In a little over 100 years, airplanes have gone from a thing of science fiction to the most important means of transportation in the world today! Nearly 500,000 individuals are in the air at any given hour! This absurd figure also goes to show how economical air travel has become. The demand for air travel is increasing at a rapid pace, but the supply by airlines is unable to keep up. One of the major bottlenecks in the supply of new aircraft to these airlines by aircraft manufacturers. This is because the conventional process of aircraft manufacture is time and resource-intensive. This problem plagues people from all walks of life – aircraft manufacturers are often unable to deliver products on time without impacting their bottom line, naturalists are worried about the tremendous wastage of energy and resources in the conventional process, and this relatively inefficient process is one of the reasons why air travel is still not affordable for everyone. However, things are looking up – recent advances in materials research have yielded novel techniques to manufacture aircraft, that promise to be cheaper, less time consuming, and more energy-efficient!
Let’s first understand the conventional manufacturing process, so that we can better appreciate the novel techniques that follow. The conventional process can be broadly divided into 4 stages:
The first stage, design, consists of engineers working with flight groups to design airplane parts individually and perform approximate computations on them using standard flight conditions.
The next stage, simulation, involves putting the various parts together in the form of a model and test in wind tunnels to test the aerodynamics of the individual parts as well as the composite structure. This structuring and arranging can take as long as 4 years for each aircraft manufactured.
In this stage, the various parts are manufactured by small companies specializing in a certain kind of aircraft parts. Parts are usually selected based on similarities to pre-existing parts.
The fact that this stage involves 4000 engineers working for a month per aircraft goes to show that assembly has little margin for error. One of the significant (and most exciting) technologies used extensively during assembly is that of autoclaving. Mechanical autoclaves are pressure vessels that process parts and make them suitable for use under high pressure and temperature conditions. In this method, individual pre-impregnated plies are placed (in bags) in the final configuration (of the parts) under vacuum, and the bags are later pressurized from the outside. This results in a uniform, high quality and consistent composite material or part. The parts are sealed together using epoxy or other thermosetting compounds. These parts can now withstand some of the harshest environmental conditions on Earth.
The biggest downside from operating more of these magic machines comes from the vast amounts of energy it takes to power the autoclaves that manufacture humongous airplane parts. The other significant disadvantage comes from the mammoth size of these autoclaves – some are over 60ft by 15ft, and hence, they bring their share of logistical nightmares.
The use of nanomaterials in airplane manufacturing promises to alleviate the downsides of the conventional process. In fact, we are on the verge of seeing nanomaterials becoming more economical than conventional materials across all industries. Let’s narrow our field of vision on one of the poster children of nanomaterial science – carbon nanotubes, and see how they can revolutionize aircraft manufacturing.
Carbon nanotubes (CNTs) are simply nano-scale cylinders made of carbon atoms. Carbon nanotubes frequently allude to single-wall carbon nanotubes (SWCNTs), which is one of the allotropes of carbon, halfway between fullerene cages and flat graphene. A good analogy to understand the structure of SWCNTs is that of a rolled-up chart paper. Just the way a chart paper is rolled up along one of its edges, an SWCNT is formed up by ‘rolling’ the hexagonal carbon lattice along its diagonal (a Bravais lattice vector). MIT engineers have utilized these hollow cylinders to devise a strategy to deliver aviation-grade composites without using autoclaves.
Figure 2: Carbon Nanotube
In 2015, Dr. Jeonyoon Lee’s group developed the revolutionary Out-of-Oven (OoO) method. This technique allowed for the manufacture of composites almost as tough as those made in conventional assembling stoves, by utilizing just 1% of the energy of an autoclave! However, this method increases the probability of voids in the nanostructure of the composite. Voids are, in essence, holes, formed between polymer or fiber layers in a composite material. Too many voids are undesirable because they adversely impact the durability of the material by decreasing shear capacity between two layers and by reducing how much a material can be stretched or compressed. Autoclaves get rid of these voids by pushing them out (as they form) to the edges and hence removing them. To overcome this issue of void formation, Dr. Wardle’s group developed the Out-of-Autoclave (OoA) strategy. Void formation probability was brought to a staggeringly low 1%, which in turn improved material life and durability. Nonetheless, the OoA strategy presently faces scalability issues when applied to manufacture core aircraft components like the wings and fuselage.
Dr. Wardle’s group also made another remarkable breakthrough! They used the physics of capillary action to create a nearly void-less composite. To get a visual picture of the physics involved, let’s imagine a dense forest, with the trees representing carbon nanotubes with very thin spaces between them (i.e., the spaces have a very high length to diameter ratio). This makes them akin to capillary tubes, which can generate pressure based on its geometry and its surface energy, which is the material’s ability to attract other materials. Now to put it all together, what if we place the carbon nanotubes between two materials and heat them? The geometry of the carbon nanotubes will cause the surface energy of the capillary to draw the material inwards as it softens on heating, thus filling any void that has formed! This is precisely what the researchers tested in the lab laying the films between layers of materials that are typically used in the autoclave-based manufacturing of primary aircraft structures. They wrapped the layers in the second film of carbon nanotubes, which they heated by applying an electric current. And voila! As the materials heated and softened, they were pulled into the capillaries of the intermediate CNT film, thus resulting in a void-free composite. The integrity of this composite was almost similar to that produced conventionally in an autoclave!
Figure 3: Cross-section of the composite
Presently, scientists are hard at work, trying to develop higher weight CNT films using this revolutionary capillary-based technique. This is essential if we are to manufacture wings or entire fuselages of aircraft out of carbon nanotube-based composites. There is another major challenge this technique will have to overcome before it can be widely used. Carbon nanotubes have a very high surface area to volume ratio, which increases the van der Waals forces, which in turn makes these nanotubes prone to material agglomeration. Quite intuitively, this leads to uneven heating properties and demands a unique heater be designed, which can compensate for this phenomenon.
Carbon nanotubes ultimately turn out to be the perfect tool that can make aircraft manufacturing more cost and energy-efficient. This could make air travel a means of transportation for the masses. The upcoming industry of space tourism could also receive a tremendous boost from such manufacturing technologies what’s more! Over the last decade, nanomaterials have also trickled into the manufacture of everyday objects. Maybe one day in the not too distant future, when we look at our household water or gas pipes, we will remember the simple physics that went into their void-less construction!