Mechjeb Rendez Vous from near target orbit

[milosh]

hello,

forgive me if this was asked already, I searched the forum but didn't find anything. While trying to build an SSTO I noticed a very odd behaviour in MJ. Wenn I launch directly into or close to the orbit of my target and then fire the rendez-vous autopilot, it increases apo- and periapsis ~200km over the targets orbit and then plots a plan to intersect from this orbit with my target. But when I select an orbit far below my target it rendez vous directly into the targets orbit.

My stations for example is at 640km. When I launch my SSTO into a 600km orbit and select the RV autopilot to reach my station, MJ would put me in an 800KM orbit first. But when I launch into a 400km orbit and then start RV autopilot, it puts me directly in the 640km orbit of my station. Same is true for targets in 270km hight, I need to launch first into a 125km orbit or otherwise I need enough DeltaV to get into a 400km orbit and back.

Am I doing something wrong? I can do manual Hohmann from 600km to get to 640km without any problems. Even with the Rendez Vous planer of MJ (not autopilot) I can select Hohmann transfer to target and I reach my station just fine, it doesnt tell me to burn up until my apo is at 800km - so why is the autopilot doing it?

This is quite bugging me as my focus lies entirely on building, I don't want to spend my time Hohmanning manually all the time, at least not yet So for now I just launch into 200km below target orbit and then auto RV to my target.

Hope someone can clarify that for me.

have fun

[blizzy78]

It does that to decrease the synodic period, which is the period the two vessels have their closest approach to one another. Of course you can always get a rendezvous with a vessel in a 640 km orbit by launching into a 635 km orbit, but if both are on opposite sides of the planet, it's going to take a really, really, really, REALLY - did I mention "really?" - long time.

[milosh]

Okay I see, so MJ does what is most time- but not necessarily fuel efficient. In this example though I didn't have to wait that long by doing it manually, only 8 hours to warp through while the AP skipped like 2 hours only and with an SSTO, fuel is more important to me than some warp'able hours. Thanks for explaining

[Justin Kerbice]

You can also use MJ with the manoeuver planner (the one below rdv autopilot), you have all the steps and you can do them on your own, with the help of nodes.

[milosh]

Yes I know, that's part the reason for the post. Because with the manoeuver planer it does not take me 200km beyond my targets orbit I was wondering, why the AP does. But as blizzy78 has correctly posted, it takes more time, depending where I end up in relation to the target after ascending to my orbit.