August 2023
Fuel Pump Dev-Build and First Bench Tests
The Bandit fuel pump uses a brushless dc motor from the electric skateboard industry to power the shaft of a gear pump designed for high pressure hydraulics systems. The devices are mounted to a sheet metal bracket and the shafts are joined with a custom coupler. The gear pump is a positive displacement pump meaning that in all cases, each rotation corresponds to a fixed, known volume of fluid. This differs from most rocket engine centrifugal pumps where the flow for a given speed depends on the head-flow curve of the pump and the system curve. The discharge pressure can be predicted for a given speed and flow rate by knowing the resistance of the downstream system. For instance, I know the design flow rate to be 0.190 Kg/s and the desired discharge pressure is about 300psia, therefore I can solve for the orifice diameter that will build 300psia behind it at the desired flow rate using basic orifice equations.
For the first bench test I tried to spin up the pump using a 4S LiPo battery and an 80A unsensored ESC. 6S is the voltage I sized but I wanted to see if it would spin without spending the time to wire 2 3S batteries in series. The motor started cogging so I inferred that the brake torque of the gear pump was too high for that configuration. I uninstalled the motor from the pump and it was able to spin up fine. I reinstalled the motor and connected the ESC to my adjustable power supply instead of the battery. I set the output to 24VDC and the motor was able to start spinning but it struggled a little on startup. Unfortunately the wattage of my power supply is too low to run the motor at full power, but once I get the 3S batteries ready I should be able to get 24V with much more current. One factor that could be contributing to the difficult startup is that, even though my motor has sensor leads, the ESC I’m using is a sensorless design. My experience with ESC-BLDC systems so far has been with drones which require very little torque to start spinning. Once an unsensored motor spins up, the BEMF generated by the fast-spinning rotor is what tells the ESC how to time the phase steps. In a system where it is hard to get the motor started and it is spinning very slow or not at all, the ESC does not really know how to power the motor. This is why low RPM, high torque motors have internal hall effect sensors that the ESC uses to time the phases even at low speeds where BEMF is not detectable.
