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Purpose
Hermes Flight 2 failed due to aerodynamic forces at approximately 7s into flight. It is speculated that pitch-roll coupling ('coning') contributed to the demise of Hermes 2. To mitigate pitch roll coupling it is necessary to maintain a frequency separation between the pitch and roll modes. Pitch modes are energetic and stochastic depending on winds at liftoff, rail angle, vehicle parameters, and more. Roll modes depend mostly on fin attachment precision. A reaction wheel that could manage roll around the long axis of the vehicle could prevent coning from occurring during the flight of Hermes 3.
Requirements
The following requirements are self imposed
| | |
|---|
| Fail Safe | System | |
| Sufficient Momentum to maintain ωz <<< ωy | System | |
Design
Let us assume Hermes is a thin tube with a diameter of .155m and a dry mass of 44kg. Mass moment of inertia is
I = mr^2 which means that Hermes has a mass moment of inertia about the long axis of
.264275 kgm^2. Using a 1.5 FoS on the Hermes 2 roll rate the maximum angular momentum capacity of the reaction wheel is
L_{max} = 2.8 hz * 1.5 *.265kgm^2 = 1.113 kgm^2/s. In other words this reaction wheel will be designed to mitigate 1.5x the maximum roll rate of Hermes 2 at burnout.
A straightforward reaction wheel design would have maximum OD of 5.5 in or .14m. The 335kv motors can obtain a max RPM of 4800. Conservatively using 3500 RPM or 583.3 hz, we can calculate the necessary moment of inertia of the reaction wheel to be
.00191 kgm^2 which is a disk of 0.8kg or a thin walled tube of 0.4kg.
These design parameters were translated into CAD. A steel tube is used to provide most of the wheel inertia, with an aluminum support and a bearing set providing constraints. A PTFE wear ring keeps the disk from riding. A thin shim stock raceway protects wires running from avionics to the payload. All components except the bearing seat mechanically mount to a new aluminum bulkhead. The system has an optimized mass of 840g. Superior performance is almost certainly possible, given the wheel mass of 390g and the motor ass of 205, which comprise the only 'essential' components of the system.
Engineering Change Request
Contol Wheel ECR.docx
Hardware
Pyxida will send I2C commands to a Teensy 3.5 which will PWM Control 60A Hobbyking ESC units that drive Quantum MT motors. The system is powered by a 4s LiPo battery.
Testing
