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Orbital Motion to model the orbit (with thrust) in the iPhone app Space Agency
from visual import *
Earth=sphere(pos=vector(0,0,0), radius =1, material=materials.earth)
R=2.75 #orbita radius in terms of radius of the planet
GM=1.47 #GM as determined from the game
v=sqrt(GM/R) #starting speed for a circular orbit
rt=0.0338 #this is the acceleration due to the rocket
#sc is the spacecraft
sc=sphere(pos=(R,0,0), radius=0.1, make_trail=True)
#other is just an object with no thrust so you can see the difference
#thrust makes
other=sphere(pos=(R,0,0), radius=0.03, color=color.red, make_trail=True)
t=0
dt=.01
sc.m=1 #spacecraft mass of 1
sc.p=sc.m*vector(0,-v,0) #initial momentum
other.m=1
other.p=sc.p
Fr=vector(0,0,0) #this is the rocket thrust force
scale=0.001 #it turns out I don't need this
#vt is an arrow to represent the rocket thrust.
vt=arrow(pos=sc.pos, axis=scale*Fr, color=color.yellow)
while t<30:
rate(100)
#here I turn on the thrust from t=5 to 10 sec
if t>5. and t<10.:
Fr=rt*norm(sc.p)
scale=10
else:
Fr=vector(0,0,0)
scale=0.001
#Calculate the gravitational force plus rocket thrust (if any)
F=-GM*norm(sc.pos)/mag(sc.pos)**2+Fr
#update momentum
sc.p=sc.p+F*dt
#update position
sc.pos=sc.pos+sc.p*dt/sc.m
#update stuff for the other orbit
other.p=other.p-GM*norm(other.pos)*dt/mag(other.pos)**2
other.pos=other.pos+other.p*dt/other.m
#update the thrust arrow
vt.pos=sc.pos
vt.axis=scale*Fr
t=t+dt
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