How to determine the orbit ejection time?

[uncle_jew]

Is there any good trick for determining when it's best to eject from a low orbit for a transfer to another planet/satellite?

The problem is, you can only plan a maneuver up to one orbit ahead. And since the orbital period of low orbits around bodies is short compared to orbital periods around the Sun, I don't see a way of checking e.g. when it is better to eject from Moho to Eve: in 60 or in 90 days?

For launches from LKO, I simply have a spacecraft in exactly the same orbit as Kerbin, but a few days ahead of Kerbin. I plan the interplanetary transfer using this spacecraft first, which allows me to select any time during a whole Kerbin year, so that I can fine tune the desired launch and LKO ejection time. Finally, I use the time of this maneuver for the LKO ejection burn.

But for launches from other bodies, this would require putting a separate spacecraft into the Sol orbit ahead of each body. Is there a more generic method of planning ejections into interplanetary orbits?

Edited July 18, 2016 by uncle_jew

[Geschosskopf]

54 minutes ago, uncle_jew said:

Is there any good trick for determining when it's best to eject from a low orbit for a transfer to another planet/satellite?

In the stock game itself?  No, there isn't.  Not unless you have absolutely nothing else going on in space and don't care about elapsed gametime.  Then you can just go to tracking center and warp months ahead, carefully watching until the planets line up the way you want.  This is just one example of the appalling lack of basic information in the game.  I mean, what's the point of knowing when to leave if there's no way to tell if you have enough fuel for the trip or enough thrust to land safely when you get there?  Don't get me started on that.

But anyway, because the game itself tells you nothing of use in designing rockets and their messions, folks use mods and websites.  For interplanetary trips, you can use a website like Alex Moon's transfer window planner.  Mods like MechJeb and this website will tell you want day you should be leaving, how much dV it will cost, how long the trip will take, etc.  MJ will even make the node for you, set however far in the future it needs to be.  Either way, you then need Kerbal Alarm Clock to keep you from warping past the departure time.

[FancyMouse]

1 hour ago, uncle_jew said:

The problem is, you can only plan a maneuver up to one orbit ahead.

nope. Right click the node, see the bottom -,+ buttons? They can adjust the node -1/+1 period.

Of course, this still doesn't work well in your situation, as low orbit is usually about half an hour period, so planning 60 day ahead would require like 480 clicks of +.

[Geschosskopf]

25 minutes ago, FancyMouse said:

Of course, this still doesn't work well in your situation, as low orbit is usually about half an hour period, so planning 60 day ahead would require like 480 clicks of +.

And this function maxes out like 5 orbits ahead anyway.  And if you're in low orbit, you can only warp like 100x anyway, so 60 days takes a LONG time.  This is why I didn't mention this.

The only viable option is to 1st know what do you need to leave (from MJ or a website) and 2nd to have KAC to take you to that point in time without going over.

Note that KAC itself also predicts transfer windows but it doesn't do a very good job.  Which is why I didn't mention this, either.

[cubinator]

This chart shows the ideal phase angles for an efficient interplanetary transfer to any planet. The delta-V values on the bottom for atmospheric bodies are outdated; use this chart instead:

For other planets, I'm not so sure, but if you have a calculator handy this might be of use, although it may be time-consuming:

https://www.faa.gov/other_visit/aviation_industry/designees_delegations/designee_types/ame/media/Section III.4.1.5 Maneuvering in Space.pdf

[bewing]

2 hours ago, uncle_jew said:

Is there any good trick for determining when it's best to eject from a low orbit for a transfer to another planet/satellite?

The problem is, you can only plan a maneuver up to one orbit ahead. And since the orbital period of low orbits around bodies is short compared to orbital periods around the Sun, I don't see a way of checking e.g. when it is better to eject from Moho to Eve: in 60 or in 90 days?

For launches from LKO, I simply have a spacecraft in exactly the same orbit as Kerbin, but a few days ahead of Kerbin. I plan the interplanetary transfer using this spacecraft first, which allows me to select any time during a whole Kerbin year, so that I can fine tune the desired launch and LKO ejection time. Finally, I use the time of this maneuver for the LKO ejection burn.

But for launches from other bodies, this would require putting a separate spacecraft into the Sol orbit ahead of each body. Is there a more generic method of planning ejections into interplanetary orbits?

You can kind-of fake this same technique with multiple manuever nodes.

Assume you are in LOBO (Low other-body Orbit):

Pretend that you are going to launch your ship to be in the exact same orbit as the Other Body, a few days ahead. You would put a maneuver node at the LOBO ejection point to figure a burn. Once you have that burn complete, you would put another maneuver node on the resulting orbit to circularize -- so put that next maneuver node out there on the yellow orbit after the first maneuver node. Once you have completed the circularized orbit, you would put another maneuver node on it and sweep it around to figure the time of the real ejection burn that you actually want to do. So create that third maneuver node and play with it. (You often need to adjust the orbit number down by 1 when you click the manuever node.) But it does basically work.

Edited July 19, 2016 by bewing