Thursday, June 07, 2018

This Next-gen Material Can Only be Made in Zero-G

It sounds crazy, but one company is trying it...and it looks like it's going to work.

At first blush, the idea of manufacturing something anywhere other than Earth's surface sounds like one of Elon Musk's fever dreams. Sure, if humans manage to establish ourselves on other worlds in the coming centuries, those colonies will need the manufacturing capabilities to maintain and expand things. But for now, human presence off-world is limited to the International Space Station: six people in a proverbial tin can just 250 miles off the ground; a toe dipped in the cosmic ocean. Given that it costs ~$10,000 a pound to send equipment and raw materials up there, aren't we getting ahead of ourselves?

Absolutely not, it turns out. For one, that "tin can" is a cutting-edge laboratory for all sorts of experiments, studying how life responds to microgravity and the inhospitable conditions of outer space. Like any lab, the ISS needs all sorts of unique equipment, and having to wait months for a gizmo to be delivered on the next supply launch can delay important research. Being able to fashion replacements for broken parts or build a new apparatus altogether using on-board equipment could be a huge benefit to the ISS's crew—there's a reason Star Trek's fictional spaceships feature hyper-advanced 3D printers that can whip up anything from martinis to machinery.

That's where Made in Space comes in. A small company based out of California, they're trying to help humanity become "multi-planetary" by creating the tools it'll take to venture beyond Earth. The first of those is a line of 3D printers that work in zero-G; one that works with plastics (like most 3D printers you'll find here on Earth) is already operational aboard the ISS, and an improved version—the Vulcan—is in development, promising metalworking capabilities once it's up and running.

Made in Space's 3D printer, currently aboard the ISS, was recently used to construct a working ratchet wrench as proof of concept.
Image Credit: NASA
Building something that can work metal safely aboard the space station is going to be a challenge: the Vulcan promises "subtractive manufacturing", which means features like drilling and cutting. But you can't just let your metal shavings fall to the ground to be swept up, like in a machine shop here on Earth; flyaway particles could cause disastrous electrical shorts, or get breathed in—there's a good reason for the infamous "space pen" urban legend. But Made in Space is going beyond overcoming the challenge of working in a microgravity environment: they're turning that challenge into an advantage, with the help of a next-generation material called ZBLAN.

Clarity

Fiber optics: it's the future of telecommunications, letting us send signals around the world at the speed of light, guided by long, thin strands of glass. It's also crucial for emerging technologies like photonic computing and fiber lasers. But not all glasses are created equal—ZBLAN (an acronym for the blend of elements that go into its creation) is a special type of glass that's unbeatable in terms of clarity at the frequencies we use for communications. Light traveling through fiber made of ordinary silica glass loses about 3% of its power per kilometer that it travels, but ZBLAN can beat that by a power of ten—at least in theory. The catch? It can only be properly made in a weightless environment.

At left, ZBLAN made in the momentary freefall of a "vomit comet" flight. At right is ZBLAN made in ordinary Earth gravity.
Image Credit: NASA. Public Domain.
Glasses are frequently described as ultra-slow-flowing liquids, but this isn't quite accurate—rather, they're amorphous solids; the defining characteristic is that they don't have a neat, orderly lattice structure the way that crystals do.

Ice, salt, and many other common materials like metals have a relatively orderly structure to their atoms, making them crystalline solids. The defining feature of a glass, on the other hand, is that its constituent atoms are bonded to one another in a more random fashion.
Image Credit: ScienceABC

This is where trouble arises; the atoms of ZBLAN want to click together into an orderly lattice, forming crystals rather than staying a glass. Unfortunately, these crystals are opaque—only the glass form has the exceptional clarity that makes it so useful, as you can see in the photo above. But how does forming the glass in zero-g help?

The key term to understand here is "geometric frustration". Imagine a crowd of people, half in red shirts, and half in blue, all mixed up randomly. Everyone in blue shirts wants to link hands with people in red shirts, and vice versa—and if they manage to, we'll say the crowd is in its crystalline state. But if the crowd is dense enough, people can't slip past one another to reach that state; everyone's kind of stuck holding hands with the person they're nearest, regardless of their shirt color. This keeps the crowd in an "amorphous" or glassy state. Once it cools, everyone's frozen in place.

The problem is that, on Earth, gravity effectively "stirs" the glass as it's cooling, with differences in density and heat causing some bits to rise and others to sink. In the crowd analogy, this motion—like Hajji pilgrims circling the shrine at Mecca—gives more red-shirts the opportunity to pass by a blue-shirt and link hands with them, creating pockets of crystalline structure. In microgravity, on the other hand, density differences don't matter and this stirring doesn't occur, so the glass stays in its amorphous, transparent form.

While the image above demonstrates that small quantities of ZBLAN can be made without going to space, the process unfortunately doesn't scale. "Parabolic flights are not feasible," explains Made in Space product strategist Harrison Pitman, "as persistent microgravity is needed to manufacture the fiber." It's nearly impossible to get more than a few dozen seconds of free-fall in one go from a parabolic flight, and heated glass is a finnicky, non-Newtonian substance. To draw it out into long fibers, you need patience and precision: tug too fast, and the strand will snap.

The Final Frontier: a Last Resort

So that seems to leave only one option, if you want any significant amount of quality ZBLAN: orbit. Partnering with German-American optics manufacturer Thorlabs, Made in Space has sent a fiber-drawing unit up to the ISS, which heats and slowly stretches rods of ZBLAN glass to produce useful lengths of optical fiber.

"In 2017 we sent the first unit to the International Space Station (ISS)," Pitman says. "Though this mission was intended to just make sure that the unit could safely operate in space, we did actually draw some fiber during this mission. Additionally, we drew fiber on the projects's second run to the ISS during April of 2018." Anyone who's ever seen a glassblower at work knows that working with glass is an art which requires a lot of practice. Even when doing it with a machine, the process takes calibration and fine-tuning—the other purpose of Made in Space's first fiber-drawing mission.

The machine itself is heavy enough that getting it into orbit alone is a serious cost, but Made in Space hopes to see a return on that investment.
Image Credit: Made in Space
Optical fiber is pretty close to being the perfect material for this kind of exploratory manufacturing. It's extraordinarily costly to ship raw materials to space, but optical fiber derives its value precisely from the fact that it's hair-thin and lightweight—a pound of it could stretch a long way. On top of that, the companies collaborating on this project will pretty much have the market cornered on the highest-quality optical fiber in the world: unless someone invents a better material, or a way to make ZBLAN here on Earth, they ought to be able to recoup that initial investment pretty rapidly. Since fiber lasers and the other kinds of equipment that would be able to best make use of this new technology are already pretty expensive to manufacture, ThorLabs and Made in Space shouldn't have a hard time finding customers who'll pay enough for a ten-fold improvement in efficiency to make the project worthwhile.

Made in Space has already seen some success in their trials, though there's no official word yet on whether the material lives up to its potential. According to Pitman, "Evaluating and characterizing the fiber can take a long time. We are actively working with companies here on Earth to qualify the optical fiber." If their production schedule is any indication, though, things are looking promising—the first unit is slated to return to the ISS this month, for its largest run yet. Stay tuned for updates on this story and the future of space manufacturing!

—Stephen Skolnick

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