cosmic craft are "constantly being barraged by all sorts of nasties," said Ian Bond, an aerospace engineer at the University of Bristol in England.

Hazards include whizzing dust and space junk and extreme temperatures, which can cause structural breaks and cracks.

Those threats and others can weaken spacecraft until they suffer catastrophic structural failures, Bond says.

So why not build shuttles, satellites, even airplanes, from materials that repair minor damage automatically?

Broken Glass

To develop a material that mends nicks and scrapes on its own, Bond and his colleagues turned to a counterintuitive solution that uses fragile glass tubes.

The thin, hollow fibers are filled with one of two liquids that quickly solidify into epoxy resin when mixed.

The engineers incorporated rows of these glass tubes into building panels similar to those used to construct spacecraft.

"As long as those fibers break during an impact event," Bond said, "the resin can bleed out into the damage and harden up."

"It effectively glues back together the damage," like a scab does on a bleeding cut, he said.

Instant Chemistry

In recent experiments, Bond's team constructed and then intentionally damaged some panels containing the fibers.

The engineers then took the panels apart to assess how efficiently they healed. Researchers determined that the process restored about half of the structural strength that the panel lost upon impact.

The team described their work in a report published this month by the European Space Agency. ESA funded the Bristol research and provided technical assistance.

"A good step has been made," said Christopher Semprimoschnig, an ESA materials scientist in Noordwijk, the Netherlands.

The results demonstrate that the resin works as it should in a vacuum, in spite of the tendency liquids have to evaporate in space, he says. "It can survive the constant extremes of temperature," Semprimoschnig said, adding that the resin withstood repeated temperature jumps between 212°F to –148°F (100°C to –100°C).

Scott White, a materials engineer at the University of Illinois at Urbana-Champaign, says an advantage of the technology is that it can be put into practice with relative ease.

"[It] can integrate … directly into current manufacturing processes," said White, who was not involved in the ESA project.

3-D Micro-plumbing

However, White says that the glass-tube approach carries certain limitations. He notes, for example, that it's difficult to run glass tubes in more than two dimensions. In other words, glass micro-plumbing could easily spider through the surface layer of a building panel but not its entire thickness.

Over the past few years White has been investigating a different approach to self-healing spacecraft.

"We build hollow vascular [channels] directly in the [building] material," he said. "Our 3-D network most closely mimics the types of vascular networks [such as blood vessels] that you see in nature."

The engineer predicts a day when such integrated plumbing would carry more than just material-repairing fluids. For example, he said, "They would actively cool [overheated material] by circulating coolants."

Vascular networks might also contain smart particles that could identify structural damage in spacecraft and immediately alert onboard computers. But White concedes that it will take years to develop such technology.

Hurdles Remain

Bond, the Bristol aerospace engineer, believes his simpler approach could be refined into a ready-to-use technology "within a five-year timeframe."

"In theory, there's no reason why it couldn't be used for aircraft," he added.

But, according to Bond, self-healing materials are unlikely to safeguard military planes hit by enemy fire or prevent disasters like the loss of the space shuttle Columbia.

"If you end up with holes in the structure, we can't do anything about that," he said. "You'd need [too] much material to be able to refill that hole."

White, the University of Illinois materials engineer, adds that resin patches on earthbound spacecraft would burn off in intense heat. "There's no way epoxy could be used to protect anything during reentry," he said.

Even for uses in orbit, current resins leave much to be desired. For one thing, Bond says, epoxy ingredients won't solidify if mixed in the wrong ratio.

The substances can also become contaminated easily, Semprimoschnig, the ESA scientist, notes.

"You need to guarantee the longevity of the materials," he said, adding that they'll have to repair damage after lying dormant "maybe one or two years in space or—if you think of an interplanetary mission—maybe years in space."

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