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The state of the art in applied materials in space, explained

What’s the state of the art in applied materials for space? For example, what would you use to make a next-gen space suit? Or the spacecraft that brought it to an exoplanet? For our purposes, let’s avoid what’s coming over the horizon; nobody wants to read about vaporware, or the kind of poorly-advised gimmick that looks shiny but ends up killing people. Here we’re only going to cover things that are in active use, or at the very least, are beta testing in the field.

There are a few different classes of technological development. Broadly, the recipes we use to make new materials have coevolved with manufacturing methods, and the things we’re trying to do with our materials have become much more ambitious. We’re courting ever greater hazards, and we have to reach a corresponding level of mastery over the composition and performance of the materials we use.

There are a couple basic kinds of materials, too. Advanced composites layer together separate materials, while alloys melt or dissolve things together to get a homogeneous finished product.

Consider ceramics. The classical definition of a ceramic is an oxide, nitride, or carbide material that’s extremely hard and brittle, which is to say that it breaks if you hit it with a big enough physical shock. Ceramics are often strong under compression, but weak under tension and shear stresses. But when ceramic materials are heated until they’re as stringy as spun sugar and then blown through nozzles into fibers, they can then be processed into soft, flexible fabrics like ceramic wool, silica felt, and “flexiramics.” These materials just flatly won’t burn, so they’re useful when there’s an application for soft, shock-absorbent padding that’s also flame-retardant.

Glass-ceramics are a little more familiar to most of us, if by another name: Gorilla Glass, which is commonly seen in smartphones today. It’s an aluminosilicate glass formed by letting molten glass nucleate around ceramic dopant particles that are only soluble at high temperatures. When it cools, this gets you somewhere between 50 and 99% crystallinity, according to Corning. The resultant material is very little like a glass except for its transparency. When tempered, the balance between tension and compression makes the stuff tough as hell. Glass-ceramics also play well with electrically conductive coatings, and engineers use that feature on spacecraft windows to keep them free of condensation and ice.

Spacecraft windows are a great application of materials science. One way of making space-worthy windows is fused silica, which is 100% pure fused silicon dioxide. Another crazy window material is aluminum oxynitride, which is actually a transparent ceramic we use to make things bulletproof. In a video produced by one manufacturer of aluminum oxynitride bulletproofing products (see below), 1.6 inches of AlON was sufficient to completely stop an armor-piercing .50 cal round. AlON and fused silica both start out as a fine powder called frit, which is tamped into a mold and then just baked at the most unearthly temperatures into a single piece of transparent, super-hard material.

Unless you’re working with 100% pure substances, which in many cases isn’t possible, the idea of doping is central to all of this. Doping means adding a pinch of something special to an otherwise mundane recipe, to take advantage of the special thing’s benefits without dealing with the flaws it has when pure. In many cases, what results from doping ends up bearing little resemblance to either of its parent materials.

Metallurgy relies a lot on doping, which in this case is called alloying. There are some pretty fantastical things we can do with metals. Aluminum-niobium alloys have melt temperatures high enough to withstand the thermal environment inside the Falcon 9’s engine nozzles. But it’s only because they also use regenerative cooling: propellant cycles through chambers in the nozzle walls, cooling the bell and warming the propellant. (It’s a heat pump.) Alloys involving gold and brass are useful because they just will not corrode, no matter the temperature or chemical extreme. Like the anti-caking additives in Parmesan cheese, there even exist metal alloys that involve silicon just because the silicon makes the molten metal flow more readily, and therefore better suited to complex casting.

Friction-stir welding, which physically melts together the two materials being welded so that they become one structural entity, solves the problem of joinery for some of SpaceX’s aluminum-alloy parts.

We see novel material chemistry a lot in semiconductor research, and lately control over the dopant has become fine enough to introduce single-atom point flaws into a diamond lattice. This manufacturing precision is also critical to so-called “high-entropy” alloys, which are hybrid mixtures of four, five, or more different elements that can yield tremendous gains in toughness, as well as making things made from them thinner, lighter, and more durable. A metallurgist from MIT has made a high-entropy steel-like alloy that’s both extremely hard and highly ductile, which are characteristics that I and everyone else thought mutually exclusive.

Of course the choice of dopant is important. Tantalum and tungsten are hard, dense, radiation-resistant metals that were stirred into the titanium to make Juno’s “radiation vault.” The vault protects the delicate circuitry in the science payload, sacrificing itself to embrittlement so that the electronics can live as long as possible.

Radiation hazards can be mitigated with shielding — basically, putting atoms between your payload and the high-energy charged particles that can flip bits, corrode metals, and short out connections. Lead is the obvious choice on earth, but lead doesn’t work for space flight, because it’s too soft to withstand the vibrations and too heavy to be practical in any case. That’s why Juno’s radiation vault is mostly titanium; it’s tougher than aluminum and lighter than steel.

It’s actually a major problem, trying to figure out how to keep electronics running as long as we can while they’re in space. You can’t make a spaceship without a computer in it. And while we keep making circuits smaller and keep cutting their power requirements, at a certain point there are physical floors of size and power consumption. Near those thresholds, it’s exquisitely easy to perturb a system. Radiation damage, thermal differentials, electrical shorting, and physical vibration all pose hazards to electronic circuits. Spintronics could help to advance computers, providing much greater computing bandwidth for use doing whatever you’d need to do on an interstellar voyage. They could also put a hard maximum on the EM hazards that are so damaging to electronics in an intense magnetic field, like the one around Jupiter. But until we make optical circuits or spintronics real, we’re going to have to figure out how to make good old electronics behave in space, and that’ll probably involve a good old Faraday cage.

Composites are tough to produce because they often require extremely specialized manufacturing facilities, huge autoclaves and the like. But when they’re good, they are very, very good.

Multi-layer insulation (MLI) is both thermally and electrically insulative, and NASA uses the stuff practically everywhere they can. MLI is what makes spacecraft look like they’re covered in gold foil. But there’s a kind of MLI for applications where the whole shebang needs to be electrically grounded, too, and that uses a metal mesh instead of the tulle-like textile mesh between its layers of foil.

SpaceX uses rigid composites in their vehicle construction, layering together carbon fiber and metal honeycombs to produce a structure that’s both very light and very strong. Foams and aerogels can do lightweight, rigid, thermally impermeable layers too.

Composites excel against physical hazards and stressors, but rigid materials aren’t the only way to go. The BEAM inflatable space hab module, which I affectionately call a bounce castle in a can, is made of flexible composite materials including a unique glass fabric called beta cloth. NASA and others have been using beta cloth and things like it since the late 90s, and for good reason: The stuff is just impossible to faze. Made of PTFE-coated glass fibers in a basket-weave fabric, it’s the love child of fiberglass and Teflon. It’s practically impossible to cut or even scratch with the hardest, sharpest blades. Because it’s flexible, it’s impact-resistant. It’s impervious to corrosion even by free atmospheric oxygen attack. Scientists shot it with lasers and that’s what finally made it start to degrade.

Similar to beta cloth, there’s also the flexible Chromel-R metal cloth, which we use in abrasion-resistant patches on spacecraft bodies and space suits. Chromel-R is like the woven glass mats of beta cloth, but made of hard, coated metal wires. Furthermore, scientists found that the “stuffed Whipple shield,” which is a layered confection of ceramic-fiber cloth and Kevlar, worked better than aluminum plating to stop hypervelocity ceramic pellets simulating space debris — by melting or disintegrating the pellets (PDF).

Space suits are actually the perfect application for flexible composites. No single material is resistant to everything. But if you sandwich together thin layers of several materials that are each resistant to most things, you get an everything-proof exo-suit that can still bend and flex with the wearer. Add a layer of Darlexx or similar, a la SpaceX’s next-gen space suits, top it off with a layer of flexiramic cloth, and you have a fireproof pressure suit. Put a layer of non-Newtonian fluid cushioning and some ceramic-alloy trauma plates in there too, and now it’s fireproof body armor. All you need then is a HUD in your helmet, and maybe some high-density memory foam in the seat cushions. This is stuff we could do just with products available today.

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