Stories of Aerospace Engineering

Read the stories of researchers and students at the Faculty of Aerospace Engineering, and discover the scientific questions they are working on and the solutions they come up with.

Fighting corrosion with algae

If algae and specifically diatoms can be used to increase the efficiency of environmentally friendly anti-corrosion coatings, we could protect all kinds of structures – aircraft, trains, military tanks and so forth – without using toxic and expensive materials. The use of coatings is one of the most widespread approaches for protecting metallic structures against corrosion. Such coatings use passive and active protective methods such as barrier against corrosive species and corrosion inhibitors. For almost 100 years, corrosion inhibitors based on chromium VI have been used in coatings to maintain the protective function even after damages have occurred. Though efficient, these particles are highly toxic and carcinogenic. As a consequence, the consumption of chromates has already been banned in many applications and is a constant target for the highly demanding aerospace sector where its banishment has been delayed several times due to the lack of sufficiently good alternatives. The use of algae, and specifically the exoskeletons of the algae group known as diatoms, might help us create the alternative the world is looking for. Along 2015, Assistant Professor Santiago Garcia at the Novel Aerospace Materials group, set up a project to explore the potential use of diatom exoskeletons (or: frustules) to protect aerospace structures. Garcia: “In 2015 we made the first proof of concept to demonstrate that algae can be used for active corrosion protection and self-healing applications. I believe this could potentially have a huge impact.” Why diatom frustules? There are already a few examples of promising corrosion inhibitors that might replace chromate. However, studies have shown that unwanted reactions occur between these inhibitors and the surrounding coating matrix thereby minimizing their inhibiting efficiency. A way to avoid this is to encapsulate the inhibitors inside carriers. Using a carrier reduces the interaction between the inhibitor and its surroundings, and in addition it can be used to control the release of the corrosion inhibitors. This strategy can theoretically result in much more efficient anti-corrosive coatings. Frustule of the Aulacoseira type diatom (diatom exoskeletons) The frustules can function as such a micro-sized carrier. Why is their specific architecture suitable for the task? PhD researcher Paul Denissen explains: “Frustules are hollow nanoporous silica microparticles referred to as ‘pill-box’ structures. They have a cell wall made of silica, a strong structure with pores. Luckily for us, these pores are big enough to allow the corrosion inhibitors in and out.” Studying individual particles After his MSc thesis on this topic Paul Denissen started his PhD research last January and is looking into the isolation and study of individual frustule particles by means of advanced characterization techniques. Garcia: “Frustules show a wide diversity in shapes, sizes and porosity (namely architecture). Dedicated tests need to be performed to find out how the individual particles behave and how we can influence that behaviour. In short, we want to: Explore the potential use of diatom exoskeletons and demonstrate that they can be used for doping and controlled release of functional species in coatings such as corrosion inhibitors. Evaluate what the effects of the architecture and geometry are on the release and efficiency of corrosion inhibitors. Modify the surface of diatom exoskeletons and apply certain triggers such as changes in the pH level to control the release of corrosion inhibitors. If we understand this, we can start making coatings that only release the required amount of corrosion inhibitors at the right time using highly available raw materials. “ Corrosion protection mechanism of coatings containing inhibitor-doped diatom exoskeletons Nature’s solution to an industrial problem The NovAM group is now specifically looking at high strength aluminium alloy 2024 used in aerospace manufacturing, which is very susceptible to corrosion but the trick can be used to protect all kinds of metal alloys. Denissen explains that this self-healing mechanism could also be used for other applications in the future: “Every inhibitor has certain characteristics which have certain effects when encapsulated in a carrier. Once we have quantified individual particles and measured how they behave, we can decide on which particle is best for a specific application, such as aircraft.” If algae and specifically diatoms can be used to increase the efficiency of environmentally friendly anti-corrosion coatings, we could protect all kinds of structures – aircraft, trains, military tanks and so forth – without using toxic and expensive materials. Garcia: “Diatoms occur in virtually every environment that contains water. Reproduction among diatoms is by spores and asexual by binary fission and they have a very high growth rate. Using frustules to make active anticorrosive coatings would not only present a healthier solution to current synthetic approaches, but it would be up-scalable, sustainable and inexpensive as well. This new concept follows the line with our ongoing research on self-healing polymeric systems and new functional micro and nano fibres for composites and coatings made out of algae.”

Glue for art’s sake

Hans Poulis (TU Delft Adhesion Institute) recently received funding from the NWO for his research proposal in the field of adhesive aging. The proposals were accepted in the first round of funding of the Netherlands Institute for Conservation, Art and Science (NICAS). Poulis: “This is the first time that the Adhesion Institute will be developing a new type of adhesive from scratch.” There are currently all sorts of standard adhesive types in use for the restoration of works of art. Synthetic adhesives are often more stable than natural adhesives, but are not always entirely suitable. They were not developed for restoration purposes and therefore never have all the necessary characteristics. So we usually don’t know how they will alter over time, either chemically or mechanically. In other words, how the substances age over time under environmental influences. They might turn yellow, for instance, or the qualities of the adhesive bond may alter. Hole in the market It is a niche market, and not really commercially viable for businesses. Recanvassing paintings requires relatively large amounts of adhesive, but you can imagine that for repasting tiny paint flakes only tiny amounts of adhesive are needed. The Adhesion Institute will be looking into that latter process, and particularly into reverse glass paintings and compositions in oil paint and gouache. These works are typical for many works of art that will need restoring in the short term if we want to preserve them for future generations. Developing a specific adhesive for this purpose is not cost-effective for a company, if only for the high costs of the research involved. Poulis: ‘Firstly we’ll take two post-doc researchers into the field. I hope to organise a brainstorm session in March with professionals who have years of experience, including curators. They know exactly which requirements the adhesive must meet. Based on the outcome of that session, we will determine the start phase of our research. That will centre on the question: what chemical substance will we base it on? That will be our main ingredient. We will then create different mixtures and depending on the requirements, adapt the recipe accordingly. Using various lab tests we will determine how the mixtures react, initially and over time.’ From test to product By carrying out aging tests, we aim to find out how mixtures react after a certain number of years. For instance, light exposure tests will enable us to see to what extent such a substance turns yellow over time. For these tests, Xenon lamps are used. They generate UV light and initiate an accelerated aging process. Poulis: ‘Ultimately, we will also test the adhesives on works of art (mock-ups). Of course, in practice you never know precisely what will happen to the adhesive, because of the numerous circumstances that could influence it. Moreover, there are no data available which can link accelerated aging tests to actual conditions.’ The Adhesion Institute has set itself a two-year time limit in which to develop an adhesive which is suitable and stable in the long term when it comes to mechanical and visual performance. The consortium for this research project includes a commercial company specialised in the manufacture of small quantities of adhesive. They would be able to put it into production. Poulis: ‘And who knows, it might generate a spin off.’ --- The NICAS is an interdisciplinary research centre aimed at the conservation of cultural heritage. It works in collaboration with the Netherlands Organisation for Scientific Research (NWO), the Rijksmuseum, the University of Amsterdam (UvA), the Cultural Heritage Agency of the Netherlands (RCE), and Delft University of Technology (TU Delft). Click here to visit the NICAS website.

A voyage of discovery through our solar system with lasers

How can we learn more about planets and moons? Dominic Dirkx recently wrote his PhD dissertation on a new method of accurately measuring the distance between the earth and satellites orbiting or on planets and moons to within between a millimetre and a centimetre. Current radio measurements are accurate to about a metre. Dirkx took laser tracking, a technique that’s currently used for measuring the distance to earth’s satellites (accurate to within a few millimetres), and extrapolated that to interplanetary distances. His research primarily shows that it’s vital that we not only measure the distance extremely accurately. Other measurements will also need to be improved in order to make the most effective use of laser tracking. If we manage to do that, this method can potentially play an important role in the exploration of our solar system. That might sound obvious, but his research has shown that focusing on making laser measurements and measurements of elements such as magnetic fields, shape or a planet’s seismic activity more accurate can really have a significant impact on your results. Influence of the clock on earth When trying to improve laser measurements, Dirkx asked to what extent the ultimate measurements are influenced by the clock we use here on earth. Dirkx: ‘When you measure distance, you’re actually measuring movement. You use a clock on earth and a clock in space to measure how long a laser pulse is en route. Because you know what the speed of light is, you look to see where a satellite is from one moment to the next. And that means that if you’re just a nanosecond out, it can easily make 30 centimetres difference.’ If you examine how something moves, you examine how gravity works and as such, you learn about the surroundings. Imagine that you measure how a satellite orbits the planet Mars: that will allow you to find out more about the composition of the planet. Last year, for example, this method was used – alongside other measurements – to discover an ocean under the surface of Enceladus, a moon of the planet Saturn. A remarkable thesis defence Dirkx graduated nominally and with distinction. David Smith (MIT) felt it was worth the trip to Delft to sit on the committee during his thesis defence. Smith is specialised in laser ranging as well as planetary sciences. Dirkx: ‘He’d worked with colleagues from our research group in the past, and I’d met him at conferences. He’s someone who’s extremely intimidating to doctoral candidates – a major figure with an endless list of significant publications to his name.’ Dirkx: ‘What really helped during my promotion was having the opportunity to work on the European FP7 ESPaCE project. This enabled me to build up a large network, so I’ve always had lots of people to discuss my research with. My tip for other PhDs is therefore to talk to people whenever you can. Not only within your own research group, but particularly outside of it.’

Understanding aircraft behaviour on final approach

PhD student Floris Herrema (Air Transport and Operations) recently won the SESAR Young Scientist of the Year Award for his MSc thesis ‘Compression on final approach and Time Based Separation (TBS) for Optimised Runway Delivery’. His work has had a direct impact on the knowledge and safety of TBS and the deployed TBS at London Heathrow Airport: the first in the world. “I never expected to win the SESAR Young Scientist of the Year Award, so I was actually quite relaxed on the day of the final round in Bologna, Italy. Single European Sky Air Traffic Management Research (SESAR) is a European funding institute focused on air traffic management (ATM). This means I got to stand in front of 400 people in the field of ATM that day, which was great.” Predicting aircraft behaviour “The objective of my study was to quantify and model the potential performance compression improvements on final approach for Time Based Separation (TBS). The concept of TBS was known, but it was too complicated to actually implement it. I have developed a new air speed profile, the Floris Friso Herrema (FFH) tool, with which we can better predict the expected aircraft behaviour and TBS. This is very relevant information for air traffic controllers. It is now easier to understand and thus implement. And it was implemented at London Heathrow Airport last year, which is now the first TBS airport in the world.” The advantages “The most important advantage is the recovery we achieve during strong headwinds. It is still estimations based on Heathrow, but we expect that during strong headwinds we can achieve two more landings per hour. In addition, TBS is on track to save 80.000 minutes of delay per year. The benefits to the airlines can add up to 7.5 million pounds per year.” Impact “I was specifically looking for a thesis subject with potential impact. It is part of the reason why I wanted to work together with EUROCONTROL on this from the start. I combined a traineeship at EUROCONTROL together with my master’s project. You see without implementation, theory is just theory. This is also why it is great now working together with Ricky Curran and Dries Visser (both Air Transport and Operations) during my PhD project. With everything I do, they ask: what is the social and academic impact? The university and EUROCONTROL each give me feedback and advice that is always slightly different. There is certainly extra value in combining these views. I hope to always keep moving around in both worlds and be an active link between the two.” Future “My PhD project, ‘Big data analyses and machine learning at airports to support decision making’, is well underway. I am working on feasible machine learning techniques. The goal is to implement it at all major airports in Europe. I really enjoy working on this. Right now we are trying to embed my research in a European research project. We are working hard on a few proposals in the context of the Horizon 2020 programme to try and obtain funding for this. If we could achieve this, that would be great of course.” The most important advantage is the recovery we achieve during strong headwinds.

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