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Revolutionizing structures in space with fabric and technology
By Kriss J. Kennedy
Editor’s note: This article originally appeared in Sept/Oct 1999 issue of Fabric Architecture. At the time, Kriss J. Kennedy was a space architect at the NASA Johnson Space Center, Houston, Texas. He is internationally known in the space industry for his accomplishments in habitat design, planetary base master planning and design, and inflatable structures for habitats. Kennedy combined more than 10 years of traditional architecture working experience with his Masters of Architecture from the Sasakawa International Center for Space Architecture (SICSA), University of Houston to help NASA get back on track using tensile fabric structures as habitats. He has worked on more than 20 advanced NASA projects dealing with the development of Moon and Mars bases, habitats and interplanetary travel facilities. He has designed numerous inflatable habitats—large and small—and has been published in many books, magazines, and conference papers. Shortly after this article appeared, and after a change in national U.S. government, the TransHab program was shelved along with the Mars program.
As old as architecture itself, fabric structures have been interwoven with human developments throughout history—and now with the architecture of the future. Cavemen created portable housing—using animal skins stretched over bones and limbs-while following herds of animals in their nomadic search for food. Subsequently, the use of these skins led to hides being sewn together and combined with erectable structures for easier deployment and breakdown. Over hundreds of years, yarn and fabric were developed to further enhance these fabric structures, or tents. Tensile fabric structures have always been at the forefront of architecture with their dynamic shapes, sweeping boldness and technological flexibility. So it’s not too surprising that a team of architects and engineers at NASA’s Johnson Space Center (JSC) have been designing and testing this ancient architecture as a way to create futuristic habitats in space and for use on other planets.
NASA has considered tensile fabric structures in the past. In the late 1960s, several inflatable structures were designed and tested for space applications. The Langley Research Center led efforts to develop and test a 24-ft. (7.3m) diameter torus space station, a Lunar Stay Time Extension Module prototype and a large space station module nicknamed Moby Dick. All of these were successfully tested, but were mothballed by bureaucrats when the Moon program was halted in favor of a new space vehicle. During the 60s, 70s, and 80s, NASA relied on metal (mostly aluminum) structures for all their habitat efforts. It was a known material, safe and reliable. So when the Lunar Base Systems Studies (LBSS) team and I began in the late ‘80s to propose an inflatable as a primary structure, we drew a great deal of criticism. It took many years of persistence, and a few failures, before the textile industry turned the technological corner for us by providing fibers like Kevlar, Vectran and Polybenzoxazole (PBO).
It was 20 years from those early inflatable habitat prototypes until the beginning of the LBSS efforts, contracted by JSC in 1987-89, when I joined NASA to help with the design of an inflatable lunar habitat and surface base design (fig. 1.)
Over the years, the idea of inflatable structures for space habitats began to catch on. Several important NASA reports, such as the Synthesis Group Report, identified inflatable structures as an enabling technology that would allow the agency to create lightweight structures at a lower cost. I continued to refine my ideas and concepts, hoping that one day I would have the opportunity to prove that inflatable structures are an “enabling” technology for advanced missions. That day came when I was asked to be part of a NASA “Tiger Team” led by Dr. William Schneider, a senior structural engineer. The small (half a dozen engineers and a space architect) team’s challenge was to design an interplanetary vehicle habitat for a crew of six to travel to and from Mars.
There was one major catch: How to deliver this habitat into space using existing launch vehicles? Due to the amount of volume required per crew member, for food, spares, etc., the logical choice was to use an inflatable structure. I had been working the Mars Mission Studies and had already defined types of habitats as transit and surface habitats. So when the team began design work on this transit habitat, I coined the name “TransHab.” The nickname caught on quickly, and soon took on a life of its own.
TransHab is a unique hybrid structure that combines a hard central core with an inflatable exterior shell. An integrated pressurized tunnel is located at one end to provide access to the space station. An unpressurized tunnel is located at the opposite end, housing the TransHab inflation system. The TransHab module currently being proposed for the International Space Station (ISS) is approximately 40 ft. (12.19m) long by 25 ft. (7.28m) in internal diameter, providing 12,077 ft.3 (342m3) of pressurized volume-about the equivalent of a three-bedroom bungalow (fig. 3.)
TransHab pushed the technological envelope beyond previous design work. The innovative engineers under Schneider’s guidance soon shaped a revolutionary concept different from the hard aluminum shell alternative. Since that project’s inception in early 1997, it has been through numerous design iterations. As an evolution of the Mars TransHab, it has transformed into the proposed alternative habitat module for the ISS. A team of architects and engineers at the JSC has been working, designing and testing this concept to mitigate the risky technical challenges that the critics continue to pose. So far, the TransHab Project team has successfully met every challenge.
TransHab Architecture
When deployed on the ISS, TransHab will provide a habitable volume nearly three times larger than a standard ISS module, yet it will be launched on the Space Shuttle. TransHab provides facilities for sleeping, eating, cooking, personal hygiene, exercise, entertainment, storage and a radiation storm shelter. TransHab also helps to develop, test and prove technologies necessary for long-duration interplanetary missions.
The habitat provides an integrated environment that creates both private and social spaces. A functional and physical separation of the crew health care area, crew quarters, and galley/wardroom area creates a home-like setting for the crew while they are in space (fig. 4.) TransHab has storage space, two means of unobstructed egress and permanently deployed exercise equipment, such as a treadmill and ergometer. Some important design objectives were to maintain a vertical configuration, separate the exercise area from the dining area, and provide adequately-sized crew quarters.
An open interior plan makes a confined volume psychologically beneficial to a crew in long-duration missions, an important factor considering the intense work and stressful environment these astronauts face. The flexible open plan will also be crucial as the habitat, station or base matures and its needs change. Flexibility is as important as the psychological benefits: the interiors can adapt and transpose to meet a new mission profile, such as an increase in crew size, or as crew members re-arrange the equipment and furnishings to accommodate their work habits.
TransHab has a unique hybrid structure that incorporates an inflatable shell and a central hard structural core, combining the packaging and mass efficiencies of an inflatable structure with the advantages of a load-carrying hard structure. The Central Core is comprised of the longerons, shelves, radiation shield water tanks, two utility chases, and integrated ductwork. Equipment shelves are placed into the Central Core for launch. The composite longerons-23-ft.- (7m-) long beams with flares at each end to attach to the bulkheads-provide the primary load path through the core and react to both pressure loads and launch loads. The 2.5-in. (6.35cm)-thick water tanks are sandwiched between inner and outer shear panels that are structurally connected to the longerons. The inflation system and tanks are incorporated into Level 0, the unpressurized tunnel.
The inflatable shell is the TransHab’s primary structure. The shell is composed of four functional layers: the internal scuff barrier and pressure bladder, the structural restraint layer, the Micrometeoroid/orbital debris shield and the external thermal protection blanket. The shell is folded and compressed around the core at launch and deployed on orbit. Its function is to contain the crew’s living space, and provide orbital debris protection and thermal insulation. Particles hitting at hyper velocity expend energy and disintegrate on successive Nextel layers, spaced by open cell foam. A thin layer of Kevlar adds an additional degree of protection. Still undergoing further development and testing, the fabric “sandwich” has withstood impacts of up to a 0.66-in.- (1.7cm-) diameter aluminum projectile fired at 15,600 mph (7kmps). Woven from 1-in.- (2.54cm-) wide Kevlar straps, the restraint layer is designed to contain four atmospheres of air pressure. Each shell restraint area is structurally optimized for that area’s load. In order to accomplish this, strap seams were developed achieving over 90% seam efficiency. An inner liner of Nomex provides fire retardance and abrasion protection. Three “Combitherm” bladders form redundant air seals; four layers of felt provide evacuation between bladder layers (necessary for launch packaging).
The ISS TransHab is divided into four functional levels within its pressurized volume. Levels 1 through 3 are for living space, and the fourth is the connecting tunnel. Because TransHab is a prefabricated, packaged and deployed habitat, it requires the crew to perform setup and outfitting activities in order to make it operational.
Levels 1 and 3 are 8 ft. (2.4m) tall at the Central Core and Level 2 is 7 ft. (2m) tall at the Core. TransHab is 23 ft. (7m) from inside bulkhead to inside bulkhead (not including the 7-ft.- (2m-) long Level 4 pressurized tunnel). After the Orbiter docks with the station, TransHab is removed from the Orbiter payload bay and berthed. Once captured by the station, TransHab is deployed and inflated to its internal operating pressure of 14.7 psia. Following inflation of the module, systems are activated for conditioning the environment for crew entry and outfitting.
Level 1 is the galley/wardroom and soft stowage area (fig. 5.) A unique aspect about this area is that it includes a clerestory above the wardroom area. The two-story clerestory, or “great room,” opening was created in response to the psychological need for open space-very important for crew morale and productivity during long-duration isolation and confinement in space. It incorporates an ISS galley rack, ISS refrigerator/freezer racks, a large wardroom table, an Earth-viewing window and a soft stowage array that incorporates ISS standard collapsible transfer bags (CTB).
The wardroom table is designed to gather all 12 crew members around it during a crew change-over. This table and area is also used for meetings, conferences, daily planning, public relations gatherings and socializing. The soft stowage area consists of the stowage array system and a handwash. The Stowage Array System (on Levels 1 and 3) has a total capacity of 880 ft.3 (24.6m3) of stowage. The Stowage Array is a framework that the CTBs can be placed into and will utilize the station coding and bar reading system.
Level 2 is the upper level of the Level 1 clerestory, the mechanical room and the crew quarters (CQ). The CQ area has six crew quarters and a central passageway located within the second-level central core structure and radiation shield water tanks. Using the architectural principle of a mezzanine level, the mechanical room is located outside the core structure and uses only half the floor space. It is designed to house the Environmental Control and Life Support System (ECLSS), and the power and avionics equipment. This area is acoustically and visually isolated from the rest of TransHab. Openings in the mechanical room floor and ceiling along the shell wall provide return airflow from Levels 1 and 3. A unique aspect of this open approach design is equipment accessibility and design flexibility. Equipment is integrated onto the shelves that are placed into the core for launch, and then the equipment shelves are moved to their final location once TransHab is inflated.
The crew quarters are surrounded by a 2.5-in.-thick by-7-ft.-long (6.35cm by 2.13m) radiation shield water jacket for radiation protection during solar flares. Access to this area is from Level 1 (below) or Level 3 (above), via the 42-in. (106.7cm) central passageway. Each of the crew quarters is 81.25 ft.3 (2.27m3) of volume and has a full height of 84 in. (213.4cm). This is larger than the ISS rack-based crew quarters. Each CQ will have personal stowage, a personal workstation, sleep restraint, and integrated air, light, data and power. An integrated soffit at the top of the crew quarters contains the ductwork, and power and data cables that feed the work station area. The acoustic wall panels will be designed for cleanability and change-out. This change-out capability could allow new crew members to bring “personalized” panels to decorate their crew quarter according to personal taste. Studies and research on long-duration isolation and confinement have shown this concept and larger private crew quarters to have a very positive impact on crew morale and productivity.
Level 3 is the crew health care and soft stowage area, which houses the two ISS Crew Health Care System (CHeCS) racks, a Full Body Cleansing Compartment (FBBC), changing area, exercise equipment (treadmill and ergometer), a partitionable area for private medical exams and conferences, and an Earth-viewing window. Also included on this level is a soft stowage area identical to Level 1.
The exercise equipment is permanently mounted in a deployed position, (fig. 6), to save crew time in deployment and stowage on a daily basis. Placement of the exercise equipment is synthesized with the window location to allow the crew Earth viewing during exercise. Two equipment shelves are placed on the floor struts as exercise equipment mounting platforms and structural integration. Four movable partitions provide visual screening of crew members for pre- and post-full-body cleansing activities and private medical exams at the CHeCS rack.
Level 4 is the pressurized tunnel area. It has two station common hatches, and avionics and power equipment. Its function is to provide a “transition” between Node 3 and TransHab, house critical equipment required during inflation and provide a structural connection to the space station. During launch it is the only pressurized volume in TransHab until inflation.
Demonstrating, Safety, Durability and Flexibility
TransHab’s design concept is based on a relatively new space inflatable structural technology. The TransHab team had to prove that this technology would work and that it was safe. There were three important goals set by the team to convince skeptics that inflatable structures can work in space:
The first goal was achieved by building a typical shell lay-up and performing hypervelocity impact testing at JSC and the White Sands Test Facility. These tests proved to be very important. If the debris shield could not stop the particle, then TransHab had no chance of surviving-literally. The 1-ft.- (30.48cm-) thick orbital debris shield took shot after shot and kept passing-exceeding all expectations. The test engineers who set up the shot normally enjoy blowing things up with their hypervelocity guns. At first, they were disappointed that the target was not failing, but got excited when they realized the tests were now a part of history. These tests turned out to be so important that the cable-TV show “Scientific American Frontier,” with host Alan Alda, included these shots as part of a television series on Mars mission technology. While testing continues, TransHab’s shell has survived the impact of a 0.66-in. (1.7cm) aluminum sphere at a hypervelocity of 7kmps.
Two shell-development test units were built and tested at JSC to prove the second and third goals. The first test unit was designed to prove that the inflatable restraint design would hold the 14.7 psia operating environment for the crew to live in. This unit was 23 ft. (7m) in diameter by 10 ft. (3.04m) tall. Since we were testing the hoop stress, it did not have to be full height. NASA used the aviation recommended safety factor of four (4), used for tensile fabric structures used in airships and blimps, as the basis for its test, which became known as the 4.0 test. This meant the restraint layer had to withstand the equivalent stress of four atmospheres. The only safe way to perform such a potentially dangerous procedure was to perform a hydrostatic test in the Neutral Buoyancy Lab at JSC. The test was successfully completed in September 1998, marking yet another historical milestone for inflatable habitat structures.
The second test unit was designed to prove that the inflatable shell design could be folded and deployed in a vacuum environment. This test unit reused the hydrostatic test article bulkheads and rebuilt a full-height restraint layer within three months-unheard of in the space industry. Also included in this test was the orbital debris shield that was proven in the impact tests. The 1-ft.- (30.48cm-) thick debris shield is vacuum-packed to reduce its thickness for folding, to enable the module to fit into the Orbiter payload bay. Once on orbit, TransHab is deployed and the debris shield is released to its desired thickness. Figure 13 shows two technicians performing a final inspection of the test unit before folding the unit. TransHab was successfully folded and deployed in the vacuum environment of Chamber A at JSC in December 1998, meeting the second goal.
With the successful completion of the Hyper Velocity Impact testing and inflatable shell development tests, TransHab has proven that the inflatable structure technology is ready for the space age. ISS TransHab’s design meets or exceeds habitation requirements for the space station. I like to think that TransHab has put the “living” into “living and working in space.” If TransHab is selected as a replacement of the hard aluminum-can habitat for the ISS, it would be launched as the last station element in late 2004.
NASA at a Crossroads
he emergence of TransHab and space architects in the space industry, NASA is at a crossroads: It is no longer an organization entirely dominated by engineering, nor limited to cylindrical hard modules. Many wonderful and architecturally-pleasing shapes will emerge to herald a new century and new era in space. Perhaps it is fitting that NASA step into the new century with inflatable structures leading the way. NASA’s space architects are ensuring that traditional architectural design principles and practices are being used in the development of habitats, space vehicles and planetary bases.
Inflatable structures have captured the imagination of many in the field and are one of the most promising new technologies for NASA. They will change how we think about designing habitats and laboratories, hotels and resorts for space. They will also revolutionize the space architecture world by opening up the possibilities of shapes and sizes to create human settlement of the solar system.
NASA has long been a leader in the research and development of new technologies for space activities. Many of these space-age technologies have spun off to benefit other critical areas, including advanced computers, new medicines, and the practice of recycling, among many, many more developments.
Numerous technology thrusts were identified for NASA technology development needs. One of these areas is Advanced Habitats and Surface Construction Technology. I co-led this activity for the Exploration Office at JSC from 1996-1998, with Marc Cohen, also a space architect from the Ames Research Center. Under the Advanced Habitats, I classified habitats into three categories: I developed a technological road map for inflatable structures that includes the development of different types of inflatable habitat structures, different technical solutions and different manufacturing approaches. Whereas inflatable structures are in the forefront of the road map, there are other important areas such as robotic construction, self-deploying structures, smart structures and self-healing structures, to mention just a few.
The vision of Advanced Habitation and Construction Technology is to begin working on the innovative technologies needed to allow space exploration and development to meet the demands of “faster, better, cheaper.” Space and planetary habitation that incorporates pressure structures and unpressurized shelters is being developed with innovative structural solutions that combine high strength and lightweight materials, along with reliability, durability, repairability, radiation protection, packaging efficiency and life-cycle cost-effectiveness. Advances in material developments and manufacturing techniques that enable a structure to “self-heal,” and the placement, erection, deployment or manufacturing of habitats in space (or on the Moon and Mars), are considered technologies that will help humans reach into space and eventually settle on Mars. Integration of sensors, circuitry and automated components to enable self-deployment and “smart” structures are necessary to permit a habitat to operate autonomously.
The objective is to create an advanced habitat that becomes a “living” structure that not only runs autonomously, but also has self-healing capabilities. A number of technologies and techniques have been proposed to allow the delivery, deployment, or manufacturing of habitats on planetary surfaces. New breakthroughs in bio-technology have opened up exciting possibilities; the use of bio-technology, combined with a fabric or matrix structure, could someday produce a self-healing surface analogous to our human skin.
NASA will be researching methods and techniques for fully-integrated inflatable “skin” and sensors/circuitry for “smart” structures to detect, analyze, and repair structural failure on their own. Manufacturing methods of integrating miniaturization technology into the habitat skins-thus reducing weight and increasing self-reliance-are being considered.
Methods for designing, manufacturing and testing inflatable structures that meet human space flight requirements are being developed for TransHab and the space station’s near-term needs. But the ground-breaking work by both architects and engineers at JSC is laying a technological foundation for structural innovation by many others for years to come.