Stories of Aerospace Engineering

Read interviews and stories of researchers and students at the Faculty of Aerospace Engineering, and discover the scientific questions on which they work and the solutions they present.

Flying formation on a few drops of water

With the trend towards miniaturized satellites, the search is on for small-scale propulsion methods. The work of Dr Angelo Cervone in the department of Space Engineering (SpE) focusses on MEMS-based propulsion systems. “Small satellites today have very limited propulsion capabilities, meaning they cannot change orbit or perform complex manoeuvres. So the development of new micro-propulsion systems is already an achievement in itself,” he says. “It also opens up the opportunity of employing constellations of satellites flying in formation.” Universities have long been leading the way in the development of small satellites. What started off as an educational tool to teach students how to build and launch satellites, has since gone mainstream. “Nano-satellites are no longer used for research and education only. A lot of companies are launching small satellites for imaging purposes and other commercial applications,” says Dr Angelo Cervone. In February 2017, a record-breaking 101 nanosatellites were launched aboard a single rocket from the Sriharikota space centre in India. These included 88 CubeSats from the US Earth-imaging company Planet Labs Inc, founded by a team of ex-NASA scientists. “Their goal is to image each point of the Earth’s surface every day, like a kind of Google Earth that is updated daily.” None of these 101 CubeSats had propulsion, however. A propulsion system would allow such nano-satellites to correct their orbit or maintain their altitude, meaning they can achieve a much longer operating time in space. “At very low altitudes, in the order of 300 to 350 kilometres or less, there is still some air which generates a drag force and slows down the satellites, so they tend to fall into the atmosphere after a few months or even days.” Propulsion is also a key enabling factor for missions based on constellations of satellites flying in formation. “Satellites flying in formation must keep very precise relative positions. This is very hard to achieve without propulsion.” Sending a swarm of small satellites into space is a long-term goal of the SpE department: in the OLFAR mission, satellites will orbit the Moon and jointly form a virtual telescope, to study the dark ages of the universe. Cervone has been working on space propulsion since his MSc and PhD research at the University of Pisa, Italy. After two years of post-doctoral research at the University of Osaka, Japan, he joined TU Delft in 2012. Here he focused his research on the development of propulsion systems suitable for the very small satellites the university is working on. That is not simply a matter of decreasing the size of existing larger systems. “Not everything scales linearly with size. Different performance factors scale in different ways with the design parameters, some of them linearly, others in a cubic or quadratic fashion. For example, reducing the size by a factor 10 will reduce the thrust by a factor 100, but also increases the chance of energy losses by a factor 10.” Resistojets While micro-propulsion is a fast-growing field, Cervone discovered that research into the thrust range of 1-10 millinewton (mN) is still a mostly uncharted territory for CubeSat-sized systems – a thrust range that would be very useful for CubeSats that have to change orbit or rotate rapidly. “Of course there are options that can provide higher thrust, but for a satellite the size of a CubeSat that would be a bit like putting an aircraft engine on a car: it’s too big for its size, causing the satellite to become uncontrollable.” This 1-10 mN range can be achieved by using resistojets, the particular propulsion option at which Cervone is currently working. “As far as we know, they are the only way to get to this thrust level with a still acceptable performance in terms of propellant and power consumption. Resistojets are not very efficient for larger satellites, so in spite of their apparent simplicity the research on them has not yet reached sufficient maturity. Looking at our requirements, however, they are the best option.” In a resistojet, the propellant is heated via an electrical resistance and then expelled through a nozzle. “We use energy from the satellite’s solar panels to drive current through the resistor. This heats up the system, providing thermal energy which in turn heats the propellant,” explains Cervone. “We are looking into water as propellant, because it is ‘green’, safe, easy to use and, surprisingly, also offers an excellent performance. In a paper recently published in the ASME Journal of Heat Transfer we have demonstrated that, among all fluids storable as liquids at nearly-ambient conditions, water is the best one in terms of volumetric propellant consumption and the second best in terms of mass consumption.” But before they got to that point, they had to create their own research instruments and facilities. “The first task of our team was to design an instrument to measure the thrust with sufficient accuracy. Consider that 1 mN is the weight force of a 0.1 grams of mass. That is approximately the same force you feel when you put a bird’s feather on your finger: practically nothing.” What the team came up with is a system based on a pendulum. “We put our propulsion system on the pendulum and then fire the thruster. That will make the pendulum oscillate a little, and we can measure the oscillation amplitude and relate it to the force.” An important development will be a vacuum chamber to test their systems under conditions comparable to those in space. “With such very small systems there is a lot of uncertainty when you don’t test them in vacuum, because normal atmospheric pressure can be more than enough to counteract the thrust. To understand what really happens in space we need a vacuum chamber.” For now they are using a vacuum oven where a moderate level of vacuum can be reached, albeit still far from the actual conditions in space. “We are also considering the option to use vacuum chambers already existing elsewhere, such as those at ESA-ESTEC, but we would obviously get only limited time slots there. We really need our own chamber to test what we want, whenever we need it.” Manufacturing Manufacturing micro-propulsion systems is another challenge, as the complete system containing the heating system, propellant channel, nozzle and even a water tank can be as small as a sugar cube. “The components of our systems have sizes ranging from a few millimetres to much smaller, sometimes as small as a human hair, and heavy integration between fluidic, electronic and structural components is required,” says Cervone. That is why the team turned to MEMS fabrication technology (MicroElectroMechanical Systems), for which they collaborate with the university’s Else Kooi micro manufacturing lab. Using MEMS offers great opportunities for further integration. “Apart from the heaters, nozzle and channels, MEMS also allows us to integrate electronics like sensors and the control system for the thruster,” says Cervone. “We are also thinking of adding a MEMS valve, a moving element that opens and closes the channel, which is now still a separate component outside the chip. The valves we are currently using measure a few centimetres, so they definitely require further miniaturization.” Micro dimensions also make high demands on the materials used. For example, too large a difference in the rates of expansion and contraction as functions of temperature is not acceptable, as even small relative changes in dimensions can have high impact. “MEMS are mainly made with silicon, a material with relatively high thermal conductivity. It dissipates heat very fast to the external environment. For propulsion that is a problem, since we need to transfer as much thermal energy as possible to the propellant without dissipating it. You want to supply the heat to the propellant, not to space”, says Cervone. “One solution is to better isolate the system by covering it with special paintings or materials.” In the long term, they are also looking at 3D printing. “With 3D printing you can design and manufacture any kind of complex shape. Unlike traditional manufacturing, which works by joining parts or removing material from them, a 3D printer makes your design in one piece without any seams or joints. The technology is not yet mature enough at the micrometre scale we need, but is progressing very fast and I expect it to advance to that level in a few years.” Satellite-on-a-chip Further miniaturisation can ultimately lead to satellite-on-a-chip concepts. “Chip-sized satellites could be disruptive in space missions. Imagine you can send such small chips into space, and do almost everything with them that we are doing now with larger satellites. Ideas that still sound like science-fiction today could then become reality, such as the Breakthrough StarShot project supported by Stephen Hawking and Mark Zuckerberg that aims to send chip-sized satellites to Proxima Centauri, the star nearest to our solar system. We are not quite there yet, though, we still need a bit more research.” Watch this space.

Earth evolving as seen from space

The trajectory calculations for satellite missions must be as accurate as possible – an area in which Dr Ernst Schrama and his colleagues have been experts for decades. They also use satellite data to monitor changes on the surface of the earth. And with new missions with cutting-edge measuring equipment soon to be launched, the challenges keep on coming. “No time to sit twiddling your thumbs in this line of work,” says Dr Schrama. Dr Ernst Schrama has been a member of the Astrodynamics and Space Missions research group of the Space Engineering department for more than 15 years. At the time, they started out calculating satellite trajectories with extreme precision. “Traditionally, our research group has been focussed on satellite trajectory mechanics. Using laser, radar and radio frequency measurements taken from the ground, we can determine a satellite’s exact location and calculate where it is headed,” explains Dr Schrama. Due to their considerable expertise, Dr Schrama and his colleagues have been involved in a number of missions, including the European Space Agency’s GOCE mission in 2009, the CryoSat-2 mission in 2010, and the SWARM mission in 2013. But that is not all. They are also experts in producing and interpreting detailed models of measurement data transmitted from the instruments on board the satellites. One significant application of this data is monitoring changes on the earth’s surface for conducting geophysical research, including changes in ice caps and sea levels. “Geophysicists observe the earth system and the changes it undergoes,” clarifies Dr Schrama. “The idea is that, without touching the earth's surface, we know its volume and shape and how it is changing. This we can do achieve both directly, using altimetry, and indirectly using measurements of the planet’s gravitational field.” Radar altimeter measurements Altimeters borne in satellites have delivered new insights into exactly what the oceans look like beneath the surface and how the sea floor has changed over the course of millions of years. Dr Schrama presents a map composed entirely of radar altimeter measurements and points out what looks like a chain of undersea volcanoes: the Hawaii-Emperor seamount chain. “That volcanic system formed above a hot spot deep in the earth – a sort of stove pipe, if you will. A number of volcanic islands currently lie in that area and more are still appearing. But the earth’s crust – the Pacific Plate to be precise – is moving extremely slowly, by a few centimetres a year, towards the north-west. So, where Hawaii now lies, the ocean floor is slowing moving in a nor’westerly direction while to the south-east, new islands are being formed. This process has been in motion for 300 million years and we can clearly recognise it on the gravity map.” Ice masses and oceans, mountains and continents make the surface of the earth irregular and, what is more, they are still moving. This in turn creates changes in the planet’s mass and therefore also in the gravitational field, depending on where and when you measure. Humans are unable to feel these gravitational variations, but we can observe them with satellites. Gravitational measurements can therefore also provide information on sea currents and glacier movements, for example. And in these cases, two satellites are better than one. “We have collaborated on developing experiments in which multiple satellites are sent into space,” relates Dr Schrama. “We are then able to measure the mutual distance between the satellites and how this is influenced by the gravitational field acting upon them, which in turn enables us to better investigate the field itself. A good example of this is NASA’s GRACE mission which was launched in 2002. Over the past 15 years, the twin satellites have provided new insights into the proportions of land and water around the globe. Rising sea levels The most important changes being observed are those in ice masses and sea levels due to their connection with climate change. Based on historical sources, we know that the current increase in sea levels is demonstrably greater than any in the past 2000 years. “The Romans built reservoirs that they used to catch fish which had an opening at the same height as the tidemark. This tells us the height of the water line 2000 years ago, which is approximately 60cm below the current sea level,” explains Dr Schrama. “This provides an indication that the sea level has risen by around the same amount over that period. We are now measuring rises of 30cm per hundred years – that is 60cm every two centuries instead of every two millennia. We also know that the ice systems are decreasing in size,” he continues. “But the question is: is the current rise in sea levels being caused solely by melting ice or are other factors at work, for example the oceans warming and consequently expanding?” Dr Schrama’s research reveals that half of the rise can be attributed to the thermal expansion of the oceans and the other half to meltwater from glaciers in Greenland, Antarctica and smaller ice systems such as Alaska and the Alps. To arrive at these results, he combined a range of different types of measurements. “Altimeter measurements taken above the ice and gravity measurements demonstrate what is happening to the ice, and we also measure fluctuations in the volume of the oceans. Subtracting the one from the other tells us what proportion of the increase in sea levels can be ascribed to which phenomenon.” He has also combined satellite measurements with other techniques. “There is a whole network of automatic buoys in place that measure, for example, the temperature and salinity of the oceans. We then check whether this data concurs with our calculations based on laser and radar measurements.” Post-glacial rebound Another factor to account for in order to effectively model current changes in the ice caps and sea levels is post-glacial rebound. The earth’s crust is still rebounding upwards following the melting of the ice caps when the last major glacial period came to an end 20,000 years ago. “The Scandinavian and North-American ice systems have now disappeared, but that rebound process is not yet complete and we are able to measure it very accurately,” says Dr Schrama. “It is considerably more difficult in the case of Antarctica, however, since that system is significantly larger. Modelling that process is another thing we do here. To this end, we combine measurement data from the GRACE satellites with, for example, GPS measurements from receivers on the ground and data relating to ocean bottom pressure.” Furthermore, a topic such as climate change continually raises new questions. “Over the past 15 years, we have observed transformation processes in progress. Are these processes accelerating, and if so, what is the underlying cause, or will it change again in another ten years? That is what we want to be able to measure and understand, and in order to do so we need to launch further satellite missions and conduct measurements with new instruments.” These missions are also in the offing. “Work is currently under way on a follow-on GRACE mission, which will be launched shortly. The current mission uses microwaves to measure the mutual distance between the two satellites, but a laser interferometer will soon supplement the microwave technology which is expected to be 20 times more accurate. Better instruments and faster data speeds also entail new challenges. How do we put this information to use? Will the new measurements correspond to the previous ones or will we need to make corrections? Do they actually deliver better quality information?” New missions and fresh challenges Another mission in the pipeline is NASA’s ICESat-2 (Ice, Cloud, and land Elevation Satellite 2). “ICESat-1 did not survive overly long: from 2003 to 2009. There was a problem with the lasers which meant that measurements could not be taken continually. The new mission promises to improve on this score,” says Dr Schrama. “Of course, this is in addition to ongoing missions, so that is quite a lot of measurement instruments all transmitting data for us to process. No time to sit twiddling your thumbs in this line of work.” In the meantime, all of the existing missions and instruments have produced a ton of data and in order to keep track of it all, the research group also manages RADS (a Radar Altimeter Data System). “We are getting more and more RADS users and are doing our best to make the system as accessible as possible.” Teaching Dr Schrama has no time for thumb-twiddling on the teaching side either. “We now have an intake of more than a hundred Master's students each year; ten years ago it was only 40 or so. And the traditional image of a cohort made up of students primarily from the Netherlands and Belgium is now well out of date. For example, we are seeing large numbers of students from Southern Europe and a great many exchange students from the Erasmus programme.” Students these days also stipulate different requirements. “Conducting teaching online is a change that we really must progress,” says Dr Schrama, who has now put together an online version of the Satellite Orbit Determination course. Another tradition within the Space Engineering department is building and launching (small) satellites – the CubeSats – which Dr Schrama also uses for his laboratory lessons. “We allow the EEMCS students to take the measurements themselves so they can see what the signal at a ground control station looks like and subsequently use it in their calculations.” The department is also busy developing the even smaller PocketQubes and Schrama is enjoying helping plan experiments with them. “This could result in useful information for us, for example concerning space weather conditions.” The research in this regard began some time ago. “One of my former professors once lobbied the ESA hard to install a gradiometer in the GOCE satellite. The design studies for the gravimeter were completed here in Delft in the 1980s, addressing questions like ‘what do we need to measure’ and ‘what sort of results are we expecting to obtain?’” Dr Schrama relates. “You then have to leave it to the industrial sector to determine how to go about building such an instrument.” In this instance, Dr Schrama entrusted the task to the hands of his colleagues. Photo: Greenland icebergs Disko Bay, Ian Joughin, Polar Science Centre, University of Washington.

Higher up and further out

Dr Axelle Viré is Assistant Professor at the department of Aerodynamics, Wind Energy & Propulsion (AWEP). Her work focuses on the numerical modelling of floating wind turbines and airborne wind energy devices. “The future of wind energy lies in moving higher up into the sky and further out at sea. This will open up new markets and sites that are still left unexplored,” she says. Models for floating wind turbines and airborne wind energy devices need to take into account the so-called fluid-structure interactions. “In the case of floating wind turbines the system is moving as it interacts with the wind, the ocean currents and the waves”, explains Dr Axelle Viré. “The structure of a kite is a flexible membrane that deforms through the surrounding wind flow. In both cases we want to know how these interactions impact on the performance of the systems.” Non-linear events Existing fast models can simulate the dynamics of such systems in real time, or over the expected lifetime of a system. However, such models usually only address linear phenomena, involving simple motions like small waves. “We are more interested in non-linear events, such as strong winds or large waves”, explains Viré. “Once you have a design for a kite or a floating turbine, you want to make sure it can also withstand severe conditions. We want to calculate for example how breaking waves impact the structure of floating wind turbines.” Another issue is the misalignment of wind and waves . “Waves do not always propagate in the direction of the wind. The combination of non-extreme winds and waves may also lead to extreme loads. Fast models usually assume that wind and waves are aligned and not severe.” “We need more refinement in our models”, concludes Viré, who is leading the development of the high-fidelity numerical tools necessary to achieve this. Among others, she is looking at the modelling of the behaviour of airborne devices, which involves interactions between aerodynamics, structural dynamics, and flight dynamics. “We already have some simple models for this, but we want to couple these with computational fluid dynamics (CFD) to refine them. This is still at a very fundamental level and has not been done yet anywhere in the world as far as we know.” High - fidelity models Such high-fidelity simulations are slow, high computational cost models, but the results from them could also be used to improve the quality of the faster models. “We cannot use a high-fidelity model to look at the entire lifetime of a system, but we can look at specific conditions and how the system behaves under them. This will then give us more accurate values that we can then put back in our fast models”, Viré explains. “For example, in our kite models we now use crude approximations of the lift and drag of the wing. With the new method we are developing, we could refine that.” That is quite another challenge, though. “You will then need some kind of representation of your detailed model in your larger model, but how do you bridge the two? That is a typical challenge with multi-scale problems.” Missing link As a postdoctoral research fellow at Imperial College London, Viré worked on the numerical modelling of floating offshore wind turbines, a subject that was still missing when she came to Delft. “I aim to develop floating wind as a research field at TU Delft. As a Wind Energy group, we should be looking into this”, she states. “Floating wind is actually closer to commercialisation than kite power is. There are already prototypes that are connected to the grid and delivering power continuously.” The world’s first floating wind park is being built by Statoil in Norway, who installed their first Hywind turbine in 2009. In Portugal, the WindFloat Atlantic (WFA) project is planning to have 3 or 4 floating wind turbines operational by 2018. “Norway and Portugal are regions where you have deep water close to shore. That is ideal for floating turbines. It is too deep to place turbines on monopiles, but close enough to shore to make it easier to connect to the grid.” Outside Europe, Japan has already installed three floating wind turbines near Fukushima, to replace the closed-down nuclear plant. Dutch interest Until recently, there was limited interest for floating wind turbines in the Netherlands. Possibly, the Dutch thought they did not need it, because the North Sea is shallow enough for monopiles. Obtaining research funding on this specific topic at a national level was difficult. “This is changing now. The Netherlands have recently published a market study on floating wind, and Dutch businesses now also see it as an opportunity”, says Viré. And rightly so, because although the market for floating wind is in deep water, you can still develop the knowledge as an export product, she says. That message seems to have sunk in now. There could be local advantages too. “With floating platforms, you can go further away from shore and avoid a lot of public opposition. It opens up a much wider range of sites than are available today.” When wind energy first moved off-shore, onshore designs and concepts were used, but that is not without its problems. “Traditionally, onshore turbines are monopiles. To install them offshore, you have to extend the pile to the seabed and know the soil condition,” says Viré. “Even if we can build something that goes that far down, it is not cost-effective. However, if you build a floating platform that you moor to the seabed, you only need cables and anchors. That is much less costly than a full structure, and has less impact on the environment.” In fact, Viré foresees both that both fields, floating wind and kite power, will merge in the future. “Kites are cheaper than conventional turbines. That is attractive when you are working offshore, where other costs are higher. You need ships to access the turbines, and in case of extreme weather that is difficult, meaning the availability windows are smaller than onshore. So launching kites from floating platforms might be the most cost-effective solution.” Collaboration Viré finds the disciplinary aspects of her research particularly interesting: “It involves so many fields. We work with the CEG faculty on the substructures, and with EEMCS and 3mE on the control and the electrical engineering. DUWIND is a good platform to bridge all these fields.” She also coordinates a course on offshore wind for professionals. This is a seven-week course that will start in May, taught by lecturers from all various disciplines. ”Because it is such a multidisciplinary field, people usually have a background in only one of these fields. The objective of the course is to get them acquainted with the other fields, and teach them how to integrate all these subjects when designing a windfarm.” Viré still maintains close links to her former research group at Imperial College, London, where she is an honorary fellow. “We try to apply for funding together, for example, and exchange staff for short visits. They can benefit from our expertise on wind, because historically the group there is more involved in ocean and tidal energy.” She also recently wrote a funding proposal for a consortium including the Norwegian and Portuguese floating wind projects. “Even though our research is still fundamental, we still have to show the societal impact and link to practice. It is becoming ever more difficult to get fundamental research funded without involving industrial partners.” However, that is not the only reason to want to involve the Portuguese and the Norwegians. “The added advantage is the data they have. In a field that is so new, it is sometimes difficult to approach commercial parties. They won’t share their company secrets with us, or their patentable knowledge. But we can still simulate the design they share with us, whether it is a more generic design or a final one. The more data we have, the better.” Further afield “My work also has strong links to applications”, Viré continues. “That always drove me, even though I am looking at more long-term solutions.” Whereas industry usually has a more short-term perspective, Viré believes you should always keep looking further ahead and at more high-risk solutions. “If you only focus on improving what currently exists, you are not going to make any step changes or breakthroughs.” The subjects she is looking at are still more about rethinking current concepts. “That way, we are opening new markets and more sites offshore. However, if we really want to move forward and have more impact we need to innovate further.” Looking back is also an option here. “Solutions that were dropped years ago may prove to be interesting again,” says Viré, citing vertical access wind turbines as an example. “These are very scarcely used due to their lower performance. But if you look at floating wind, they might be interesting, as they have a lower centre of gravity,” she explains. “You can imagine that a moving turbine on a floating platform is dynamically not very stable. If you can move the mass of the system further down, the stability will increase.” Unfortunately, industry will not be cueing up to adopt this idea yet, as all current turbines, and thus all design and production facilities, are based on horizontal technology. “We are now researching what is the best scale of floating platform for the existing wind turbines. We can take knowledge from the oil and gas industry but scales in our field are very different. The question is how does the platform scale with the wind turbine rating? What is the optimal floater design? Changing the whole concept might provide the answers. That is still applied research, though it may never be put into practice. That is why we need more funding for high-risk research.”

A sabbatical in the R&T industry: ‘It’s not all that different’

From October 2016, TU Delft assistant professor Roeland De Breuker spent four months working for Airbus Group Innovations in Munich. Having taken the ‘academic route’ through TU Delft (student – PhD – lecturer), it was time for a break. De Breuker took the sabbatical to broaden his horizons and it proved to be a positive experience. A double interview with De Breuker and his former manager at Airbus, Andreas Wildschek. Roeland de Breuker is an assistant professor in the Aerospace Structures and Computational Mechanics group at TU Delft’s faculty of Aerospace Engineering. ‘I’ve been at the faculty for some time. I studied here, took my doctorate here, and now work here as a lecturer. I’d been working for the faculty for over ten years, and in my view it was time to gain some new personal and professional experience. I thought it would be interesting to take a look behind the scenes at a company, but I didn't want to leave TU Delft.’ Roeland de Breuker TU Delft offered the opportunity to take a sabbatical. De Breuker: ‘There’s a clear procedure in place. First you talk to Human Resources and your direct supervisor, and in the end it’s approved by the dean. TU Delft recognises that sabbaticals can be valuable for staff and TU Delft alike. No one had ever done one at Airbus – not in Munich at least – so it took a few months to arrange everything. My Airbus badge was a different colour from everyone else’s, and they didn’t really know where to put me.’ New territory for Airbus Group Innovations, in other words. But now that it’s been done once, it can be done again, explains Wildschek. ‘Roeland brought lots of new ideas with him, and it was refreshing to have an external perspective on our work. He proved to be a valuable addition to our research team.’ Different discussions De Breuker discovered that working with industry is quite different from working for industry. ‘I have developed a better feel for what they consider to be important and how they see the future. Once I was actually there, I had different discussions, and that’s interesting. Evidently, it makes a difference whether you’re at the table as a scientist or as a fellow engineer.’ De Breuker was also struck by the way in which Airbus Group Innovations works: ‘They always have the end product in mind. At first, I thought that their approach would be, “it doesn’t matter how it works, so long as it solves the problem”. But I have a different impression now. The research is fairly applied, but they really want to know how something works and they take the time to develop a long-term vision. It doesn’t all necessarily have to be part of an aeroplane in five years’ time. Just like at Delft, attention is also paid to innovations with a lower TRL [Technology Readiness Level; this indicates the stage that a technology is at, from idea to application]. It’s not all that different.’ Wildschek: ‘We have joint research topics and joint research objectives. By combining our expertise and forces, we can make more rapid progress and work more efficiently. It’s also led to an agenda for future research cooperation.’ ‘And a shared doctoral student,’ mentions De Breuker. ‘There was a doctoral candidate at Airbus who had previously been involved in one of our joint European projects. We had further discussions about this when I went to work at Airbus, and now Chiara [ed.: Bisagni, professor of Aerospace Structures and Computational Mechanics at TU Delft] is his promotor and I am his co-promotor.’ 'Collaboration is truly the key to progress' What lessons have been learned? De Breuker: ‘We could also pay more attention to the end product at times. Fundamental questions are important, but as engineers – for in the end, we are primarily engineers – we should keep asking ourselves: why do we need this answer? We should avoid situations in which we do research and publish the results, and then they languish in a drawer. We have a duty to maintain a balance between discovering and creating.’ Would he recommend it to others? Wildschek has no doubts: ‘Collaboration is truly the key to progress. And it really does work best if you actually sit round a table together, at least for a certain amount of time.’ De Breuker agrees: ‘And I would advise anyone who wants to do a sabbatical to think carefully beforehand about what they want to get out of it, besides personal development. If you keep that in mind from day one, you may create a truly sustainable and valuable partnership in very little time at all.’ Do you work for TU Delft and would you also like to find out more about opportunities for taking a sabbatical? Check out the Employee Portal.

PocketQubes offer much to scientific progress

Artist impression of Delfi-PQ With Google Earth just a click away, we’ve gotten used to satellites. In fact, around 3,600 are currently orbiting the Earth. In 2018, Jasper Bouwmeester is hoping to add some of his tiny PocketQube satellites to that total. WorldView-4, one of the many satellites that produces those Google Earth images we’re so familiar with, weighs 2,500 kilograms and is 7.9 x 5.3 meters. In contrast, Bouwmeester’s satellites are 5 x 5 x 18 centimetres. They are part of a satellite group known as PocketQubes. CubeSats were originally developed by California Polytechnic State University and Stanford University in 1999. Space agencies such as NASA and ESA launch large satellites but universities often don’t have the financing or the need to put such big contraptions into space. CubeSats aimed to allow graduate students to design and test satellites and, for that end, a much smaller scale was needed. CubeSats fit into defined spaces on rockets carrying larger satellites launched by ESA and other organisations. A CubeSat consists of one or more standard units of 10 x 10 x 10 centimeters. Since their development a decade ago, CubeSats have become popular with universities and, more recently, space technology companies. “TU Delft can’t compete,” said Bouwmeester, who is part of Space Systems Engineering at the Faculty of Aerospace Engineering. Space technology companies, especially those based in the US, get up to hundreds of millions in venture capital funding. Universities research groups don’t have access to that sort of money and the field of CubeSat development is currently dominated by private industry. According to Bouwmeester, TU Delft had three choices: leave the field, develop only very niche satellite applications or go smaller. The 5 x 5 x 18 centimetre plastic prototype on his desk is the outcome of that decision. PocketQubes are yet smaller miniature satellites with a similar design model as CubeSats. At TU Delft, they are being developed in a method often found in software companies. “We took on a more agile approach,” said Bouwmeester. “Rather than very extensive testing and documentation, space itself serves as the ultimate test facility.” Some Earth-based testing is, of course, done on the satellites, with function, performance and for example on launch vibrations. “It would be extremely risky to other on-board satellites to have parts break off during the launch,” says Bouwmeester. Delfi-n3Xt model Despite their tiny stature, PocketQubes offer much to scientific progress. The simplistic design and inexpensive price tag means that young researchers and even students can work on them, creating an environment open to creative endeavours. PocketQubes are currently being used to test components and systems which will hopefully pan out for use on larger satellite applications. But, to some, their small size has also relegated them to the category of educational toys, rather than serious scientific instrument. Bouwmeester wants to change that perception. Bouwmeester’s work focuses on the platform itself. “We want to make this platform capable and reliable,” he says. Understandably so, as conducting repair work in space is generally impossible. “You can’t manually turn them off and turn them back on again,” Bouwmeester says of his satellites. His research focuses on developing satellites that won’t require hard resets or other repairs. 3D printing technology has allowed satellite designs to be easily prototyped. “If a student has an idea about how to improve the structural design, we can print it here on our 3D printer,” he says. “If we test it and it fails, that’s still good. It shows us the value of the existing design.” The majority of the work on PocketQubes at TU Delft is currently taking place within the Faculty of Aerospace Engineering, but Bouwmeester would like to broaden the research base. “We talking to researchers at other faculties and even companies about bringing their expertise to these projects,” he says. His research group plans to launch their first PocketQube in early 2018. After that, they anticipate launching these satellites one to two times per year. Photo by Marcel Krijger .

TU Delft students design new aircraft for last resort option of geoengineering

If global efforts to reduce carbon emissions fail and temperatures will rise, a last resort could be to turn to geoengineering methods, like the injection of aerosols in the stratosphere. This will produce stratospheric clouds which re fl ect part of the incoming sunlight . Students of the TU Delft have looked into the practical aspects of this option, including the design of a new aircraft to deliver the aerosols into the stratosphere and a rough cost estimation: 11 billion dollars per year. Solar Radiation Management Current models of the climate show that a possibility exists that the response of society to counteract rising global temperatures is not implemented fast enough or does not become effective fast enough to ensure temperatures remain within safe bounds. In that case, an intervention might be necessary to temporarily halt temperature increase until preventive long-term solutions are effective. Stratospheric geoengineering, more specifically, Solar Radiation Management (SRM), offers such a temporary solution. A possible implementation of SRM is the injection of aerosols in the stratosphere, producing stratospheric clouds which reflect part of the incoming sunlight. It mimics volcanoes by spraying sulfuric acid into the stratosphere. This thin, high, long-lasting haze reflects a little sunlight, keeping us cool. Sunset over Hong Kong a year after Mt Pinatubo erupted in the Philippines, showing how aerosols released by volcano’s can affect our environment (Credit: JackyR, Wikipedia) Last resort Students of the TU Delft (faculty of Aerospace Engineering) have produced a research report* to describe the preliminary technical and operational design of a fleet of purpose-built Stratospheric Aerosol Geoengineering Aircraft (SAGA) to deliver five megatons of aerosol per year to altitudes between 18.5 and 19.5 km to gain insight in the cost and impact of such a system. Dr. Steve Hulshoff, who supervised this student project: ‘I want to stress that this is not a solution we advocate to ‘solve’ the global warming issue. First and foremost, we need to reduce carbon emissions. If these global efforts fail, and temperatures reach dangerous levels, a last resort could be to use geo-engineering methods, like SRM, in order to temporarily halt the temperature increase. But only as a last resort.’ ‘There will also be severe drawbacks to SRM, like acidification of the oceans’, Hulshoff warns. ‘We will not see a blue sky as often as we do now, and ozone depletion, deposition through precipitation and climate effects other than temperature reduction, will inevitably affect the environment.’ Stratospheric Aerosol Geoengineering Aircraft Nevertheless, the group of students have looked into some practical aspects of the last resort option of SRM. The design of an aircraft delivery system for stratospheric aerosol geoengineering has provided valuable insights in the practical aspects of geoengineering. Their aircraft is designed for the job of geoengineering, and nothing else. It’s designed to go a lot higher but won’t be required to fly huge distances – so its range is only a little over half that of a jumbo. The high altitude and high payload requirements drive the design of all aspects of the SAGA mission. First of all, the operational scenario, employing a fleet of 344 unmanned aircraft, enabling 572 flights per day, is established with focus on the most efficient delivery of the required 5 Megatons of sulfuric acid aerosol to the stratosphere, which would theoretically be enough to halt temperatures rising. Isometric view of the SAGA aircraft, as proposed by the students High altitude In the plan of the TU Delft students, sulfuric acid will be ejected from the aircraft in gas phase to facilitate efficient aerosol particle formation. Transport of the aerosols at elevated temperature in combination with on-board evaporation enable gas phase dispersion, introduce a compelling power requirement. Aerosol dispersion will occur in the tropical region at altitudes between 18.5 and 19.5 km. The high altitude at which dispersion takes place, governs the aircraft design. This altitude demands efficient lift generation and considerably high thrust. The need for efficient lift generation results in a design featuring a combination with a wing surface area of 700 m2. The structural integrity of this long and slender wing is ensured with the help of a strut-braced wing design. Purpose-built engines are proposed to efficiently provide thrust and power to SAGA aircraft. Four engines, each providing over 600 kN thrust at sea level, facilitate this. Costs The costs of SAGA are estimated to amount to an initial capital cost of 93.9 billion US dollars and a yearly operational cost of 11 billion dollars, which are acceptable amounts considering the costs of global warming, according to the students. Prepare for the worst TU Delft Climate Institute is the place within TU Delft where climate researchers and climate research are brought together, ultimately to develop new scientific knowledge. Prof. Herman Russchenberg, director of the institute, applauds and stimulates the type of out-of-the-box thinking shown by the students, ‘because the world community is still not doing enough to keep climate change in check.’ According to Russchenberg we have to be very reluctant in applying techniques like these, as we don’t know their influence on the earth system, we haven’t considered legal en ethical aspects enough, and we may even increase the problem by temporarily masking temperature rise. ‘But there may come a time when we will actually be needing techniques like these, like it or not. The sooner we start investigating practicalities, potential pitfalls and consequences, the better prepared we will be.’ More information *The Design Synthesis Exercise (DSE) is the conclusion of the undergraduate part of the education in faculty of Aerospace Engineering in Delft. This report on SAGA was produced by one of the groups of students in the last edition of this DSE. Supervisor Dr. Steve Hulshoff, +31 15 27 81538, S.J.Hulshoff@tudelft.nl , http://staff.tudelft.nl/S.J.Hulshoff/ Director TU Delft Climate Institute Prof. Herman Russchenberg, +31 15 27 86292, H.W.J.Russchenberg@tudelft.nl, Science Information Officer TU Delft Roy Meijer, +31 15 2781751, r.e.t.meijer@tudelft.nl The report is available to journalists, please contact Roy Meijer.

Looking backward and forward with Gijs van Kuik

On 7 December 2016 professor of Wind Energy Gijs van Kuik gave his farewell speech titled 'Wind verwacht: zet je schrap' ('Wind expected: brace yourself') in de Aula (Auditorium) of TU Delft. During his nearly forty year career, Gijs van Kuik has worked all over the Netherlands but he has started and will end his career at TU Delft. "It is my university," he says. Gijs van Kuik came to TU Delft in 1969 to study aerospace engineering. “It was the only place where I could study airplanes.” He finished his master’s degree in 1976, a few years later than originally planned. “It was the 70s” he says “so I took some time off from studying to be part of the student movements.” But he soon returned to his studies and, after graduation, he began working in the newly formed wind energy consortium under the tutelage of Theo van Holten. “I was in the right place at the right time,” says van Kuik. The oil shocks in the 1970s brought about an interest in what we now call renewable energies but were then referred to as alternative energies. However it wasn’t long before a PhD position at TU Eindhoven called him to leave Delft. He completed his PhD in 1991 and he wondered, as many recently matriculated PhDs do, if there was life afterwards. Fortunately for van Kuik, there was. He spent nearly fifteen years working in industry before returning to academia and TU Delft. In fact, for several years prior to returning to the university full time he was working simultaneously at TU Delft and Stork Product Engineering. But ultimately, two agendas proved to be too much and he decided to make TU Delft his full time home. His research while at the university has focused on the development of rotor technology for use in wind turbines. The goal of which was, according to van Kuik, “to build more intelligence into the rotor.” Wind turbines, especially those off shore, are tremendously difficult to access and improvements in their longevity can reduce the cost of maintenance. Van Kuik had the opportunity to see for himself just how difficult maintenance can be when he spent time in the nacelles of some turbine prototypes while checking certification processes. “I was much younger than,” he says of his time climbing the sixty metre high structures. Although this research may have predominantly focused on wind turbine technology, van Kuik has had a few pet research projects over the years, including a nearly decade long quest to see a Russian scientist rightly credited for discovering a constant foundational to aerodynamics. Betz’s Law, so named for German physicist Albert Betz, shows the maximum power that can be extracted from the wind. Van Kuik thought that the law had been discovered at the same time by a Russian scientist, Nikolay Zhukowsky , and, after years of digging through Russian scientific texts, was able to prove Zhukowsky had published the law the same year as Betz. Van Kuik does not speak or read Russian and could only read the maths in the articles but that was sufficient to credit Zhukowsky with the discovery as well. During his tenure at the university, he has served as the scientific director of DUWIND, a multidisciplinary research institute focussed on wind energy. Interest in wind energy has increased dramatically, At his start the introductory wind energy course would see 5-10 students per semester. Now that course attracts over 200 students. Van Kuik plans to spend his retirement, in part, sculpting. He’s been sculpting since 2001 and got into the craft after deciding he needed to do something different from his everyday work. He wanted to do something with his hands, took a sculpting course and has been creating the large stone creations ever since. He’s even given one to the Faculty of Aerospace Engineering which was unveiled in October 2016. “It was a huge thing and someone must have it. There isn’t a better place for this one than the Faculty,” he says of the work which now sits in the lobby of the faculty building. Van Kuik looks back on his time with TU Delft fondly. “I’ll miss the students, being around young people keeps you young.”

‘We want to build aircraft as well as design them’

Aerospace expert Joris Melkert from TU Delft is one of the first four Education Fellows at TU Delft. The Delft Education Fellowships are awarded annually to lecturers who have made a substantial and valuable contribution to teaching at TU Delft. As part of the Fellowship, Melkert is going to involve students in building a real aircraft. Melkert explained, ‘Most of the problems facing the aerospace industry today concern the production side, so this is what we are going to focus on.’ The elective module that Melkert is developing fits into the faculty's ‘Pioneering Innovations’ project called ‘Building Aircraft’ and will be taught within the Master’s track Flight Performance and Propulsion and the ASM track. Melkert continued, ‘There's quite a gap between all the fantastic ideas thought up at this university and their actual implementation. As things stand at the moment, the aerospace industry focuses more on production than on design. I think lecturers should take this into account, so that our students are fully prepared when they graduate. They should hit the ground running .’ For the duration of the new module, Melkert is devising, the mezzanine in the Aeroplane Hall will be rearranged to simulate an aircraft factory. The Faculty is purchasing a construction pack for a VAN RV12. But this isn't just a game – a real aircraft will be built that can actually fly, and could be sold to a private party once it is finished. Melkert added, ‘It's a unique experience whereby students will feel what it's like to work in the strictly controlled environment of an aircraft factory. They will have to comply with airworthiness requirements, quality controls and the dynamics of working with colleagues with differing interests, while also trying to be highly innovative. In addition, they will eventually have to pass ‘their’ project on to another group.’ A challenging project The aim is to teach the module three times a year. It will last for 20 weeks and generate 6 ECTS. It is expected to start in February, providing places for around 15-20 students per round. Melkert clarified, ‘Students will be selected on the basis of a motivation video, their marks and an interview. We will also pay attention to the composition of the team: diversity is very important.’ Working as members of the team, students will not only develop good technical skills, but also gain a better understanding of the safety culture involved in producing and certifying an aircraft. In addition, they will be confronted with organisational issues, such as project and certification administration and ensuring that the project passes smoothly to the next team. Melkert concluded, ‘In short, it's a project to get your teeth into.’ As yet, no companies are involved in developing the module. ‘But,’ says Melkert, ‘that would be beneficial to both parties. We are at the beginning of a highly ambitious project and would welcome any ideas, questions and support. An initial investigation revealed that there is already a great deal of interest.’ Photo by Marcel Krijger

A guitar for the future

When Max Roest started guitar lessons at the age of four, he had no idea that, two decades later, he’d make one for his master’s graduation project at the faculty of Aerospace Engineering. Roest had long been a guitar player, even producing an album called ToneWood, which was released in 2012. He’s competed nationally in the Netherlands but ultimately, the guitar was a hobby and not a professional pursuit for Roest. After completing his bachelor’s degree in aerospace engineering at TU Delft and moving on to the master’s programme, Roest was in need of a thesis project. He was inspired by the winner of the 2010 TU Best Graduate Award. Maarten Kamphuis, an Industrial Design Engineering student, who created a training sword for Historical European Martial Arts (HEMA.) Kamphuis was a proficient HEMA longsword practitioner and Roest was attracted to the idea of combining work and his hobby. His first step was to contact Dr. Otto Bergsma, a professor in the Structures and Materials Department, who agreed to serve as Roest’s thesis supervisor. As acoustic guitar players, like Roest, are aware, while wooden guitars may produce a warmer sound, this comes at a cost. Wood is very sensitive to changes in temperature and humidity. Touring musicians know their acoustic guitars will sustain damages, even when they are being well-cared for. There are guitars made from composite materials available on the market already. But, as Roest describes, “Their sound is brittle and lacks the character of a wooden guitar.” So, inspired by a sword making IO-graduate, Roest proposed to complete his thesis on developing a composite material which would emulate the sound of a wooden guitar without the downsides of the fickle wooden material. “Wood is a lot lighter than most composites,” Roest says, “so that was the most difficult criteria to match.” He also needed a material that would match in stiffness and internal damping. He started with a polethylene material but had to abandon it due to issues with bonding. He also tried existing composite materials with foam layered in between but the damping wasn’t high enough. After three months of experimentation, he discovered that fibre-reinforced foam appeared to meet his criteria. Then, however, he had to design a testing method to verifying the acoustic properties of this new material. Fortunately, he met Farbod Alijani, a professor in 3mE who just so happened to have a master’s student starting in his group who was designing a similar testing method for another project. “I was very very lucky to meet Luka Marinangeli.” The two created a production method that resulted in panels which were very, very similar to wood. And not just any wood, moon spruce which is felled according to lunar cycles and is Roest's preferred material for his guitars. Merely producing the material did not prove sufficient for Roest’s own exacting standards. He wanted to building a complete guitar. With an estimated price tag of €10,000, however, Roest first needed to talked to Dr. Rinze Benedictus, Head of the Structural Integrity Group. “He supported me in exchange for also making the faculty a guitar.” Roest did and it now sits in his office in the Faculty of Aerospace Engineering. Once they secured funding for the project, Roest approached the technicians in the Delft Aerospace Structures and Materials Lab. “These technicians are underappreciated at the university, I never would have completed the project without their help.“ With their help, he created a guitar form out of a strong plastic and was able to attach the panels. In the interest of science, Roest tested his new guitar in the anechoic chamber also known as the “dead room” at the Faculty of Applied Sciences. It wasn’t his first trip to the space. He played in the room previously, using his traditional guitar. You can watch a video of that performance on YouTube. Museum of Sound II - Anechoic Room - Max Roest The project was more than just a fun challenge. Much of the high quality wood that is used in guitars is harvested in Alaska, in the United States, where deforestation has reduced the availability of wood. “At the current rates, we might run out of this high quality wood in ten years,” says Roest. While guitar-making is a fairly small portion of overall wood use, finding a viable alternative would be good for the industry.

Clark Borst: ‘Research and education need each other’

During the opening ceremony of the academic year 2016-2017 in the Faculty of Aerospace Engineering, Clark Borst, lecturer and MSc track coordinator of Control and Operations, was voted AE Teacher of the Year by the students. So what sort of teacher is he? How does he combine his teaching duties with his research? And how does he envisage the future of education? ‘Automation is a bit scary. It's impersonal, emotionless. Automation can help us to progress, but it can also make us stupid. Take graphic calculators. Pupils don’t always learn how particular answers are calculated. They skip that step. I want to create automation that makes people smarter , not stupid. This is what drives me. Learning to ask question Automation plays a major role in education. We have online education, and these days, students have fast access to all the information they need and are able to navigate their way skilfully through the digital world. But do they actually think about the information they receive? This is where our task as teachers lies. It is our job to help our students develop a critical attitude. It’s so important for their future, whether they choose a career in academia or leave university. A critical attitude will always stand you in good stead. One of the first things I try to teach students is that there's no such thing as a stupid question. To me, the stupid thing is not to ask questions. This is the atmosphere I try to create in the lecture room: one of openness. And they're allowed to laugh, too!’ Lectures becoming more valuable ‘Teachers can do great things with online education and media, although I have to say I'm not a great fan of Collegerama. I don't understand why you'd want to record a lecture just to broadcast it online. Personally, I think that lectures, and the interaction they generate, are more important than ever in today's digital, impersonal world. In a world where we are constantly bombarded with information, people are crying out for context and explanations. Students can learn books by heart, but does this mean they understand what they've read? I think that the most valuable aspect of media in education is the huge range of images we can access: we can make theory visual, which helps to make things clearer. I always thought that the best teachers were the ones who managed to simplify highly complex concepts. So this was my own ambition when I started teaching. I enjoy looking for simplicity in complicated material. For the course in Avionics, for example, I make films with animations. It's very time-consuming, but I enjoy doing it, and the films are really useful. I choose difficult subjects for these ‘tutorials’. Fortunately, the students seem to appreciate them, too.’ Boosting each other ‘Education is so important. This is a university after all! We are training the engineers of the future, and it is up to us to inspire a new generation. But education needs research to prevent it from losing touch. Your research can ‘feed’ your teaching with the latest relevant developments in the field. So it's the combination that actually generates added value. Conversely, teaching can also boost your research. We mustn't forget that we can learn from students by listening to the questions they ask. What do I do if I don't have the answer to a question? I say that I don't know, but I go away and start searching and come back with an answer at the next lecture. Difficult questions like this keep me on my toes, as a teacher and as a researcher.’ Dr. ir. C. Borst c.borst@tudelft.nl