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Precision Orbital Insertion


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I've done many trips to Minmus (and some to Mun, Duna, Ike, Jool) and one thing which I've not developed the knack for, is precisely inserting my spacecraft into a planned orbit. Currently they are somewhat random. I can get an intersection, then tweak it to a low value, but its still a bit unpredictable...until my last attempt. I tried this (for the Mun):

1. Fly as close as possible to 0deg inclination (for the Mun) LKO (for Minmus, its a bit more involved)
2. The burn from Kerbin orbit is typically done so the apoapsis is ~45deg ahead of where Mun currently is. So, start there and use the maneouvre planning tool to burn prograde so the Ap touches the orbit of Mun. You should achieve an intersection - ie Mun approach, Pe, escape all light up in different colours. Highlight the Pe
3. Then tweak the 'ejection angle' (its not really an ejection.....because you're not leaving the SOI of Kerbin, just intersecting another thing orbiting it) to minimise the Pe. If you got the burn length right, you should have a big amount of angles where there is no Pe, ie you would hit the Mun. So, choose the angle in the middle of the range. It is important (see later) to get this 'no Pe, strike the Mun' condition.
4. Then reduce the burn time until you no longer strike the Mun, instead aim for ~10,000m. This is a good target altitude consisting of ~7500m terrain clearance and another ~2500m to give a bit more "flight time" when slowing from a low orbit to prepare for landing without striking that terrain at high (horizontal) speed (during the slowdown you'll lose a bit of altitude because the burn takes time). By reducing, you'll arrive behind the Mun, it will swing you round anticlockwise (as viewed from above) and put you into a prograde orbit
5. Half way towards the Mun, do another maneouvre plan. This time adjust only the normal/antinormal (pink triangle icons) to influence the Pe to a lowest achievable amount. What this does is puts you "onto the equator" of the Mun, ie makes your eventual inclination (nearer) 0deg.

Then optionally, I did this, just as a tryout:

6. Try tiny amounts of prograde burn to confirm it takes you closer (ie lowers Pe).
7. Burn prograde/retro as appropriate to lower Pe to nothing (ie striking Mun), but keep burning and burning until Pe appears again. Instead of arriving "late" and being behind the Mun, you arrive "at the same time" (hit it), or "before" (in front of it) and its enough to influence your eventual orbit into a retrograde one.
(8. I should have made tweaks normal/antinormal again to get myself onto the equator, but I didn't)

Its difficult to visualise it, I think KSP does the best it can but its difficult to see. I did the above and my resultant orbit was 171deg ie retrograde and nearly equatorial.

KSP%20-%20Mun%20retro.png

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AAhhhh I forgot to add the question. That's the theory for how to get it more/less on 0deg or 180deg inclination.....but how would you set yourself up for, example 30deg inclination? And how would you hit that 30deg with a precise Longitude of Ascending node - I guess you'd need to figure out the current 'lunar day of the month' (ie to know the longitude angle you'll be coming towards the Mun at), then factor in your travel time to know precisely the arrival and subsequent orbit you'll be put in?

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20 hours ago, paul_c said:

AAhhhh I forgot to add the question.

Ah, okay.  Sorry 'bout that.  Moving back to Gameplay Questions.  ;)

20 hours ago, paul_c said:

how would you set yourself up for, example 30deg inclination? And how would you hit that 30deg with a precise Longitude of Ascending node

So, aiming for a particular inclination exactly is kinda tricky.  If you're willing to do a little bit of trigonometry you can work it out more exactly, but generally what I do is just eyeball it, which I find gets me within a couple degrees with a bit of practice; and then I can just manually adjust the inclination once I'm in orbit, which isn't too expensive if it's just a small adjustment.

The easiest way, for me at least-- both for the inclination and for getting the longitude of ascending node-- is to eyeball it in map view, but with the view set to Kerbin centric (instead of the default ship-centric view).  A common use case for this is when I've taken a contract to put a satellite into a particular orbit, and the orbit is inclined by a fair amount, like 30 degrees or whatever.

In such situations, here's what I typically do:

  1. Get the target inclination.  It's generally specified in the contract wording.  Or, alternatively, I can just switch control to a satellite I have in LKO in a perfectly equatorial orbit (I always have at least one), and then check out what the AN/DN says in map view.
  2. In the VAB, design my satellite that's going to be heading to the target orbit.
    • (Side note:  For launching into equatorial orbit, I always design my craft facing east, rather than the game's default of north, because that makes way more sense to me-- I do my gravity turn by pitching down, rather than yawing right.  I used to do this by manually rotating the root part by 90 degrees in the VAB each time, but eventually that got too tedious so I made a mod, VABReorienter, that makes that the default for me.  The remainder of this discussion assumes that the craft is facing east to start with, for launching to zero-inclination orbits.)
  3. For a zero-inclination launch, I normally face east.  Since this is going to be an inclined orbit, after I finish building my craft, I use the rotate tool to rotate the root part (and therefore the whole ship) left or right a bit to match the desired inclination.  For example, if I'm going to be launching at the ascending node and the target orbit has an inclination of 30 degrees, then I would rotate my craft to the left (i.e. north) by "a bit more" than 30 degrees.
    • Why it's "a bit more" than the target inclination:  This is because Kerbin is rotating, so even when you're at zero velocity relative to the ground, you're already moving eastward at around 174 m/s.  So therefore you launch a little bit farther to the north (for launching at the AN) or south (for launching at the DN) than the target inclination.  Just eyeball a smidge different, like 3 or 4 degrees.  Close enough.  ;)
  4. Okay, now my rocket is facing the correct direction to launch into the desired inclination, or pretty close to it, anyway.   Now launch it to the pad but do not yet take off.
  5. You want to get the longitude of ascending node correct.  The way to do this is to launch your ship when it's directly under the AN or DN.  How to do this:
  6. While you're sitting on the pad, switch to map view.
  7. By default, the map view is centered on your ship.  Change this by double-clicking on Kerbin, so that the view is centered on Kerbin, instead.
  8. Rotate the camera to latitude zero, i.e. so you're precisely over the equator looking "horizontally" at Kerbin.
  9. Now rotate the longitude of the camera until you're looking precisely edge-on at the target orbit.  From this viewpoint, it's collapsed to a straight line passing through Kerbin's center.
  10. Note also that at this viewpoint, you'll see the pointy tips of the AN/DN markers just about touching each other, right at Kerbin's center.
  11. Now, without moving the camera, start to time warp.  You'll need to warp forward a few minutes or hours until your spacecraft (sitting on the launch pad) is directly over Kerbin's center in the camera view, i.e. directly between the AN/DN markers.
  12. Congratulations!  Your ship is now sitting directly under the AN or DN of the desired orbit.  Now's the time to launch.
  13. Launch the craft and immediately begin your gravity turn by pitching down, same as usual.
  14. Climb to LKO and circularize as you would usually do.
  15. You'll have pretty close to the desired inclination, but likely you'll be off by a degree or two.  So, after you're well underway (e.g. when you're going mostly horizontally and you've reached the point in your gravity turn when you're pointed, say, 30 degrees or less above horizontal), you can look at the map view and check your AN/DN with respect to the target orbit, and adjust your aim slightly to the left or right for the remainder of the burn in order to correct it.

Anyway, that's what I do-- it's not mathematically perfectly accurate, but it gets pretty close and requires only a very small fine-tuning correction burn to make it exactly accurate once in orbit.

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Many thanks for the answer.....however that's the answer for how to put the ship at a particular inclination/LAN of a Kerbin orbit.

I was after the way to do an orbital insertion (to say, Mun or Minmus - but could equally apply for interplanetory) to a precise inclination (which isn't 0 or 180) and LAN.

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Your LAN is set by which moment you cross the SOI boundary. You can only adjust that arrival time at the Mun by a few hours, out of a 6 day orbit. So, generally it's not something that you can have much control over. And trying to control it is very fuel-inefficient. It's much easier to just adjust your LAN after arrival, by leaving yourself with a very high Ap after your capture burn, and making sure the Ap is near your ascending or descending node for your target orbit.

For adjusting inclination at your arrival, that's done by adjusting how far above/below the equator you are when you cross the SOI boundary. This means you need to either be in a non-equatorial orbit when you leave Kerbin, or you need to do a normal/antinormal burn as a mid-course correction. KSP does not give you any tools to do this except for your eye, your patched conic, and the Mun's orbital path.

 

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