Free return trajectory to Mun/Moon - how to?

[michal.don]

Hi guys,

I recently started a modded playthrough with a historical mission contract pack that simulates the real progression of the space race. I reached the part where I'm sending the first satelites to the Moon. I'm aware that most of the missions sent to Moon used the free return trajectory in case something went bad (which it does quite often in KSP), but I'm not sure how to plot my trajectory so that it sends me back to Earth atmosphere. I read some stuff on wiki, including the height of munar periapsis, but it doesn't work for me, the final Eart orbit is still pretty far off. What am I missing? Does any of you know how to plot a free return trajectory, either in RSS or the original stock system?

Thanks,

Michal.don

[Spricigo]

@NecroBones Explain it way better than I may dream to do.  Hope it helps you. (definitely helped me) 

[michal.don]

3 hours ago, Spricigo said:

Hope it helps you

Yes, thank you very much. It seems the key is going a bit earlier and faster, I'll just have to get used to plotting the course by trying it a few times. Also, thank you, @NecroBones for a great tutorial

Michal.don

[Van Disaster]

Scott Manley also did a video on it once I think. Apollo style figure of 8 is pretty satisfying to set up

Spoiler

 

[strider3]

On ‎8‎/‎9‎/‎2016 at 4:59 AM, Spricigo said:

@NecroBones Explain it way better than I may dream to do.  Hope it helps you. (definitely helped me) 

That...was...AWESOME! I had no idea. This will really help with those "fly by" contracts and also science gathering.

Thank you Spricigo for the link...and @NecroBones for the excellent video!

Is a free return possible on any close (to Kerbin) planets? I imagine that the more orbits you cross, the more likely bodies between you and the target will interfere.

Edited December 17, 2016 by strider3

[FullMetalMachinist]

1 hour ago, Victor3 said:

Is a free return possible on any close (to Kerbin) planets? I imagine that the more orbits you cross, the more likely bodies between you and the target will interfere.

I'm no astrophysicist, but I'm pretty sure free returns only work for Mun and Minmus. 

[WanderingKid]

Watched the Nutshell video.  There's a way to do it without maneuver nodes.  At Munrise, burn prograde until your apoapsis is ~14.5m from Kerbin.  You'll do a high flyby of Mun and come back for free (or nearly free, depends on exactly how well you hit it).

[Spricigo]

On 17/12/2016 at 0:45 AM, FullMetalMachinist said:

I'm no astrophysicist, but I'm pretty sure free returns only work for Mun and Minmus. 

Is at least highly impractical.

-The game don't predict the t trajectory after a few SOI changes;

-kerbin is orbiting so the timing need to be correct

-small deviations will have bigger effects on longer trajectories.

 

[Chakat Firepaw]

On 16/12/2016 at 9:45 PM, FullMetalMachinist said:

I'm no astrophysicist, but I'm pretty sure free returns only work for Mun and Minmus. 

Others exist but they are harder to plot and generally have limited launch windows.

Heck, in RL there have been proposals for _interstellar_ free returns that involve visiting two or more stars before returning.

[tab]

1 hour ago, Chakat Firepaw said:

Others exist but they are harder to plot and generally have limited launch windows.

Heck, in RL there have been proposals for _interstellar_ free returns that involve visiting two or more stars before returning.

Visiting two or more star?! 

[GoSlash27]

The free return trajectory is actually useful in early career mode in KSP, especially when you're doing caveman mode.

 You need to do a Munar flyby for science. You want to be able to mission plan with minimal parts and acceptable launchpad mass, but you don't have the luxury of patched conics. You don't want to haul the fuel required for a full orbit and return burn, so how do you make it all work? The free return, or at least something close to it.

I just wait until my prograde is precisely aligned with the Mun and burn for an apoapsis of 16 Mm. This puts me into something very close to a free return that requires very little DV to get reentry.

To do this, I keep the nose on prograde and wait until the Mun is aligned along my ship then burn, switch to map view, and wait for Ap.

It's not perfect, but it works out close for me.

Best,
-Slashy

[Chakat Firepaw]

On 22/12/2016 at 2:56 PM, tab said:

Visiting two or more star?! 

You use the first to swing you around to the second and the second to swing you around to home.  For more than two stars you add in the other ones in the middle.