Stefanos Fasoulas

Methodology for the development of combined energy and attitude control systems for satellites

The Combined Energy and Attitude Control System (CEACS) is a promising technologically advanced concept for space applications which could replace the conventional separate Electrical Power System (EPS) and Attitude Control System (ACS). In the past years this system was proposed for larger spacecraft but until today neither the feasibility of CEACS on small spacecraft, such as the Micro/Nanosatellites, nor end-to-end system demonstration was performed. Additionally, the existing literature does not emphasize extensively mathematical models for a precise analysis of the CEACS. Therefore, the study presented here deals with an initial investigation of useful models that allow for a systematic analysis of CEACS independent of the satellite types. The concept discussed is based on a double counter rotating flywheel assembly in the pitch axis. For the presentation of the methodology, a reference mission is chosen to demonstrate the feasibility of CEACS on smaller satellites. The capabilities of energy storage and attitude control are especially demonstrated. The critical limitations are analyzed and the simulation results are presented. Finally, the general feasibility of CEACS on Micro/Nanosatellites is discussed.

The asteroid and comet impact hazard: risk assessment and mitigation options.

The impact of extraterrestrial matter onto Earth is a continuous process. On average, some 50,000 tons of dust are delivered to our planet every year. While objects smaller than about 30xa0m mainly disintegrate in the Earth’s atmosphere, larger ones can penetrate through it and cause damage on the ground. When an object of hundreds of meters in diameter impacts an ocean, a tsunami is created that can devastate coastal cities. Further, if a km-sized object hit the Earth it would cause a global catastrophe due to the transport of enormous amounts of dust and vapour into the atmosphere resulting in a change in the Earth’s climate. This article gives an overview of the near-Earth asteroid and comet (near-Earth object-NEO) impact hazard and the NEO search programmes which are gathering important data on these objects. It also points out options for impact hazard mitigation by using deflection systems. It further discusses the critical constraints for NEO deflection strategies and systems as well as mitigation and evacuation costs and benefits. Recommendations are given for future activities to solve the NEO impact hazard problem.

Approach for Combining Spacecraft Attitude and Thermal Control Systems

Coupling of existing spacecraft subsystems could be an alternative to reduce the emerging cost, decrease the system volume/mass and complexity, increase the reliability, and enhance the overall performance of future spacecraft. Therefore, a new concept to combine the attitude and thermal control systems is presented. The concept is based on electric conducting e uids used in a closed loop for the attitude and thermal control systems. The potential coupling is discussed for two alternative setups incorporating electromagnetic pumping and thermoelectric phenomena, respectively. The corresponding equations are derived and the end-to-end system demonstration is performed. The results demonstrate that the new concept could replace the existing subsystems using current available technology, especially the thermoelectric option, when used as a combined attitude actuator and thermal radiator.

PANIC – A surface science package for the in situ characterization of a near-Earth asteroid

This paper presents the results of a mission concept study for an autonomous micro-scale surface lander also referred to as PANIC – the Pico Autonomous Near-Earth Asteroid In Situ Characterizer. The lander is based on the shape of a regular tetrahedron with an edge length of 35 cm, has a total mass of approximately 12 kg and utilizes hopping as a locomotion mechanism in microgravity. PANIC houses four scientific instruments in its proposed baseline configuration which enable the in situ characterization of an asteroid. It is carried by an interplanetary probe to its target and released to the surface after rendezvous. Detailed estimates of all critical subsystem parameters were derived to demonstrate the feasibility of this concept. The study illustrates that a small, simple landing element is a viable alternative to complex traditional lander concepts, adding a significant science return to any near-Earth asteroid (NEA) mission while meeting tight mass budget constraints.

New Miniaturized and Space Qualified Gas Sensors for Fast Response In Situ Measurements

This paper gives a general overview of the development of new miniaturized gas sensors for in situ measurements of molecular and atomic oxygen, carbon dioxide, carbon monoxide and hydrogen applicable for space experiments and instruments. Due to space driven miniaturization and a reference-free measurement principle, these screen printed ceramic gas sensors enable in situ measurements of partial pressures and flow rates in physicochemical processes and environmental control systems (air renewal/control, fuel cells, control of medical/biological processes, human respiratory investigations, etc.). Also, first experiences have been gained in low pressure, under high vacuum and other extreme/unconventional environments, i.e. in pure hydrogen or pure oxygen conditions. The first space application is the space qualified sensor system FIPEX (Flux-φ(Phi)-ProbeExperiments) that has been in operation on ESA’s external platform EuTEF on the International Space Station for about 18 months. Based on some selected results, this paper is intended to illustrate the performed development steps and the broad applicability of these systems. Hence, measurement principles of the sensors, typical calibration characteristics and measurement ranges are presented for the different terrestrial and space related applications.

Development of a DSMC code on planar unstructured grids with automatic grid adaptation

A Direct Simulation Monte Carlo package on planar unstructured grids was developed at the Space Systems Institute. The new package includes grid generation tools a n d visualization aids to ,accompany the main DSMC code. The code can handle flow field as well as reservoir simulations of ‘gas mixtures of arbi t rary mona n d diatomic species with relaxation processes a n d chemical reactions. The general approach employed in this work alloivs a rapid problem statement for arb i t ra ry geometries. The solution process is supported by routines for the automatic adaptation of grids and boundary conditions. This article presents a brief summary of the software package and first qualification results.

Turbulent free jet theory for prediction of high-enthalpy and low pressure flows

This article presents an extension of the well-known turbulent free jet theory to high enthalpy and low pressure flows. These flows are produced in plasma wind tunnel facilities in order to simulate the first phase of the reentry of space vehicles. It is shown that the semi-empirical results obtained from turbulent free jet theory of cold gases are partly transferable to the high enthalpy flows. The theory is simplified in that way that it is possible to obtain analytical equations for the description of the jet behavior. The theoretical prediction according to this turbulent free jet theory is compared with different experimental test cases showing generally a good qualitative agreement An extensively studied high enthalpy nitrogen test case is taken for the quantitative comparison showing also a good agreement between theory and experiment

Numerical Simulation of Nozzle Flow into High Vacuum Using Kinetic and Continuum Approaches

Laminar nitrogen flow expanding through a conical nozzle into high vacuum is numerically reproduced and compared to available experimental data. As the gas density varies quickly by several orders of magnitude, leading to high rarefaction and thermal non-equilibrium, standard (continuum) CFD tools are not sufficient to accurately model the expanding flow. In the work presented here, the efficiency of Navier-Stokes solvers is to be exploited where applicable, supplying the boundary conditions for a kinetic Direct SimulationMonte Carlo (DSMC) solver to handle the domain of rarefaction and non-equilibrium. The hypersonic character of the flow suggests to attempt a pure downstream coupling. The validity of this approach is to be verified.

ATHOS DEFLECTION MISSION ANALYSIS AND DESIGN

An asteroid deflection mission is analyzed and designed based on a fictitious threat scenario. The mission objective is to prevent the collision of the virtual binary asteroid Athos with Earth on February 29, 2016. Two alternative techniques are investigated both aiming on the diversion of Athos trajectory rather than on its destruction. These techniques are an inflatable solar collector and a kinetic energy projectile. Based on the results of a precursor mission to Athos and the outcomes of technology feasibility studies, one technology will be selected for mitigation. Mission constraints are identified as the time of collision, the physical and orbital properties of Athos, and the technology readiness level of the envisaged mitigation techniques and underlying launch system and spacecraft technology. The last point is of high priority because of the short time span from time of detection (February 22, 2005) to launch date of the deflection spacecraft. Detailed mission schedules, ∆V analysis, mass budgets, payload analysis, and cost estimates are derived to assess the feasibility of both mitigation techniques. We show that Athos can be deflected with non-nuclear concepts. Further, the threat posed by Athos satellite DeWinter, which might separate during mitigation, is assessed in terms of Earth atmosphere entry analysis.

DSMC SIMULATION OF FLOW FIELDS WITH ADAPTIVE BOUNDARY CONDITIONS

An adaptive methodology to determine unknown boundary conditions, especially at outflow boundaries, within the Direct Simulation Monte Carlo method (DSMC) has been developed at the Space Systems Institute. The new algorithm has been implemented into the DSMC package LasVegas and has been successfully tested. A theoretical basis of the method, details of its implementation and results of computations using this approach are presented here.