Welcome to the NLR Reports Repository
Royal NLR conducts research, which is published in reports. Only a minor part of these reports is made public. This repository holds these public, so called Technical Publications (TP), mainly produced from 1996 up till now.
Technical publications published before 1996 may be obtained using the request-form. Other types of NLR-reports are Contract Reports (CR) and Technical Reports (TR), which in general are not public. You may however request a copy by using the request-form. In some cases permission may be granted, depending on specific properties and contractor of the report. Reports will be supplied as pdf-file.
Recent Submissions
Item type:Item, End-to-end predict-and-optimize dynamic predictive maintenance planning integrating prognostics - the case of short-range electric aircraft with lithium-ion batteries(Elsevier, 2026) Oosterom, S.J.M. van; Mitici, M.Modern assets are continuously monitored by sensors. As a result, large datasets on the health condition of these systems are often available. Using supervised machine learning, recent studies have leveraged such data to generate remaining useful life (RUL) prognostics. Here, the focus of the machine learning regressors is on achieving prognostics of high accuracy. Once obtained, in a second stage, these prognostics are usually integrated into maintenance planning optimisation models. However, aiming for high accuracy prognostics in a first stage does not guarantee that the maintenance costs are also minimized in a second, maintenance planning stage. To address this, we propose an end-to-end, dynamic framework for the predictive maintenance problem that integrates the planning stage into the prediction stage. For this, the maintenance costs are directly estimated from sensor data, instead of being derived based on RUL prognostics. We apply our end-to-end framework for the maintenance planning of a fleet of electric Vertical Take-Off and Landing (eVTOL) aircraft equipped with Lithium-ion batteries. We show that, when compared to state-of-the-art prognostics-based maintenance planning, the proposed framework reduces the number of battery failures by 24% and the total maintenance costs by 9.4%. Overall, our framework proposes an effective, data-driven paradigm for an end-to-end predictive maintenance planning.Item type:Item, Assessing the Impact of Uncontrolled Space Object Re-entries on Air Traffic Management Operations(IAA, 2026) Chalabi, W.A.A.; Haagsma, A.; Badea, C.A.; Kok, J.T.A.With the growing number of activities in space, higher airspace, and conventional Air Traffic Management (ATM) domains, the likelihood of uncontrolled space object re-entries impacting airspace operations is increasing. Such events pose a potential safety threat to airborne traffic and challenge the efficiency of current ATM procedures. With the uncertainty of the exact time and location of a space object re-entry, and the velocities involved in orbit, it is proven difficult to have a specific and exact warning for an expected re-entry event. Recent real-world cases have shown inconsistent responses, ranging from complete airspace closures, causing significant disruptions and delays, to inaction, which may become untenable as space activity expands. This research addresses the question: How should ATM deal with uncontrolled re-entries from space? The study develops a use-case scenario in which a returning space object is detected shortly before its expected entry into Dutch airspace above Amsterdam Schiphol Airport, one of Europe's busiest hubs. A traffic simulation was developed to explore possible operational responses within this limited timeframe. Based on the simulation, a set of procedures is proposed to enable ATM to safely and efficiently manage such events. For instance, flights are categorized into critical and non-critical groups to facilitate prioritization, ensuring that aircraft most at risk or constrained by operational needs received immediate attention. In addition, the study identified the importance of providing controllers with visualization tools depicting the predicted impact zone and uncertainty area and time window, allowing for better situational awareness and decision-making under time pressure. The study concludes that a structured approach combining improved space object tracking capabilities, predefined risk assessment zones, and collaborative decision-making between ATM and space operations stakeholders can significantly enhance both safety and efficiency during uncontrolled re-entry events. Recommendations are proposed for integrating these procedures into existing ATM frameworks to better prepare for the increasing interaction between the space and aviation domains.Item type:Item, Measuring the spreadability of pre-treated and moisturized powders for laser powder bed fusion(Elsevier, 2020) Cordova, L.; Bor, T.; Smit, M.J. de; Campos, M.; Tinga, T.For AM processes—specifically Laser Powder Bed Fusion (L-PBF) processes—powder flowability is essential for the product quality, as these processes are based on a thin layer spreading mechanism. However, the available techniques to measure this flowability do not accurately represent the spreading mechanism. Hence, this paper presents two novel applicator tools specifically designed to test the spreadability of l-PBF powders in thin layer application. The results were checked by running standard tests to analyze the powder morphology, moisture content, chemical composition and flowability using the Hall-flowmeter. For this study, four common l-PBF metal powders were selected: Inconel 718, Ti6Al4V, AlSi10Mg and Scalmalloy. From the as-received state, drying (vacuum and air) and moisturizing treatments were applied to compare four humidity states and investigate the feasibility of pre-treating the powders to remove moisture, which is known to cause problems with flowability, porosity formation and enhanced oxidation. The tests reveal that AlSi10Mg is the most susceptible alloy to moisture and oxygen pick-up, considerably decreasing the spreadability and relative density on the build platform. However, the results also reveal how challenging the direct measurement of moisture levels in metal powders is.Item type:Item, Effects of powder reuse on the microstructure and mechanical behaviour of Al–Mg–Sc–Zr alloy processed by laser powder bed fusion (LPBF)(Elsevier, 2020) Cordova, L.; Bor, T.; Smit, M.J. de; Carmignato, S.; Campos, M.; Tinga, T.Laser powder-bed fusion (LPBF) technology is one of the additive manufacturing (AM) processes that uses metal powder to produce parts for various industry sectors such as medical, aerospace, automotive and oil & gas. As an ‘additive’ based process, the material is selectively melted by a focused laser. By this working principle material is added in a layer-by-layer approach only where is needed. Therefore, this technology enables a high reduction of waste by avoiding chips typically generated in ‘subtractive’ based processes such as milling and drilling. However, to ensure lower waste consumption the metal powder surrounding the solidified part must be reused in subsequent build jobs. Current knowledge on the effect of powder reuse on LPBF builds is mostly limited to titanium- and nickel- based alloys. The aim of this paper is to study the effect of powder reuse on Al–Mg–Sc–Zr, a high strength aluminium-based alloy, manufactured by LPBF. Here, powder properties such as morphology, composition, particle size distribution are studied of virgin (pristine) and reused Al–Mg–Sc–Zr powder. The mechanical properties of specimens made of virgin powder and after four build cycles are analysed and compared to assess the influence of a mixture of virgin and reused powder material on the consolidated material properties. In general, the powder does not present large differences in composition and morphology, only the reused powder presents coarser particle size distribution (PSD) as previously observed in other alloy compositions. The microstructure of the studied specimens is very similar unlike the porosity. The specimens built with reused powder show a few small micro-sized pores which do not show significant differences in the mechanical properties. In fact, the ultimate tensile strength (UTS) and elongation to break of specimens, respectively built with virgin and reused powder are 565 MPa, 13% and 537 MPa, 11%. Based on the obtained results, it is concluded that it is feasible to reuse Al–Mg–Sc–Zr powder in four subsequent build jobs with proper powder sieving and a rejuvenation step mixing 40% of virgin powder.Item type:Item, Promising first deployments of hydrogen-powered aircraft operations in the European commercial flight network(NLR, 2026) Scheers, E.; Andringa, J.Y.