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Army blows up building

technical & practice

While the '60s was the heyday for inflatable architecture, new R&D ensures that it is applicable to modern demands

For the US army, the development of air beams has been like a search for the Holy Grail. In the 1960s they developed a number of prototypes that highlighted the potential for these structures, but which also demonstrated the significant technical challenges that would need to be resolved. The prototypes showed that it could be possible to build structures using air beams, thereby eliminating any hard structural components. The structures are composed almost entirely of fabric elements that could be tightly folded into compact packages for easy storage and transportation, yet could be rapidly assembled and self-erected within hours in the field.

In very simple terms, air beams are analogous to pre-stressed concrete beams, but in this instance the fabric has stiffness and strength only when it is in tension, rather than in compression. Therefore, an inflated tube of fabric has some inherent stiffness so that it can resist applied loads up to the point where they exceed the inflation stresses.

The bending capacity at the onset of wrinkling - or failure - known as the 'wrinkling moment' capacity, is proportional to the radius and to the pressure cubed, which means that if you want to reduce the tube diameter by 50 per cent, as is desirable to avoid very bulky tubes, the inflation pressure needs to increase by a factor of eight to provide a tube of equivalent wrinkling moment capacity.

This leads to the next concerns, of which the army had first-hand experience through its previous trials:

how to manage safely the vast amounts of stored energy in the tubes, and how to maintain the high inflation pressures. If mistreated, a tear could propagate very quickly, causing an inflated tube to explode, with the potential of severe personnel injury or property damage. To assess the risks, the army initiated two recent studies with Buro Happold and FTL Design Engineering: one study to design and build a prototype structure using existing technology, and the other to encourage the development of braiding and weaving technologies.

Light beams

Initial findings revealed that the final weight of an air beam of a given stiffness was almost independent of the chosen diameter and inflation pressure. However, any decrease in diameter resulted in a large increase in inflation pressure that increased the fabric stresses, which, in turn, necessitated a heavier fabric. This increased weight was more significant than any decrease in air beam size.

An exploration into suitable lightweight fibres highlighted the potential of aramid (a polypropylene blend) and extended long chain polyethylenes (known by the trade names Kevlar and Spectra). Both of these fibres have higher modular strength - their strength to weight ratios are nearly 2.5 times higher than polyester - although this was to the detriment of other properties, such as susceptibility to abrasion, reduction in strength from repeated flexing, UV degradation, cost etc. However, it was felt that the potential gains outweighed the disadvantages.

Because the yarns are oriented diagonally to the axis of the tubes, the principal stresses within the inflated air beam no longer align with the fibres. This has two primary benefits.

If the air beam is damaged, the interaction of yarns prevents the tear propagation that would occur in traditional woven fabrics, greatly reducing the risk of explosion. This allows a reduced factor of safety and, hence, more efficient use of the material. Secondly, the interaction of the yarns allows the inflated air beam to flex. Bending stiffness is provided by additional longitudinal yarns that are applied to the finished tube.

Making light work

Using the braiding technology developed by Advanced Fiber Innovation, straight length braided tubes were manufactured from Spectra yarns.

Given the loose weave and slippery nature of the fibres, these braided tubes are very unstable and require coating in polyurethane to fix the fibre matrix; so as to stabilise the fibres, but also to provide protection against abrasion and UV degradation and create the air-proof coating needed to maintain inflation pressures.

A braided air beam structure, known as the LANMaS (Large Area Night Maintenance Shelter, used 300mm-diameter air beams, which could be inflated to 60 psi; however, the working pressure was set to 30 psi, which provided sufficient stiffness to resist most working loads.

The maximum inflation pressure of 60 psi is equivalent to about 4 atmospheres - typical bicycle tyre pressure. This induced permanent circumferential stresses in the tubes of about 6 tonnes/metre. This pressure could be maintained for extended periods without the need of further pressurisation.

Building on the success of the LANMaS project, and using additional investment, the Aviation Inflatable Maintenance Shelter (AIMS) structure was developed using 750mm-diameter air beams that span more than 30m. A prototype of this structure, completed in 2002, covered almost 1,000m be deployed by eight people in one day, taking only one hour to inflate.

These two projects start to demonstrate the feasibility of air beams to replace aluminium or steel beams for the primary frames of deployable structures. The only drawback to their acceptance for more conventional structures is price. However, as the number of manufacturers who can provide this technology increases, and costs start to come down after initial investments are realised, it is hoped that the use of air beams may become feasible in non-military applications.

Sealed and delivered

A further example of braided highstrength fibres for use in deployable pressurised structures was the Advanced Inflatable Airlock (AIA), developed as part of NASA's search for weight and cost-saving technologies for the next generation Space Shuttle.

This two-phase, two-year research and development programme comprised:

Phase 1: a focus upon the overall system design, basic human factors, and technology; and

Phase 2: a demonstration of the structural capacity to 4 atmospheres of pressure, thus demonstrating the capability for emergency hyperbaric operations.

The first year of development focused upon solving some of the basic architectural questions as well as proposing an approach to the complete system. While a number of alternative scheme designs were proposed, NASA stressed that the program was primarily about technology development and not about design.

Maintaining the shape and functionality of the airlock during EVA (extra-vehicular activities, ie space walks) was addressed by employing high-pressure air beam rings inside the airlock. These air beam rings would pretension the innermost fabric scuff layer in the hoop direction during EVA.

The air beams would also be outfitted with handholds and foot straps to aid the mobility of the astronauts.

The design included a micrometeor orbital debris shield (MMOD system). Folding the MMOD layers presented unique challenges. Ideally, all the fabric layers would compact both in volume and footprint. Experiments using origami techniques proved unsuccessful because of low fault tolerance: if one fold goes wrong, the whole system fails and NASA could not accept such unregulated folding behaviour of the fabric.

An active system using the lesserknown technology of pneumatic muscles was adopted. Fabricated by Shadow Robotics in London, pneumatic muscles, just like air beams, are tubes composed of a structural outer layer and an inner bladder layer. With the pneumatic muscles though, the structural layer is applied on a bias so that when the tube is pressurised the structural sleeve expands in diameter, which causes a contraction in length.

In principle, the folding behaviour of the MMOD system would be controlled by air beams pushing outwards and air muscles pulling inwards.

The Phase 1 program culminated in the demonstration of a full-scale test article that proved the viability of the concepts of deployment and retraction, and the folding behaviour of the different systems (MMOD, restraint layer and bladder).

Having satisfied the requirements for Phase 1, NASA committed funding for a further year to proof-load the Advanced Inflated Airlock to 4 atmospheres of pressure. The structural system selected consisted of a braided structure nearly 3m tall with a maximum diameter of 2.1m and a minimum diameter of 1.6m. The braid geometry was controlled throughout this structure to generate the appropriate strength and stiffness properties required for the airlock. A single layer tri-axial braid was composed of ribbons handling hoop stresses, and axial ribbons principally used to maintain braid stability. This was then coated in an elastomeric coating that was developed to ensure that the braid would remain stable throughout the fabrication processes.

A secondary system of belts was used to restrain the axial loading.

Upon completion, the braid, still on its mandrel, was shipped to California for final assembly. Subsequent operations of mandrel removal, braid termination, securing top and bottom aluminium plates (each weighing in at 0.75 tonnes), installation of the bladder, trial inflations and belt adjustment were all unprecedented procedures. These operations had been carefully planned as there was only going to be one chance to pass the pressure test. The airlock was then crated and shipped to a bomb-testing facility in Santa Clarita, California.

Overall, the test article carried the proof pressure as designed with minimal degradation. While this shows the ability of air-inflated structures to exceed expectations, unfortunately the EVA programme was dropped by NASA and the follow-on program of development for the Advanced Inflatable Airlock was deemed unnecessary.

The potential for this type of technology is vast - from storage facilities to emergency disaster zone accommodation. In the meantime, the airlock is on display in Houston at Johnson Space Center's New Technology Exhibit.

Angus Palmer is group director, Buro Happold, and Robert Lerner is associate at FTL Design Engineering

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