Moho to Kerbin - how?

[Lunar Sea]

Trying to plan the return leg of my Moho mission. Since Moho's orbit is inclined 7 degrees to Kerbin's, can I account for that by my Moho parking orbit inclined by the same amount before making the transfer burn (at the relative node)? 

[Palaceviking]

No! 

And don't bother trying to get it right within moho soi, due to oberth it will cost a lot of delta, any plane changes are best done as far from gravity sources as poss, best done halfway back to kerbin.Also- though your manoeuvre may not show an intercept with that inclination, try to hit kerbin as close to ascending/descending node as possible to minimise plane changes.

[Streetwind]

3 hours ago, Palaceviking said:

And don't bother trying to get it right within moho soi, due to oberth it will cost a lot of delta, any plane changes are best done as far from gravity sources as poss, best done halfway back to kerbin.

Sorry, that's not correct. I can roughly guess what you think you're trying to say, but you're mixing up three or four different individual aspects of orbital mechanics.

1.) The Oberth effect actually improves burns done deep in a gravity well, and makes those out in solar orbit less efficient.

2.) Deflection burns (normal or radial) are best done when your speed is lowest, because your forward momentum resists the deflection. This means that deep in a gravity well, those burns indeed cost more dV. But solar orbit is not necessarily better than the outer reaches of a SOI... especially a SOI you just arrived in. Finally, while in solar orbit, you are slowest not halfway to Kerbin, but rather when you are just arriving at Kerbin. By then, it is too late for matching inclinations.

3.) Combining burns greatly reduces costs. If you can include the required normal component in your Moho ejection burn, your combined total burn cost will be significantly less than the sum of normal + prograde would suggest. Oberth helps too, here. For most transfers, this is by far the cheapest method - but see below for the big caveat.

4.) Most of the time you don't let dV cost decide where you match inclination. You either do it on the ejection burn if you can get away with it, or you do a mid-course correction. And in the latter case, you do it at the AN/DN locations, regardless of where they happen to be. The reason for that is that those are the only two points that are simultaneously on your current orbital plane and also on the orbital plane you want to achieve. If you make the burn anywhere else (like during your ejection burn, while not being right on top of AN/DN), you will be physically unable to match inclinations completely. Sometimes the error is small enough to still graze the target SOI anyway, especially if it's big like Jool, and the required plane change is small like for most of the stock solar system. But the larger the required change gets, the more religiously you need to stick to the crossing nodes. Even if they happen to be in inconvenient, expensive burn locations.

 

You are however correct in saying that having an inclined departure orbit around Moho doesn't make it any easier to hit Kerbin. In fact, unless the AN/DN happens to fall within Moho's SOI, even the "don't bother getting it right within Moho's SOI" part is correct too. Moho/Kerbin is one of the very few truly significant plane changes you'll need to perform in stock KSP, and the only way to get it right without a midcourse correction is to already be sitting on the AN/DN when you want to leave. Most of the time, the departure window won't give you that privilege.

Edited August 25, 2016 by Streetwind
Typos, typos everywhere!

[cantab]

Theoretically there's an ideal inclination for your Moho parking orbit to make your ejection burn from. But working out that inclination is not easy, it's more complicated than just "match Moho's orbit" or "match Kerbin's orbit" and it depends on the full details of your planned transfer. Easier to just park in an equatorial orbit. Or if you have a separate orbiter and lander, just anything reasonably low inclination.

[Lunar Sea]

I think I will try to meet Kerbin at a node and avoid the plane change, which is what I did to reach Moho. 

[Snark]

+1 to @Streetwind's excellent explanation.

There are a few different ways to slice this, depending on what tradeoffs are important to you.  Are you looking to save dV?  Or save calendar time?  Or save personal time?

For example, f you want to save dV and personal time, but don't care much about calendar time:

1. Do your launch when you're at Moho's AN/DN with Kerbin's orbit.  Don't even bother worrying about where Kerbin is, or trying to get an intercept.
2. Just do a prograde departure burn (bonus points if your orbit around Moho is lined up to be coplanar with Moho's orbit around the Sun, but frankly it doesn't make all that much difference).
3. Make the burn such that you raise your solar Ap up to Kerbin's orbit.  You want your Ap to be kissing Kerbin's orbit right at the opposite node from where you're launching.
4. Unless you're very lucky, Kerbin will be nowhere near that point on your first orbit.  So you need to tinker.
5. Set up a maneuver node at your Ap to boost your solar Pe just enough to get a Kerbin intercept.  There's a good chance you can't do that on the first orbit, unless you're lucky.  So plan ahead.  If you're willing to wait several orbits, you can do this with a very modest dV expenditure.  In other words, you can do a small burn now in order to get a Kerbin intercept three (or four, or five) orbits from now.  The farther into the future your intercept is, the less dV you need to get that intercept.

You may be wondering "how do I plan maneuver nodes for an intercept multiple orbits in the future?"  There's a simple trick for that (see spoiler section, in case you're not familiar with it).

Spoiler

The "trick" is based on the fact that KSP always shows closest approach markers for a point in time after the latest maneuver node.  So to plan an intercept several orbits ahead, what you do is this:

1. Set your maneuver node where you want it to be (in this case, at your solar Ap), but don't actually give it any dV yet-- just leave it at 0.0 m/s for now.
2. That will cause a dotted-line path to show up, with your post-maneuver trajectory on it.
3. Somewhere on that dotted-line trajectory (doesn't matter exactly where, I usually just use the opposite point from my main maneuver node), drop another maneuver node.  This is your "dummy" node.  You're not going to give it any dV, it's just there as a "bookmark in time".
4. Click the "+" button on your dummy node, to move it one orbit ahead in time.  Note that when you do this, the "closest approach" marker for your target planet (Kerbin) jumps around.  This is because the game is showing you the closest approach that happens after your dummy node.
5. You want the target's-position marker to be just slightly behind the ship's-position marker.  If it's not, then click the "+" button on the dummy node again.  Do this a few more times if need be.  Sooner or later you're going to get something that's at least in the general ballpark.
6. Now go back to your real maneuver node, and drag its prograde handle to give it some dV.  Notice that when you give it just a little bit of dV, the closest-target-position marker moves by a lot-- that's a multiplier effect due to the intercept being multiple orbits in the future.
7. So all you have to do is to do a moderate bit of dV to get yourself a Kerbin intercept at the future time, and you're set.

A different approach you can take, which will get you to Kerbin sooner on the calendar and is very simple to set up but spends a lot more dV:  just wait until Moho's at the appropriate position for the launch window to Kerbin, and launch then.  Unless you're really lucky, this will happen when Moho is not at the AN/DN, which means you'd need to do a plane-correction burn en route to Kerbin, which will be pretty expensive (likely in the neighborhood of 1000-2000 m/s, IIRC).

A third approach is to wait for the launch window, but directly launch into the intercept orbit without doing a mid-course correction.  This requires some tricky tinkering with maneuver nodes.  The total dV ought to be lower than the second method described above (since you're combining the burns)-- it's a question of dV-versus-skull-time. 

Yet another technique, if your return craft is really strapped for dV:  Don't go to Kerbin.  Go to Eve instead, where you have previously set up a convenient refueling station on Gilly.  It takes quite a bit less dV to get from Moho to Eve than from Moho to Kerbin.  So you send your ship from Moho to Gilly, refuel, and then then on to Kerbin.

[Palaceviking]

I stand corrected.

[Superfluous J]

4 hours ago, Lunar Sea said:

I think I will try to meet Kerbin at a node and avoid the plane change, which is what I did to reach Moho. 

I pretty much know all the tricks to get to and from inclined orbits, and this is by far my favorite. Simple (easy to set up and understand), effective (ALWAYS works), and efficient, as you're doing your "alignment burn" at Kerbin (furthest from Sun) AND pairing it with your encounter burn (getting the benefit of "walking the diagonal" and combining two near-perpendicular burns into one burn).

The only problem is that it sometimes takes a Sun orbit or two to meet back with Kerbin, but time's free in this game and orbits so close to Sun go by quick.

[Plusck]

Moho is one of my favourite destinations, probably because of the satisfaction of getting the timing and plane changes right. Plus it is so quick to get there and back.

The most vitally important thing, to keep dv costs down, is to arrive at and leave from Moho when it is on the AN relative to Kerbin that is closest to the sun.
It is true that to get to Moho's orbit furthest from the sun, it's cheaper from Kerbin but the capture burn is massively more expensive - about 1200m/s more expensive IIRC. And leaving from Moho farthest from the sun will also cost about 1000m/s more.
Leaving from Moho, you don't really care about incoming speed as long as you are properly shielded for a sub-30km Pe at Kerbin. It's hot but both the standard heavy-duty command pods (Mk1 and Mk1-2) are fine with about 50% ablator with a Pe at 28km.

And yes, partly correcting the plane as part of the departure burn is vital. It feels like it's inefficient but it really only increases the departure burn by a few hundred m/s.

[juanml82]

Well, the first question is whether you need to reach an equatorial orbit at Kerbin or not. If you aren't keeping your interplanetary transfer stage and you just want to recover a lander (and crew) in Kerbin after a successful landing at Moho, then don't bother with the inclination.

Just get an intercept with Kerbin, enter Kerbin's SOI at any inclination, capture and deorbit. If you want to land near the KSC, once you're in a (probably inclined) LKO, you can time your deorbit burn to hit near the KSC.

[Lunar Sea]

8 hours ago, Plusck said:

The most vitally important thing, to keep dv costs down, is to arrive at and leave from Moho when it is on the AN relative to Kerbin that is closest to the sun.

Oh, I arrived at the close one but was going to leave at the far one...  I guess they can just wait a little long then!