How do you get a free return trajectory around the moon?

[JEB'S DESTINY]

I've been wanting to replica an Apollo Mission with a free return trajectory, but I have a problem, I don't know how to make one!

[Geonovast]

Get in a prograde orbit around the Mun.

With 12 o'clock pointing directly at Kerbin, set a maneuver node at approximately 1 o'clock.

Pull the node's prograde handle until the projected course leaves the Mun's SOI.

Right click on the Pe in the purple line, and keep pulling the prograde handle until it's about 30-35km.

Execute the node.

 

That's how I do it anyway.  I've also been getting into the habit of landing in that general area of the Mun since it's tidally locked, just burning straight out from the surface, and not bothering with achieving orbit first.

 

Edit - Post before coffee error: Terms confused.

Edited February 2, 2018 by Geonovast

[Streetwind]

@Geonovast Pretty sure that's not what he means  "Free Return Trajectory" is a specific term that denotes a figure-8 transfer orbit that goes from the Earth around the Moon and back to Earth. Apollo missions used free return trajectories and propulsively braked out of them to enter lunar orbit. This turned out to be a wise decision, as it allowed Apollo 13 to make it back home.

@JEB'S DESTINY Plan a normal Hohmann transfer to the Mun. Then add an additional 10 to 20 m/s prograde. Finally, pick up your maneuver node and slowly slide it forward along your Kerbin parking orbit. You will lose your initial encounter. Carefully slide it further forward - at some point you will get a new encounter. This one can be refined into the proper figure-8 shape by careful management of the maneuver node position and prograde/radial dV vectors. If this doesn't work, try again from step 1 and add 10 m/s more than last time.

Edited February 2, 2018 by Streetwind

[Starman4308]

30 minutes ago, Geonovast said:

Get in a prograde orbit around the Mun.

With 12 o'clock pointing directly at Kerbin, set a maneuver node at approximately 1 o'clock.

Pull the node's prograde handle until the projected course leaves the Mun's SOI.

Right click on the Pe in the purple line, and keep pulling the prograde handle until it's about 30-35km.

Execute the node.

That's not a free return trajectory. 

A free return trajectory is a specific type of translunar injection that leaves you doing a figure-8 around the Moon, returning (for free) back to Kerbin/Earth. You typically need to burn a bit earlier than usual and a bit more than usual to get a free return.

Usually in KSP, even a rough translunar injection is good enough for flyby missions, since you can make corrections, often at perilune, to put your Kerbin periapsis back at ~30 km.

The reason they went with a free return for the Apollo missions was in case the service module engine failed, something that isn't a concern unless you use a parts failure mod.

EDIT: First, what Streetwind said. Second, a cislunar injection puts you into a prograde orbit around the Moon; your lunar periapsis is behind the Moon, and you wind up orbiting eastwards. A translunar injection puts you into a retrograde orbit, with lunar periapsis in front of the Moon. Translunar injections are generally a bit more expensive, and landing from a retrograde orbit is slightly more expensive, since when starting from a retrograde orbit, the Moon's rotation is working against you, not with you.

Cislunar injections, if you don't brake into an elliptical orbit, will cause you to slingshot out at high velocity,/energy helping you escape Kerbin/Earth. That's what the Apollo planners wanted to avoid. Translunar injections help you stay in Kerbin/Earth orbit if you don't brake, with free returns being a specific case where you wind up returning to Kerbin/Earth atmosphere.

Edited February 2, 2018 by Starman4308

[MaxwellsDemon]

The trick is to "lead" the Mun, so you get into a retrograde orbit.  That way, the Mun is (loosely speaking) slowing your ship down and slinging it back toward Kerbin.

I've found it's very hard to do without MechJeb (though Streetwind's answer is interesting and I want to give it a try).   With MechJeb, though, it's quite easy-- start with a Hohmann transfer to the Mun, and then do a fine-tune approach to target with a negative value (say, -200 km).   Don't incline your orbit.  It will take some trial and error to get it just the way you want it. 

Edited February 2, 2018 by MaxwellsDemon

[Zhetaan]

@JEB'S DESTINY:

@Streetwind has the right of it, and I will only add that when we say 'figure 8', don't expect it to look much like the numeral with the two loops being of roughly the same size.  It's a very lopsided 8 where the orbit crosses itself very near to the Mun.  About 90% to 95% of the figure is Kerbin's loop, and the Mun gets the tiny bit left over.

[Kryxal]

If you want to try a bit of a neat trick, it's also possible to set up a cislunar FRT (so you'll capture into a prograde orbit).  It ends up looking a bit like a Valentine heart, since you'd go PAST the Mun's orbit and catch it on the way back down, be redirected BACK up, and return to Kerbin, not to mention it takes longer...

[sevenperforce]

For all my Mun-SOI missions, I take basically the same steps, assuming that I start from a roughly circular, low-or-no-inclination orbit.

• Create a maneuver node that won't take you anywhere near the moon, and tug on the prograde marker until your apoapse just touches the Mun's orbital path.
• Grab the node and tug it around your orbital path until you get the kind of Mun encounter you want (prograde for normal landing or gravity assist; retrograde for free-return).
• Pull back on the retrograde marker until you get your trajectory as low as possible.
• ???
• Profit!

[Norcalplanner]

And if you do it right, the trajectory should look like this when your view is focused on the Mun...

https://m.imgur.com/gallery/MCcbm

 

Edit - on mobile and can't do things well, but it's the ninth or so image in that album 

Edited February 2, 2018 by Norcalplanner