#!/usr/bin/env python3
# -*- coding: utf-8 -*-
"""
Created on Thu Jul 16 09:41:54 2020

@author: ashesh
Example: Simulate the parachutist's speed by ODE - Raw

"""

# Import 
import matplotlib.pyplot as plt 
import numpy as np 


# Problem parameters
mass = 100          # Mass of the parachutist 
time_deploy = 6    # Time at which the parachute is deployed 
air_drag_coeff = 12.5     # Drag coefficient for air
para_drag_coeff = 80      # Drag coefficient for parachute 
speed0 = 0  # Initial speed 
height0 = 4000 #Initial height

# Simulation parameters
time_start = 0
time_end = 20
time_increment = 0.25

# Create a list of regularly spaced time instants
# [0,0.25,0.5,0.75,...,20] 
# 
time_array = np.arange(time_start,time_end+time_increment/2,
                       time_increment)

# Constants 
g = 9.81

# determine time increment
dt = time_array[2]-time_array[1]
    
# Simulation output
speed_array = np.zeros_like(time_array)


for k in range(0, len(time_array) - 1) :
    time_now = time_array[k]
    
    if time_now <= time_deploy :
        drag_coeff_now =air_drag_coeff
    else :
        drag_coeff_now = para_drag_coeff
        
    speed_array[k+1] = speed_array[k] + (g - drag_coeff_now * speed_array[k] / mass) * dt 
        




# Plot the figure 
fig1 = plt.figure() 
plt.plot(time_array, speed_array, label = 'ODE (speed)')
#plt.plot(time_array, height_array, label = 'height')

plt.legend()
plt.xlabel('time [seconds]')
plt.ylabel('speed')
#plt.ylabel('height')
plt.grid()
plt.show()
plt.savefig('para1.png')