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.

Leo Veldhuis

Pollution caused by aviation is increasingly affecting the climate. In order to improve the sustainability of the sector, there is a real need for more energy-efficient aircraft designs. This is the domain of Leo Veldhuis, Professor of Flight Performance & Propulsion (FPP).

Mirjam Snellen

If you can precisely identify the noise that an aircraft produces, you can make smart use of this information in designing sustainable aircraft. This is the aim of Associate Professor in Acoustics Mirjam Snellen and her colleagues in the Aircraft Noise and Climate Effects (ANCE) research group, who find it fun to explore acoustic puzzles.

Arvind Gangoli Rao

Flying with 65% lower CO2 emissions than the current generation of Boeings? And even 80% lower NOx emissions? This is possible, and Arvind Gangoli Rao, Associate Professor of Propulsion & Power, and his team have proved it.

Airplane maintenance at the speed of data

While your airplane is on its way to reach your destination, all airplane systems and components are also on their way, very slowly, from a healthy state to one of malfunction. Four tenure trackers from TU Delft envision that the terabytes of data a modern airplane generates each day can be used to determine the health condition of all airplane parts, from wheels and brakes to air-conditioning to structural integrity. They built a multi-industry collaboration resulting in a 6.8 million euros Horizon2020 grant for their ReMAP project proposal. It may pave the way for a paradigm shift in airplane maintenance and save up to 700 million euros in maintenance costs per year for Europe alone. Adaptive Maintenance Schedules When a light indicates something to be wrong with an airplane system, it often may not be clear which part of a (sub-)system is causing the error. “Using historical and actual airplane data, we can help pinpoint the root cause, saving time and money,“ says Bruno Santos, project leader. “More importantly, most maintenance in aviation is preventive, meaning that many systems and components are inspected while they are still in good health. We want to use health diagnostics and prognostics to switch to real-time condition-based interventions.” It is this switch to condition-based maintenance (CBM) that drives the researchers. They state that the thousands of sensors in a modern airplane, the accessibility and fast communication of the vast data obtained from these sensors, and the increasing capability of data analytics provide the ideal context its implementation. Bruno Santos: “We think the data generated can provide a reliable estimate as to the remaining useful lifetime of all airplane parts, reducing the need for manual inspections and allowing adaptive maintenance intervals.” Part of their research plan is to increase the amount of data generated even further by adding sensors to monitor the structural integrity of airplanes. Currently, airplane maintenance is pre-scheduled according to fixed intervals, with the intervals determined by flight hours, flight cycles or calendar days, whichever comes first. The maintenance effort varies from quick daily checks to several weeks for complete airplane overhaul. Deviations from the strict regulatory maintenance schedules are only allowed for non-flight-critical components. Even then it takes a lot of effort to convince the regulatory authorities that the same high safety level will be maintained using an alternative maintenance schedule. The researchers’ vision is, however, shared by the Advisory Council for Aeronautical Research in Europe (ACARE) which envisages CBM to be the standard for all new airplanes by 2050. “The idea of condition-based maintenance is not new,” says Bruno Santos, “but its application in aviation is minimal and there is no roadmap as to its implementation. We want to provide this roadmap and prove that the current safety level can be maintained or even improved.” Smart diagnostics and prognostics The ReMAP proposal states an estimated benefit to European aviation of more than 700 million Euro per year due to a direct decrease in maintenance costs, reduced unscheduled airplane maintenance events, and increased airplane availability. “There simply is too much information when we extend airplane health monitoring to a set of systems and structures. What we create are decision support systems for a reliable and consistent application of CBM. It will still be the human operator making the final decision.” The researchers want to develop off-line algorithms to enhance both diagnostics (something is broken, and it is most likely due to this sub-system) and prognostics (this sub-system or component may fail within a certain timeframe), dealing with the vast amount of data generated by an entire fleet of airplanes. These will be complemented by lean on-board versions of the algorithms that may allow for a quick check of the airplane conditions while flying, even supporting unscheduled maintenance planning during a regular stop-over. Increased understanding of the deterioration of systems and structures can furthermore lead to a significant reduction in airplane weight and systems’ complexity. In the long-term, the number of on-board backup systems may be reduced, and load-bearing structures may become less overdimensioned as a result of new airplane design philosophies supported by CBM. TU Delft as project leader The four tenure trackers submitted their proposal to the Horizon2020 program, which encourages research institutes and industry to bring innovative technologies to higher maturity levels. Bruno Santos, Wim Verhagen and Mihaela Mitici are from the group of Air Transport and Operations and Dimitrios Zarouchas is from the Structural Integrity & Composites group, all from the Faculty of Aerospace Engineering. “Our group focusses on all aspects of air transport operations and planning, from support to maintenance to crew,” says Bruno Santos. “The common denominator being that the research has to involve an airplane. Our view is that we need to deal more and more with data, reacting swiftly. The world is not deterministic, stochastic uncertainties need to be taken into account in the decision process.” The TU Delft will lead the project as they have already run several projects on this topic. The TU Delft researchers quickly assembled collaborators, involving major players in the European airline industry, such as the airline company KLM, the airplane manufacturer Embraer Portugal, the research center from UTC (the largest airplane systems manufacturer), and the multinational IT company ATOS. Several universities and research institutes joined the consortium as well, together with the support of three small and medium enterprises (SME). Most parties are new collaborators for the TU Delft team, but they were eager to join, often after the first contact. “The companies would not have joined if they didn’t believe we, as a consortium, can do the research and get it close to the market,” says Bruno Santos. “On the other hand, their input was very valuable for assessing the industry needs and the added value of the solution proposed. They also provided valuable contributions to the sections of the proposal related to the economic impact.” ReMAP is Bruno Santos’ first proposal as a project leader, and with immediate success. Less than ten of the more than 100 proposals submitted to the specific Horizon2020 call received funding. Four to five new PhD students will be involved at the TU Delft alone. The project will extend the concept of CBM currently being explored by AIRMES, an ongoing European project in which TU Delft is also participating, with Verhagen and Santos as involved researchers. “With ReMAP we provide a full package. We cover both systems and structures, we apply machine learning techniques to build a decision support tool for optimizing maintenance, and we perform a safety risk analysis to validate that maintenance reliability will at least be preserved if not improved compared to current standards.Even though we’ll put a flag on the horizon, there are still some challenges limiting the application of CBM in practice as envisaged in our project.” Lab tests and field tests The researchers will focus on twelve airplane systems, such as cabin air-conditioning, the auxiliary power unit, wheels and brakes. Systems data is provided by the collaborating parties. Together they will develop and apply machine learning techniques and physics-based models to build diagnostic and prognostic algorithms from the terabytes of data they’ll collect. During the project run-time these predictive algorithms will be validated in the lab, continuously improving them until final verification during an unprecedented six months field test. These field tests will focus on the KLM fleet of Boeing 787 airplanes (twenty per 2020) and KLM City Hopper Embraer 175 (ten per 2020). For structures the researchers will perform lab tests at TU Delft and the University of Patras, Greece, initially using basic stiffener panels and then increasing their complexity to curved multi-stiffener panels with ribs and fasteners. The focus will be on the composite elements of the structure of the airplane. They will continue the development of sensor technologies and optimize their placement with respect to the panels. Bruno Santos: “Structural health monitoring is an underdeveloped concept in practice. This project is a trial for how it could evolve in the future.” Workshops and a whitepaper The researchers plan to involve more airline companies and other stakeholders by organizing regular workshops, sharing their results and raising interest. “The scale factor is important for CBM,” says Bruno Santos. “The same airplane type is used by multiple airlines. Without sharing the actual data, the ICT-framework we will build can use all of it to increase the predictive power of our models.” At the end of the project they will publish their findings in a whitepaper. “It’s a roadmap. What we expect to be able to show is that it may be safe to relax some maintenance constraints, and how to safely implement CBM. Then it is up to the worldwide regulatory aviation authorities to discuss current regulations and the way to proceed towards the implementation of CBM.” The researchers will involve the European Aviation Safety Agency (EASA), local authorities, Airbus, Thales, and many other relevant stakeholders in the discussion of this whitepaper. Such regulatory changes take time. In twenty years’ time, however, your in-flight entertainment system may be working not because it isn’t broken or prematurely overhauled, but because its condition and remaining useful lifetime are meticulously monitored.

Fiery romance: a risk-model for sky lanterns

When it comes to romance, few spectacles can compete with a swarm of gently floating sky lanterns. They are especially popular on New Year’s Eve. Michiel Schuurman is also mesmerized by the sight. But as assistant professor in the Structural Integrity and Composites Group and as an instructor of the Forensic Engineering course he also sees the risks. Schuurman: “According to our measurements they reach much higher altitudes than allowed. They can even reach altitudes aircraft fly. And there is the risk of wildfires.” He and his colleague Derek Gransden performed experiments to model these risks. It can help authorities in evaluating existing sky lantern legislation. Not near an airport In the Netherlands a sky lantern is considered a toy and as such it is regulated by the Netherlands Food and Consumer Product Safety Authority (NFCPSA). The authority investigated sky lantern safety but according to Schuurman they limited themselves to the risks for the user and his/her direct surroundings. According to the recommendations of the NFCPSA, sky lanterns may only be launched when the wind force is less than three Beaufort and when the nearest (glider) airport is at least 15 km away. Schuurman: “This means that their use is limited to at most 20 days per year and to about one-third of the Netherlands. However, your wedding is today and ‘here’.” The fire brigade would like to see a nationwide ban on sky lanterns because of the fire hazard for forests, dunes and houses with thatched roofs. Such incidents have been reported but retailers claim to stick to the NFCPSA safety regulations. “What this discussion needs,” says Schuurman, “is clear data about the risks associated with the use of sky lanterns.” He and colleague Derek Gransden got to work. Hot air Schuurman: “Up to now our research was aimed at the vertical flight profile of sky lanterns. What altitude can they reach and how long do they remain airborne?” Archimedes’ principle dictates that a balloon is positively buoyed upwards by a force equal to the weight of the air it displaces. After correction for gravity, the vertical acceleration of the balloon follows from Newton’s second law: force equals mass times acceleration (F = m×a). The balloon furthermore experiences drag. The three key parameters in balloon performance are the shape and volume of the balloon and the weight of the fuel cell. The fuel cell determines the amount of energy available to heat the air inside the balloon. The volume and temperature of the heated air determine the lift. “Our model is based on formulas for hot air balloons published in the seventies. We applied a number of simplifications such as the assumption of a uniform pressure and temperature inside the balloon.” Bean bag beads Schuurman and Gransden performed experiments with five types of balloons. First they determined their volume. This proved to be more difficult than anticipated as sky lanterns are very fragile. They can’t be filled with water. “The solution was to pack them with polystyrene beads from a bean bag.” In addition, the minimal lift of sky lanterns is a challenge to measure. Schuurman: “We experimented for six months before settling on a simple approach that provided the best consistency. Sky lantern lift is best measured by attaching them with a light string to known masses atop a digital scale. Reproducibility was still an issue due to varying workmanship. We observed large variations in the flight profile due to the little extra weight of excess adhesive surface.” Schuurman and Gransden performed their experiments in a fireproof laboratory free from horizontal airflow. They monitored the temperature inside the balloon with an infrared camera while using a regular video camera to log the moments of ignition, lift-off and landing. “’Landing’ in our setup corresponds with the balloon reaching its pinnacle during an outdoor flight.” Surprized pilots “All balloons land within ten minutes after lift-off. But contrary to the findings of the NFCPSA, our model shows that a cruising altitude of more than 300 metres is common for sky lanterns rather than an exception.” On average they reach an altitude of 500 metres. That is the altitude where they interfere with small airplanes and with larger airplanes taking off and landing. “Many pilots told me about their encounters with sky lanterns. A single balloon will cause little to no damage to an airplane, but an entire swarm of sky lanterns can startle a pilot at a time when he/she needs full attention for a safe landing.” Fire hazard “Our first results indicate that the sky lantern discussion needs additional research.” Schuurman and Gransden would now like to validate their model in the open air, tracking the balloons using a drone. This would require special permission. The next step is to model the horizontal flight profile. They already have the data of wind force and wind direction in the Netherlands needed for this. Building a risk model is the final step. Even though manufacturers use impregnated paper, the greatest risk of sky lanterns remains starting a wildfire. “If I launch my balloon from this location,” says Schuurman, “where will it go, what kind of vegetation and buildings will it come across and can it still be aflame when landing?” Their experiments and those by the NFCPSA show that the latter is indeed possible. Together they used only a few dozen balloons while sales in the Netherlands were already in the order of a few hundred thousand balloons in 2009. Add to that the rise in sales at firework shops and online. Scientific and playful In the summer of 2017 Schuurman presented their research at the AIAA Balloon Systems Conference in Denver, Colorado. The venue was packed with NASA employees and other balloon researchers, their eyes twinkling. “Just imagine,” says Schuurman, “these people operate balloons for researching earth and possibly other planets. Our research into sky lanterns may appear playful, but it certainly isn’t less scientific. Our completed model could be used by the authorities to re-evaluate existing sky lantern legislation.” Read tips about using sky lanterns (Dutch only) https://www.brandweer.nl/brandveiligheid/wensballonnen

Kevin Cowan: ‘Rote memorization is not thinking’

Space Engineering lecturer Kevin Cowan wants to teach his students the essence of understanding. Since this September, the Aerospace Engineering Teacher of the Year Award 2016-2017 shines on the left corner of his desk. ‘You know, that’s not there to show off,’ Cowan says. ‘It’s there to remind me that it actually happened, that what I do is appreciated.’ Cowan earned his bachelor’s degree in Texas, USA and his master’s at the faculty of Aerospace Engineering at TU Delft, where he now teaches courses in Space Engineering. He also earned an MBA and worked in strategic and financial advice for numerous organisations, amongst which the British government. ‘It was interesting work, but at some point I started wondering: how can I make a difference in this world?’ He remembered how certain lecturers during his studies (amongst whom: Ron Noomen) inspired him in the classroom. ‘They didn’t just explain, they exposed the magic of a subject. It sounds cliché, but if you know how to light that kind of fire in somebody’s mind, they can face any challenge later on.’ Cowan decided he would do his part by teaching. Curious like Newton Cowan teaches among other courses the MSc courses Astrodynamics I and II with his colleague Eelco Doornbos. The main textbook, titled ‘Fundamentals of Astrodynamics’, is full of formulas and text. How do you go about teaching such a heavy-duty subject? ‘I start with something fundamental that all students can relate to. Then we peel the formula apart. Do you really understand what it is about? Or have you simply memorised the formula? Observing and memorising is useful, but it cannot help you to predict anything, to create something new. If we take an idea apart and understand all the different building blocks that it is composed of, we can use those blocks to build anything.’ Cowan refers to Newton. ‘Imagine you are sitting beside Newton. He is sitting in his office behind his desk with his paper and quill. There is a candle on the corner of his desk, lightning up the room. He gazes outside, up towards the stars, and wonders: ‘how does it all work?’ I try to teach students this ‘feel for physics’, the wonder and the essence of understanding it. You don’t actually need Google or a computer to grasp it.’ Bells and whistles ‘At first I was not very fond of online education,’ Cowan says. ‘It was just another way of delivering material to students, although for that it can be useful. In the meantime, I’ve come to appreciate its surprising benefits. Nevertheless, it can’t replace the human-to-human interaction in the classroom. We should be careful not to get distracted by all the bells and whistles of online tools.’ What is the added value of teaching in a classroom? Cowan: ‘There are three pillars in education: knowledge, skills and motivation. If someone has only some of the first two but tons of motivation, he or she can get very far. You can move mountains with motivation. Motivating students, actually connecting with them and getting into a real, thoughtful conversation cannot happen without direct contact. If you, as a teacher, are just transferring information to students, something a computer can do as well, you will be out of a job soon. No, you should be out of a job soon.’ Cowan doesn’t see a world in which education is only offered online. ‘That would be suboptimal,’ he says. ‘If we want to tackle the challenges we are now facing in the world, we need to work together. If we don’t, we are in a heap of trouble. Universities are the cauldrons of change. We need to step up and accept responsibility — for guiding students to be active, life-long learners rather than passive recipients of knowledge — and for guiding students to take a pro-active role in improving society and our stewardship of the planet, to use their knowledge, skills, and motivation to be a force for good.’

Being part of a DreamTeam: ‘This is a unique opportunity’

Team Ecorunner VII Aerospace Engineering student Paul Hulsman (23) is the Team Manager of the Eco-Runner Team Delft, one of the many DreamTeams at TU Delft. The team’s aim: to build the most fuel-efficient car possible. A new team is now being put together, and Paul reflects on his time with the Eco-Runner. ‘Not everyone at TU Delft realises it, but the D:DREAM Hall is a truly wonderful place’. The Eco-Runner Team Delft dates back to 2005. The first Eco-Runner Team was founded by three students from the Faculty of Aerospace Engineering. Since then, a new team has been put together every year, which then spends the following months continuing work on the Eco-Runner in the D:DREAM Hall (Delft: Dream Realization of Extremely Advanced Machines). The Delft team subsequently takes the improved Eco-Runner to compete in the Shell, a student competition held in June every year. Studying and a DreamTeam Paul Hulsman is Team Manager of the Eco-Runner Team Delft. He is studying the Wind Physics track of the European Wind Energy Master’s (EWEM) at the Faculty of Aerospace Engineering. Paul has already completed the first year of his Master’s. He then decided to take a year out to concentrate on the Eco-Runner (VII) and in September, he departs for Denmark to continue with the second part of his Master’s. Paul: ‘I was previously also involved in another DreamTeam: the Formula Student Team Delft. That was part-time, but I enjoyed it so much that I wanted to join another DreamTeam on a full-time basis. The aim of the Eco-Runner (building the most fuel-efficient car possible) really appealed to me, so that is the team that I applied to join’. Unique opportunity ‘I think you learn a lot in a DreamTeam that you are unlikely to learn elsewhere’, says Paul. ‘At the Faculty of Aerospace Engineering, we have the Design/Synthesis Exercise at the end of our third year, during which you also learn certain soft skills such as teamwork. But working together with students from other faculties is a different kettle of fish. Also because you realise that everyone tackles certain things slightly differently. During my studies, I had absolutely nothing to do with PR – now I am learning a lot about the field. Alongside learning to take a problem-solving approach to technical issues, you also have to learn how best to communicate with companies. And once I get a job after graduating, I cannot imagine my boss saying: “go and build a car”. This is a unique opportunity’. Challenges While every Eco-Runner Team continues to build on the design and expertise gained by the previous team, each new team is faced by fresh challenges; especially when a new Eco-Runner is built, as was the case this year. Paul: ‘One of our greatest technical challenges was to tailor the design of the Eco-Runner VII to a more dynamic track with tighter corners than on previous tracks. That makes fuel-efficient driving more difficult. The drivetrain had to be completely redesigned, which meant that the suspension also needed to be adjusted. So while you are always taking the Eco-Runner to the next step, some things just have to be developed from scratch’. The trial The Shell Eco-marathon was held in London in late May of this year. Each team is given four attempts as standard, and each attempt consists of ten laps. During the first attempt, the Eco-Runner VII suffered a puncture. During the second attempt, the top cover flew off due to strong winds. A screw came loose during the third attempt and in the fourth, the fuel cell began to leak. Unfortunately, the team were therefore unable to complete any of their attempts. Paul: ‘Luck was certainly not on our side this year. Nevertheless, it was fantastic to experience the atmosphere and meet the other teams. And I am hugely proud of our team’. The current record stands at 3,771 km, which is roughly the distance from Amsterdam to Rome and back. For the next generation Paul: ‘You can work on the Eco-Runner either part-time or full-time, but I would recommend full-time. In retrospect, I think that it is better to take a year out between the Bachelor’s and Master’s, instead of during the Master’s. Personally, I missed a degree of work experience during the first year of the Master’s. But that is perhaps because EWEM is home to lots of students who already have some work experience. In Denmark, for example, it is quite common to work for three or four years after completing your Bachelor’s before starting your Master’s, but that seems a bit inconvenient to me. A year out is perfect. You learn so much in such a short space of time, and it is an unforgettable experience. How many people can say that they have designed and built their own car?’ ---. Want to learn more about the Eco-Runner? Follow the Facebook page or visit the website . Interested in joining the team? Click here to apply.

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 ME 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.”