NASA's plans to send astronauts back to the Moon and, ultimately, to Mars raises an important sartorial question: What ever will they wear? The zero-gravity spacesuits that astronauts currently don when leaving the shuttle to manhandle satellites and other large objects won't do when it comes to walking around the surface of a planet for hours at a time. "The biggest issue is mobility. On a planet surface, they'll have to walk, move and bend," says Lara Kearney, program manager for advanced extravehicular activity (EVA) suits at NASA's Johnson Space Center. Planetary suits will also have to be lighter. The shuttle suits weigh in at just more than 100 lbs with their life-support backpacks. Even when contending with Mars' lower gravity, that kind of weight would impose a big burden.
And because planetary missions will last longer than a shuttle flight, the next generation of spacesuits will also have to be more robust. Kearney predicts that planetary suits may have to hold up for a hundred or so outings between major maintenance teardowns. The zero gravity shuttle suit, by contrast, gets an overhaul after four or five uses. Making maintenance matters worse, both the Moon and Mars have dust problems. Both are covered with dust ranging in size from something like a fine talcum powder to something like coarse sand. On the Moon that dust is "about as abrasive as glass," notes Joseph Kosmo, one of NASA's senior engi-neers for advanced spacesuits.
In preparation for our next big leap forward in space, NASA engineers recently asked two makers of the soft portions of the suits, ILC Dover and D. Clark Co., to come up with prototype planetary suits. The two prototypes intentionally take slightly different approaches to creating lightweight, mobile suits that can still function as pressure vessels. Kosmo stresses that neither of these prototype suits should be considered ready to go. "We don't have a Mars suit yet," he says. "What we do have is a group of technologies that we hope will be useful on Mars."
Yet even in their current form, the prototype suits do offer some engineering lessons for engineers who work on earthbound products. Look at ILC's I-Suit for a case in point. Cutting-edge materials and smart mechanical design of the suit's rotating bearings reduced some of the usual trade-offs between strength, weight, and flexibility. "In our world, high strength and mobility usually conflict with one another," says David Graziosi, ILC's lead design engineer for the I-Suit. "We've managed to minimize those conflicts."
Strong Yet Flexible
That spacesuits have to work in extreme environments should come as no surprise. What may be less obvious is that these high-tech garments also have to withstand significant structural loads. "Our structural loads are a lot higher than many engineers would expect, especially from fabrics," Graziosi says.
He breaks these loads down into three classes: Pressure loads consist of the axial and hoop stresses that result from inflating the suit to 4.3 to 8.3 psi, a range that covers its operating pressure on the low end and its proof pressure on the high end. Next come "isometric manloads" or forces the astronaut exerts from within the suit. These forces can result from actions as simple as locking the knees or trying to squeeze into a glove that's too small-or what NASA calls a "negatively sized glove." Finally, there are "satellite loads" that result from astronauts manually handling large objects in space. Zero gravity aside, these objects "still have plenty of mass and momentum," Graziosi points out. And since astronauts tend not to let go of expensive space equipment without a fight, significant strain forces can be transferred through gloves and foot restraints into the rest of the suit.
These three kinds of loads can add up. The wrist joint on shuttle suits, for example, has to endure an 860-lb load during a push-out test. The finger crotches of the glove have to hold up to slightly lower forces. And Graziosi says in worst-case scenarios the satellite loads on the suit can even approach 2,000 lb. On top of all these forces, ILC designs the suits with a safety factor that's two times the ultimate load. It's true that as a planetary suit, the I-Suit might be able to get away with a less robust structural design, but Graziosi notes that ILC designed the suit to accommodate loads similar to the shuttle suit, "even though NASA didn't require it."
Balanced against the need for structural strength, the I-Suit has to allow enough mobility for walking and working on Mars. For ILC engineers, this requirement translates to minimizing the torque required to move the joints through various ranges of motion that NASA specifies (see table). Even though the suit mostly consists of fabric components, satisfying the torque requirements is tricky. As Graziosi explains, flexible fabric quickly turns rigid when inflated to the suit's operating pressures. And the structural restraints that help tie the suit's various components together likewise grow taught under pressure-think of them as bridge cables rather than shoelaces.
-Combination of hard fiberglass and fabric structural body sections
-Nylon structural restraints
-Rotating bearings made from stainless steel
-Stainless steel brackets and swivels
-Combination hard fiberglass and fabric structural body sections
-Polyester structural restraints at first
-Stronger Spectra webbing in current versions
-Rotating bearings made from stainless steel
-Stainless steel brackets and swivels
-Soft structural body sections made primarily from high-tenacity polyester
-Some Spectra restraints. Vectran cord used in temperature-critical areas
-Rotating bearing housings made from aluminum and composites
-Titanium brackets and swivels
Less Torque, More Strength
Since the first Apollo spacesuits that ILC built in 1961, the company's engineers have approached the torque problem in two ways. They've added gores, convolutes, pleats, and other fabric features that make the taut fabric easier to bend. They've also come up with sealed rotating bearings for the wrists, arms, shoulders, waist, and hips.
Rather than taking an entirely new tack, the I-Suit features refinements to both these hard and soft joints, and it permits lower torque movement than any of the company's previous suits (see chart). According to Phil Spampinato, ILC's manager for space systems, some of the torque improvements have even become so fine that they exceeded our ability to feel them. "You wouldn't always be able to tell the 'before' from the 'after'," he says. But the elimination of even the tiniest "torque risers" adds up. "You end up with a suit that gives crew members the ability to work for longer periods of time," Spampinato says.
Historically, the drive toward lower torque and higher structural strength has come down not just to mechanical design but to materials selection as well. Structural webbing has evolved from nylon during the Apollo era to polyester on the first shuttle suits.
A few years ago, the shuttle suits incorporated Spectra. This long-chain-polyethylene fiber offered three to four times more tensile strength than polyester and a fivefold reduction in elongation. ILC engineers used these properties to produce restraints that were strong yet thin enough to drive down torque. "Thinner webbings equate to a thinner beam," says Graziosi, getting back to some engineering fundamentals.
With the I-Suit, ILC has again upped the ante on materials, this time replacing some of the Spectra restraints with braided cords of Vectran, a liquid-crystal-polymer fiber. Graziosi says the Vectran doesn't necessarily beat out Spectra from a strength or elongation standpoint, but it does have "better warm-side properties." It resists temperatures up to about 300F versus 150F for the Spectra. Vectran also exhibits better creep performance, he says. Another selling point for Vectran comes down to experience; ILC has used variants of the material extensively in the inflatable landing systems it has made for the Mars Rover and other spacecraft.
Still, ILC doesn't use Vectran throughout the suit. On the suit's body, or structural restraint layer, it has stuck with high-tenacity polyester similar to what it uses on the shuttle suits. "The highest strength fabric is not always the best," says Graziosi. In the suit's soft upper torso, for example, you might want some elongation to smooth out the load transfer and avoid stress concentrations.
Another key element of the I-Suit design involves its weight. Zero gravity suits for the shuttle "are heavy because it's okay for them to be heavy," Spampinato notes. But for a walking suit every ounce counts, and ILC engineers have shaved about 40 percent of the weight off the suit. Their I-Suit prototype weighs in at a svelte 65 lb without its life support backpack or thermal layer, compared to 107 lb for the shuttle extravehicular suits.
This huge weight loss came mostly from a switch to lighter weight materials. ILC engineers started by replacing the hard fiberglass upper torso found on previous suits with a lighter fabric torso. On the suit's many rotating bearings, they replaced large stainless steel housings with aluminum, keeping stainless steel only for the bearing races and balls-which see high contact loads. They finally crafted the suit's 40 or so swivels and brackets, which form the attachment points for the structural restraints, out of titanium rather than stainless steel.
Graziosi notes that titanium didn't play a role in past suits because ILC was able to meet its mass budgets without using an expensive lightweight alternative to steel. "There were also some questions about titanium's stability in a pure oxygen environment," he says. With the I-Suit's premium on weight, titanium started to make sense. What's more, titanium components have also become less expensive to produce over the past few years, according to Graziosi. He explains that advances in machining techniques have closed the cost gap between titanium and specialty grades of stainless steel. "The cost differences between the materials themselves has become a minor difference when you consider the cost of machining," he says.
The I-Suit also provided an opportunity to prove out concepts for another kind of light-weight material. ILC engineers fashioned one of the suit's large shoulder bearing housings from a graphite epoxy instead of aluminum, saving about one pound. Making all the bearings housings from the same material would save about ten pounds, Graziosi reports. "One of our goals with the I-Suit was to prove that we can use lightweight materials at a low cost, and I think we've done that," he says.
|Under pressure: Once pressurized, the suit's soft fabric and restraint webbing become too taut to bend easily. So spacesuit designers have come up with an elaborate system of moveable joints based on fabric features and rotating sealed bearings.|
More Work to Do
Though the I-Suit may look very much like a finished spacesuit, it will still take years before NASA arrives at a final design for its next planetary suit. As Kosmo notes, NASA engineers still have to make fundamental design decisions such as the number and location of rotating bearings. D. Clark's prototype suit, for example, uses a rotating bearing only at the wrist and arm while the I-Suit has them at the hip and shoulder too. "It's clear we'll need some rotating joints," Kosmo says, "but how many is still undecided." NASA also requires more testing of any new materials used in the prototype suits, not just for strength and thermal performance but also against an extensive battery of chemicals. The suit designs themselves also need their share of testing in the lab and in the desert proving grounds.
Finally, the suits that ultimately go to Mars will likely feature technologies not found on the I-Suit but currently in development by NASA and its suppliers. Kearney says the next generation of suits will likely include "multi-function" materials. Materials that perform a structural and thermal role, for example, could reduce the number of layers used to make a finished suit. Kearney says another keen area of interest involves "textile switches," or fabric that integrates touch pad controls. ILC is already at work on just such a technology and plans to have it in the I-Suit this month. These textile switches will be used for tasks as simple as turning on the suit's helmet lighting or as complex as remotely controlling robotic systems. ILC is also working to integrate gestural, or movement-based, controls into the suits.
Contact Senior Editor Joseph Ogando at [email protected].
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