Image 1: Artist’s rendition of an outpost on the moon. ILC Dover is designing habitats for astronauts similar to the cylindrical structures pictured at right, using Abaqus finite element analysis (FEA) software from SIMULIA. Image courtesy of NASA

ILC Dover uses realistic simulation to design habitats for astronauts hoping to create new addresses on Mars and the moon.

The 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.
• Inflatable houses designed for future outposts on the moon—or even Mars.

ILC may need to come up with a new street name.

Delaware-based ILC has provided solutions to the National Aeronautics and Space Administration (NASA) since the early days of the U.S. 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 you are living on the Moon for months on end, where is everyone going to sleep?

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 and the Johnson Space Center, the company has been developing ideas for different configurations of lightweight space habitat structures (see Image 1).

“There’s a keen interest among the Constellation management and engineers for softgoods solutions,” says 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 (metal or composite) endcaps and a deployable softgoods section in the center (Image 2).  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 with a higher volume than a similar hard material configuration would be. The endcaps are where doors, airlocks and other structures are mounted.

Unique Risks Of Lunar Environment

The Moon environment contains a host of external hazards, including extreme temperature fluctuations—which softgoods withstand much better than metals—radiation, dust, and low gravity.

While all these are taken into account by engineers designing the lunar habitat (multiple external fabric layers are built up to protect the structure from such outside threats), it was the inflation pressure on the two innermost layers of the structure that presented the biggest challenge to ILC’s design engineers. 

“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,” says 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 has a very large radius.”

In the zero-atmosphere Moon environment, not only do you need to control for oxygen leakage through the habitat walls, 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 fabric was deliberately oversized so that it would bulge out 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 nine psi (an acceptable pressure for humans living on the Moon) while meeting NASA’s safety standards for space construction.

Physical Prototyping

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 a million dollars for a test like that,” says Willey.

“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.”

Realistic Simulation

So the group turned to Abaqus finite element analysis (FEA) software, from the SIMULIA brand of 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,” said Timmers. “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 3D geometry of the design as the basis for the simulation. The group then ran their simulation models with 2 CPUs 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,” says Timmers. 

FEA Helps Identify Safe 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,” says Timmers, “since the out-of-plane curvature of each unit cell was negligible relative to the full radius of the entire habitat.”

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,” says Timmers.

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 nine psi of pressure was applied to the model and Abaqus calculated the resulting stress in the material (see Image 3). With a center load result of 74 lbs/in, and an edge load of 84, the material was well within NASA’s required safety factor of four (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,” says Timmers. 

“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.

Keeping The Web Of Safety Intact

In addition to low stress in the fabric restraint system, another important contributor to the stability of the habitat 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 (end-to-end) length of the webbing (hoop webbings around the circumference are more isolated from each other and are less sensitive to any uneven lengths between 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,” says Timmers.

The team set up their model with all 26 axial webbings fixed to a flat plate representing the hard endcap of the habitat (Image 4). 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 3600 pounds on the rest of the webbings. But since the breaking strength of the webbing chosen for the habitat (also made of Vectran) was 24,000 pounds, the safety factor of four was still exceeded.

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

With their habitat design complete, ILC teamed with NASA to build a prototype of it 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,” says Timmers.  “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.  What’s the address there—One Icewalker Road?  The engineers at ILC hope that One Marswalker Road is not that much farther away.

For more information visit www.ilcdover.com