The propulsion system has three objectives. 

• The first objective is to provide the UW Dawgstar with attitude control, which involves torsional disturbance rejection and angular positioning. 

• The second objective of the propulsion system is formation keeping. To maintain a formation with the other satellites, the propulsion system must have the ability to reject translational disturbances and reposition the UW Dawgstar to the correct position when the satellite drifts out of the formation. 

• The third objective is to provide the necessary DV for the orbital maneuvers throughout the mission as determined by the mission-plan, such as changing from one formation to another.

The propulsion system requirements are determined by mission requirements and derived requirements imposed by other subsystems. The requirements may be divided into three categories; mass, power, and performance requirements. A propulsion system must be chosen such that the mass and the power consumption of the propulsion system are minimal and the performance of the system is adequate for the mission

Mass.  The mass requirement for the UW Dawgstar is less than or equal to 10 kg. Of the 10 kg, the Propulsion system is allocated a mass of 1.5 kg. Since propulsion is also used for attitude control, a portion of the Attitude Determination and Control System (ADCS) mass budget of 1 kg may also be used for propulsion.

Power.  Peak power, the duration of power consumption, and energy consumption are the important elements to the overall power criteria. The power requirement is one of the satellites’ limiting factors.

Performance.  Three parameters determine the performance of a propulsion system. These three parameters are thrust (T), minimum impulse bit (I_bit), and specific impulse (I_sp). Given propulsion system hardware, the thrust can be measured experimentally. From these experimental data, the average thrust can be calculated. 

Although a wide variety of propulsion systems exist, only two types of propulsion systems are suited for the UW Dawgstar due to the mission requirements. These two options are micro pulsed plasma thrusters (mPPTs) and cold-gas propulsion systems.

Micro Pulsed Plasma Thrusters.  A typical pulsed plasma thruster consists of two electrodes, a solid Teflon© propellant bar, an igniter (spark plug), a feed spring, a power supply, and a capacitor. The power supply charges the capacitor, which is connected to the two electrodes. When a small plasma puff from the spark plug is released between the electrodes, the puff creates a low-resistance arc path, discharging the energy stored in the capacitor. This arc ablates a small amount of the Teflon© propellant bar and turns part of it into plasma. The current flowing through the arc also creates a magnetic field, and the resulting J×B force accelerates the plasma away from the thruster, thus generating thrust.

Cold-gas Propulsion System.  Cold-gas is a more traditional propulsion system. It provides thrust by expanding high-pressure gas through a nozzle. Some of the components required for the system are tank, tubing, filter, pressure regulator, valves, and thrusters. The high-pressure gas is stored in a tank.

Micro PPTs were chosen as the design choice for the UW Dawgstar propulsion system.  It currently consists of eight thrusters, four capacitors, eight discharge initiation circuits with igniters and one power-processing unit.  For the mPPT that is used on the UW Dawgstar, several performance characteristics are based upon scaling down the performance of larger PPTs. The specific impulse of a full-size PPT is approximately 800 seconds. Due to the reduced size and power of the mPPT, the I_sp is assumed to be about 500 seconds, a conservative estimate. 

The effective thrust of a single thruster pulse is about 70 mN at a 1 Hz firing rate.

Each thruster’s dimensions depends on the limits of the satellite dimensions and the amount of propellant in the thrusters. Four thrusters are mounted on the sides of the nanosatellite, and four thrusters are mounted at corners. The configuration was chosen to enable three-axis attitude control that is described in detail in Section 7.4.5. For the four thrusters mounted on to the sides of the nanosatellite, the total thruster length is 7.5 inches, as shown in Fig. 6.3. For the corner thrusters the length is 8.3 inches, as shown in Fig. 6.4

The volumes of each DI circuit and capacitor housings are estimated from the capacitor dimensions, which are approximately 2 ´  1 ´  0.5 inches. The volume of the two DI circuits is unknown, but is estimated by adding two inches to the length and height and 1.5 inches to the thickness of the housings. The DI circuit and capacitor housing thus results in a box that is 4 ´  3 ´  2 inches made of aluminum sheets of 1/16 inch thickness.

For the centralized power-processing unit, the dimensions are estimated from the PPU that Primex used for the Earth-Orbiting 1 (EO-1) mission. The EO-1 PPU consisted of the converter and two DI circuits. The PPU for the Dawgstar will not have the DI circuits in the PPU, but will have four high voltage switches. The footprint of the PPU board is estimated at 4 ´  6 inches and the height of the box will be determined by the transformer’s height, estimated at 2.5 inches. The housing material will be made of aluminum sheets of 1/16 inch thickness.

The mPPT was chosen over the cold-gas thruster due to its superior characteristics for the UW Dawgstar’s mission. The mass of propellant required per DV is low for the mPPT system, which means that the total propellant mass required over the mission lifetime is lower than it would be for a cold-gas system. The fine thrusting capability of the mPPT allows the satellite to perform fine control for attitude control and formation flying. The mPPT technology is in its early stages of development, but in the future mPPT technology may become an industry standard on nanosatellites, due to mPPT’s small size and low mass.