Planet Transfer with plane change: How to ballistic instead of mid-course plane change

[CrashyMcCrashFace]

I use a window planner and do a mid-course plane change normally but I know it can be done direct because I've done it a few times (by mostly luck). So my core question is, Is there a better way than just making guesses and playing with the maneuver node? I have no idea and just pull the node in random directions. A more procedural process? Making minor mid-course adjustments is fine, but just not full-on plane changes.

Also something about the most efficient window AND when the planes cross or something like that. Like when you wait for the right time to launch directly into Minmus orbit. Or does planes crossing matter for interplanetary? I'm super fuzzy on this.

I dare ask for an Explain it Like I'm 5, but that might not be possible. Or 5 year olds would be working at NASA.

Edited June 13, 2018 by CrashyMcCrashFace
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[TheFlyingKerman]

With the Alexmoon transfer planner you just select ballistic transfer and it tells you the normal component of the ejection burn.

There are indeed particularly favorable transfer windows if you can eject at the plane crossing and you arrive while your target is at the plane crossing.

[kerbonara]

It is the same as for Minmus, the problem is that you will have to wait (much) longer for a launch window that matches a plane crossing.

However, if you are transferring to a higher orbit (to Duna and beyond) it may be cheaper to do the inclination change burn later on.

[Kesa]

If you aim at Minmus, the KSC is i Minmus rotation plane twice a day. You can inject in LKO with the right inclination and do a hohman transfer without needing to wait.

For interplanetary transfer, it's more complicated because launch windows are every years, not every 20 minutes (once in LKO), and chances are Kerbin is not in the tilted orbital plan of your target. If it is, then you won't have plan change to do (but you probably shoud, partially at least, especially if your target has no aerobraking or is significantly less massive than Kerbin).

 

If you do have a plan change to do, then a few methods can be used (which also work for trying to rendez vous two ships efficiently) :

1. Matching plan. Combine your ejection speed with a bit of plan change. Eg. if you're few tens of days before ascending node, burn down in order to reduce as much as possible relative inclinason (it will rise again without reaching zero). then do a plan change at the new Ascending node.
2. Getting an encounter. Push your AN/DN to the rendez vous. Inclinaison change have no impact on where your craft will be half an orbit later, and have maximum effect for 90-270°. So this method is not usable without a midcourse correction burn in a "minimal" Hohman transfer. If the AN/DN manoeuver nodes are close enough to you and to the rendez vous, rather than trying to match plan in two burn as in 1, you can do a plan change mid burn that will pull or push the DN/AN to the rendez vous and should give you an encounter. You benefit mainly from aerobraking, oberth effect and burn combinaition (when you get captured by the planet, you effectively get to match its inclination, but potentially for cheaper than by only a burn midflight).

If the AN/DN are midcourse, none of method above work, but that's because you're in a worst case scenario, and a full change plan is actually your only option if you don't wan tto wait for the next launch window, or use gravity assist shenanigans to change plan (EVE is great for that, and it also can be useful to sling you to reach higher or lower bodies).

I suspect you're using 1 currently, and I expect 2 to be better though it likely depends a on the situation, and it's worth plotting them both to see which is the best. For Moho for example, the absence of aerobraking, the poorer Oberth effect than Kerbin ad it being closer to the Sun mean orbital plan change might very be more efficient closer to Kerbin and mid course, favoring option 1.

Edited June 14, 2018 by Kesa

[Xd the great]

Theorectically, if you are hofman transferring your way to everywhere, there is always a plane intersection on your way there. Always. Assuming you are not starting from a polar orbit. Therefore, mid course plane matching? Of course!

P.S. planetary transfers require more accurate nodes. But they are almost the same as a minmus transfer.

P.S. the furthur you are from a gravity pull, the less fuel you use.

[blakemw]

Ideally you want to leave and/or arrive at the ascending/descending node between Kerbin's orbit and the target planet's orbit around the sun, allowing the plane change to be performed at the same time as the ejection/insertion burn.

Of course, this requires the planets to line up right, when using transfer window planner these will manifest as transfer windows with particularly low deltaV requirements and is most obvious with Dres, Moho and Eeloo.

Of course you can also change the plane of the transfer orbit at time of ejection to give a ballistic trajectory, this might be more or less expensive than a mid course plane change. How you do this is you need to make the AN/DN relative to the sun, line up with the rendezvous with the target planet by adjusting the normal/anti-normal component of the ejection burn, this will often involve a steeply inclined ejection, the cost of this can be greatly reduced firstly by launching directly into that inclination (i.e. by doing the planning with a seperate craft, possible cheated), or by first raising your apoapsis around Kerbin to a 1day+ orbit, then doing the inclination change at apoapsis, then completing the ejection burn at periapsis, when the required inclination change is large this results in fuel savings.