...
The pipe diameters and the source pressure are decided by us also, and the flow velocity is determined from the pipe area.
Helios Pressure Drop Calculations
We know the C_v constant for the feed system components (valves, solenoids, and injector) and can calculate the pressure drop across those components. Since we want to achive a stagnation pressure at the chamber, there is a certain required source pressure.
Helios Tank Calculations
We know the weight flow rate and the burn time, so we know how much total propellant weight we need.