FIX: changes Generic Motor exhaust velocity to cached property

[giovaniceotto]

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 (black rocketpy/ tests/) has passed locally
• All tests (pytest --runslow) have passed locally
• CHANGELOG.md has been updated (if relevant)

Current behavior

GenericMotor.exhaust_velocity is being decorated by funcify_method, which makes it (a float essentially) into a lambda defined Function. This causes everything that depends on it algebraically to become lambda defined Functions as well.

More specifically, this contaminates GenericMotor.mass_flow_rate, which in turn contaminates GenericMotor.propellant_mass (which actually includes an integral_function of the lambda GenericMotor.exhaust_velocity 🫣). Well, in the end, this contaminates Rocket.total_mass, which is used extensively by the Flight class during a simulation.

In summary, this use of funcify_method causes the Flight class to take forever (literally) to run.

New behavior

What can I say? Changing the decorator from funcify_method to cached_property makes the sun shine once again, solving everything.

Breaking change

• No

[phmbressan]

Thank you for going through the journey to discover the bottleneck in this class. I was postponing for some time. Great finds!

Won't even start about snowball effect that the Function is currently generating that slows down the code so much. I will only say that the sooner this is fixed, the better.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (8d6f244) 70.95% compared to head (e4e83ae) 70.92%.
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #497      +/-   ##
==========================================
- Coverage   70.95%   70.92%   -0.04%     
==========================================
  Files          55       55              
  Lines        9262     9262              
==========================================
- Hits         6572     6569       -3     
- Misses       2690     2693       +3     

Flag	Coverage Δ
unittests	70.92% <100.00%> (-0.04%)	⬇️

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[Gui-FernandesBR]

I swear that I tried but I honestly have no idea of how you discovered this error in such a chain of calls.

[Gui-FernandesBR]

Before merging, do you see any possibility for new unit tests here? It's quite complicated because the error was only happening due to a "snowball" effect.

[Gui-FernandesBR]

LGTM