ending up in retrograde orbit around Jool

[seyss]

Can anyone explain why this is happening?

A retrograde orbit around Jool makes impossible to land on its moons.

Thanks.

Edited April 13, 2015 by seyss

[Superfluous J]

When you come in to orbit around Jool, before you've even gotten there, focus on Jool and look at your orbital path. You can tell where you're entering and exiting Jool's SOI because the exit end has a little circle on it.

Make sure that path goes around Jool in the same direction as the moons. You have do do this intentionally on purpose the first few times, but after that it'll become second nature and you won't even think about it.

[Starhawk]

When you set up a maneuver to another planet, or when you make course corrections during the trip, you can set your trajectory so that you come in on either side of the target. One side will put you in a prograde orbit, the other side in a retrograde orbit. Once you have an encounter set up, focus on the target body to see which way your encounter will leave you orbiting, and adjust if necessary.

Hope this helps.

Happy landings!

[Spheniscine]

You have to be careful with the interplanetary transfer, and make course corrections to ensure that you enter at the correct side of your destination. It is helpful to focus view on the destination (double click, or click → Focus View) so that your trajectory is shown relative to it. Ensure your hyperbolic trajectory passes the right side before the exit symbol.

Even now, just after you enter the SOI, you might be able to reverse your orbit by burning anti-radial for a while; it would require more fuel than a midcourse correction, but less than after orbital insertion/aerobraking.

Or you could capture to an extremely elliptical orbit, then reverse your orbit at the apoapsis, where your velocity is low.

Edited March 19, 2015 by Spheniscine

[impyre]

"Can anyone explain why this is happening?"

If you're using the optimal transfer orbit (which I assume you generally are), then you stop the burn the instant you have intercept. Since Kerbin orbits at a lower altitude than Jool, this means you'll approach Jool from the lower (sun-facing) side. If you stop your transfer burn as soon as intercept is achieved, you'll most likely have a Jool periapse on the sun-facing (radial-in) side. If you then perform your capture at that periapse without changing it, your normal will be opposite that of the moons.

There are three ways you can fix this: 1) burn a little extra during the initial transfer burn, 2) perform a mid-course correction (likely burn prograde a bit, or some combination of prograde and radial out) or 3) establish capture first (but only enough to just barely bring the apoapse into Jool's SOI) then burn to flip your normal at apoapse.

The first option is arguably the most efficient, and the most difficult (as even the slightest variation can cause a significantly more difficult capture).

I believe the second and third options are both equally viable; however, option three will be more difficult to plan if you're performing an aerocapture (which seems likely).

So my recommendation is a mid-course correction, a handful of m/s (at the right time) is enough to make drastic alterations to your encounter profile, and you'll have more than enough time to tweak it precisely. Midcourse corrections are also the best way to establish a good aero capture, so this makes the most sense. The only downside is that mid-course corrections are... less than intuitive with the large distances and orbital forces at work... but I assume you are familiar with this type of maneuver if you're trying to get to Jool.

[mrclucks]

I apologize for being naive and/or a noon, but how does a retrograde orbit make landing impossible?

[Xaver]

If you go in direction of a pillar and put out your hands to the left and right. If you go the left side you will go right around the pole and the other way around.

Sometimes you have to look in space if the pillar comes to you instead of you come to the pillar. Jool is i.r. fastet then you when you doing a homann transfer.

Edited March 19, 2015 by Xaver

[Vanamonde]

I apologize for being naive and/or a noon, but how does a retrograde orbit make landing impossible?

Not impossible, but much more difficult. In prograde, you approach at (your speed) minus (moon's orbit speed), while in retrograde, you approach at (your speed) plus (moon's orbit speed). Approaching retrograde requires you to loose much more dV in order to avoid smashing into the surface.

[Deutherius]

Alternatively, you can use Jool's moons to reverse your orbit via gravity slingshots (especially with Tylo) and/or aerobraking+sling at Laythe. It would be a pain to plan, though, so just avoiding the retrograde orbit/reversing your orbit at very high Ap would be better - but the possibility is there.

I apologize for being naive and/or a noon, but how does a retrograde orbit make landing impossible?

It's not impossible, it's just way harder.

Say you want to land at Vall. Vall orbits Jool at 2558.8 m/s. You are coming at Vall from a retrograde Jool aerobrake. Your velocity relative to Vall when the two of you meet will be about 4 km/s (just a rough guess, don't quote me on that. Point is it's higher than Vall's orbital velocity around Jool), which you have to (almost) cancel out using precious fuel (and/or Vall's gravity) just to get into orbit around Vall. Comparatively, the traditional transfer from Jool prograde aerobrake to Vall orbit will take much less dV due to both the craft and Vall going in the same direction.

Landing on a celestial body from a retrograde orbit around that body will also be harder than landing from a prograde orbit, because of the body's rotation. You want to match the velocity of the surface for landing, and being in retrograde orbit increases your velocity relative to that surface. It's like trying to exchange something between two cars, one of which is driving at a constant speed. If the cars are driving in the same direction, you just have to match speeds. If the cars are driving opposite to each other, one of them will have to slow down, stop, turn around and then match speeds. Aplied to spacecraft and surface of a celestial body, stopping and turning around costs precious fuel.

[thadisjones]

If yer goin' to Laythe, that marble's coastin' at a hot little 3.22 km/sec. So yer approach to that orbit's gonna be sneaking up on each other like a kitten chasin' a baby, or be standin' still an tryin' ter grab hold of a truck gunnin' by ya at 60 miles per and pulling yerself on board (or maybe smacking into the lithosphere at 3 klick a second). Dependin' on yer choosin' pro or retrogradin'.

[cantab]

You know, a Laythe aerobrake would do the trick to reverse your orbit round Jool. But you need to get it *just* right, you want to almost be captured by Laythe and when you escape you'll be going prograde round Jool. It might not be practical to be precise enough.

[thadisjones]

Laythe aerobrake would do the trick to reverse your orbit

The worst case scenario for this BTW is an inescapable capture trajectory plowing into Laythe at a relative velocity upwards of 6400 m/s. So what I'm saying is, make sure you are set up to record a video.

[seyss]

Man I'm following that illustrated guide and I always ending up in retrograde.... I've landed on Jool fine before without using the guide, but with a little luck due to planetary alignment

[Kryxal]

Landed on Jool? I doubt that ended well...

[seyss]

Haha sorry I meant orbited.

[seyss]

right after entering Jool's SOI I burned anti-radial and my orbit turned prograde! right after it turns prograde you should switch to burning radial instead of anti-radial, to keep increasing your periapsis.

thanks!

- - - Updated - - -

You have to be careful with the interplanetary transfer, and make course corrections to ensure that you enter at the correct side of your destination. It is helpful to focus view on the destination (double click, or click → Focus View) so that your trajectory is shown relative to it. Ensure your hyperbolic trajectory passes the right side before the exit symbol.

Even now, just after you enter the SOI, you might be able to reverse your orbit by burning http://i2.wp.com/www.lets-play-sphen.com/wp-content/uploads/2014/11/ksp-antiradial.png?resize=24%2C24 anti-radial for a while; it would require more fuel than a midcourse correction, but less than after orbital insertion/aerobraking.

Or you could capture to an extremely elliptical orbit, then reverse your orbit at the apoapsis, where your velocity is low.

this guy. the anti-radial burn was VERY small

[KerikBalm]

Well this is relatively easily fixed if you are using stock, at least until 1.0.

You can't burn up on reentry, so get an intercept with laythe, and use this:

http://alterbaron.github.io/ksp_aerocalc/

Basically, what you should have done is made sure you passed Jool with it on your "left" (ie it is between you and the sun).

A course correction long before the encounter is much much cheaper than the course correction you need to do now...

Except you can use aerobraking at laythe, so use aerobraking at laythe.

Normally, I directly aerobrake at Laythe, its even cheaper than a Jool aerobrake, as far as visiting the other moon is concerned (and much cheaper for a laythe visit).

This may not be viable later when reentry heating is added - or I may at least have to ensure I encounter laythe as it is travelling in the same direction, and not the opposite (as you would face now with the retrograde orbit)