Thesis (MScEng)--Stellenbosch University, 2013. / ENGLISH ABSTRACT: The use of electrodynamic tethers on-board satellites is an exciting scientific prospect. These conductive tethers provide the means for satellites to generate power and to do propulsion by electrodynamic interaction with the geomagnetic field. Although well researched in theory, the concept has not enjoyed much success in practice. This study aims to utilise low-cost CubeSats as experimental tool to verify many of the theoretical principles that govern the behaviour of conductive tethers in orbit. The study provides a theoretical background of the concept by evaluating past tether missions and analysing existing theory. A feasible application of an electrodynamic tether within the size and weight limitations of a Nano-satellite is formulated. Existing theoretical work is adapted to model the dynamics and electrodynamics of specifically Nano-satellites. Using these mathematical models, control and estimation algorithms are designed which would provide stable deployment of a tethered CubeSat pair and stable control of the orientation of the tethered system. To be able to implement these algorithms on a satellite mission, a prototype of a sensor capable of measuring the angle of the tether using a CMOS camera is designed and built. A hardware platform is built to test the deployment of the tether using an electric motor. Electronics are designed to control the operation of the camera, to do motor control, and to run control and estimation algorithms. Using the results obtained from the practical tests done on the hardware, and using the theoretical models and control algorithms designed, a full orbital simulation of the deployment was done. This simulation includes the performance of the deployment system, the electrodynamic performance of the tether in earth‟s plasmasphere, and the estimation and control algorithms to control the system. Different deployment strategies are analysed and their performance are compared. / AFRIKAANSE OPSOMMING: Die gebruik van elektrodinamiese toue aanboord satelliete is 'n opwindende wetenskaplike vooruitsig. Hierdie geleidende toue verleen aan die satelliete die vermoë om krag op te kan wek en propulsie deur elektriese interaksie met die geomagnetiese veld te kan doen. Alhoewel dit goed nagevors is in teorie, het die konsep nog nie veel sukses in die praktyk geniet nie. Hierdie studie het dit ten doel om lae-koste CubeSats aan te wend as 'n eksperimentele instrument om baie van die teoretiese beginsels wat geld vir die gedrag van geleidende toue in wentelbane te verifieer. Die studie bied 'n teoretiese agtergrond van die konsep deur die evaluering van vorige tou-missies sowel as die analise van bestaande teorie. 'n Uitvoerbare toepassing van 'n elektrodinamiese tou binne die grootte- en gewigsbeperkinge van 'n Nano-satelliet is geformuleer. Bestaande teoretiese werk is aangepas om die dinamika en elektrodinamika spesifiek van toepassing op Nano-satelliete, te modelleer. Deur hierdie wiskundige modelle te gebruik, is beheer- en afskattingsalgoritmes ontwerp wat stabiele ontplooiing van 'n verbinde CubeSat-paar en stabiele beheer van die oriëntasie van die verbinde stelsel sal verseker. Om hierdie algoritmes te implementeer op 'n satelliet-sending, is 'n prototipe van 'n sensor wat in staat is om die hoek van die tou met behulp van 'n CMOS kamera te meet, ontwerp en gebou. 'n Hardeware platform is gebou om die ontplooiing van die tou met behulp van 'n elektriese motor te toets. Elektronika is ontwerp om die kamera te beheer, motor beheer te doen asook om beheer- en afskattingsalgoritmes uit te voer. Deur gebruik te maak van die resultate wat verkry is tydens die praktiese toetse wat gedoen is op die hardeware, en deur gebruik te maak van die teoretiese modelle en beheeralgoritmes wat ontwerp is, is 'n volle wentelbaan-simulasie van die ontplooiing gedoen. Hierdie simulasie sluit die gedrag van die ontplooiingstelsel, die elektriese gedrag van die geleidende tou in die aarde se plasmasfeer, en die afskatting- en beheeralgoritmes om die stelsel te beheer in. Verskillende ontplooiingstrategieë word ontleed en hul gedrag word vergelyk.

Thesis (MEng)-- Stellenbosch University, 2013. / ENGLISH ABSTRACT: The next generation of CubeSats will require accurate attitude knowledge throughout orbit for advanced science payloads and high gain antennas. A star tracker can provide the required performance, but star trackers have traditionally been too large, expensive and power hungry to be included on a CubeSat. The aim of this project is to develop and demonstrate a CubeSat compatible star tracker. Subsystems from two other CubeSat components, CubeSense and CubeComputer, were combined with a sensitive, commercial image sensor and low-light lens to produce one of the smallest star trackers in existence. Algorithms for star detection, matching and attitude determination were investigated and implemented on the embedded system. The resultant star tracker, named CubeStar, can operate fully autonomously, outputting attitude estimates at a rate of 1 Hz. An engineering model was completed and demonstrated an accuracy of better than 0.01 degrees during night sky tests. / AFRIKAANSE OPSOMMING: Die volgende generasie van CubeSats sal akkurate orientasie kennis vereis gedurende 'n volle omwentelling van die aarde. 'n Sterkamera kan die vereiste prestasie verskaf, maar sterkameras is tradisioneel te groot, duur en krag intensief om ingesluit te word aanboord 'n CubeSat. Die doel van hierdie projek is om 'n CubeSat sterkamera te ontwikkel en te demonstreer. Substelsels van twee ander CubeSat komponente, CubeSense en CubeComputer, was gekombineer met 'n sensitiewe kommersiële beeldsensor en 'n lae-lig lens om een van die kleinste sterkameras op die mark te produseer. Algoritmes vir die ster opsporing, identi kasie en orientasie bepaling is ondersoek en geïmplementeer op die ingebedde stelsel. Die gevolglike sterkamera, genaamd CubeStar, kan ten volle outonoom orientasie afskattings lewer teen 'n tempo van 1 Hz. 'n Ingenieursmodel is voltooi en 'n akkuraatheid van beter as 0.01 grade is gedemonstreer.

Thesis (MEng)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: The main on-board computer in a satellite is responsible for ensuring the correct operation of the entire system. It performs this task using flight software. In order to reduce future development costs, it is desirable to develop generic software that can be re-used on subsequent missions. This thesis details the design and implementation of a generic flight software application for CubeSats. A generic, modular framework is used in order to increase the re-usability of the flight software architecture. In order to simplify the management of the various on-board processes, the software is built upon the FreeRTOS real-time operating system. The Consultative Committee for Space Data Systems’ telemetry and telecommand packet definitions are used to interface with ground stations. In addition, a number of services defined in the European Cooperation for Space Standardisation’s Packet Utilisation Standard are used to perform the functions required from the flight software. The final application contains all the command and data handling functionality required in a standard CubeSat mission. Mechanisms for the collection, storage and transmission of housekeeping data are included as well as the implementation of basic fault tolerance techniques. Through testing it is shown that the FreeRTOS scheduler can be used to ensure the software meets hard-real time requirements. / AFRIKAANSE OPSOMMING: Die hoof aanboordrekenaar in ’n satelliet verseker die korrekte werking van die hele stelsel. Die rekenaar voer hierdie taak uit deur van vlugsagteware gebruik te maak. Om toekomstige ontwikkelingskostes te verminder, is dit noodsaaklik om generiese sagteware te ontwikkel wat hergebruik kan word op daaropvolgende missies. Hierdie tesis handel oor die besonderhede van die ontwerp en implementering van generiese vlugsagteware vir ’n CubeSat. ’n Generiese, modulêre raamwerk word gebruik om die hergebruik van die sagteware te verbeter. Ten einde die beheer van die verskillende aanboordprosesse te vereenvoudig, word die sagteware gebou op die FreeRTOS reëletyd bedryfstelsel. Die telemetrie- en telebevelpakket definisies van die “Consultative Committee for Space Data Systems” word gebruik om met grondstasies te kommunikeer. Daarby is ’n aantal dienste omskryf in die “Packet Utilisation Standard” van die “European Cooperation for Space Standardisation” gebruik om die vereiste funksies van die vlugsagteware uit te voer. Die finale sagteware bevat al die bevel en data-hantering funksies soos wat vereis word van ’n standaard CubeSat missie. Meganismes vir die versameling, bewaring en oordrag van huishoudelike data is ingesluit sowel as die implementering van basiese fouttolerante tegnieke. Toetse het gewys dat die FreeRTOS skeduleerder gebruik kan word om te verseker dat die sagteware aan harde reëletyd vereistes voldoen.

Attitude control remains one of the top engineering challenges faced by small satellite mission planning and design. Conventional methods for attitude control include propulsion, reaction wheels, magnetic torque coils, and passive stabilization mechanisms, such as permanent magnets that align with planetary magnetic fields. Drawbacks of these conventional attitude control methods for small satellites include size, power consumption, dependence on external magnetic fields, and lack of full control authority. This research investigates an alternative, novel approach to attitude-control method for small satellites, utilizing the noncommutative property of rigid body rotation sequences. Piezoelectric bimorph actuators are used to induce sinusoidal small-amplitude satellite oscillations on two of the satellites axes. While zero net change occurs on these signaled axes, the third axis can develop an average angular rate. This noncommutative attitude control methodology has several advantages over conventional methods, including scalability, power consumption, and operation outside of Earth's magnetic field. This research looks into the feasibility of such a system, and lays the foundation for a simple control system architecture.

Variations in ionospheric plasma density can cause large amplitude and phase changes in the radio waves passing through this region. Ionospheric weather can have detrimental effects on several communication systems, including radars, navigation systems such as the Global Positioning Sytem (GPS), and high-frequency communications. As a result, creating models of the ionospheric density is of paramount interest to scientists working in the field of satellite communication. Numerous empirical and theoretical models have been developed to study the upper atmosphere climatology and weather. Multiple measurements of plasma density over a region are of marked importance while creating these models. The lack of spatially distributed observations in the upper atmosphere is currently a major limitation in space weather research. A constellation of CubeSat platforms would be ideal to take such distributed measurements. The use of miniaturized instruments that can be accommodated on small satellites, such as CubeSats, would be key to acheiving these science goals for space weather. The accepted instrumentation techniques for measuring the electron density are the Langmuir probes and the Plasma Impedance Probe (PIP). While Langmuir probes are able to provide higher resolution measurements of relative electron density, the Plasma Impedance Probes provide absolute electron density measurements irrespective of spacecraft charging. The central goal of this dissertation is to develop an integrated architecture for the PIP that will enable space weather research from CubeSat platforms. The proposed PIP chip integrates all of the major analog and mixed-signal components needed to perform swept-frequency impedance measurements. The design's primary innovation is the integration of matched Analog-to-Digital Converters (ADC) on a single chip for sampling the probes current and voltage signals. A Fast Fourier Transform (FFT) is performed by an off-chip Field-Programmable Gate Array (FPGA) to compute the probes impedance. This provides a robust solution for determining the plasma impedance accurately. The major analog errors and parametric variations affecting the PIP instrument and its effect on the accuracy and precision of the impedance measurement are also studied. The system clock is optimized in order to have a high performance ADC. In this research, an alternative clock generation scheme using C-elements is described to reduce the timing jitter and reference spurs in phase locked loops. While the jitter performance and reference spur reduction is comparable with prior state-of-the-art work, the proposed Phase Locked Loop (PLL) consumes less power with smaller area than previous designs.

<p>Secondary payloads have a rich and successful history of utilizing cheap rides to orbit to perform outstanding missions in Earth orbit, and more recently, in cislunar space and beyond. New launch vehicles, namely the Space Launch System (SLS), are increasing the science opportunity for rideshare class missions by providing regular service to the lunar vicinity. However, trajectory design in a multi-body regime brings a host of novel challenges, further exacerbated by constraints generated from the primary payload’s mission. Often, secondary payloads do not possess the fuel required to directly insert into lunar orbit and must instead perform a lunar flyby, traverse the Earth-Moon-Sun system, and later return to the lunar vicinity. This investigation develops a novel framework to construct low-cost, end-to-end lunar transfer trajectories for secondary payload missions. The proposed threephase approach provides unique insights into potential lunar transfer geometries. The phases consist of an arc from launch to initial perilune, an exterior transfer arc, and a lunar approach arc. The space of feasible transfers within each phase is determined through low-dimension grid searches and informed filtering techniques, while the problem of recombining the phases through differential corrections is kept tractable by reducing the dimensionality at each phase transition boundary. A sample mission demonstrates the trajectory design approach and example solutions are generated and discussed. Finally, alternate strategies are developed to both augment the analysis and for scenarios where the proposed three-phase technique does not deliver adequate solutions. The trajectory design methods described in this document are applicable to many upcoming secondary payload missions headed to lunar orbit, including spacecraft with only low-thrust, only high-thrust, or a combination of both. </p>

<p>  </p> <p>  </p> <p>An approach to modify surface flashover of insulators in vacuum by limiting duration of its high-current stage responsible for the damaging effects of a classic flashover was developed. The flashover assembly was made by TorrSeal-gluing copper electrodes (10 x 10 x 0.5 mm) to both side of an alumina ceramic sheet (0.635 mm thick). The modified flashover, referred to as low energy surface flashover (LESF), was achieved by utilization of a high voltage (HV) nanosecond pulser or addition of a resistor in series with the LESF assembly when HV DC was utilized. The duration of LESF was visualized by ICCD fast photography to be 100 – 200 ns accompanying electrical characteristics measurements, which gave insight of a way to control the flashover duration by inserting additional capacitor in parallel with the LESF assembly to increase the stored energy prior to breakdown. The LESF assembly was tested for > 1.5 million consecutive pulses and remained operational, while operation in high energy regime with parallel capacitor (4nF) lead to significant damage after 200 pulses.</p> <p>The igniting capabilities of LESF assembly was demonstrated via successful triggering of vacuum arc and a prototype pulsed plasma accelerator. The plasma plume propagation speed and angular distribution was measured via Langmuir probes. Efforts were made for temporally resolved spectroscopy measurements. </p> <p>The LESF assembly was improved by replacing TorrSeal-gluing with direct bonding of copper to alumina ceramic and changing the configuration from parallel plate to coaxial. The improved assembly was demonstrated to be operational throughout and after an extended test of 10 million pulses. A higher resolution ICCD photography revealed finer LESF discharge features including initial bright line across the insulator developing into a double-jet plasma plume propagating at around 10⁵m/s and later-on point-like attachment of the discharge column to the electrodes. The composition of the plasma and erosion pattern on the LESF assembly was studied via SEM/EDX analysis, which supported the predominant ceramic erosion over copper electrodes erosion.</p>

<p>    </p> <p>Additive manufacturing (or 3D printing) is one of the manufacturing processes which is currently being explored for its applicability under space boundary conditions, also known as in-space manufacturing. The space boundary conditions specifically affect material properties which in turn affect the printability of materials in space. Printing of soft materials in space is a novel application and the intent of this research was to print the softest of materials: edible materials, as a case study. 3D food printing is a novel food delivery method of using food products to either reproduce as a more aesthetically pleasing product or to print more nutrient-diverse foods. Launch of payload carrier and the boundary conditions of low Earth orbit including a vacuum environment, microgravity, temperature fluctuations, etc. These conditions make printing difficult, and my thesis is to overcome the boundary conditions (except microgravity) using a 3D soft material printer operating within a CubeSat. A CubeSat is a small satellite usually launched as an auxiliary payload used for basic Earth observation and radio communication. The printer must be able to survive launch and operation conditions, print within a simulated space environment, and adhere to the American Society for Testing and Materials (ASTM) specific definition of additive manufacturing. The 3D soft material printer was designed, fabricated, and tested using space and CubeSat boundary conditions for determining optimal design. Testing conditions including: (1) printing under Earth conditions showing it follows ASTM standards, (2) surviving NASA standards for vibration testing for microsatellites under launch conditions, (3) completing a print under a vacuum setting. The results of the testing would prove a small microsatellite could print in the vacuum of space and survive launch parameters. Further work would provide insight into the design of food printers being readily available in smaller sizes and its operability in microgravity condition. </p>