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UT ASE 463Q - Structural Subsystem

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6.0 Structural Subsystem The CubeSat Structural Subsystem is made of a lightweight material that provides adequate interfaces to each other subsystem to ensure safe passage through all phases of the mission. The ease of fabrication and assembly, light-weight, and free space for the payload sensors, circuitry, and batteries are the key features of the CubeSat structural subsystem design. The structural subsystem also has the ability to accommodate multiple payload sensors integrated in the subsystem in a simple manner. This section begins with a discussion of the previous CanSat structural subsystem designs. Subsequent subsections will discuss the requirements and constraints for the new structural subsystem, followed by the options and evaluation of materials, and finally, the modifications to the current structural design. AutoCAD drawings of the structural components can be found in Appendix A. 6.1 Background The previous CanSat (summer 2002) was a cylindrical-shaped structure, 12.3 centimeters tall and 6.6 centimeters in diameter, and weighed only 166 grams, as shown in Figure 28. The structure alone accounted for 50% of the total weight of the CanSat and was made from aluminum because of its light-weight and high tensile characteristics. The structure consisted of two sub-assemblies: a cover and a frame. When assembled with 3 mm stainless steel countersink bolts, the structure became a monocoque design that provided rigidity. The top plate had holes for parachute lines and an antenna. The parachute chords were attached directly to a bolt connected to the main frame. The circuit boards were mounted on the frame and the transceiver was placed between thewalls of the frame. The frame of the structure provided extra protection for the expensive transceiver. The structural subsystem tests were conducted using various methods of vibration analyses, including static loading, and others. The final launch also proved that the CanSat design was able to withstand about 50 g’s of load. Figure 28: Previous Coke-Can size CanSat [Campbell and others, 2003]. The exterior of the CanSat design was strong; however, the interior setup lacked some planning. For example, incorrect temperature data was recorded because the temperature sensor was located next to an integrated microchip, see Figure 29. Furthermore, the location of the antenna (located on top of the CanSat, see Figure 30) created communication interruptions between the CanSat and the ground station. The communication interruptions occurred because the transceiver works on high frequencies that require line of sight communication. In addition, the off-center location of the centerof gravity and improper setup of the parachute resulted in a continuous spin of the CanSat during the descent phase of the mission. The spinning of the CanSat also aggravated the communication interruptions. Next, since the structure was not properly sealed on all edges, an excessive amount of dust entered the CanSat when the parachute dragged the CanSat on the floor of the desert. The dust in return contaminated the circuit boards and the sensors. In addition, on the final project day, the parachute deployment rate was approximately 60%, according to the previous CanSat group; this resulted in a free fall of several CanSats, from different Universities, and caused their total destruction on the launch day. Furthermore, the CanSat did not have any external ports or peripherals; therefore, the previous group had to open the CanSat frequently to change the batteries and to upload and download data. Figure 29: Location of Temperature Sensor Next to an Integrated Chip [Campbell and others, 2003]. Temperature Senosr Integrated ChipFigure 30: Location of Antenna on Previous CanSat [Campbell and others, 2003]. 6.2 Requirements and Constraints The objective of the structural subsystem for the CubeSat project is to provide a simple, sturdy structure that will survive launch loads, while providing an easily accessible data and power bus for debugging and assembly of components. Because of the size constraints of the CubeSat and small expense budget, this must be done with the philosophy of maximizing usable interior space, while minimizing the complexity and cost of the design. The design of the CubeSat conforms to the structural and launcher requirements set by the Stanford/Calpoly CubeSat program. The shape of CubeSat is essentially a cube, with outer dimensions of 10 x 10 x 10 cm, with 3.0 mm clearance above each face of the cube for mounting exterior components such as antenna, data link and power charger inlet port. The satellite must have four launch rails along four edges Antennaof the cube, allowing for easy ejection from the P-POD (Poly Picosatellite Orbital Deployer) launch tube, shown in Figure 31. To maintain spacing and prevent sticking with other CubeSats, standoff contacts or feet must exist at the ends of these rails; therefore the four rails are extruded by 5 mm on all ends. The center of mass of the CubeSat must be within ±2 cm of the geometric center. The maximum allowable mass of CubeSat is 1 kg, and it is desired that the structure be no more than approximately 30% of the total CubeSat mass, and should be able to withstand a minimum of 50 g’s load [Wells, Stras, and Jeans, 2003]. The structural subsystem shall have an external power-off switch, such that when pressed should lie flush with the surface. The structure should be assembled with flat head metal screws and all sides should be sealed properly. The structure should also be able to pass harmonic and random vibration tests. There must be two holes, one on each diagonally opposite guide rail to connect the parachute chord. A hole will be carved on the lower surface to place the flexible antenna. The suggested material for the main satellite structure is Aluminum 7075 or 6061, Stainless Steel, Titanium, Composites, and Honey Comb. If other materials are used they must have the equal or more value for thermal expansion and yield strength as the aluminum.Figure 31: Poly-PicoSatellite Orbital Deployer [“About AAU CubeSat,” 2003]. 6.3 Material Options and Evaluation As suggested in the previous subsection, several materials were considered before selecting the final material. The criteria for selection were based on characteristics listed below:  Strength  Weight  Machinability  Cost Table 15 lists several materials along


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UT ASE 463Q - Structural Subsystem

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