ENH: Tank Fluids with Variable Density from Temperature and Pressure

[phmbressan]

Pull request type

• Code changes (bugfix, features)

Checklist

• Tests for the changes have been added (if needed)
• Docs have been reviewed and added / updated
• Lint (make format) has passed locally
• All tests (pytest tests -m slow --runslow) have passed locally
• CHANGELOG.md has been updated (if relevant)

Current behavior

Only constant valued densities are supported by the Fluid class, which hinders the Liquid and Hybrid motor simulation accuracy.

New behavior

This PR aims to bring support for defining a Fluid with a density value that can be a function of Temperature (K) and Pressure (Pa). For instance:

>>> Fluid(name="N2O", density=lambda t, p: 44 * p / (8.314 * t))
>>> from CoolProp.CoolProp import PropsSI # external library
>>> Fluid(name="N2O", density=lambda t, p: PropsSI('D', 'T', t, 'P', p, 'N2O'))

So as to model the density evolution in time in the Tank simulation, the optional parameters temperature and pressure were added to the tank classes, which correspond to simulation/static fire data as a function of time one may have of these parameters.

Breaking change

• Yes
• No

Additional Information

Documentation could probably be expanded with a more throughout example of this feature and a description of the new modeling assumptions, but this does not block the review of the implementation.

[codecov]

Codecov Report

❌ Patch coverage is 87.59124% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.19%. Comparing base (f17893b) to head (568661d).
⚠️ Report is 24 commits behind head on develop.

Files with missing lines	Patch %	Lines
rocketpy/motors/tank.py	90.12%	8 Missing ⚠️
rocketpy/plots/fluid_plots.py	27.27%	8 Missing ⚠️
rocketpy/motors/fluid.py	97.50%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #852      +/-   ##
===========================================
+ Coverage    80.02%   80.19%   +0.17%     
===========================================
  Files           98      103       +5     
  Lines        12004    12730     +726     
===========================================
+ Hits          9606    10209     +603     
- Misses        2398     2521     +123     

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[Copilot]

Pull Request Overview

This PR adds support for variable density fluids in tank simulations by enabling the Fluid class to accept density as a function of temperature and pressure. Previously, only constant densities were supported, limiting simulation accuracy for liquid and hybrid motor tanks.

Key changes:

• Enhanced the Fluid class to support density functions based on temperature and pressure
• Added temperature and pressure parameters to tank classes for time-dependent modeling
• Updated tank inertia and volume calculations to use variable density functions

Reviewed Changes

Copilot reviewed 14 out of 15 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
rocketpy/motors/fluid.py	Converted Fluid from dataclass to regular class with variable density support
rocketpy/motors/tank.py	Added temperature/pressure parameters and variable density calculations
rocketpy/prints/fluid_prints.py	Updated print methods to handle variable density functions
rocketpy/plots/fluid_plots.py	Added density function plotting capabilities
tests/	Added comprehensive test coverage for variable density functionality
docs/user/motors/tanks.rst	Updated documentation with examples of variable density usage

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[Gui-FernandesBR]

very good PR, great addition.

I have a few minor comments.

[Gui-FernandesBR]

LGTM, great addition!!