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Interplanetary flight - apoapsis of ship trajectory is above/below plane of planet


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I struggle so often finding a periapsis when I try to travel interplanetary. Eve as well as several other planets are not in the same plane as Kerbin, so sometimes the orbit of the ship and that of the planet are so far off that the ship does not go through the planet's SoI. Instead, my Ap/Pe is above/below the planet's plane. I end up dragging the maneuver node along the orbit at all kinds of velocities, hoping that the "Pe" or "Ap" symbol appears near the desired location, incidating that I found a trajectory. It is a rather aimless iterative process by me and I hope that I can learn how to improve that. 

For reasons unknown to me, the tools that are available (e.g. https://alexmoon.github.io/ksp/) cannot be trusted blindly - required velocities are often off by a bit, and the angles at which I launch to find a Pe/Ap directly are often quite different than predicted. Maybe this is because my orbits are not exactly equatorial (how do you even do that?), maybe because they are not exactly circular. Maybe because executing a maneuver node is not an exact science either. 

How do you guys launch your interplanetary missions? Just blindly follow the above tool, and correct for the plane mid-flight to catch the Pe/Ap? Or are there tricks to adjust the plane of the planned trajectory while still in a safe orbit around your planet of origin? And how do you do that when leaving for example Gilly back to Kerbin? First go equatorial, then circular, then launch back to Kerbin? And do you just eyeball it to go equatorial / circular, or is there some science to it? 

The worst fear of my Kerbals is to miss their encounter, and drift around the Sun with empty tanks. 

Btw, installing mods to help with this is a no-go for me until I understand it myself. :) 

 

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15 hours ago, Magzimum said:

For reasons unknown to me, the tools that are available (e.g. https://alexmoon.github.io/ksp/) cannot be trusted blindly - required velocities are often off by a bit, and the angles at which I launch to find a Pe/Ap directly are often quite different than predicted. Maybe this is because my orbits are not exactly equatorial (how do you even do that?), maybe because they are not exactly circular. Maybe because executing a maneuver node is not an exact science either. 

Yes, it comes down to precision - especially the ejection angle, which is incredibly hard to just eyeball. However, for your results to be significantly off, it seems like you could be more precise in your execution. If you ever do feel like installing additional mods, the transfer window planner you linked is available as an ingame tool that has a button to display the exact point where you need to place your node. It helps quite a bit with making the burn. Also note: the planner can spit out both ballistic (direct) trajectories, and trajectories with mid-course correction maneuvers, depending on what you set it to. If you set it to "optimal", you may sometimes get one and sometimes the other, depending on which is cheaper. If you overlook this, you may end up trying to treat a trajectory with a mid-course correction as a direct trajectory and end up confused because you arrive nowhere near your target.

Finally, you're probably planning your node wrong. The website displays you a transfer dV cost, but in 99% of all cases that is not a simple prograde vector! it is a combination of prograde and normal or antinormal, and you can only see this if you click the little blue info button next to the ejection dV reading. Yes, that's kind of stupid to hide important info like that, I know, but it's what it is. The larger the normal/antinormal component is, the more massively wrong your result will be if you just treat it all as prograde dV.

 

As far as understanding the problem goes: try to visualize and understand each of the following bullet points in order of presentation, before moving on to the next one. If you have questions on any of them, don't hesitate to ask.

- An orbit around a central focus point defines a plane in which it resides, the so-called "orbital plane". This plane always passes through the focus point. Ergo, the planes of all possible orbits around the same central focus always pass through it.

- Because of this, that means that any two given orbital planes of orbits around the same focus point will intersect with each other. The orbital parameters are irrelevant; they are guaranteed to intersect somewhere. The only exception to this rule is for orbits that are coplanar - that occupy the exact same orbital plane. But in that case you don't have a problem in the first place :P The intersection takes the form of an axis of infinite length passing through the focus point as well as the orbital paths of both orbits.

- In order to assume a specific target orbit, you must be in that target orbit's orbital plane. In order to enter an orbital plane through a maneuver, the maneuver must be executed in a place where that orbital plane actually is. Think of it like: in order to drive from a rural road onto a highway, you must first follow the rural road to the place where the highway actually is.

- If you are on orbital plane A, and you want to go to orbital plane B, you must therefore find a spot which is simultaneously located on both orbital plane A and on orbital plane B. That is the spot you must make the maneuver at - the only spot where you physically can make this maneuver at. This maneuver is called a "plane change", unsurprisingly.

- The only spots where these conditions are fulfilled are those where the two orbital planes intersect. Any spot along the intersection axis is valid. That means that even when the two orbits don't touch each other anywhere, you can still find a suitable spot. in fact, since the intersection is an axis of infinite length that passes through the orbital path of each involved orbit, that means that there are always two such perfect spots on any given orbit. You literally cannot make an orbit that doesn't have exactly two such spots.

- But keep in mind that these two spots do not exist on a lone orbit. They only exist in reference to another orbital plane, which is why you don't normally see them in KSP if you are just minding your own business in your own orbit. They only show up if you have another orbiting object targeted. And they will be in different locations for different target orbits. This is why guides about rendezvous or traveling to another celestial body always start with "select x as your target". Only then you will see all the information you need.

- These two spots have special names. They are called the "ascending node", where your orbit rises from below through the target orbit's plane, and the "descending node", where your orbit falls through the target orbit's plane from above. Ingame, they are marked in neon green as AN and DN markers. Mousing over them will show you how many degrees of inclination you are off from the target orbital plane. (Bonus trivia: the game will also show you a dotted line leading from each node linearly towards your target orbit. This is the axis of intersection between the two orbital planes that I keep talking about.)

 

If you have understood all of the above, you should now know what you must do in order to correctly arrive at a target orbit without ending up above or below. You can check your answer inside this spoiler.

Spoiler

You must target the object you want to go to, and travel to either the ascending or descending node on your current orbit. There, you must perform a plane change maneuver that zeroes your inclination relative to the target orbit, as displayed while mousing over the AN/DN. Which of the two nodes you pick is not important - the only difference is the direction in which you must burn to get the effect that you want. At the ascending node you must burn antinormal, and at the descending node you must burn normal. (Those are the purple maneuver markers, if that wasn't clear.)

If you are sufficiently close to one of the nodes at the time you make your transfer burn, or the inclination difference is very small, you can usually include a normal/antinormal component in your ejection burn and be done with it. This is not only incredibly fuel efficient, but it also means you can avoid coming back for a mid-course correction later. Alexmoon's planner almost always gives you a trajectory that does this in some form, as mentioned near the top.

At other times, this will not be possible or sufficient, and you must make a separate plane change. You can either do this before your ejection burn while still in your original orbit (plane matching), or after your ejection burn while you coast towards your target (mid-course correction). The latter is almost always cheaper, but requires you to remember when you need to stop timewarping and/or doing other stuff and come back to this vessel.

 

Edited by Streetwind
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What I do create a node at the "mathematically accurate" parameters (alexmoon et al) and then I put a second node somewhere on the projected orbit (An/Dn if they happen to be early enough to be dV effective) and play with the normal / anti-normal on that node. If things are correct, then I'll get an encounter. If the mathematically accurate solution doesn't work (usually because small inaccuracies not in the math but on my part) then I tweak the first node and rinse and repeat.

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One thing that can make and encounter easier is to target the planet you're trying to get to  and plan your maneuvers from the ascending or descending nodes. It can be tricky, because they won't always line up with the proper phase angle for your target as given by the alexmoon planner, but if you have enough time and dV you can adjust your orbit to get close to a match. That way, at least your intercept will be crossing the target's plane and you'll be close enough to see the encounter and adjust from there. 

I know you said mods are a no-go, but KER can also help. It has a readout for orbital inclination. So, if you're goint to say.. .Minmus, with a 6 degree inclination, you could use KER to adjust your Kerbin orbit to 6 degrees from equatorial, and then you're orbiting in the same plane. At that point, getting a Minmus encounter should be no more difficult than getting to the Mun.

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9 hours ago, Guber K. said:

I know you said mods are a no-go, but KER can also help. It has a readout for orbital inclination. So, if you're goint to say.. .Minmus, with a 6 degree inclination, you could use KER to adjust your Kerbin orbit to 6 degrees from equatorial, and then you're orbiting in the same plane. At that point, getting a Minmus encounter should be no more difficult than getting to the Mun.

I don't consider KER a mod, lol. It's an essential and should be stock. So yeah, I have KER:) I meant stuff like MechJeb which will just do all the flying. 

How do you figure out where the ascending/descending nodes are for interplanetary travel? They only show up after you leave the Kerbin system and you enter a Solar orbit, but I usually do all the acceleration while still in LKO, to get some Oberth effect. Eyeballing it doesn't seem to work for me. 

Also, it is possible to change the inclination to equatorial? On Kerbin that's easy, as you can just target the Mun and change at the ascending/descending node. But on Eve, Gilly is not useful for that. How do I find out where my ascending/descending nodes are to go to an equatorial orbit around Eve? 

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15 minutes ago, Streetwind said:

Finally, you're probably planning your node wrong. The website displays you a transfer dV cost, but in 99% of all cases that is not a simple prograde vector! it is a combination of prograde and normal or antinormal, and you can only see this if you click the little blue info button next to the ejection dV reading. Yes, that's kind of stupid to hide important info like that, I know, but it's what it is. The larger the normal/antinormal component is, the more massively wrong your result will be if you just treat it all as prograde dV.

This here (quote, above) is mostly new to me, and may therefore be the key information that I needed. I'll try later today to see if I can use this to get my ship back from a high and inclined Eve orbit to Kerbin.

In the meantime, I understand that you follow the tool sometimes blindly. What I mean is that you do the ejection burn without finding the confirmation when the Pe at the destination planet pops up at the end of the burn... since sometimes you only get that at the mid-flight correction burn.

I may consider installing the transfer window planner mod - getting accurate information is not cheating, in my opinion, so I am open to such mods.

Thanks for a great reply!

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9 minutes ago, Magzimum said:

In the meantime, I understand that you follow the tool sometimes blindly. What I mean is that you do the ejection burn without finding the confirmation when the Pe at the destination planet pops up at the end of the burn... since sometimes you only get that at the mid-flight correction burn.

Well, since I can make maneuver nodes on the predicted path from another maneuver node, I can pre-plan a mid-course correction and see the result of it ahead of the ejection burn. But yes, I tend to just trust the tool, since it has never let me down in the past. The only times I have missed my encounter were those when I had a low TWR spacecraft that was not able to execute the burn quickly enough to achieve sufficient precision. In such a case I must make my own mid-course correction, even if none was planned; the tool will not help me with this. Or I should just have gone and done some periapsis kicking beforehand, like you're supposed to with low-TWR spacecraft :wink:

Edited by Streetwind
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13 hours ago, Magzimum said:

I don't consider KER a mod, lol. It's an essential and should be stock. So yeah, I have KER:) I meant stuff like MechJeb which will just do all the flying. 

How do you figure out where the ascending/descending nodes are for interplanetary travel? They only show up after you leave the Kerbin system and you enter a Solar orbit, but I usually do all the acceleration while still in LKO, to get some Oberth effect. Eyeballing it doesn't seem to work for me. 

Also, it is possible to change the inclination to equatorial? On Kerbin that's easy, as you can just target the Mun and change at the ascending/descending node. But on Eve, Gilly is not useful for that. How do I find out where my ascending/descending nodes are to go to an equatorial orbit around Eve? 

I agree about KER. I realized after I posted that Minmus wasn't a good example, since you asked about planets...sorry. The concept I was trying to point out is the same though.  You can find the info on nodes in the wiki, if I remember right, but I personally don't. To explain my earlier comment, what I used do is to look and see that Eve, for example, has an orbit that is inclined 2.1 degrees. So, while in Kerbin orbit, I'll make small normal/antinormal burns until KER tells me that I'm  plus or minus 2.1 degrees. There's no specific readout for it, but if you watch the numbers once or twice it becomes apparent (90 + or -, IIRC, I'd have to look). This basically does what TWP does (I think) in compensating for the normal/antinormal dV, but it does it while in orbit around Kerbin. Then, you don't have to worry so much about the plane change that Streetwind pointed out in his very complete reply, and if you pull prograde while approximately at the correct phase angle you should get some sort of encounter. I have no idea if there is any dV savings when considering the Oberth effect; it probably costs more to do the inclination change in Kerbin orbit than it does in space, but you get some extra from Oberth?  These days I usually just launch and adjust when out in space because it's easier than fiddling with orbits trying to get lined up. I did use TWP for a while too, and before that just scrolled way out on the map in the plane of Kerbin orbit and tried to eyeball where orbital paths crossed and set maneuver nodes there. That didn't work so well, but seemed like a  very Kerbalish solution. :)

Edited by Guber K.
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18 hours ago, Magzimum said:

In the meantime, I understand that you follow the tool sometimes blindly. What I mean is that you do the ejection burn without finding the confirmation when the Pe at the destination planet pops up at the end of the burn...

As far as I'm concerned - absolutely never.

I use the transfer planning tools the other way around - to find out when (roughly) to go and what my best effort should be at that time.

If it's the right time, I'll either get an encounter with a straight prograde burn, or I won't. Since the cheapest encounter should always involve a direct prograde/retrograde escape, I simply drag up a node until it has roughly the right number and my ejection path is parallel to Kerbin's orbit.

Then I look at map view to see whether I have an encounter or not. Add and subtract on the prograde vector until it's hitting the target or is as close as can be. Right click on the target chevrons so that I can always see whether the intercept distance is rising or falling (or if there is an encounter, right click on the destination Pe to see how close it's getting). If there is still no joy, I drop a node on AN/DN* to reduce the difference in inclination.

If it still isn't hitting the target, that means that the ejection angle needs correcting. So I zoom back on my first node and gently drag it along the orbit. At this stage, it really has to work... The only reason it could fail to hit now is if the prograde vector isn't high enough and I'm not reaching the target orbit at all.


Anyway, all that was a long-winded way of saying I like eyeballing.
Also, if you are trying to hit a target which is not on or very near to Kerbin's orbital plane when you encounter, a ballistic trajectory is never going to be optimal. After all, your encounter is supposed to be half an orbit away so a normal burn now will not make any difference to your location in space then, just your angle compared to the sun. Since I want to change my arrival location, I'll need to correct a quarter of an orbit later anyway, and I want to know in advance whether that correction is possible and reasonable. So I need to see it with my own eyes :D

 

* as for AN/DN showing up on the projected path... the game is a bit odd about that. I haven't really noticed when it's properly visible and when it's not. Sometimes it disappears...

It's easy enough to eyeball, though, just by rotating the map view. You can see more-or-less where the paths cross, so that's the place to drop a (temporary) course-correction node while you fine-tune the ejection burn.

Edited by Plusck
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6 hours ago, Guber K. said:

... To explain my earlier comment, what I used do is to look and see that Eve, for example, has an orbit that is inclined 2.1 degrees. So, while in Kerbin orbit, I'll make small normal/antinormal burns until KER tells me that I'm  plus or minus 2.1 degrees....

I'm not sure if I follow, but if I've understood you right, you were doing it wrong :D

The angle of your orbit around Kerbin will affect your orbit around the sun, but your sun orbit angle will always be much less than what it was around Kerbin because of Kerbin's orbital speed being added only in one direction: prograde. I've never really bothered much for Eve, but for Moho, for example, you can match its inclination of about 7° by making your ejection burn on a Kerbin orbit that's about 25° (iirc).

But maybe I understood you wrong.

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2 hours ago, Plusck said:

I'm not sure if I follow, but if I've understood you right, you were doing it wrong :D

The angle of your orbit around Kerbin will affect your orbit around the sun, but your sun orbit angle will always be much less than what it was around Kerbin because of Kerbin's orbital speed being added only in one direction: prograde. I've never really bothered much for Eve, but for Moho, for example, you can match its inclination of about 7° by making your ejection burn on a Kerbin orbit that's about 25° (iirc).

But maybe I understood you wrong.

Yes, almost certainly wrong. I'm no rocket scientist. :) 

  It did get me a lot closer, though, and make it easier to get encounters. A lot easier when paying attention to the nodes. In effect, I was trying to match inclinations before ejecting. Now, I pretty much do as you described above and correct half way to target. 

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For planets with orbits only a few degrees inclined, I don't worry too much about hitting the AN/DN, I just drop a plane change about halfway there.  It may not put you in the same plane as the target, but it WILL allow you to get an intercept without too much effort.  If it's worth moving the plane-change by all THAT much, it's worth adding a normal/antinormal component to your ejection burn (which will also move your AN/DN towards that midpoint).

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On 26.10.2016 at 6:27 PM, Magzimum said:

Maybe this is because my orbits are not exactly equatorial (how do you even do that?),

Let me answer this one :)

Set Mun as your target, and match orbital inclination (burn normal/antinormal at descending/ascending node.)

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1 hour ago, Sharpy said:

Let me answer this one :)

Set Mun as your target, and match orbital inclination (burn normal/antinormal at descending/ascending node.)

And around Eve?

***

In the meantime, I would like to thank you all. I got my crew back on a trajectory to Kerbin thanks to a lot of very useful tips. My method which I tested only once so far:

Get the data from the transfer window planner. Realize that this is a prograde and a (anti)normal part (that was new to me, amazing I missed that thus far). Place two nodes, one of which is the prograde, one has the (anti)normal part. Then execute them one at a time, and actually hit the mark. 

I still don't understand which I should execute first. My Spidey-sense tells me that first 500 m/s prograde, then 83 m/s normal is not the same as going going first 83 m/s normal, then 500 prograde, as in the 2nd case I actually changed which direction prograde is pointing. However, luckily both got me to a Kerbin Periapsis, although not the same. For now, my methods improved a lot, and I think I can use a little eyeballing to get that periapsis down to the very low value that I am looking for, to make my friend Mr. Oberth a happy man.

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1 hour ago, Magzimum said:

I still don't understand which I should execute first. My Spidey-sense tells me that first 500 m/s prograde, then 83 m/s normal is not the same as going going first 83 m/s normal, then 500 prograde, as in the 2nd case I actually changed which direction prograde is pointing. However, luckily both got me to a Kerbin Periapsis, although not the same. For now, my methods improved a lot, and I think I can use a little eyeballing to get that periapsis down to the very low value that I am looking for, to make my friend Mr. Oberth a happy man.

When it comes to inclination changes, mr. Oberth is your deadly enemy.

You want to turn your current trajectory by 45 degrees in the normal direction.

If you're going at 200m/s, that means a normal burn of 200ms sharp.

If you're going 2200m/s, the same change takes exactly 2200m/s of delta-V.

Inclination change is best performed 90 (or 270) degrees from the encounter point - that way you are getting "most bang for the buck", because a normal burn shifts the point of orbit 90 degrees from current position the most. When your encounter is at the apoapsis, burn at periapsis won't move it at all.

That may not be the case if your orbit is strongly eccentric - low speed near apoapsis can still count for more than getting most distance out of the inclination change. But the 'ballistic' transfer is often really wasteful in terms of normal delta-V, the only redeeming factor being you're done with all burns until arrival - and having the encounter assured instead of guessing if the normal burn will be just right. If you have Kerbal Alarm Clock though, you may be better off with mid-route plane change and just deferring the normal burn until you're 1/4 the orbit away from the encounter. Besides, you can usually correct any inaccuracies within some 30-50m/s of radial/antiradial burn there.

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@Sharpy

I think I may not have explained my point so well. What I meant to say is that thanks to the answers in this thread, I am now able to get back to Kerbin from even (slightly) inclined orbits. The science was explained quite thoroughly already, and the tools handed to me by all of you get me a periapsis of between 7000-20000 km above Kerbin, which is not so bad. I meant to say that I can do the final finetuning myself, so that I arrive around a 100 km periapsis, so I can do the final burn at a nice low altitude, so that Mr. Oberth is happy. 

As for the inclination of my departure orbit: I quite understand the disadvantages of having to do an inclination change. I did not choose to be in the inclination that I am in... Gilly did. I understand the mechanics / physics of changing inclination: the further out, the cheaper it gets. 

I still have 2 gaps in my knowledge - at least I feel like I do (there's probably more):

  1. My first problem is that I would like to know how to be on a 0.00000 degree inclination around Eve. How can you do that? There is no convenient Mun to target. Gilly is useless. Is that only possible with additional mods, or is there a way to do it in stock+KER too?
  2. My second problem is that the Alexmoon tool (transfer window planner) tells me to go 500 m/s prograde, and 83 m/s normal. I assume that the tool kinda expects me to do a little Pythagoras, and some more trigonometry, and then aim somewhere off the actual prograde marker at something like the sqrt(500² + 83²), and blast until the correct dV is reached. In Stock+KER I have no markers to aim for, so I have to do them separate: either normal first, then prograde, or prograde first, then normal. If there is a way to execute both in one single burn, rather than two separate burns, I would love to know how to do that in practice.

Let me emphasize that by now, I think I am confident that I understand the physics, but I still need to know some more tricks in the game to put that knowledge to use. 

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16 hours ago, Magzimum said:
  • My first problem is that I would like to know how to be on a 0.00000 degree inclination around Eve. How can you do that? There is no convenient Mun to target. Gilly is useless. Is that only possible with additional mods, or is there a way to do it in stock+KER too?
  • My second problem is that the Alexmoon tool (transfer window planner) tells me to go 500 m/s prograde, and 83 m/s normal. I assume that the tool kinda expects me to do a little Pythagoras, and some more trigonometry, and then aim somewhere off the actual prograde marker at something like the sqrt(500² + 83²), and blast until the correct dV is reached. In Stock+KER I have no markers to aim for, so I have to do them separate: either normal first, then prograde, or prograde first, then normal. If there is a way to execute both in one single burn, rather than two separate burns, I would love to know how to do that in practice.

1.) You can do it with KER. I'm not sure if the default setup includes the necessary fields, but you can configure the Orbit display to include them. What you need is "inclination", and "time to AN" or "time to DN" or both. You then remember that on the ascending node you come up from below, so you need to burn antinormal to flatten out, and with the DN it's the other way. Wait until the time to AN or DN shows only a few seconds left, then burn into the appropriate direction while watching your inclination go down.

2.) Conveniently somebody else asked about the finer details of setting up a Transfer Window Planner node just the other day, and I wrote a wall of text about it, and now you can benefit :)

 

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23 hours ago, Magzimum said:

@Sharpy

I still have 2 gaps in my knowledge - at least I feel like I do (there's probably more):

  1. My first problem is that I would like to know how to be on a 0.00000 degree inclination around Eve. How can you do that? There is no convenient Mun to target. Gilly is useless. Is that only possible with additional mods, or is there a way to do it in stock+KER too?
  2. My second problem is that the Alexmoon tool (transfer window planner) tells me to go 500 m/s prograde, and 83 m/s normal. I assume that the tool kinda expects me to do a little Pythagoras, and some more trigonometry, and then aim somewhere off the actual prograde marker at something like the sqrt(500² + 83²), and blast until the correct dV is reached. In Stock+KER I have no markers to aim for, so I have to do them separate: either normal first, then prograde, or prograde first, then normal. If there is a way to execute both in one single burn, rather than two separate burns, I would love to know how to do that in practice.

Let me emphasize that by now, I think I am confident that I understand the physics, but I still need to know some more tricks in the game to put that knowledge to use. 

I don't have KSP here but I'm pretty sure KER in the Orbit section can have 'Inclination' added, that would be your clue as to how much off you are. As to when to burn, it provides Latitude, so you burn at the equator, when latitude is about to pass through 0.

Alexmoon's calculator is meant to be used with PreciseNode, or its newer alternative Precise Maneuver.

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