Flexible composites in defence and aerospace

Materials World magazine
,
1 Dec 2008

Giacomo Giovangrossi, Space Business Unit Technical Director, and Marco Adami, NBCR Business Process Manager, of international manufacturer Aero Sekur, describe two of the composite flexible material developments it has undertaken for the aerospace and defence markets.

When most people hear the word composite they think of either low cost resin-based systems like glass-reinforced plastic or high-tech materials such as metal matrix composites. However, there is a group of materials known as flexible composites which tend to be used in sheet form for a variety of applications.

Cool application

The failure of the ceramic heat shield on the Columbia Space Shuttle in 2003 prompted engineers at Aero Sekur to consider the potential of a flexible lightweight structure that would resist the high temperatures generated during atmospheric re-entry while offering a low launch volume and mass.

The key challenges faced were:
• Identifying a flexible coating that would resist temperatures of up to 1,400ºC.
• Providing sufficient insulation to keep the payload temperature to <40ºC during re-entry.
• Developing a supporting structure that would be low volume when packed, but provide sufficient rigidity to support the thermal protection layers during re-entry.
• Keeping the total system weight to a minimum.

Layered approach

The solution consists of a compressed nitrogen gas system which, when activated, inflates a multi-sectioned bladder, comprising both gas impermeable and structural elements based on nylon and Kevlar.

On top of this, a lightweight thermal barrier was required. Alumina was chosen due to its low thermal conductivity and weight. The final layer, which is directly exposed to high re-entry temperatures, had to withstand the thermal conditions, as well as be flexible to ensure compact packing. The team selected a composite design of silicon rubber and alumina blankets. The structured layer approach provides an ablative outer surface where the silicon layer pyrolises during exposure to extreme heat. This ‘sacrificial burnt outer’ is a necessary feature of the design as it protects the structure underneath.

Recent tests carried out in Italy at the Centro Italiano Ricerche Aerospaziali Plasma Wind Tunnel Facility have proven the thermal integrity of the system. An inflated development model SPacecrew Emergency Module (SPEM) unit has been introduced into the wind tunnel and the structure was exposed to temperatures of around 1,400ºC. Wind generated loads have been successfully resisted and a number of thermocouples located within the structure have confirmed the design temperatures are close to those predicted – the inner temperature remains below 40ºC.

Having successfully completed laboratory validation work, Aero Sekur is now looking to secure a trial to prove the system on an actual re-entry.

Chemical protection

Images from the recent Gulf wars brought to public attention the use of ‘Noddy’ or nuclear, biological and chemical (NBC) suits. For specialist and short-term NBC hazard protection, heavy non-permeable suits are worn by personnel. However, for generic military/combat use a more comfortable permeable NBC is specified which can be worn for longer and is less restrictive.

The permeable combat suit has a host of different and often conflicting requirements. It must offer protection from NBC agents, but it should also be easy to wear and minimise restriction on mobility. It needs to enable service personnel to operate in hot climates without heat exhaustion, and must stand up to the rigours of military life. Additionally, it must be washable and re-usable to provide a realistic service life.

The suit design, which is a composite fabric consisting of three layers, has not fundamentally changed for decades. This comprises an:
• Inner layer – worn next to the skin/inner clothing to provide support.
• Active layer – allows body moisture out but prevents ingress of NBC agents.
• Outer layer – provides mechanical strength and tear resistance.

What has changed, however, is the design of the suit shape/fit and, in particular, the materials used. The original suits consisted of a simple cotton inner with a charcoal foam active layer and polyester cotton outer. While this met basic combat requirements, the suits were restrictive and could only be washed five or six times before the active layer was destroyed. Improvements were needed in the following areas:
• Protection from NBC agents.
• Permeability.
• Greater wearer comfort.
• Enhanced ‘washability’ without impairing the active layer.
• Flame tear resistance.
• Reduced weight.

As with many products, design is a compromise and the NBC suit is no different but, with the adoption of more advanced materials in the active and outer layers, suit performance has significantly improved.

Today, active layers are still based on a charcoal composition, but the mechanism of holding/using the carbon ingredient has improved. Radioactive dust contamination and biological threats are kept out of the suit because the active layer acts as a physical barrier, while chemical agents are neutralised by the carbon held in the material. The outer layer is a Kevlar construction which improves flame and tear resistance. Compared to a suit of the 1970s, there has been a weight saving of typically 30%.

So what of the future? Improvements will continue to be made to the suit’s construction materials. The company is also working on an ‘intelligent clothing concept’ which will incorporate a simple lightweight and low cost gas alarm into the suit to advise the wearer when they are under NBC attack.

Further information: Aero Sekur