How to Reach Polar Orbit Around Kerbin

[xXIndestructibleEVAXx]

I recently made a SCANsat satellite to put in orbit around kerbin, and I've reached the appropriate altitude of 500 km's, but when I place a maneuver node and drag on the normal or anti-normal node, it inclines the orbit, but also changes the apoapsis/periapsis (depending on where the maneuver node is placed.) The only way I could think of countering this is to burn normal as well, which seems rather inefficient. So, I've come to you for help. Oh great kerbal masters, what is the most efficient way to reach a 500x500 polar orbit?

Edited July 8, 2015 by xXIndestructibleEVAXx

[TheTom]

Have you tried just starting it into polar orbit? Like just right from the launchpad. Just start it with a 0 degree heading, not 90 degree. Finished.

[xXIndestructibleEVAXx]

yeeaaaaaaaahh probably should've done that... but, it's too late now and I don't really want to revert... Plus, I need experience in orbital maneuvers like this. But next time, I will just launch to the north.

[Xannari Ferrows]

If you have the Delta-V, all you really need is a normal/anti-normal kick. This means always burn perpendicular to your trajectory until you've reached 90 degrees [or 0]. I recommend maneuver nodes for that, as managing your SMA will be difficult.

You could also edit the save if you wanted.

[xXIndestructibleEVAXx]

Is there any particular place in the orbit to burn that is most efficient?

Edit: also, when I tried to place a maneuver node to do that, it changed the apoaps and periaps significantly, so much so that the scanner wouldn't work. How would I counter this?

[Montag]

Inclination changes are most efficient at higher altitude - but you may wish to do them at the ascending / descending nodes to avoid other orbital changes. For Kerbin, this will be where you cross the equator.

[xXIndestructibleEVAXx]

How do you see where your ascending/descending nodes are?

[Reddeyfish]

How do you see where your ascending/descending nodes are?

If you have a sat contract, it will show up in the map.

If you don't, then it doesn't matter for polar orbits.

If you want to do it in a single burn, you'll need to burn both normal/anti-normal and retrograde. The retrograde part of it kills your previous orbital direction, and the normal/anti-normal part recreates your orbital velocity for a polar orbit.

(the same applies for any other 90 degree-ish change)

If you're moving into a polar orbit and don't particularly care about which polar orbit, then burning at apoapsis will work perfectly well and be the most efficient. If you care about which polar orbit (i.e. for a sat contract), then burn at the inclination points relative to the target orbit.

Edited July 8, 2015 by Reddeyfish

[Xannari Ferrows]

They aren't listed if you don't have a target. You would need to eyeball it, or use something that tells you inclination.

EDIT: Sneaky...

Also worth mentioning that you would want to burn prograde at your descending node, and burn normal at your ascending node.

[xXIndestructibleEVAXx]

could I set kerbin as a target? I don't currently have anything in LKO.

[justidutch]

They are the pink triangle-type thingies on the navball. And a long burn like that, those nodes will move over time, so if you keep your heading centred on the moving ascending or descending node, your api/peri will NOT change at all.

[xXIndestructibleEVAXx]

I know what the target node is, I just don't have anything to target.

[fchurca]

How do you see where your ascending/descending nodes are?

Quite conveniently, in the Kerbin system, the Mun has a perfectly equatorial orbit. Targetting the Mun will thus show you the ascending and descending nodes of your orbit with respect to an equatorial one.

A less-stock but quicker option can be to use the Kerbal Engineering Redux mod.

[justidutch]

You don't need to target anything. Just find one of the pink triangles and burn, baby burn!

[Xannari Ferrows]

I know what the target node is, I just don't have anything to target.

Normal changes are more or less independent of current inclination. You can start the burn from anywhere, only that the farther away you are from either node, the more Delta-V you'll need.

EDIT: Extra Delta-V is needed for SMA.

Edited July 8, 2015 by Xannari Ferrows

[xXIndestructibleEVAXx]

Xannari ferrows, when I do that it changes the apoapsis, which is why I need to know where the ascending/descending node is.

justidutch, I now what normal is. I need to keep my apoapsis/periapsis the same, if I do that it will change it.

fchurca, thank you. that helps a lot. I will do that.

[fchurca]

Glad to help! Also, if you have an eccentric orbit and the ascending and descending nodes are at different altitudes, burn at the highest one. You will be travelling more slowly there, so the same Dv will change your trajectory by a greater angle (or you will need less Dv for the same intended deflection).

Edited July 8, 2015 by fchurca

[xXIndestructibleEVAXx]

okay, I got it figured out. I set mun as the target, and burned normal. when my apoapsis got too high, I burned retrograde. I continued to do this until I ran out of fuel. My end inclination was 80.57 degrees. unfortunately, it appears that I misread the scanners description, so the sat's entire orbit was too high. So I terminated that one, and soon I will launch another into a lower orbit, which will save pleny of dV and allow for a better inclination. Over all, this thread has been very helpful and I have learned a lot. Thank you.

[FancyMouse]

If you have the Delta-V, all you really need is a normal/anti-normal kick. This means always burn perpendicular to your trajectory until you've reached 90 degrees [or 0]. I recommend maneuver nodes for that, as managing your SMA will be difficult.

You could also edit the save if you wanted.

Maneuver node burn in this case is actually different from always burning normal/antinormal. Maneuver is a little bit more efficient (dV ratio between two approaches is sqrt(2):pi/2)

[ajburges]

Two pages of thread and no mention of gravity assist or vector addition? [Facepalm]

First, without a SoI change, if you are willing to sacrifice some orbital altitude, the most efficient plane change is to calculate the direction of your velocity difference. For the simple problem of 90° plane change, you just want to be going the same velocity orthogonally. Just burn halfway between retrograde and normal until your plane is correct (don't change heading during burn). Pathagerous tells us that such a manuver costs a tad more than 1.4 times your current velocity in dV. You will lose a little altitude, but the linear approach is cheaper than the integral approach. Altitude loss is proportionate to TWR.

If your SoI has any nearby Muns-- I mean moons, you can perform cheaper plane changes with a gravity assist. 850 m/s for a Mun encounter 10-50 m/s for a mid course inclination change, a little more for corrections and aerobrake (above 50 km) and you can be in any Kerbin inclination you want for less than 1 k/s and a few Kerbin days!

Edited July 8, 2015 by ajburges

[Hcube]

Pro tip : Just set the mun as target, its orbit is perfectly equatorial. You could try a bi-elliptic transfer to save some deltaV.

[Skalou]

the best is to plan it before

I launch direct in the right direction,

because landed on the launchpad you have yet a speed of 175m/s from the orbit point of view,

you need to launch a few degrees more west (5° from experience)

or you can drive your rocket to a pole with wheels to save this Dv

be carefull of the direction of the wanted orbit (launch to 95° or 355° heading)

I launch 2 min before crossing the target plane.

stock scanner need to be at 0° but with scansat you can have some angle to cover faster all the area without scanning many times at poles.

Edited July 9, 2015 by Skalou

[Sharpy]

Set the Mun as target. Then fly to it and fly by it in such inclination that it ejects you into Kerbin polar orbit (fly-by through nearly polar trajectory of Mun).

Although launching directly into polar orbit IS preferable. Just click the "speed" indicator to change it to "Orbit" to know where your Prograde *really* is, to lose that 100 or so m/s of equatorial speed coming from the planet's rotation.

[Wemb]

I'd suggest you pop on something like Kerbal Engineer when you're learning - the data it produces will really inform your experience.

As for doing plane changes (especially major ones), the higher your altitude when you them, the cheaper they'll be - so you might want to try getting into a much larger orbit, doing the plane change, and then transfering back to 500km altitude.

Though, as others have said launching north (or south) from the start is the way to go - you won't get a perfectly polar orbit cause you're moving sideways due to the planet's rotation - but it'll be near enough.

Wemb

[ajburges]

Set the Mun as target. Then fly to it and fly by it in such inclination that it ejects you into Kerbin polar orbit (fly-by through nearly polar trajectory of Mun).

Although launching directly into polar orbit IS preferable. Just click the "speed" indicator to change it to "Orbit" to know where your Prograde *really* is, to lose that 100 or so m/s of equatorial speed coming from the planet's rotation.

Actually, its not simple as fly through a polar Munar trajectory. To get polar inclination, you need to ensue you leave Munar SoI with a component of around 500 m/s in the direction of Mun retrograde and around 200 m/s normal. You want a trajectory to enter the Mun SoI in retrograde escape trajectory (I recommend free return as it will place inital Mun Pe near where you want), establish a 20° inclination enroute, and prograde burn at Munar Pe for the required escape velocity.

Edited July 9, 2015 by ajburges