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Concepts and Considerations for Re-entry Experiments to inform Fragmentation Prediction for Destructive Entry
Abstract
The accurate prediction of major spacecraft fragmentation events is a main driver for the evaluation of the expected ground casualty risk posed by an uncontrolled destructive re-entry. It is attractive to study spacecraft fragmentation by flight experiment due to difficulties associated with studying the phenomenology on ground; many of these difficulties stem from the limited scale achievable in ground tests. The purpose of the present work is to establish a baseline set of requirements and associated mission concepts for European destructive re-entry experiments to inform predictive fragmentation. This study builds on European heritage for flight recorder technology (BUC and DOC) as well as experiences gained from recent ground test campaigns of spacecraft materials, subsystems, joining technologies and Design for Demise (D4D) concepting. Here we report on the selected concepts as well as the underlying justification.
... However, the processes effecting the space craft during re-entry are not yet fully understood, causing substantial uncertainties in predictions of demise and reentry trajectories. Referring to this it is evident that a synthesis between the well-experienced small satellite IRS group [2] and the IRS group dealing with atmospheric entry, aerothermal demise, in-flight sensors and relevant projects on ESA level is of high sustainability for a project as such [3] [4] [5] [6] . ...
The Institute of Space Systems (IRS) at the University of Stuttgart, which has successfully flown the small satellite Flying Laptop, and the Small Satellite Student Society (KSat e.V.) are developing a 3U+ CubeSat for innovative technology demonstration, atmospheric research and investigation of satellite demise. The Stuttgart Operated University Research CubeSat for Evaluation and Education (SOURCE) will be realised by master and bachelor students supported by PhD candidates, who gained experience in satellite design with FLP and is currently in its detailed design definition phase. The project includes graded lectures and seminars for each subsystem, workshops and bachelor and master theses. It is expected to be launched NET 2020 using a piggy-back flight opportunity. In particular, SOURCE will perform atomic and molecular oxygen measurements during its 1-2 years of operation and heat flux density distribution analysis shortly before its demise. This enables students to take part in an innovative hands-on project with scientific relevance regarding the demise of satellites. The re-entry sensor setup is one of the student developments and is designed with the aid of the numerical simulation "PICLas", developed by IRS and the Institute of Aero and Gas Dynamics at the University of Stuttgart. On the basis of the simulations, commercial and self-developed heat flux density sensors are selected. To complete the sensor package, photodiodes are used to analyse the tumbling frequency and detect traces of aluminium in plasma flows. The acquired data will then help to validate the numerical models. This enables an innovative approach to illustrate the correlation of theory and practice in space related engineering processes. This paper presents the structure of the project with a focus on student work in all aspects of the mission, in particular the payload subsystem. Finally, an outlook is given on future educational missions, which are currently under development.
An overview of the results of an extensive experimental campaign is given, in which eight spaceflight-relevant materials as well as segments cut from a Composite Overwrapped Pressure Vessel (COPV) were characterised with regards to their aerothermal demisability, using simulated uncontrolled atmospheric entry conditions generated in the Institute of Space Systems' (IRS) plasma wind tunnel facilities. The facilities, measurement techniques and procedures as well as the various test conditions employed are presented. Data extracted from the experiments include spatially and temporally resolved heating histories of exposed samples, visible and near-visible range boundary layer spectra, temperature-dependent spectral and total surface emissivities as well as extensive visual and physical observations and measurements. Due to the extensive scope of the activity, only an overview containing a limited selection of representative results is presented. On the basis of these findings, a brief discussion of each specimen type's specific demise-relevant behaviour and some observations of note are provided.
Four typical aerospace materials, including type 316L stainless steel, aluminium Al7075, grade 5 titanium Ti6Al4V and silicon carbide SSiC, have been subjected to high enthalpy air flows in the plasma wind tunnel facility PWK1 at IRS. The generated conditions are comparable to those experienced by spacecraft and space debris re-entering Earth’s atmosphere in an uncontrolled manner. Using the emissivity measurement facility (EMF) at IRS, temperature-dependent total and spectral and/or band emissivities are determined for each material prior to and following the respective experiments, employing a range of different contactless temperature measurement devices. Temperature ranges are selected to be relevant for destructive atmospheric entry scenarios. A general tendency is observed for surface emissivities to increase significantly as material specimens are subjected to oxidation, erosion and melt. For grade 5 titanium, the influence of the beta phase transition is found to be highly relevant to its emissivity.
Defunct, manmade objects in orbit regularly reenter Earth's atmosphere in an uncontrolled manner causing risk of both personal injury and property damage. To reduce uncertainty and improve our ability to predict surviving debris, impact time and impact location, reentry breakup dynamics and aerothermodynamics data is needed. The Reentry Breakup Recorder has demonstrated the ability to obtain inertial and thermal measurements during reentry that are pertinent to spacecraft breakup. Building on this concept, the present investigation explores the design space for this device and matures a smaller, lighter and more operationally flexible system, termed RED-Data2. This paper documents the conceptual design, modularity and operational benefits of RED-Data2.
An experimental and computational investigation of the unsteady separation behaviour of two spheres in Mach-4 flow is carried out. The spherical bodies, initially contiguous, are released with negligible relative velocity and thereafter fly freely according to the aerodynamic forces experienced. In experiments performed in a supersonic Ludwieg tube, nylon spheres are initially suspended in the test section by weak threads which are detached by the arrival of the flow. The subsequent sphere motions and unsteady flow structures are recorded using high-speed (13 kHz) focused shadowgraphy. The qualitative separation behaviour and the final lateral velocity of the smaller sphere are found to vary strongly with both the radius ratio and the initial alignment angle of the two spheres. More disparate radii and initial configurations in which the smaller sphere centre lies downstream of the larger sphere centre each increases the tendency for the smaller sphere to be entrained within the flow region bounded by the bow shock of the larger body, rather than expelled from this region. At a critical angle for a given radius ratio (or a critical radius ratio for a given angle), transition from entrainment to expulsion occurs; at this critical value, the final lateral velocity is close to maximum due to the same ‘surfing’ effect noted by Laurence & Deiterding (J. Fluid Mech., vol. 676, 2011, pp. 396–431) at hypersonic Mach numbers. A visualization-based tracking algorithm is used to provide quantitative comparisons between the experiments and high-resolution inviscid numerical simulations, with generally favourable agreement.
The recently introduced discipline of design-for-demise (D4D) is looking for technical solutions on different levels to promote the atmospheric demise of spacecraft and respective components in order to reduce the casualty risk on ground. Previously performed studies revealed that opening the outer satellite structure during re-entry as early as possible helps to improve the overall demise. Therefore, technologies to open and/or release external structural elements and spacecraft modules are needed. In order to get a better understanding of the behavior during re-entry of current structural joining technologies, tests have been performed in high enthalpy wind tunnel and static heat chambers. These were setup to be representative of a number of joining configurations utilized within satellite designs. Samples representing a broad range of options were prepared and tested in both chambers. An overview of test procedures and findings are presented here along with early conclusions and future activities. The samples exhibited a broad range of phenomena and it was seen that a number of different failure scenarios are possible dependent upon joining technology used along with heat flux profile and mechanical loads applied, among other influencing factors. The results from these tests will feed into the development of new demisable joining technologies for bread-boarding development and assist in designing similar tests in the near-future. These on-ground activities will help to raise the current understanding of satellite demise and the role that joining technologies play, therefore leading to more informed decisions regarding the ways to increase satellites break-up altitude in the future reducing the on-ground casualty risk.
Equations are presented which provide heating rates for objects reentering the Earth's atmosphere. The relationships are appropriate for estimating the survival or aerothermal breakup of a reentering vehicle. The relationships presented in this report predict heating which is an order of magnitude less than predicted by existing theory. The results presented in this report are based on numerous satellite reentry experiments conducted over the last three decades.
The breakup of two satellites (Vehicle Atmospheric Survivability Project) during atmospheric reentry is documented. The satellites were deboosted from orbit, and their subsequent reentries were observed by surface-based radars and optics as well as airborne optics. Numerous pieces of satellite debris were tracked, and, in some cases, their heritage identified. The breakup process substantiated the heating relationships derived from the VAST test, which is an order of magnitude less than traditional heating relationships above an altitude of 30 nmi.
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