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New Address On The Moon -- Or Even Mars

ILC Dover uses realistic simulation to design habitats for astronauts.

Lynn Manning

T he street sign at ILC Dover's headquarters -- One Moonwalker Road -- gives a strong hint about what goes on inside the 260,000 square feet of office, development and manufacturing buildings located there: Spacesuits for the Apollo astronauts in the 1960s and '70s; gear for the space shuttle crew that repaired the Hubble telescope in May 2009; and now, inflatable houses designed for future outposts on the Moon -- or even Mars. ILC may need to come up with a new street name.

A leader in the development of flexible material systems that withstand extreme environments, Frederica, Del.-based ILC has provided solutions to the National Aeronautics and Space Administration (NASA) since the early days of the US space program. The company designs both hardware and softgoods for the wide-ranging challenges of space exploration -- from the high heat of re-entry, to the profound cold of a lunar night, to the airbags that cushioned the landings of the Mars Rovers. ILC makes a multitude of earthbound commercial products as well, from innovative containment systems for packaging powder pharmaceuticals to highly advanced protective military gear.

Still, it's the inflatable lunar habitat idea that grabs one's imagination. From the first Moon landing in 1969 to the last trip there three years later, no one ever spent more than three days on the surface, and they took the lunar module with them when they left. In the 21st century, NASA's Constellation program -- to return to the Moon, set up a permanent base, and from there send people to Mars - started taking shape. This program created a host of new challenges, including the most basic one: if a crew is living on the Moon for months on end, where is everyone going to sleep?

ILC Dover is designing habitats for astronauts similar to the cylindrical structures pictured in this artist’s rendition of an outpost on the Moon, using Abaqus finite element analysis software from SIMULIA. Image courtesy of NASA.

Launching A House Into Space

ILC's engineers are working on the answer to that. In partnership with several different branches of NASA, including Langley Air Force Base and the Johnson Space Center, the company has been developing ideas for different configurations of lightweight space habitat structures.

"There's a keen interest among the Constellation management and engineers for softgoods solutions," said Cliff Willey, ILC program manager of space inflatables. "When you are launching equipment into space on a rocket, everything needs to be as lightweight as possible, packed densely. In the case of a habitat, you want to be able to deploy something that can expand to be much bigger on the surface of the Moon without a lot of mechanism. An inflatable, soft item is very good for that."

ILC recently completed the design work on one such project, a "mid-expandable" habitat with two hard endcaps and a deployable softgoods section in the center. The endcaps for the current prototype are being built by NASA Langley Research Center out of low-cost metal, but a lightweight aerospace composite is envisioned for lunar deployment to reduce mass. For transport, the softgoods section packs into the endcaps. During deployment, it is unfolded and inflated by air pressure, more than doubling in length. The midsection's unique fabric lobe system allows for a structure that is much lighter in weight but has a higher volume than a similar hard-material configuration would be. The endcaps are where doors, airlocks and other structures are mounted.


Harsh Lunar
Environment Poses Unique Risks

The Moon's environment contains a host of external hazards, including extreme temperature fluctuations -- which softgoods withstand much better than metals -- plus radiation, dust, bombardment from micrometeoroids, and low gravity. All these are taken into account by engineers designing the lunar habitat, which has as many as 10 outer protective attenuation fabric layers built up over two internal structural layers. Although exact material specifications are still under study, the outermost attenuation layer will likely be constructed of Ortho-Fabric, which consists of a blend of Gore-Tex®, Kevlar®, and Nomex® materials. The thermal micrometeoroid portion of the attenuation layers might be constructed of layers of aluminized Mylar®, laminated with Dacron® scrim.

But the biggest challenge ILC's engineers faced when designing the multi-layered habitat was not external: It was the inflation pressure on the two innermost layers of the structure. "You have to come up with a pretty clever design to handle the high loads inside a dwelling that is pressurized to a level in which astronauts can live," said Ric Timmers, ILC senior analysis engineer. "The skin load on the internal layers is proportional to the internal pressure times the radius, so you need to find a material that's able to handle the pressure on a big structure like this one, which is 3 meters in diameter."

In the zero-atmosphere Moon environment, not only do you need to control for oxygen leakage through the habitat walls, but also, any significant fabric failure would result in a devastating outward explosion of the structure. ILC's solution was to design an interlocking webbing net over a gas-impervious, coated fabric. The 2-inch-wide webbing is constructed of Vectran® in a plain weave and has an advertised breaking strength of 24,000 pounds. The fabric was deliberately oversized so that it would bulge slightly between the webbing panels, transferring the pressure load to the webbing. This unique combination of fabric and webbing working together would allow the habitat to be inflated to 9 pounds per square inch (psi), an acceptable pressure for humans living on the Moon, while meeting NASA's space construction safety standards.

Physical Prototyping
Would Send Costs Out Of Orbit

Testing the integrity of the design on the Moon's surface was obviously impossible. Building numerous prototypes out of custom fabric, and pushing habitat models to destruction, would also be prohibitively expensive, as well as time-consuming. 

"Earlier, we were contemplating building a test rig and physically measuring the pressure load on the fabric, the tension in the webbings, the pressure behind the windows -- all simultaneously -- but we were looking at well over $1 million for a test like that," Willey said. "That's when we backed off and decided to go with realistic simulation. We couldn't be Edisonian about this, relying on trial and error. We had to be able to build a reliable, finished product design the first time out."

ILC’s mid-expandable habitat prototype is stored in two hard endcaps during rocket transport to the Moon and then deployed on the lunar surface using air pressure, more than doubling in length.

Realistic Simulation
Provides Down-to-earth Answers

So, the group turned to Abaqus finite element analysis (FEA) software, from the SIMULIA brand of France-based Dassault Systèmes, to test virtual models of the fabric and webbing under varying load scenarios. They also used FEA to evaluate the robustness of some minor structural components, such as the metal brackets holding the webbings. "We relied heavily on analysis with Abaqus for this project," Timmers said. "It would have been pretty risky to do this without FEA -- you had to sleep at night!" 

Abaqus/CAE, the pre- and postprocessor for the Abaqus Unified FEA product suite, was used to model the 3-D geometry of the design as the basis for the simulation. The group then ran the simulation models with two central processing units on a Linux machine using Abaqus/Standard, which provides all the material, geometry, and loading nonlinearity needed to simulate fabric structures. "Our models were fairly straightforward, so static loading was appropriate for what we needed to know," Timmers said. 

FEA Helps Identify
Safe Fabric Yield Strength Limits

ILC began its analysis of the fabric/webbing system by modeling a unit cell of fabric constrained by a square of the webbing net. "We used a simple planar approach for this analysis since the out-of-plane curvature of each unit cell was negligible relative to the full radius of the entire habitat," Timmers said. When setting up the model, the group measured the sides of the cell from webbing center to webbing center instead of from webbing edges. "We used the midpoints rather than the edges because we wanted to be more conservative in our analysis by imagining that the webbing wasn't there, as a sort of worst-case scenario," Timmers said.

To model the fabric itself, membrane elements were selected, and all degrees of freedom at the perimeter of the unit cell were held fixed. Then, the model was oversized slightly, using what Timmers calls "a neat thermal expansion coefficient technique" that raised the temperature until the fabric expanded to a set percentage, to simulate the bulge of fabric between webbings. Finally, the nominal 9 psi of pressure was applied to the model, and Abaqus calculated the resulting stress in the material. With a center load result of 74 pounds per inch (lbs/in), and an edge load of 84, the material was well within NASA's required safety factor of four, as the ultimate tensile strength of the fabric was approximately 500 lbs/in.

"Using Abaqus FEA to identify the allowable limits of the fabric's performance was very useful because with this type of structure you have to be really sensitive to total mass," Timmers said. "When we found one material that worked, we could use Abaqus to virtually test another, lighter material to see how much we could save on total weight and still provide the right factor of safety." The final fabric selected for the lunar habitat was a 0.0075-inch-thick, 200-denier Vectran with a urethane coating impermeable to gas leakage. The weave is plain, with a yarn count of 50 by 50 threads per inch. The ultimate tensile strength is 551 lbs/in in the warp direction, and 520 lbs/in in the fill direction.

Keeping The Web Of Safety Intact

In addition to low stress in the fabric restraint system, another important contributor to the habitat's stability was evenly balanced loading of the ring of webbing itself. To test this part of the design, the ILC team used Abaqus to simulate just the critical axial, or end-to-end, length of the webbing. Hoop webbings around the circumference are more isolated from one another and are less sensitive to any uneven lengths among them. The purpose of the model was to simulate "manufacturing uncertainties" that might unexpectedly shorten the length of a single webbing.

"Our biggest concern this time was that any deviation in the length of one webbing could foreshorten the whole system, concentrating 100 percent of the load on a single section and leading to a cascade of breakage," Timmers said. The team set up their model with all 26 axial webbings fixed to a flat plate representing the hard endcap of the habitat. The usual 9-psi pressure load was applied to the surface of the plate to simulate conditions in an inflated habitat. When one webbing was shortened by just 0.125 percent, the analysis results showed that the load on it jumped to 4,815 pounds, versus 3,600 pounds on the rest of the webbings. But since the breaking strength of the Vectran webbing chosen for the habitat was 24,000 pounds, the safety factor of four was still met.

How long will such a well-designed structure last on the Moon? "The intended design life of the lunar habitat is at least 10 years," Timmers said. While accelerated aging studies on the component materials and creep testing on webbing material have already been carried out, additional long-term durability studies are still pending.

Camping On The Moon, Mars
-- Or Even Just The Antarctic

With their habitat design complete, ILC teamed with NASA to build a prototype for "Camping on the Moon," now on display at NASA Langley. Real-world verification tests of a full model -- including a deployment run-through, a high-pressure test, and tear-resistance evaluation -- are pending further funding.

"We may very well run these tests ahead of time with Abaqus," Timmers said. "It's ideal to use a combination of modeling and testing back and forth, applying FEA to dial into just a few possible scenarios."

Whatever the timeline for deploying astronaut habitats on the Moon or Mars, ILC's unique approach to such structures has applications closer to home as well: potentially as hyperbaric chambers for health clubs or hospitals; or, already, as dwellings for polar- or desert-based scientists. A similar habitat designed with Abaqus Unified FEA has been tested in the harsh environment of the Antarctic and will be going to the Arctic as well. The polar habitat model had requirements around wind, snow and ice loading, all of which are absent on the Moon, so the material selected was several layers of polyurethane-coated nylon separated by layers of Thinsulate™.

What will the address be in the Arctic -- One Icewalker Road? The ILC engineers hope One Marswalker Road is not that much farther away.

Editor’s Note: Lynn Manning is a science and technology writer based in Providence, R.I.