Ph.D. Dissertation Defense
Ananthalakshmy Krishna Moorthy
Co-Advisor: Dr. John J. Blandino, Associate Professor, WPI
Co-Advisor: Dr. Nikolaos A. Gatsonis, Professor, WPI
Co-Advisor: Dr. Michael A. Demetriou, Professor, WPI
Committee Member: Dr. Zachary Taillefer, Busek Co. Inc.
Grad. Committee Representative: Dr. Nikhil Karanjgaokar, Assistant Professor, WPI
A wide variety of scientifically interesting missions could be enabled by orbital flight altitudes of 150 – 250 km. This range of altitudes is defined as extremely Low Earth Orbit (eLEO) for the present work. The use of low-cost nanosatellites (mass < 10 kg) has reduced the cost barrier to orbital flight over the last decade and the present study investigates the feasibility of using primarily commercial, off-the-shelf (COTS) hardware to build a nanosat specifically to allow extended mission times in eLEO. CubeSats flying in the lower thermosphere have the potential to enable close monitoring of the Earth’s surface for scientific, commercial, and defense-related missions. The research proposes that the proper selection of primary and attitude control thruster combined with precise control techniques results in significant extension of the orbital life of a CubeSat in an eLEO thus allowing detailed explorations of the atmosphere. An estimate is made of the primary disturbance torque, due to aerodynamic drag using a high-fidelity calculation of the rarefied gas drag based on a Direct Simulation, Monte-Carlo simulation. The primary propulsion system consists of a pair of electrospray thrusters providing a combined thrust of 0.064 mN at 1 W. A trade study to select the best attitude control option reveals pulsed plasma thrusters operating at 1 W. Extended Kalman filter is used for orbital position and attitude estimations. The attitude determination system consists of sun sensors, magnetometers, gyroscopes serving as attitude sensing devises. The mission consists of two phases. In Phase I, a 4U CubeSat is deployed from a 414 km orbit and uses the primary propulsion system to deorbit to a mean target altitude of 244 km. Phase I lasts 12.73 days with the propulsion system consuming 5.6 g of propellant to deliver a of 28.12 m/s. In Phase II the mission is maintained till the remaining 25.2 g of propellant is consumed. Phase II lasts for 30.27 days, corresponding to a of 57.22 m/s. Using this approach, a primary mission life of 30.27 days could be achieved, compared with 3.1 days without primary propulsion.