ENH: Introduce Net Thrust with pressure corrections

[kevin-alcaniz]

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 tests -m slow --runslow) have passed locally
• CHANGELOG.md has been updated (if relevant)

Current behavior

RocketPy currently does not take into account the thrust dependence on the ambient pressure.

It's a important behaviour considering two situations: correction for thrust curves obtained from a static hot fire test performed in a altitude much different from the launch or in-flight corrections for high target altitude flights.

This PR is related to an Issue: ENH: Add pressure thrust #769

New behavior

The theory applied to this method is explained in the image:

• Motor classes:

A new optional attribute, reference_pressure, has been introduced. If not defined, RocketPy's behavior remains unchanged. Given the reference pressure and the thrust curve, the vacuum thrust curve can be computed using the new get_vacuum_thrust function.

• Flight class:

If a reference pressure is provided, a new method is implemented to calculate the net thrust. Otherwise, the net thrust remains equal to the current thrust.

Additionally, a new funcify_method has been created for net thrust, and thrust power is now calculated based on net thrust as well.

Breaking change

• Yes
• No

Validation

The results obtained after implementing this new feature are shown below:

The Y-axis represents Thrust (N) and the X-axis represents Time (s). As you can see, at the very beginning, both curves (raw and net thrust) are the same because the reference pressure is set to sea level pressure. As the rocket ascends, the net thrust increases because the negative term in equation 5 decreases.

I have also attempted to validate this behavior with ASTOS, and the results obtained are shown below:

In this plot, the vacuum thrust curve (blue) is input directly into ASTOS. The net thrust curve (black) is evaluated by ASTOS. Finally, the calculated thrust curve (red) was evaluated by me using the same method implemented in this PR.

[kevin-alcaniz]

There are other modifications related to another PR (BUG: fix the wind velocity factors usage and better visualization of uniform distributions in Stochastic Classes #783)

[codecov]

Codecov Report

Attention: Patch coverage is 82.14286% with 5 lines in your changes missing coverage. Please review.

Project coverage is 79.10%. Comparing base (4df0b38) to head (e8215df).
Report is 1 commits behind head on develop.

Files with missing lines	Patch %	Lines
rocketpy/motors/motor.py	58.33%	5 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #789      +/-   ##
===========================================
- Coverage    79.11%   79.10%   -0.02%     
===========================================
  Files           96       96              
  Lines        11575    11589      +14     
===========================================
+ Hits          9158     9167       +9     
- Misses        2417     2422       +5     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[Gui-FernandesBR]

Another great contribution made by @kevin-alcaniz , thank you!!

We are going to review it completely as soon as possible

[phmbressan]

Amazing work @kevin-alcaniz ! I made some comments regarding the split into two thrust attributes that I would like to hear some second opinions from the team before proceeding; of course, feel free to also participate on the discussions.

[phmbressan]

Good work addressing the review comments @kevin-alcaniz ! I plan to give this PR a final look tomorrow, sorry for the delay.

[kevin-alcaniz]

@phmbressan Thank you! Everything is ready on my end for the merge. Let me know if you find any issues.

[Gui-FernandesBR]

LGTM.

Waiting for @phmbressan 's review

[kevin-alcaniz]

One minute, I'm going to update de CHANGELOG. I always forget 😂

[kevin-alcaniz]

Done!

[phmbressan]

After reviewing a bit of literature on the topic and the excellent info that @kevin-alcaniz provided in the PR's description, I believe the root of my questions regarding the exhaust_velocity are related to these two concepts:

• exhaust velocity: the actual exhaust velocity of the gases exiting the nozzle, denoted by $V_9$;
• effective exhaust velocity: a measure of efficiency that can be computed by $\frac{T_{\text{net}}}{\dot{m}}$ (or $\frac{T_{\text{ref}}}{\dot{m}}$ in case of a static fire).

If I understood correctly, I believe the current exhaust_velocity method for liquid motors actually computes the effective exhaust velocity of the reference thrust curve (e.g. static fire). A similar reasoning apply to the exhaust_velocity methods of the other motors, but assuming the mass flow rate as constant.

In my view, this behavior is ok, but it should be documented (likely a Notes section in the docstring) that the value being computed is not the $V_9$ in the formula, but the effective exhaust velocity of the reference curve provided.

What do you think @kevin-alcaniz , did I misunderstand anything?

[phmbressan]

To be clearer on the outcome of the method, I would mention in the message that the vacuum thrust will be equivalent to thrust. Maybe a user warning would be better.

[kevin-alcaniz]

I’m not sure I fully understood what you meant by that.
Would you like me to clarify in the print that the vacuum thrust refers to the actual thrust the rocket would produce in a vacuum?
Or are you suggesting that I should add a warning indicating that this thrust value won’t be used in the actual simulation?
Could you let me know what you had in mind, please?

[phmbressan]

My suggestion is that, since we cannot compute the vacuum thrust when the reference pressure is None, we should raise a warning in this case to inform that the vacuum_thrust method will simply return the thrust.

[Gui-FernandesBR]

If I understood correctly, I believe the current exhaust_velocity method for liquid motors actually computes the effective exhaust velocity of the reference thrust curve (e.g. static fire). A similar reasoning apply to the exhaust_velocity methods of the other motors, but assuming the mass flow rate as constant.

In my view, this behavior is ok, but it should be documented (likely a Notes section in the docstring) that the value being computed is not the V 9 in the formula, but the effective exhaust velocity of the reference curve provided.

What do you think @kevin-alcaniz , did I misunderstand anything?

@kevin-alcaniz what do you think about it?

[kevin-alcaniz]

Sorry, @phmbressan , for the delay — I’ve been quite busy these past few days.

Yes, I think you're right. To accurately calculate the actual exhaust velocity, we’d need the thermodynamic properties of the gas upstream of the nozzle as well as the nozzle geometry. That’s not feasible with the current set of parameters used in RocketPy.

If you'd like, I can improve the documentation for this parameter. I propose adding a disclaimer in both the class attribute descriptions and within the exhaust velocity functions themselves.

Would that work for you?

[Copilot]

Copilot reviewed 7 out of 7 changed files in this pull request and generated no comments.

Comments suppressed due to low confidence (1)

rocketpy/motors/motor.py:1063

• [nitpick] Consider replacing the print statement with a logging mechanism to better control output in production environments.

print("Reference pressure is not set, cannot calculate vacuum thrust")

[phmbressan]

If you'd like, I can improve the documentation for this parameter. I propose adding a disclaimer in both the class attribute descriptions and within the exhaust velocity functions themselves.

Would that work for you?

Yes, a description in the style you mentioned should be good to clarify everything.

[kevin-alcaniz]

@phmbressan I've already applied the requested changes. I added a warning for the vacuum thrust when no reference pressure is defined, and improved the documentation for the exhaust velocity, both in the class attributes and the related functions.

If you come across anything else that needs attention, feel free to let me know!