Hermes II Avionics Redesign

Semester project completed with the EPFL Rocket Team in 2022.

Hermes II is a compact avionics architecture developed for a small experimental rocket intended for validation flights and data generation. The project focused on redesigning the electronics of the original Hermes platform into a smaller, more modular system with telemetry, GPS, multiple sensor boards, and improved integration inside the rocket.

Project summary

The avionics stack is built around one main PCB and three auxiliary PCBs placed in different parts of the rocket:

• Main PCB in the avionics bay
• Nosecone PCB for total pressure and wall temperature
• Static port PCB for static pressure and wall temperature
• Load cell PCB for thrust measurement signal conditioning

The design integrates:

• Teensy 3.5 microcontroller
• GPS
• XBee telemetry
• IMU and accelerometer
• pressure sensors
• thermocouple interfaces
• load cell instrumentation amplifier
• battery management and power regulation

System overview

Source in repository: docs/Hermes_Presentation.pptx

Main PCB

The main board had to fit inside a very constrained avionics bay while interfacing with the GPS, telemetry radio, IMU, pressure sensors, thermocouples, Altimax, Wildhorn sensor board, and auxiliary boards. It uses a 4-layer stack-up with dedicated ground and power planes.

GPS layout constraints

One of the most layout-sensitive parts of the board was the GPS section, especially antenna routing and the controlled-impedance microstrip connection.

Auxiliary PCBs

Nosecone PCB

Measures total pressure and wall temperature.

Static port PCB

Measures static pressure and wall temperature.

Load cell PCB

Conditions the load cell signal used for thrust measurement.

Main technical points

• Compact avionics redesign for a constrained rocket airframe
• Separation of avionics into one main PCB and three auxiliary PCBs
• Integration of telemetry and GPS
• Pressure, temperature, acceleration, and inertial sensing
• Load cell instrumentation for thrust measurements
• 4-layer PCB design under mechanical, thermal, and routing constraints
• Manufacturing through a combination of external PCB assembly and manual soldering

Results

According to the final presentation, the following subsystems were functional during project testing:

• GPS worked outdoors
• telemetry radio worked
• thermocouples worked
• pressure sensors, IMU, and Altimax were functional
• the accelerometer worked after replacing a faulty capacitor

Main issues and remaining work included:

• power supply malfunction
• some MAX31855 thermocouple interfaces failed
• connector orientation errors that were later corrected
• remaining validation work for the load cell amplifier and some Wildhorn sensor software

See slides “RESULTATS” and “CONCLUSION” in docs/Hermes_Presentation.pptx.

Repository structure

Hermes-II-GitHub/
├── README.md
├── .gitignore
├── docs/
│   ├── Hermes_Presentation.pptx
│   └── report.pdf
├── images/
│   ├── rocket_overview.png
│   ├── avionics_bay.png
│   ├── system_block_diagram.png
│   ├── main_pcb_top.png
│   ├── main_pcb_bottom.png
│   ├── main_pcb_signal_routing.png
│   ├── main_pcb_power_planes.png
│   ├── gps_layout_guidelines.png
│   ├── gps_microstrip_stackup.png
│   ├── nosecone_board.png
│   ├── static_port_board.png
│   └── load_cell_board.png
└── hardware/
    ├── main_pcb/
    ├── nosecone_pcb/
    ├── static_port_pcb/
    └── load_cell_pcb/

Opening the hardware files

The hardware design files are provided in KiCad project format. Open the corresponding .kicad_pro file in each hardware subfolder.