Kerbal Polar Orbit - Need some help getting there!!

[dpraptor]

Hi,

I am attempting to set up a MapSat in a polar orbit, but I keep failing to get that North South orbit. Everything I tried has failed - with orbits going totally wacky.... I looked for any tutorials and could not find one that helped. I use the maneuver mode, since I am not sure how to do it manually. I tired once without it, and set the direction between A & P on the navball - right around 0 degrees, but when I accelerated, the it just did not work. Using the maneuver mode, I used the purple arms to bring the orbit closer to N-S, but the orbit went far out on one side, I brought that in with the green arms, but when I tried to execute the maneuver, it brought both A & P inward.

So can someone let me know the best way to a N-S orbit. Please be as simple as possible, as I rarely fully understand most of the ways people have described it. It was a bit over my head!!! I even searched You-Tube, but found nothing that can help....

Appreciate any help!!

[Goldham]

You go North during launch

[Xeldrak]

Well, can you achieve a non-inclined orbit by hand with any efficiency? It's really the same thing, just keep your rocket on the orange line on the navball.

Start staight up, then do a gravity turn, starting at something between 10 and 15 km hight.

[The Jedi Master]

You go North during launch

Pretty much, yeah. If you want it to be CIRCULAR, though, that's a bit harder...

[Pursuedtank]

If you have a large amount of Delta V, you could sort out the inclination first, by targeting the Mun and getting your ascending/descending node to 90 degrees. Then you could circularise the orbit. Otherwise, just plan using maneuver nodes and try to get it as close as possible.

[SRV Ron]

For polar orbit;

1. Orient your camera view 90* from its default. This is optionsl but will give you a better visual of the turn.

2. When you do the gravity turn, either use the W or S key to turn to 0 or 180 on the nav ball. Orbit as normal.

Do expect to take a little more fuel to orbit then normal as you will no longer have the rotational boost from Kerbal.

[Boris_T_Roach]

Head north on the nav ball when you start your gravity turn at about 10km

Keep an eye on the yellow path marker, it will drift away due to the planet's rotation being added to your flight path so you have to compensate by starting to turn away to about 345 degrees at about 30km up, then just fly upto your favourite orbital height exactly as you would when you fly east from the launch pad.

You'll need a touch more deltaV though because the planet's rotational speed is'nt being added to your orbital speed, so if your craft is marginal for a 250km orbit... MOAR Boosters!

Boris

<<has a comm sat ina 50 000 km polar orbit

[Maddmatts]

Mapsat doesn't have to be in an perfectly polar orbit to map the poles either. I think you can be off by about 7 degrees either way and still get 100% coverage.

[killer420]

I could be wrong but I don't think a perfect north south orbit is the best I think you want it cocked a few degrees. Side note try to land on the exact north pole and watch the nav ball and camera go nutts.

[dpraptor]

Thanks for the great advice! I realized how easy it was when I gave it a shot, and established a perfect polar orbit!!

Its funny how you tend to do things a dozen hard ways and want to slap yourself when you find the correct and direct way is often the easiest!!

Thanks Again!

[dpraptor]

Mapsat doesn't have to be in an perfectly polar orbit to map the poles either. I think you can be off by about 7 degrees either way and still get 100% coverage.

Yes - you are right - I am off by a little - not exactly perfect - but that is fine, I was not looking to be 100% perfect, but now at least I understand what it takes to do that. Eventually I will have to figure out how to do that from space, like at Mun or other planets, but I will learn in steps, and through error, as is always the case!

[bracknellexile]

Hi,

<snip...>

Using the maneuver mode, I used the purple arms to bring the orbit closer to N-S, but the orbit went far out on one side, I brought that in with the green arms, but when I tried to execute the maneuver, it brought both A & P inward.

As others have said, achieving it from launch by aiming North when you start your gravity turn is probably the easiest way to do it on Kerbin but, as you say, for other bodies (and Kerbin too if you have stuff already in space) you'd have to do the inclination change from orbit and that, I guess, is where you're hitting the problems with the manoeuvre nodes. I think I can explain why the apoapsis and periapsis fluctuate so much and maybe help with those orbital inclination changes.

In orbit too, to make inclination changes from an equatorial orbit you aim North and burn. What you're actually doing is burning perpendicular to the plane of your orbit, so you're aiming 90 degrees from your prograde marker which, in a equatorial orbit is pointing roughly east.

I may be wrong here but if I understand it right, I think that the reason for your orbital burns not working properly is that the perfect burn for changing inclination should not be in a single fixed direction ("North") and the manoeuvre node doesn't allow for this, it just shows what will happen if you burn in your target direction for a certain burn duration.

When you burn prograde or retrograde, you're working in two dimensions, burning in the plane of the orbit so the only change is in that plane - e.g. extending apoapsis. All of your burn is parallel to the plane of your orbit working towards changing apoapsis and there is no component perpendicular to the orbit, working to change your inclination. Once you start burning perpendicular to the plane of the orbit (the purple arrows on the manoeuvre nodes) to do inclination changes, there is, in theory, no component of your burn going towards changing apoapsis and it's all going into changing inclination.

In short, the component of your burn that's parallel to your orbital plane changes apoapsis and the component that's perpendicular changes inclination. If you were to burn in some random direction you'd have your burn affect both to varying degrees.

The problem comes in that when you burn perpendicular to your orbit to change inclination you're instantly changing the plane of that orbit and thus the angle between the direction you're burning and the plane of your orbit has now changed, it's no longer perpendicular.

Imagine you're in an equatorial orbit. To increase the inclination you aim 90 degrees from your prograde marker, which is likely pointing east at 90 degrees if you've launched normally, point north - aiming the navball at 0 degrees on the artificial horizon - and burn. Now imagine your orbit was already inclined at 30 degrees. To further increase the inclination there, you'd burn perpendicular to that orbital plane so you'd point your navball at 330 degrees on the artificial horizon, not at north which is perpendicular to the equator. It's 330 because: 90 (east) - 30 (current inclination) = 60 degrees, then we subtract 90 to get a perpendicular to our current orbit: 60 - 90 = 330.

In an extended burn as your inclination changes, the plane to which you have to burn perpendicular is constantly changing so your target direction shouldn't be fixed.

What happens to the manoeuvre node when you use the the purple arrows is that you're adding a component to the burn perpendicular to the original orbit, you're effectively telling it to set up a manoeuvre to burn North for x seconds.

The instant you start that perpendicular burn, no matter how accurate you are on the target marker, you're not burning perpendicular to your current orbit - you're burning perpendicular to your original one. Your current orbit changed the instant you started the burn. The further from an equatorial orbit you get, the further off perpendicular from your current, constantly changing, orbit your burn is.

Now, if not all of your burn is going towards changing inclination (because it's not perpendicular to your current orbit) then some component of it must be doing something else and it is; it's either burning prograde or retrograde which is why the apoapsis and periapsis change too (and why they go crazy when dragging the manoeuvre nodes around).

One solution is to do a series of small burns, changing inclination by a few degrees each time and then adding a new manoeuvre node so that each burn is roughly perpendicular to the orbit that resulted from the last one - this shouldn't affect the periapsis and apoapsis too much.

The other solution is to get a feel for how the direction of burn changes as the inclination changes and do it manually!

Assuming you're in a circular equatorial orbit, your direction of burn should track smoothly from North (0 degrees) to West (270 degrees) along the artificial horizon of the navball as you go through a 90 degree inclination change (assuming you can complete the burn while you're pretty much on the equator - it'll track off the artificial horizon as you move away from the equator). This second solution is a lot trickier though and would take some practice.

In theory, if I could adjust my direction to precisely match my inclination (so when my inclination reaches 20 degrees my burn direction is 360-20 = 340 degrees; when it reaches 45 degrees my direction of burn is 315 and so on) then I could execute a perfect inclination change without affecting periapsis or apoapsis. In practise it's damned difficult! I just had a trial run and even with Flight Engineer telling me what my current inclination was I still ended up with a periapsis at 40Km having started from a circular 90Km orbit.

Hope that makes some sort of sense

[spikeyhat09]

step 1: launch going north, or the red line on the navball.

step 2: achieve a relatively circular and stable orbit

step 3: set the Mün as target, and adjust inclination at the descending/ascending nodes until it reads 90°

this is possible because the Mün has exactly 0° inclination, and is therefore can be used to mark the exact equator.

[Kimberly]

All of the advice in this thread is good, but for mapping purposes, you don't want an exact polar orbit. This is because a perfectly polar orbit always passes over the poles, meaning you're scanning the same area over and over again--ideally, you want to have as little overlap during your scan as possible, so you can map the whole planet in the fastest time. There are a few very specific orbits that are good for this; I suggest you check out this post to look up good orbits for Kerbin. There is an explanation of the method on the first page, but be sure to use the numbers on the second page, which are up to date for the latest version of Mapsat.

[Mephane]

Having sent satellites in near perfect polar orbits (displayed as 90°, periapsis/apoapsis within >10km difference of each other) around Kerbin and Duna - how do you do this on bodies that do not have a natural companion in a perfect equatorial orbit? Actually, the same applies for equatorial orbits there, too. Is there any way to get an indicator relatively to the bodies rotation?

[MockKnizzle]

Using the +NRM setting in MechJeb's SmartASS helps immensely, since it will continuously adjust your orientation as your orbital inclination changes.

[killer420]

Well if you're using mechjeb just launch to 0 degrees and it should be perfect.

[Kimberly]

Having sent satellites in near perfect polar orbits (displayed as 90°, periapsis/apoapsis within >10km difference of each other) around Kerbin and Duna - how do you do this on bodies that do not have a natural companion in a perfect equatorial orbit? Actually, the same applies for equatorial orbits there, too. Is there any way to get an indicator relatively to the bodies rotation?

If you want it in terms of numbers, you can use Kerbal Engineer to show your inclination. (I don't recommend using MechJeb if you want to learn how to do this yourself.) To change your inclination, you should burn normal or anti-normal; these headings are always 90 degrees between prograde and retrograde. The ascending or descending node are the places where your inclined orbit intersects the orbital plane of the body you want to change your inclination relative to (where you pass the equator, if you want to change relative to the body you're orbiting). If you don't have a body to target, you'll have to eyeball where the ascending and descending node are.

Simpleton method: use Kerbal Engineer. Make your heading 90 degrees between prograde and retrograde, and burn a teeny bit. Did your inclination change in the wrong direction? Flip to the other side of the Navball, still 90 degrees between prograde and retrograde. Burn another teeny bit. Is your inclination still going in the wrong direction, or going in the wrong direction after going in the right direction? You're not at your ascending or descending node, take another look at your orbit and try again later.

Edited July 14, 2013 by Kimberly

[Mephane]

Well I know how to do it manually, both by changing planes (expensive!) and by launching North or adjusting planetary encounter for capture directly into a polar orbit. My problem is that most bodies do not have a handy indicator of your inclination relative to their equator. I will try Kerbal Engineer, thanks for the tip.

[Zephram Kerman]

To get a polar orbit from space, simply mess up the inclination a bit during the transfer orbit. Roughly 1/2 to 2/3 of the way from Kerbin to your destination, burn Normal or Antinormal (North or South) a bit, so that your encounter happens to be north or south of the destination.

[lammatt]

fly north once you launch and till you achieve orbit.

then correct the inclination; the trick doing this is to set Mun as target (since Mun's orbit is on the coplanar to Kerbin's orbit and rotation)

[Kimberly]

To get a polar orbit from space, simply mess up the inclination a bit during the transfer orbit. Roughly 1/2 to 2/3 of the way from Kerbin to your destination, burn Normal or Antinormal (North or South) a bit, so that your encounter happens to be north or south of the destination.

Is this more efficient than establishing an equatorial orbit first and then changing inclination?

[lammatt]

no.

definitely not.

the directional vector East and North are normal to each other.

so you would be required to waste a lot of energy to kill the eastward velocity and waste the same amount again to gain a northward velocity to maintain the same orbit height

and as i said before... the trick here is to set Mun as target so that you know where the ascending/decending nodes are. (Mun's orbit is co-planar with Kerbin's rotation

Is this more efficient than establishing an equatorial orbit first and then changing inclination?

Edited July 14, 2013 by lammatt

[Tw1]

For a more imaginative approach, use the Mun!

Then aerobrake, and then circularise at apoapsis.

Not sure you save anything at all this way....

(as you can see, I was trying to land on the pole via a munshot.)

[Scorpi15]

Is this more efficient than establishing an equatorial orbit first and then changing inclination?

I'm pretty sure it is much more efficient, because from a long way out ~10 m/s could ruin your encounter. A little bit from a long way out will be a lot when you get there.

A inclination change is very time- and fuel- consuming, if I remember correctly.