STRUCTURAL DESIGN AND LAYOUT
This satellite is designed with an open layout, without an outer shell. This is due to the weight limit, as well as ease of access to the different component sleds. As well, certain sensors need access to the open air for greater accuracy, such as the barometer and temperature sensor.
Internal Structure and Configuration
The satellite is designed in a modular fashion, with three component sleds mounted onto the main internal structure. These sleds are the avionics, camera, and battery mounts. They are attached to the inner structure with various M3 bolts. The internal structure itself consists of three carbon fibre rods connected to the mounting points on the top and bottom plates, with the bottom plate being designed to additionally hold a mount for the DC motor.
Our main on-board computer will be a single, proprietary board designed for this mission. We believe using proprietary avionics is the best solution as it is the most space and cost efficient. Additionally, using proprietary electronics will give us the best understanding of the systems on the satellite. The computer is a 50mm x 30mm, 4 layer PCB manufactured by JLCPCB. Outer copper layers are 1oz, inner layers are 0.5oz. The PCB is manufactured using a high temperature, flame resistant FR4 material.
Power is sourced from a 7.4V 2S 900mAh LiPo battery. A Diodes Inc. AP62300 3A synchronous buck converter is used to step down 7.4V to 5V. The telemetry transceiver runs off of the 5V supply. A 3.3V linear regulator is used to regulate the 5V down to 3.3V which is used for all of the 3.3V logic level devices. The flight computer has solder bridge pads which give us the option to select 5V or 3.3V for telemetry, and 7.4V or 5V for the camera. This allows us to be able to work with a wider range of devices, if needed.
Descent & Recovery System
After considering many designs, we landed on a simple 12’’ parachute attached externally to the forward plate of the satellite. A passive design was chosen due to the reduction of potential ejection problems that may be introduced by an active deployment system. The Adafruit PA1010D GNSS module will be used to record and transmit position, velocity and coordinate data for recovery operations. This GPS module is capable of communicating with GPS, GLONASS, GALILEO and QZSS constellations. The module is positioned on the top of the RotaSat to put the built-in patch antenna in the orientation most optimal for satellite connectivity.
The Bosch BMP388 is our chosen pressure, temperature and altitude sensor. This sensor has a pressure accuracy of ±0.08 hPa and a temperature accuracy of ±0.5 °C. The pressure data can be used to estimate altitude to ±0.5 m. The STMicroelectronics LSM6DSLTR inertial measurement unit (IMU) contains three low G accelerometers (±16G Max Range), three gyroscopes (±2000°/s Max Range), and a temperature sensor with lower accuracy compared to the Bosch BMP388. The data from this IMU will be recorded and transmitted, the temperature data used as a backup to the Bosch BMP388. The Rohm Semiconductor KX134 high G accelerometer unit is included on the computer to measure acceleration during the peak Gs of the powered ascent phase. The accelerometers in this unit have a max range of ±64G. MRobotics SiK telemetry transceivers on 915mHz are being used for communications with RotaSat. The transceiver on the satellite is connected to a 2dBi 915mHz whip antenna. A Micro SD card will be used to store flight data in a .csv file.
The reaction wheel will be driven by a F130 DC motor from OSEPP Electronics, with a calculated 0.0014875 N*m of torque, it will have more than enough authority to exert control over the 0.35kg craft in 40km/hr winds. The reaction wheel is a 30-gram aluminum wheel, turned on a manual lathe to a 52mm diameter. It will be fixed to the shaft of the DC motor using set screws and incorporated into the bottom of the satellite.The motor will be powered by the Texas Instruments DRV8871 H-Bridge onboard the flight computer using 7.4v and commanded by the active control software. A RunCam Hybrid 2 will be mounted on RotaSat to capture 4K @30fps video in flight. The RunCam will be powered with 7.4v through the main computer. The computer has the ability to turn the RunCam on and off.
The required primary mission of RotaSat is to log temperature and air pressure data to an SD card in flight after separation from the launch vehicle. This data will also be transmitted to the ground at 915mHz to be displayed on our ground control station (GCS). RotaSat will also log and transmit data in the powered ascent phase of flight. In addition to the required temperature and pressure data, we are extending our primary mission to transmit additional data points including orientation, angular rates, accelerations, avionics voltage, GPS coordinates and system state.
The secondary mission is based on the active attitude control systems of every satellite that is launched into orbit. Having control authority over the craft is integral to many missions, even ones in the atmosphere. Here, we aim to be able to redirect where the camera is pointing in real-time. To do so, we have designed a thirty gram aluminum reaction control wheel to be incorporated into the bottom of the satellite. With a calculated 0.0014875 N*m of torque, it will have more than enough authority to exert control over the 0.35kg craft in 40km/hr winds. We originally had the idea of recording wind speed and developing an atmospheric model, but that has been relegated to a “tertiary” mission.