Gravity Simulator
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General >> Discussion >> Lunar travel gone wrong
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Message started by Thagur on 03/25/08 at 08:44:29

Title: Lunar travel gone wrong
Post by Thagur on 03/25/08 at 08:44:29

Hello!

Again, this is one great piece of software.

Today I tried to simulate an Apollo mission to the moon. I fired the thrust either too soon or to late (don't remember), and the result was that the lunar module became an asteroid around earth. I noticed the orbit of the lunar module constantly changing, so I just sat back and enjoyed the show. Suddenly, something happened. Poor crew on that lunar module...

Colors:

Earth: Blue
Moon: Gray
Lunar Module: Red

Title: Re: Lunar travel gone wrong
Post by Tony on 03/25/08 at 10:50:42

A moon-crossing orbit is very chaotic.  It looks like your Lunar module was ejected into interplanetary space.

Try this:
  • Start with the Earth and Moon
  • Create your Lunar Module at a distance of 6700 km from Earth.  This is an altitude of about 320 km.  
  • Open a Thrust Box from the View menu.  Set it to Lunar Module with respect to Earth.
  • Open an Orbital Elements Box from the View menu.  Set it to Lunar Module with respect to Earth.
  • Keep a slow time step, so the seconds are ticking by in real time.
  • When the Lunar Module appears to be about 80 or 90 degrees behind the Moon, accelerate prograde until Q (apogee) in the Orbital Elements Box reads ~390000 km.  (You may miss the Moon the first few times you try it.  Keep practicing and playing around with different numbers)
  • Set your Thrust box to Lunar Module with respect to the Moon.
  • Set your Orbital Elements Box to Lunar Module with respect to the Moon.
  • Open a Distance and Velocity Box from the View menu.  Set it to Lunar Module and Moon.
  • When the Lunar Module gets close to the Moon, keep your eye on radial velocity in the Distance and Velocity Box.
  • When radial velocity reaches 0, you are at your closest approach.  Make retrograde burns until q (perilune) in the Orbital Elements box is about 3000 km.
  • Again, watch your radial velocity as the Lunar Module drops towards the Moon.  When it is 0, make retrograde burns until e (eccentricity) in the Orbital Elements box is as small as possible.
  • You are now in a stable Lunar orbit.
  • Want an ecliptic orbit (inclination = 0)?
  • Click "Custom" on the thrust box and set your altitude to -90 degrees.
  • Set the Scroll Bar to the middle by pressing the [+] button underneath the scroll bar.  This gives you an ecliptic view.  To go back to top-down view, drag the scroll bar to the top.
  • When the Lunar Module is crossing the middle of the Moon in an upward direction, thrust (10 m/s) until your inclination on the Orbital Elements box goes to 0.  But be careful.  Keep an eye on q (perilune), which is your closest approach to the Moon.  Don't let it get under 2000 km, or you risk crashing into the Moon.  If it gets too low before your inclination is zeroed out, wait until apolune, your furthest point from the Moon.  You'll know you're there when your radial velocity is 0.  Then do prograde burns to raise your perilune.
  • From there, just tweak the orbit to your liking.

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