Stable rockets with negative stability margins? #115
Description
Describe the bug
Rockets with slightly negative stability margins seem stable in RocketPy
To Reproduce
Run the following code:
# Helper class
from rocketpy import Environment, SolidMotor, Rocket, Flight, Function
# Prepare Motor
Pro75M1670 = SolidMotor(
thrustSource="../../data/motors/Cesaroni_M1670.eng",
burnOut=3.9,
grainNumber=5,
grainSeparation=5/1000,
grainDensity=1815,
grainOuterRadius=33/1000,
grainInitialInnerRadius=15/1000,
grainInitialHeight=120/1000,
nozzleRadius=33/1000,
throatRadius=11/1000,
interpolationMethod='linear'
)
# Prepare Rocket
Calisto = Rocket(
motor=Pro75M1670,
radius=127/2000,
mass=19.197-2.956,
inertiaI=6.60,
inertiaZ=0.0351,
distanceRocketNozzle=-1.255,
distanceRocketPropellant=-0.85704,
powerOffDrag='../../data/calisto/powerOffDragCurve.csv',
powerOnDrag='../../data/calisto/powerOnDragCurve.csv'
)
Calisto.setRailButtons([0.2, -0.5])
Nose = Calisto.addNose(length=0.55829, kind="vonKarman", distanceToCM=0.71971)
FinSet = Calisto.addFins(4, span=0.1, rootChord=0.120, tipChord=0.040, distanceToCM=-1.04956)
Tail = Calisto.addTail(topRadius=0.0635, bottomRadius=0.0435, length=0.060, distanceToCM=-1.194656)
# Prepare Environment
Env = Environment(5.2, 9.8)
Env.setAtmosphericModel(type='CostumAtmosphere', wind_v=-10)
# Simulate Different Static Margins by Varying Fin Position
simulation_results = []
for factor in [0.1, 0.3, 0.5, 0.8, 1.0]:
# Modify rocket fin set by removing previous one and adding new one
Calisto.aerodynamicSurfaces.remove(FinSet)
FinSet = Calisto.addFins(4, span=0.1, rootChord=0.120, tipChord=0.040, distanceToCM=-1.04956*factor)
# Simulate
TestFlight = Flight(rocket=Calisto, environment=Env, inclination=90, heading=0, maxTimeStep=0.01, maxTime=2.5, terminateOnApogee=True)
# Post process flight data
TestFlight.postProcess()
# Store Results
staticMarginAtIginition = Calisto.staticMargin(0)
staticMarginAtOutOfRail = Calisto.staticMargin(TestFlight.outOfRailTime)
staticMarginAtSteadyState = Calisto.staticMargin(TestFlight.tFinal)
simulation_results += [(TestFlight.attitudeAngle, 'Static Margin of {:1.2f} c'.format(staticMarginAtOutOfRail))]
Function.comparePlots(simulation_results, lower=0, upper=2.5, xlabel='Time (s)', ylabel='Attitude Angle (deg)')Expected behavior
Negative stability margins should be unstable.... right?
Screenshots
Here is a plot of the angle that the Rocket makes with the horizontal plane after leaving the rail. Notice how the green and orange trajectories are stable even thought their stability margin is negative,
