Fluid_Aspect_Course_4_1

Fluid Aspect Course 4.1: Network Scope

Overview

• We try to keep networks as small (node count) as possible, because CPU demand increases as the cube of # nodes.
  • Break the vehicle’s fluid systems into separate networks.
• On the other hand, we can’t arbitrarily split fluid systems anywhere we want, because network-network interfaces can be unstable
  • Tightly-coupled interfaces are unstable.
• User preference also comes into play
  • Documentation & vehicle subsystem diagrams imply a certain organization, and they’ll typically prefer the models to match
• So there is a trade between small networks, stable interfaces, user expectations

Coupling & Stability

• Separate fluid networks are interfaced together so they can flow between them.

• The separate networks are in themselves stable because they’re solved as simultaneous systems
  • this is the main reason why we use simultaneous system solvers: for stability
• But the 2 separate networks are not solved together as one system. Instead they handshake pressure & flow information between them across a data interface with lag. This introduces instability.

• “Tightly-coupled”: System A has a large effect on B and vice-versa.
  • Relatively high flow rates between relatively small volumes:
    • volume (m3) / flow rate (m3/s) = (s), so you can think of the interface as having a time constant that relates to how fast the two networks equalize. Higher volume or lower flow rate increases the time and thus stability.
  • Example: a hatch between two cabin volumes. Hatches have large flow area and thus the cabin volumes are always at almost the exact same pressure. Any big pressure change to volume A causes a large flow and B instantly follows.
  • Tightly-coupled = BAD.
• “Loosely-coupled”: System A has a small effect on B or vice-versa.
  • Low flow rates between two big volumes, or between one big & one small volume
  • Example: O2 supply valve to the cabin. O2 flows at a relatively slow rate and so causes small changes in cabin pressure
  • Loosely-coupled = GOOD.

Vehicle breakdown examples

• ATCS:
  • All separate coolant loops as separate networks
  • Gas supply systems to gas-pressurized accumulators, if they’re simple and belong to a single loop, can be included in that loop’s network
  • Complicated gas supply systems that pressurize multiple loops should be a separate network
  • Loop-to-loop leaks should not be a consideration for combining two loops into one drawing — use external interfaces for that
  • never split a liquid loop into separate networks

• ECLSS:
  • ARS/THC ventilation loops typically in same network as cabin because splitting across loops is a no-no
  • Combined cabins of multiple docked vehicles:
    • Small passive vehicles (crew transport to ISS) usually owned by one big “master” network (ISS stack), smaller networks slaved to the master via shadow links
    • Multiple active vehicles that can’t be slaved/shadowed require special interfaces similar to external supply/demand but with extra logic to enforce stability. TS-21’s ISS US segment <→ RST is an example
  • PCS typically separate network(s) from cabin. If O2 & N2 are separate w/ no x-over, they should be separate networks
  • waste water busses & storage systems typically separate
  • Regen systems typically in separate networks because they deal with low flow rates & loosely-coupled interfaces

• Prop:
  • Fuel & Oxid sides typically together in one network because rocket links need to mix them in the combustion chamber
  • Ullage pressurization can be a separate network if its complicated but with a loosely-coupled interface in the tank supply

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