Figure 8 or Bicycle Chain to Mun?

[MrOsterman]

I've been currently trying to do the figure eight plan when sending crafts up to the Mun or to Minmus.  Flying "ahead" of the body, then either slowing into an orbit while on the far side, or accelerating slightly to set up a return flight.

However on a recent "rescue from orbit" mission I had to do a major correction enroute because the kerbals floating where orbiting the mun the "other way".  I used to do the "Bicycle chain" which was to pass the body from behind, and then when I hit the AP of my transfer orbit turn around and decelerate into an orbit of the mun that way.  

I can see that taking off after a munar landing I would have a SLIGHT advantage on speed doing the Bike Chain approach since I'll be orbiting the way that the mun is turning, though such a benefit feels negligible.  Do I see any other benefits doing one approach over the other?  I ~feel~ like the return trajectory is easier when I'm pushing off of the moon coming around it's far side from the figure 8  and accelerating back into a Kerbin orbit.  Doing the bike chain, if I recall correctly, I had to set  up a launch that had me flying out while between the mun and Kerbin and then pushing off back into a kerbin orbit that way.

 

[bewing]

Whether you pass ahead of, or behind your target body should make almost no difference. The only important factor is the altitude and position of your Pe. The lower it is, and the closer you are to it when you do your ejection burn determine the efficiency.

If you end up in a high prograde orbit, it may take forever for you to reach the proper ejection point in order to return to Kerbin -- because as you rotate around, say, the Mun ... the Mun is orbiting Kerbin and your ejection window keeps moving ahead of you.

If you put yourself retrograde (with a figure 8), you will get ejection windows much more often. When you eject, you will be at a slightly higher altitude over Kerbin, which is slightly less efficient.

[Geschosskopf]

13 hours ago, MrOsterman said:

I can see that taking off after a munar landing I would have a SLIGHT advantage on speed doing the Bike Chain approach since I'll be orbiting the way that the mun is turning, though such a benefit feels negligible.  Do I see any other benefits doing one approach over the other?  I ~feel~ like the return trajectory is easier when I'm pushing off of the moon coming around it's far side from the figure 8  and accelerating back into a Kerbin orbit.  Doing the bike chain, if I recall correctly, I had to set  up a launch that had me flying out while between the mun and Kerbin and then pushing off back into a kerbin orbit that way.

I don't quite understand your problem....

If you are trying to rendezvous in Munar orbit, then you need to arrive at Mun so you're going in the same direction as the target.  This is determined by which side of Mun your Pe is on when you enter its SOI.  Set your encounter up so you don't have to change directions once in Mun's SOI, which wastes beaucoup fuel.

Now, as to getting home....  If you're in a retrograde Munar orbit, you still get home the same way as if you're in a prograde orbit.  You still burn when starting to fly over the near side of Mun.  The only difference is that you come home with a retrograde Kerbin orbit, too.  This means you hit the air slightly harder.

In the general case of landing on Mun and returning, the dV for the mission is pretty much a wash whether you go prograde or retrograde around Mun.  You just burn slighly more or less fuel for the various burns, but they all add up to about the same amount.  Compared to a prograde approach, a retrograde approach is slightly less expensive on the capture burn (gravity assist) but slightly more expensive on the landing (more horizontal velocity relative to the surface to kill).  Taking off to the west and circularizing are slightly more expensive than going east, but the transfer back to Kerbin is slightly less expensive, as is capturing into Kerbin orbit without aerobraking.