I think, I got the main function of the tank farm nailed down pretty solid now, which is to fill the rocket with subcooled liquids. There are a lot of simplifications and guesstimates though. Feedback to improve something here, which doesn't take huge amounts of coding, are welcome. I probably won't make it much more complicated though. I wonder, if it's too much already for most people... anyway here are a few points regarding the current state of the simulation:
- Liquids go directly from one tank to one other tank. (you can only fill ship OR booster at a time)
- Liquids are moved by pressure differences, taking into account gas pressure and hydrostatic pressure, but no pressure losses due to flow.
- The pump is modeled very crudely. It just supplies a pressure head and its power is calculated with that and the flow rate. Not quite correct. But it only serves as an indicator anyway.
- The tanks have thermal resistance guesstimates which lead to hopefully a somewhat reasonable temperature increase over time or boil-off rate.
- In the sub-coolers nitrogen is boiled, cooling it and the liquid flowing through it down. This is pretty cool actually, and I should write something like a deep dive about it sometime.
- The methane subcooler is always automatically filled with nitrogen at 90 K, which makes it easier to keep the methane above freezing point. This is independent of what the nitrogen storage tank shows as temperature. (I wonder, how they actually do it. Are they running the incoming methane through a nitrogen heat exchanger to pre-cool it a bit? Would save a lot of nitrogen at least...)
- For simplicity the storage tanks have a fixed pressure setpoint which keeps their pressure always constant. A pressurization system using the water tanks and vaporizers might come later.
- I was wondering about automatic controllers for storage tank and methane subcooler pressure as well as the subcooler fill valves, but it might be too much. Also, without the controllers I feel you can learn much more about the way how everything operates.
- I made a helpful google sheet for the liquid properties.
- I used the sea level boiling point as default temperatures for the storage tanks. Is this reasonable or does anyone know if they have this stuff cooled and pressurized in there? Could save a lot of nitrogen if they pre-cool it!
One note regarding the visuals. You can quickly underestimate how long stuff takes. I spent hours just to make the subcooler vents look somewhat realistic, so that they react dynamically to the pressure and volume of emitted gas - and it still looks barely ok-ish now, and it runs terrible when you got close to it with all the overdraw. It's fun to work with this stuff though. But it's a full time job in itself!
Known issue (but not a bug!): Methane flow is very low. That's because below 90 Kelvin Methane freezes! The longer you are below that temperature the more the pipe freezes up and the lower the flow gets until it almost stops (it doesn't quite get to zero to be able to unfreeze it a bit quicker). Use the vent valve to adjust the boil-off pressure and thus temperature of the nitrogen in the subcooler.