Trajectory question (figure 8 / free return)

[Pedro29]

Hello,

I have a rather noob question, maybe someone will point me to some tutorial that help figuring this out

Apollo 11 trajectory is shaped like a 8, which means that after take off and escape from earth, the ship passes in front of the moon on it's orbit insertion (just type " Apollo 11 moon trajectory " in google image you'll see what I mean).

However I tried to do it like this in KSP but I can only manage to get the Mun encounter (or Kerbin, on the return trip) from behind (which is great for slingshots).

Whatever point I choose for my starting burn (relative to the Mun) I always end up behind...

Is there some clever mid-course correction to obtain this? And is it efficient compared to the "classic" insertion?

Thanks!

Edited August 24, 2014 by Pedro29

[Master Tao]

You're thinking of a free-return trajectory. Short version: push your AP higher than normal so your intercept is on the far side of the Mun. It will always be a little less efficient than the trajectories you've achieved so far unless your goal is a flyby.

[Jimbimbibble]

You also need to aim farther in front of it. Start your burn when you are about 60 degrees behind the mun (as opposed to the typical 90 degrees you would use for best efficiency). It's definitely less efficient, but it's safer in real life because if the engines fail on the way there you won't get stuck in space.

[Awass]

Even without a free return trajectory, you can still make a figure 8 like the Apollo missions did. Just burn so your apoapsis reaches the Mun's sphere of influence, but keep the apoapsis inside the Mun's orbit of Kerbin instead of going around the far side of the Mun. Once you enter the Mun's sphere of influence, you need to burn prograde to set up an orbit around the Mun. You will end up orbiting the Mun clockwise whereas you (assumably) launched from Kerbin counter-clockwise, and you will have yourself a figure 8. I believe it's actually slightly more efficient too because you have to cancel less excess velocity when setting up your Munar orbit than if you went around the far side.

[Claw]

You also need to push the maneuver node a little later than you normally would. That, with a little extra dV, will get you a figure-8. Then it takes a little tweaking to get a good Mun and Kerbin PE.

Cheers,

-Claw

(Edit: Delayed posting. This is similar to what Jim said above.)

[Lheim]

Yep, you want to approach the moon so that it is `behind` you. That way the dominant way it pulls you is retrograde to your Kerbin orbit - making you come back slowed down and into the atmosphere. Go slightly above the moon`s orbit with your maneuver node, and fiddle with the timing, and you`ll get it.

[Kasuha]

Here is how a free return trajectory (maneuver) looks like from 100 km orbit. It takes a while to set up because the trajectory is quite sensitive to changes - NASA used it not because it is comfortable, but because it is safe. And that paid off in case of Apollo 13. You don't need such safety in KSP because you can always send a rescue mission and your Kerbals won't die even after years - unless you use mods to do it or have some self-imposed rules.

[Pedro29]

Cool, great info!

I guess I'll try to achieve it at least once or twice then I'll stick to the simpler stuff hehe.

[Red Iron Crown]

You will need to switch to conic patch mode 3 to get a free return to look like a figure eight as pictured above.

[Mr Shifty]

Here is how a free return trajectory (maneuver) looks like from 100 km orbit. It takes a while to set up because the trajectory is quite sensitive to changes - NASA used it not because it is comfortable, but because it is safe. And that paid off in case of Apollo 13.

None of the Apollo missions after Apollo 11 used a free-return trajectory. After the accident on Apollo 13, they used the LM's descent engine to put the spacecraft into a free-return trajectory.

[SRV Ron]

You will need to switch to conic patch mode 3 to get a free return to look like a figure eight as pictured above.

No conic patch was used for this.

Perhaps, for viewing in tracking station mode.

[Red Iron Crown]

Er, I was thinking of when mode 1 used to be the default, it is now mode 3. I barely notice as I use mods that can change it in game, but I remember having to change it manually before that. Thanks for the correction SRV Ron.

[UberFuber]

None of the Apollo missions after Apollo 11 used a free-return trajectory. After the accident on Apollo 13, they used the LM's descent engine to put the spacecraft into a free-return trajectory.

You are partially correct.

http://www.braeunig.us/apollo/hybrid-profile.htm

Apollo mission after 11 uses a "hybrid" trajectory. The initial trajectory did put the ship in a free return trajectory. After completing various system check, an additional burn is made to get the space ship out of free-return and "optimize" it for a lunar orbit capture (basically, they "slow down" a little bit early on so they save some delta-V when putting themselves into a lunar orbit).

http://en.wikipedia.org/wiki/Apollo_13#Mission_highlights

Apollo 13 needs to correct their course after the explosion because the explosion happen after the additional burn that took them out of free return. In short, the sequence went like this.

1. Apollo 13 on free return trajectory.

2. Apollo 13 perform burn to optimize for lunar orbit insertion.

3. O2 tank exploded.

4. Apollo 13 has to perform a burn to put them back on a free return trajectory.

[Pedro29]

A little feedback: I'm not familiar with this conic stuff, but anyway it worked quite well.

Accuracy is not extreme as I once crashed on the Mun instead of going around as planned because I did not monitor the trip and I wasn't close enough from the planned trajectory.

The technique I used was to setup an Ap at 13500 or so and after that, fine tune the maneuver position to get the good shape (as seen on a tutorial on youtube, very similar to SV Ron's captures).

I even saved a crew with that, on my way to the Mun I ran out of electricity (early career... no solar panels) before entering Mun orbit and without touching anything the ship got back directly to Kerbin. Neat ^^