4 IN 1 Posted November 7, 2011 Share Posted November 7, 2011 So I have been flying between Karth and Mun and landing on it numbers of time, one thing that left in my mind is that every time I will have to do a numbers of burn to correct the escape trajectory into a parking orbit, it seems that I don\'t hold the skill and knowledge to do a free return trajectory so here is the, big question: How to, if possible, get into a free return trajectory? Link to comment Share on other sites More sharing options...
Jarnis Posted November 7, 2011 Share Posted November 7, 2011 It is possible to get a trajectory that, after entering moon\'s sphere of influence, you will fling back towards Kerbin and enter orbit around it (that requires very little to change it to a re-entry).Trick is to time the entry to lunar influence so that you are further away from the Kerbin than the Mun is. That way Mun gravity, when it starts to affect the trajectory, will pull you towards the right direction. If you enter Mun influence from the Kerbin side, you are bound to 'slingshot' even further away (you 'fall' towards Mun, adding velocity and the trajectory becomes hyperbolic without further braking). So 'fall' towards Mun so it is also a 'fall' towards Kerbin and/or against your trajectory at that point. You will fling around the moon on a trajectory that heads back towards Kerbin. A 'free return' to Kerbin influence. It will not be a free return to Kerbin atmosphere, but it should be close enough to be fixable without too much deacceleration. You can minimize this by trying to get Mun encounter that flings you 'backwards', towards the direction where the Mun came, so when you look at your relative speed to Kerbin, it is low (you want it to be sub-100m/s at 11000km if you want direct re-entry - remember that Mun is doing 542m/s so if you fling past it 700m/s relative to Mun, you would be then doing still 200m/s to the direction Mun is travelling, in relation to Kerbin...)Just make sure you actually miss the Mun - it is also easy to get a trajectory that will result in a fresh new crater (assuming no course corrections). Link to comment Share on other sites More sharing options...
Senji Posted November 7, 2011 Share Posted November 7, 2011 OK, how do I do a free return from Mun-surface then? Link to comment Share on other sites More sharing options...
Jarnis Posted November 7, 2011 Share Posted November 7, 2011 OK, how do I do a free return from Mun-surface then?It isn\'t 'free', you do have to make burns, but to use least fuel, lift off and turn against the orbit of the moon. That minimizes the speed around Kerbin and once you are outside Mun influence, the orbit should have periapsis close to Kerbin. Either brake then or wait until you are at apoapsis (going by Kerbin once) and burn at apoapsis for minimum fuel usage. Link to comment Share on other sites More sharing options...
4 IN 1 Posted November 7, 2011 Author Share Posted November 7, 2011 So my Ap have to be around 11000Km and I have to get there just in time the mun reach that point ? Link to comment Share on other sites More sharing options...
Jarnis Posted November 7, 2011 Share Posted November 7, 2011 So my Ap have to be around 11000Km and I have to get there just in time the mun reach that point ?Yes. Actually tad more than that is usually better, it allows more time to enter Mun sphere of influence. I usually aim to 11600-11800. It puts the elliptic transfer slightly beyond Mun orbit but that\'s perfectly fine (and actually the preferable angle for the capture)The way to time it is simple; While on low Kerbin orbit, look at the map from above. When the Mun is approximately at 'two o-clock' and your capsule is at 'six o-clock' in relation to Kerbin, start your burn. Your orbit periapsis will head towards 'tweleve o-clock' (opposite of your position around Kerbin). Stop burn when periapsis is at around 11600km and wait/time warp. Once your craft gets to 11000km+, Mun will be there. Trust me - I\'ve got math and science that says it is so... Link to comment Share on other sites More sharing options...
4 IN 1 Posted November 7, 2011 Author Share Posted November 7, 2011 Ok, I will give it a try Link to comment Share on other sites More sharing options...
foamyesque Posted November 7, 2011 Share Posted November 7, 2011 The general problem with free-returns is hitting them intentionally and not by accident. Most of my lunar missions would result in crashes without further burns... Although I have managed direct insertion into munar orbit by pure chance once or twice. Link to comment Share on other sites More sharing options...
Jarnis Posted November 7, 2011 Share Posted November 7, 2011 The general problem with free-returns is hitting them intentionally and not by accident. Most of my lunar missions would result in crashes without further burns... Although I have managed direct insertion into munar orbit by pure chance once or twice.Indeed. Mun is a big object that wants to pull you in for a little episode of 'lithobraking', so the margin between 'you\'ll miss it' and 'you weren\'t even captured by it' is fairly small. Most Mun captures require further course correction burn in order to enter the orbit. Link to comment Share on other sites More sharing options...
Hypocee Posted November 7, 2011 Share Posted November 7, 2011 As, of course, did the real thing. They were 'free' return trajectories in the sense that the main engine was not necessary for midcourse corrections. If you can do a single burn into a free-return, I\'m scared of you. Link to comment Share on other sites More sharing options...
4 IN 1 Posted December 17, 2011 Author Share Posted December 17, 2011 Hum...for some reason I still can\'t get it right. I always got there too early and cannot sling back to kerbin, I just try any point of the moon orbit from 45 to 75 degrees relative to kerbin when the ship is 180 degrees relative to kerbin...I think I really did something very wrong here :-\ Link to comment Share on other sites More sharing options...
Hypocee Posted December 17, 2011 Share Posted December 17, 2011 More discussion here. Conclusion, it may juuust be possible in KSP. Edit: I failed to notice this topic was old and I\'d already said basically the same thing. Link to comment Share on other sites More sharing options...
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