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fix the sign of Flight.potential_energy #1204

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fix the sign of Flight.potential_energy #1204

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2 changes: 1 addition & 1 deletion rocketpy/simulation/flight.py
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Original file line number Diff line number Diff line change
Expand Up @@ -3824,7 +3824,7 @@ def potential_energy(self):
return (
standard_gravitational_parameter
* total_mass
* (1 / (self.z + self.env.earth_radius) - 1 / self.env.earth_radius)
* (1 / self.env.earth_radius - 1 / (self.z + self.env.earth_radius))
)

# Total Mechanical Energy
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25 changes: 25 additions & 0 deletions tests/unit/simulation/test_flight.py
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Original file line number Diff line number Diff line change
Expand Up @@ -553,6 +553,31 @@ def test_max_values(flight_calisto_robust):
assert pytest.approx(285.94948, rel=rtol) == test.max_speed


def test_potential_energy_increases_with_altitude(flight_calisto):
"""Test that the potential energy grows as the rocket climbs. Between
burn out and apogee the mass is constant, so the change in potential
energy must match m * g * dz.

Parameters
----------
flight_calisto : rocketpy.Flight
Flight object to be tested. See the conftest.py file for more info
regarding this pytest fixture.
"""
test = flight_calisto
burn_out_time = test.rocket.motor.burn_out_time
apogee_time = test.apogee_time
mass = test.rocket.total_mass(apogee_time)
delta_z = test.z(apogee_time) - test.z(burn_out_time)
initial_potential_energy = test.potential_energy(burn_out_time)
final_potential_energy = test.potential_energy(apogee_time)
delta_potential_energy = final_potential_energy - initial_potential_energy
expected_delta = mass * 9.80665 * delta_z

assert delta_z > 0
assert pytest.approx(expected_delta, rel=1e-2) == delta_potential_energy


@pytest.mark.parametrize(
"flight_time_attr",
["t_initial", "out_of_rail_time", "apogee_time", "t_final"],
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