Setting Initial Rotational State Using Aerodynamic Angles Only

[luigigisolfi]

Question

User: Hey everyone, I was wondering if there is a way to make the initial rotational state dependent on the aerodynamic angles only. From the documentation I get the feeling that it is not possible. It seems like I need to input the quaternion from body to inertial. Is this always the case?

Thanks a lot!

[luigigisolfi]

Answer

Tudat Team: That's right, by definition the initial rotational state is the body-fixed<->inertial one. But, you can construct this in any way you like, although manually

Note that we have this function to convert a rotation matrix to a (Tudat-compatible) quaternion: https://py.api.tudat.space/en/latest/element_conversion.html#tudatpy.astro.element_conversion.rotation_matrix_to_quaternion_entries

[luigigisolfi]

User:

But, you can construct this in any way you like, although manually

I expected this. Is there a way to obtain two matrices, C_ I_V and C_V_TA or the corresponding angles (longitude, latitude, heading and flight path) before the simulation starts? I understand that both of the previously mentioned matrices depend on the translational state and simulation start epoch, which I have defined. I have been unable to do get the values so far tho.
I was also thinking of setting an initial ephemeris, getting the values and then deleting the ephemeris before the start of the simulation but if you have any other suggestions I'm all ears.

[luigigisolfi]

Tudat Team:

I expected this. Is there a way to obtain two matrices, C_ I_V and C_V_TA or the corresponding angles (longitude, latitude, heading and flight path) before the simulation starts? I understand that both of the previously mentioned matrices depend on the translational state and simulation start epoch, which I have defined. I have been unable to do get the values so far tho.

I think the easiest way is to just calculate them yourself. All required rotation matrices are in Erwin's little green book. I'm not sure if the associated low-level Tudat functions are exposed to Python (don't think so). If you think that doing the calculation manually wil be cumbersome, let me know and I can see if we can expose some of these functions.

I was also thinking of setting an initial ephemeris, getting the values and then deleting the ephemeris before the start of the simulation but if you have any other suggestions I'm all ears.

This could work, although I'm not sure the FlightConditions/AerodynamicAngleCalculator can be updated properly outside of a propagation from Python.

[luigigisolfi]

User:
Ok perfect. Thanks a lot. I will give it a shot and see how it goes!