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Matching Inclination While in Orbit


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Hi all,

Most of my missions these days revolve around exploring Eve's system; but I still don't feel particularly confident in my ability to get there (or back again) efficiently or reliably. I've completed a few missions, including a flyby, placing a satellite in orbit around Eve, and more recently, a Kerballed mission to orbit and back.

My method for getting to Eve tends to be to wait until the correct angle, getting into orbit around Kerbin, and set a maneuver that gets the 'closest encounter' marker to be 'fairly close', then at one of the Dn or An nodes in orbit around the Sun, set another maneuver to match inclination... plus a bit of fiddling to try and put me into a fairly close encounter. I might then add another maneuver a little closer to Eve to fine tune.

This has worked... reasonably well... for the missions I've been on; but it does feel like I'm winging it a bit. Returning from Eve is even more unreliable, and essentially devolves into a lot of time spent fiddling with maneuver nodes to try and get some kind of encounter with Kerbin. It all feels 'brute forced', wasteful, and incorrect. Mainly because I don't know what I'm doing.

I don't like to feel like I'm crossing my fingers and hoping for the best, especially as I don't have quicksaves or reverts on, so I wondered if there was any advice - particularly about matching inclinations while I'm in orbit of Kerbin, as theoretically I might be able to plan a maneuver that gets me an encounter while still in Kerbin/Eve orbit. That means I can feel a bit more confident about the maneuvers I'm planning, rather than sending poor Jeb off into interplanetary space, without knowing if he'll ever make it there, or back again.

I don't use mods, so I'm just working with what I've got. Thanks for your help!

Edited by Chequers
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TL;DR: It may feel like you're winging it, but as it turns out - it's largely the way things work. If you really want to skip making mid-course corrections, you can employ an external tool to precalculate a ballistic transfer for you.

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Eve's orbit is inclined compared to Kerbin's orbit. Eve's orbital period is different from Kerbin's. Even the eccentricity differs, although not by much. But, to put it in highly unscientific terms, these differences are not "the same", not "in sync". As in, If you fly transfers at various recurrences of the correct phase angle, then you will not always have the same transfer with the same plane change to do. Each time the planets line up in the correct angle, the alignment of orbital planes and parameters will be different. This means there is no magical solution to the way you should do your Kerbin departure burn that works in every case. You literally have to find a new unique solution each time.

As far as matching inclination directly through your Kerbin departure burn Kerbin goes - the general rule of the thumb is "you cannot". There are special alignments where it is possible, but outside of them, it flat-out doesn't work. If you think about it for a moment, the reason becomes clear. Where do you normally do your mid-course corrections again? The ascending/descending node, I hear you say? Yes, that is the correct spot. At these points (or rather, along the axis that goes through both of these points and the parent body), the orbital planes intersect. So you can go from one to the other. If you are not where they intersect, you cannot go from one to the other. Any attempt to do so will invariably lead you to go into a different orbital plane instead (even if only slightly different). Try and imagine it in your head - two paper discs stuck on top of each other at an angle, intersecting along a line. And you can only travel along the rim of any given disc.

This means that, in order to match inclination with Eve's orbit directly with your Kerbin departure burn, Kerbin itself needs to be at AN or DN with respect to Eve's orbit. Then, and only then, this maneuver works. You'll simply be folding your normal AN/DN course correction into your departure burn. Of course, since the difference in orbital periods, inclinations, and eccentricities are not "in sync", Kerbin won't do you the favor of being anywhere near the AN/DN on most phase angle matches. People still use the so-called "apoapsis transfer" to go to Moho, because a Hohmann transfer to Moho is the single most dV-expensive and precision-dependant transfer you can make in stock KSP, and most people would rather have a cheaper and less finnicky option even if it takes a few years longer to arrive. For Eve though, which is already one of the cheaper transfers to other planets, this seems unpractical.

You can, however, still typically get a direct Eve encounter from Kerbin orbit anyways. The lack of sync just means that this will be fairly easy to do for some opportunities, and fairly hard for other opportunities.

The next thing you need to understand is that, by being in Kerbin orbit, you are also in solar orbit, because Kerbin itself is in solar orbit. KSP does a bit poorly at representing this fact, due to the way you can only be in one sphere of influence at a time. But orbital mechanics won't let you cheat either way. Try launching a spacecraft into a perfect 90° polar Kerbin orbit. Now make a retrograde escape burn like you normally would when going to an inner planet like Eve. What's your inclination around the sun going to be like? 90° too? Hah, not even close. It's likely going to be less than 9°, even. The maximum solar inclination you can achieve by leaving Kerbin is the inclination of the upper (or lower) edge of Kerbin's spherical SoI with respect to Kerbin itself. 84,159,286 meters above Kerbin's own solar orbit. You cannot go any higher without a plane change in solar orbit. And most of the time, you will be far lower than that, even when leaving Kerbin from an inclined orbit.

This means that, in order to attain even a few degrees of solar inclination through your Kerbin departure burn, you'll need a highly inclined departure orbit. And these things are... uncomfortable. For starters, doing plane changes in low Kerbin orbit is so expensive that you'd be spending far more dV trying to enter that inclined orbit after launching into an equatorial one than you would save by not making a course correction in solar orbit. No, in order to make this worth the effort, most of the time you'd have to launch directly into the inclined orbit. This also costs more dV than an equatorial launch, because you're not getting the full eastward rotation bonus; it's difficult to fly, since you need to actively steer rather than just letting the rocket fall over towards the east; and it's even more difficult to time it right. In contrast to an equatorial orbit, which has most of its orbital parameters nulled out and irrelevant, an inclined orbit needs to be inclined correctly. Not just the correct amount, but also the correct direction: the so-called longitude of ascending node (LAN). To get this one right, you have an instantaneous launch window - you need to launch at exactly the right time of day, when the launch site is directly under where you want your inclined orbit to be. You may or may not have some practice with this kind of launch window through exploring Minmus, where you can launch directly into a 6° Kerbin orbit in the moment where the launch site passes under Minmus' AN or DN with respect to the equator, and save yourself a plane change that way. But, in contrast to Minmus, here you have Kerbin's motion around the sun to consider. That means that for any given LAN value you select, the interplanetary transfer window is strictly speaking also an instantaneous window. If you wait too long past your opportunity, your laboriously achieved inclined departure orbit will slowly drift out of alignment with Eve's orbit.

That, anyway, is the "perfect" solution. An instantaneous launch window leading directly into an inclined departure orbit for an instantaneous transfer window that's been precalculated to represent the total trip dV minimum for this particular, unique planetary alignment. And if you're now sitting back and going "that's way too complicated", then at least 95% of the playerbase will eagerly agree with you.

Heck, even most third-party navigation tools agree, too :PEven Alexmoon's mighty Transfer Window Planner, which I absolutely recommend to each and every player, makes the default assumption that your departure orbit is perfectly equatorial and perfectly circular, because deviating from that assumption makes the calculation so much more complicated and the execution ingame so much more prone to imprecision. And it can still compute you ballistic (no course correction) transfer windows to Eve for every phase angle opportunity. You may ultimately be paying a few percent of extra dV to fly these... but considering how effortless it is to go around the stock system, that's honestly negligible.

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The most useful thing I've found to help me with mission planning is this Alex Moon's Launch Window Planner:

https://alexmoon.github.io/ksp/#/Eve/200/Moho/100/false/optimal/false/10/80

For larger or more eccentric initial orbits I do have to tweak my initial burn by comparison, but otherwise you can usually get an encounter straight way if you use the information correctly.

I'd started with the 'Mid course plane-change' method that this tool will calculate for you, so tended to stick to what I knew once I found it.  However, since I've grown to love the 'Optimal' mode it offers and getting those burns right in 3d really makes sense when theres dV to be saved.

I like the fact that it doesn't railroad you into a specific transfer, you can explore around the graph working with the trade-offs of departure date, transfer time, and dV.  Even when customising like that, you can usually get a slightly better suggestion by hitting the 'refine transfer' button without loosing what you wanted.

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@Streetwind, thanks for your detailed and thorough explanation. The principles you've described make perfect sense, and put me at ease about the way I am planning my manoeuvre. Perhaps I have gotten so comfortable with Mun and Minmus missions (I'd done about 10 to each before considering anything outside of the Kerbin system), that I was wanting to get as comfortable with Eve. It's fun learning this new challenge, where it requires a bit of thinking!

Appreciate all your help too, @Linkageless, I'll check it out! 

Edited by Chequers
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Here are a few in-game tools that might help you:

  • Astrogator. Tells you the time to the next transfer window, delta-V required and can plot nodes to get there, but you may have to adjust them a bit. Works for the stock solar system, OPM and MPE, JNSQ, RSS (I think) and will give you transfers to other stars if you have additional solar systems installed. A useful tool that can be used in flight as long as you’re in a stable orbit.
  • MechJeb’s maneuver planner. In advanced transfer mode it gives you a porkchop graph showing the delta-V needed at various times and with various transfer speeds- sometimes waiting a little while gives you a shorter and more efficient transfer. It can plot pretty much any kind of maneuver you like and execute them too, works for virtually any system at any scale (but longer burns are less accurate so larger systems will need more corrections) and is automatically included in any command part- crew pods or probe cores. Different features are tied to nodes on the tech tree but you can disable that feature if you like.
  • Where Can I Go. Particularly useful when building your rockets, it tells you where you can go and if you can get back again, using average values- it doesn’t give you transfer times or nodes, but it’s a useful tool for planning missions. It gives you values for all the stock bodies (planets and moons) and will also give values for planets and stars from modded planet packs including upscaled systems like JNSQ or RSS. Works in editors and in Kerbin orbit but from what I remember not anywhere else.

I use all three regularly and they make interplanetary flight so much easier compared to just eyeballing it.

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One thing i want to add is that when you make the plane change manuever on the ascending/descending node, that's also the perfect place to make a correction manuever. the reason is pitagora's theorem. if you have to burn 400 m/s for plane change, and 400 m/s prograde, you burn at 45 degrees and you end up spending 400*square root of 2 m/s, which is less than 600 m/s. A net saving over having to make the two burns separately.

 

For the same reason, a small inclination correction when leaving kerbin SoI can be convenient, because you are already bunring 1000 m/s, so you can slip a 2-300 m/s in another direction almost for free (1000 m/s prograde + 300 m/s normal will result in just 1044 m/s total). Of course it won't fix your inclination completely, because you are not in an orbital node. However, you can use that to move an orbital node. For example, you can minimize your inclination. or you can push the node closer to kerbin, so it will happen when your ship moves slower, and it will be less expensive to change orbital plane.

those tricks may save a little bit of fuel, though they won't make a huge difference

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10 hours ago, king of nowhere said:

One thing i want to add is that when you make the plane change manuever on the ascending/descending node, that's also the perfect place to make a correction manuever.

 Quite the contrary in many cases. Prograde/retrograde burns are cheaper when your speed is high, normal/antinormal burn are cheaper when your speed is low. That means if you combine both is not the ideal moment for at least one.

Edited by Spricigo
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10 hours ago, Spricigo said:

 Quite the contrary in many cases. Prograde/retrograde burns are cheaper when your speed is high, normal/antinormal burn are cheaper when your speed is low. That means if you combine both is not the ideal moment for at least one.

yes, but if you want to match your orbital plane, you have no choice on when making the normal/antinormal burn. and while you are there, the savings for pitagora's teorem for adding a prograde or radial component to the burn generally far outweight the loss of efficiency. especially when going to closer planets, like eve and duna, where the difference between different parts of the orbit is not huge.

furthermore, the best place to make a prograde manuever is the apoapsis, but you cannot make a correction burn there. if you are going to eve, then by apoapsis you are already on eve, and if you are missing, you must make your correction before that. if you are going to outside planets, then apoapsis is when you start and of course it would be great if one could make a precise burn and be already on a perfect intercept upon leaving kerbin, but good luck on that. they are called correction manuever exactly because you wouldn't need them if the original manuever was 100% accurate.

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1 hour ago, king of nowhere said:

yes, but if you want to match your orbital plane, you have no choice on when making the normal/antinormal burn.

Of course you have choice, the ascending node or the descending node. And you can, during a previous maneuver, change where those will be if that means lesser overall deltaV expenditure.

 

Anyways, I'm not saying that is never a good idea to combine burns, just to be aware that sometimes is not.

 

 

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Despite playing on and off for many years, I've only just realized the benefit of something I've never done before. 

Transferring between Minmus and the Mun is exactly the same as transferring between different planets, and even involves an inclination change, but with much smaller distances, shorter burns, and is therefore an overall faster process.  As I'm doing everything with kOS, this is proving an excellent way to develop my interplanetary scripts without the time and expense of interplanetary missions.  I've never bothered transferring between them before, and I guess lots of other people don't do it either,  but I'd say it's well worth doing for new players to get the hang of the maneuvers needed for interplanetary missions.

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