Anyone do this kind of Mun orbit?

[Blackbird]

Sorry if this sounds like a stupid question, but now I'm curious: what is the actual advantage of this 8-shaped free return trajectory? Does it save any kind of Delta-V at any point or do they "just" do this to get savely back to earth/kerbin if the engines or something else went nuts. I did this maneuver once but I just can't find a use for it since we don't got those "errors" in KSP.

-BB.

[Flixxbeatz]

Sorry if this sounds like a stupid question, but now I'm curious: what is the actual advantage of this 8-shaped free return trajectory? Does it save any kind of Delta-V at any point or do they "just" do this to get savely back to earth/kerbin if the engines or something else went nuts. I did this maneuver once but I just can't find a use for it since we don't got those "errors" in KSP.

-BB.

Free return trajectory. In case something bad happens on-board, or you just happened to go the Moon for a flyby, you won't need to spend even a droplet of fuel to get back and re-enter to your home planet, thus saving you some fuel. It's a fail-safe strategy actually, particularly in real life.

[rodion_herrera]

Sorry if this sounds like a stupid question, but now I'm curious: what is the actual advantage of this 8-shaped free return trajectory? Does it save any kind of Delta-V at any point or do they "just" do this to get savely back to earth/kerbin if the engines or something else went nuts. I did this maneuver once but I just can't find a use for it since we don't got those "errors" in KSP.

-BB.

KSP-wise, no advantage at all, save for a little bit less DeltaV due to grav-assist. The main reason why it was ever done at all (real-world Apollo-wise), I already stated in some of my posts above:

a) To allow for a retrograde orbit, so the Apollo CM/LM could orbit the moon from east to west. This was essential because they had to see SHADOWS in the near the terminator, near their landing site--the shadows of the rocks and craters on the ground helped them to gauge/judge their distance from the surface (depth/distance perception).

While they could have done the above in a prograde orbit (west to east, which would happen if they didn't do a figure 8 and just "loop" around the moon), that would mean that landing would have to occur on the FAR SIDE of the moon, facing away from the earth, which was of course, err, stupid (i.e. no line of sight for comms and TV).

I only wanted to do it because it "felt" right doing something that Apollo did--it's not one of those "a must" things.

-RODION

Edited May 22, 2013 by rodion_herrera

[Blackbird]

yeah, seems to makes sense. Don't want to loose eye of sight to earth or they can't watch TV anymore ;P . Thanks guys^^

-BB.

[Flixxbeatz]

As of 0.20, I think I've just found a use for free-return trajectory: Munar sightseeing.

[kiwiak]

I use free returns in eary career mode. Thanks to that, i can get lot of science using craft with only ~1000 dV.

First i achieve retrograde orbit around kerbin. Than i make mun insertion per cost of 800 dV. I set it so, my apoapsis is about 40km, to achieve low mon orbit science results.

At this point, my final orbit around kerbin has peryapsis of about 200 km.. So after mun encounter i spend rest of fuel to lower it, to make ship hit kerbin atmosphere. Last bit of fuel is usef few seconds before touchdown to cushion impact.

Edited February 4, 2014 by kiwiak

[kBob]

I did this by accident on my second Mun orbital mission before I knew how to use maneuver nodes...I thought wow this is easy, I'll just hope over to Minmas now....ended up so far from Minmas I couldn't even see it, then I decided to learn about m nodes.

[Rakaydos]

Hmm... I wonder if you could set up a

calibrated to launch a given mass at the right speed to get a free return. For munar sighseeing, it would mean the craft literally has no way to maneuver.

[John Crichton]

Granted I've only been playing for around 2 weeks...but my first moon landing worked out this way by accident. Worked well

[RoboRay]

First i achieve retrograde orbit around kerbin.

Why? The direction of your orbit around Kerbin doesn't make a bit of difference to your orbital direction around the Mun. You just adjust your start-time for the burn so that you "lead" the Mun instead of "trailing" it.

Launching retrograde is just throwing away 300 m/sec.

[Ravenchant]

You just adjust your start-time for the burn so that you "lead" the Mun instead of "trailing" it.

This.

I've done some free return flybys in early career and with SSTOs, it works like a charm if you adjust your periapsis with RCS. Since my Mun station is in low retrograde orbit, though, I have to do it like that all of the time anyway.