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dbookstaber / py_ballistics

Point-mass ballistic trajectory calculator with support for custom drag models
GNU Lesser General Public License v3.0
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TrajectoryCalc zero_angle vs. trajectory #11

Closed dbookstaber closed 6 months ago

dbookstaber commented 7 months ago

Presently TrajectoryCalc._zero_angle() and ._trajectory() use a similar but not equivalent implementation. In the end, the zero_angle doesn't care about windage, and it ignores other details that are handled in the "full" trajectory calculation:

  1. Winds
  2. twist (spin-drift)
  3. cant_angle
  4. Settings.USE_POWDER_SENSITIVITY
  5. altitude-adjusted density_factor

Otherwise, the only difference is that trajectory() is checking each iteration for various flags and recording TrajectoryData rows, whereas zero_angle is checking for range_vector.y hitting the zero.

Doing the full trajectory calculations while finding searching for zero_angle shouldn't (TODO: verify?) be much more compute time than what it does at present. Benefit: zero_angle will match trajectory under all conditions, whereas now there will be breaks due to winds and/or cant_angle.

Therefore: The two methods should share the same initialization and step logic.

dbookstaber commented 7 months ago

One critical break found between the two with the .50BMG 30° look angle Example:

Where zero() only sets these initial values:

        mach = shot_info.atmo.mach >> Velocity.FPS
        density_factor = shot_info.atmo.density_factor()

trajectory() updates them each iteration via:

density_factor, mach = atmo.get_density_factor_and_mach_for_altitude(alt0 + range_vector.y)

After 2000' trajectory the zero() velocity is 20m/s higher than the trajectory() calculation! (Which seems backwards, so I'm digging into the calculation and use of density_factor and mach.)

dbookstaber commented 7 months ago

Actually the big break in previous comment's example was that zero() doesn't apply powder temperature correction, if that has been selected.

The reduced drag at higher altitudes is a smaller effect, but still visible: If we hold the density_factor and mach number static (at their initial values) in trajectory() then for a high-angle shot we do get velocity and drop values that are increasingly lower downrange, as expected.

dbookstaber commented 6 months ago

Done! https://github.com/dbookstaber/py_ballistics/commit/15cde78ecb724df269fed2e387ee1d4bf6549359