Help with Jool transfer?

[box_brain]

I've been having a lot of trouble lately getting to Jool with my ground base, space station, two planes, and a couple of rovers (Getting to laythe from Jool orbit should be easy). Each of these have their own nuclear tug (the tugs have 3240 units of fuel and two nuclear rockets). It took me a long time to assemble this in kerbin orbit and now I want to see if this was worth it. I wanted to do a test first so that nothing went wrong, so I took the tug with the ground base and waited for the window to Jool (using ksp transfer calculator). I used the maneuver node to see how much I would need to burn to get to Jool, the best i could get was 1934m/s for a Jool periapsis of about 1,400,000,000. I started my burn, keeping the header on the blue vector. I stopped there cause it seemed like something wasn't right. So first question is, should I keep the header on the prograde vector? Next question is how to lower that huge periapsis before actually getting into Jool orbit? Hope you guys can help me

[K^2]

You don't really have to lower the periapsis before getting there. The difference in fuel will be fairly minimal. If you aren't planning aerobraking, it might make things easier. But otherwise, once you're on transfer orbit, just set up a new node and play with the controls. See which ones make periapsis go down. If inclination is the reason you aren't getting good periapsis, best place to adjust that is at ascending/descending nodes.

But yeah, once you set the navigation node, it's just a matter of burning in direction of blue marker. The only reason you might be getting off course this way is that your "control from" part of the ship might not match the direction of your engines. That can happen if your overall ship is a multitude of docked parts. Just find a docking port, capsule, or probe that points the right way, click on it, select, "Control from here," and you should be set.

[numerobis]

I beg to differ. If you wait until you enter Jool's sphere of influence, it'll cost you about 1km/s, which is very important. The further out you reduce the periapsis, the better. The typical place to do that is at the ascending (or descending) node, but if you can do it from your exit burn you're in even better shape. Keep in mind that if you timewarp through the SoI switch out of Kerbin to solar orbit, some numerical error will creep in and your periapsis will change.

But if you're actually going to Laythe, you should probably aim for Laythe directly.

[entaran]

Any entry into jool can aerobrake into a direct laythe capture after a max of about 5 orbits (depending on where laythe was when you entered)... I've even had entries directly encounter laythe before I got anywhere near jool but it's way harder to aerobrake on laythe when you're coming in at 6kms or more (laythe is very small!)

[TMS]

For simplicity's sake when bringing in fleets, I sometimes place them in parking orbit around Jool, just outside of Laythe's SOI. Then it's just a case of nudging them into Lathe capture.

Not as efficient as direct capture, but not exactly expensive on delta V either.

[K^2]

I beg to differ. If you wait until you enter Jool's sphere of influence, it'll cost you about 1km/s, which is very important. The further out you reduce the periapsis, the better.

That's only true if you want to park low in Jool's orbit. If your goal is transfer to Laythe, it's going to be considerably less.

Suppose, you are heading to Jool on Hohmann transfer orbit from Kerbin. You'll hit Jool's SOI at 1,750m/s. If your periapsis is right there at the edge of the system, transfer to Laythe will cost you 1,700m/s and you'll reach Laythe SOI going 1,310m/s. If your goal is parking in Laythe orbit, you'll have to burn these 1,310 and then some, but that's an absolute minimum for parking anywhere in Laythe SOI. So your total Delta-V is 3,060m/s.

Now, suppose you are going to pass right above Jool's atmo. You'll buzz the giant at 9,740m/s and it will cost you 1,080m/s to slow down to Laythe transfer. You'll reach Laythe's SOI going 1,270m/s relative to it. So your total Delta-V is going to be 2,450m/s. So you do save 600m/s with extremely low periapsis. Of course, if you are going to drop that low, you might as well aerobrake and save a lot more. A more typical periapsis of a few thousand km over Jool will save a lot less.

Finally, if you decide to hit Laythe directly, by carefully adjusting your periapsis from a distance, you'll enter Laythe SOI at 4,860m/s. So this is definitely not the best option if you are planning to put a station there. On the other hand, if you are planning a direct landing on Laythe, there is no reason not to do that. You'll hit Laythe atmo going 5,450m/s, and that's perfectly survivable even in Laythe's thiner atmosphere. (Though, once they enable over-G damage, you'll have to chose re-entry angle very, very carefully. If you come in too steeply, you can easily hit 50G+.)

[Luis]

To answer the first part of your question, the blue node vector shows the direction you should be pointing as your craft reaches the manoeuvre node. For a simple transfer burn, that will line up with the prograde marker at that point. But if you start your burn earlier, they don't match. Because the blue marker is point at your prograde vector at some future point along your orbit, it actually angles downwards towards Kerbin a bit. And the earlier you start your burn, the bigger the discrepancy. This means that a lot of your thrust is wasted by firing in the wrong direction. If your burn produces exactly the same amount of delta v on either side of the node, the average vector should still line up with the projected orbit trajectory. But your orbit will be a lot more circular - less of a narrow elipse - because of the extra thrust you have expended that wasn't prograde. And because exactly balancing the delta v across the manoeuvre node is hard to do, you will probably also miss your intended intercept point to some degree.

I find it is generally best to start your burn when the prograde marker is no more than 5 degrees from the node marker on the nav ball. You still burn towards the blue marker, and you burn for as long after the node as you did before the node. The resulting transfer orbit will still differ from the projected trajectory a bit, but it's relatively easy to correct for mid-course.

Most of the time, your engines won't be powerful enough to make a transfer burn within this narrow +/- 5 degree window though. In which case you have 2 options. You can gradually raise you orbit by burning at the node over several successive orbits (these are called periapsis kicks), or you can just burn prograde for the whole time. The latter is much less accurate and will require more mid-course correction.

A compromise that I have used effectively on some missions is to use more engines for the first part of the burn. At the beginning, when your orbit is highly curved, the prograde and node vectors diverge more quickly, so you want to burn hard to get enough useful thrust for the short window that they line up. Later as your orbit straightens out, you can shut down everything except for your most efficient engine and make the rest of the manoeuvre by simply burning prograde.

TLDR: The blue manoeuvre vector only points in a useful direction when you are close to the node. At other times, use the prograde vector.

[Rune]

To answer the first part of your question, the blue node vector shows the direction you should be pointing as your craft reaches the manoeuvre node. For a simple transfer burn, that will line up with the prograde marker at that point. But if you start your burn earlier, they don't match. Because the blue marker is point at your prograde vector at some future point along your orbit, it actually angles downwards towards Kerbin a bit. And the earlier you start your burn, the bigger the discrepancy. This means that a lot of your thrust is wasted by firing in the wrong direction. If your burn produces exactly the same amount of delta v on either side of the node, the average vector should still line up with the projected orbit trajectory. But your orbit will be a lot more circular - less of a narrow elipse - because of the extra thrust you have expended that wasn't prograde. And because exactly balancing the delta v across the manoeuvre node is hard to do, you will probably also miss your intended intercept point to some degree.

I find it is generally best to start your burn when the prograde marker is no more than 5 degrees from the node marker on the nav ball. You still burn towards the blue marker, and you burn for as long after the node as you did before the node. The resulting transfer orbit will still differ from the projected trajectory a bit, but it's relatively easy to correct for mid-course.

Most of the time, your engines won't be powerful enough to make a transfer burn within this narrow +/- 5 degree window though. In which case you have 2 options. You can gradually raise you orbit by burning at the node over several successive orbits (these are called periapsis kicks), or you can just burn prograde for the whole time. The latter is much less accurate and will require more mid-course correction.

A compromise that I have used effectively on some missions is to use more engines for the first part of the burn. At the beginning, when your orbit is highly curved, the prograde and node vectors diverge more quickly, so you want to burn hard to get enough useful thrust for the short window that they line up. Later as your orbit straightens out, you can shut down everything except for your most efficient engine and make the rest of the manoeuvre by simply burning prograde.

TLDR: The blue manoeuvre vector only points in a useful direction when you are close to the node. At other times, use the prograde vector.

I can actually consistently hit Minmus' SOI with 10 minute burns from very low orbits (~70kms), just pointing at the blue marker, and at the start of one of those, it's like more than 30 degrees offset with the prograde marker. Almost no deviation form the projected final orbit or total delta-v, too (wobbling uncertainty is often more of an issue). So I'd say it's not much to sweat about, as long as you have T/W over 0.1. I suspect following the prograde vector should be slightly more efficient... but I'm afraid that not easier, and it is definitely much more tedious than timing things and going for coffee.

Rune. Minmus is a decent interplanetary travel analogue, very tiny SOI very far away, so practice on it.

[Luis]

That might work for Minmus but Jool is a different story. It's *much* further away and the transfer burn requires around 1900m/s dV. If your Minmus burn is taking 10 minutes, a Jool transfer burn will need at least 20. At LKO, your orbital period is only 30 minutes, so starting 10 minutes early will put you 120 degrees away from the manoeuvre node. If you just point at the blue navball marker from that far away, your final trajectory will deviate hugely from the idealised manoeuvre node projection. You will waste a huge amount of dV and quite possibly miss the encounter altogether.

[box_brain]

Thanks guys for all the info, sorry I haven't replied at all, but I decided this was all a bit too much for me and I cancelled the operation. I took runes advice and am currently doing missions around Minmus and the Mun. Whatever this might mean to you guys, I've done 3 missions to mars, all of them sloppily done. So I don't think I'm ready for journey to Laythe yet.

[Matrix Aran]

Thanks guys for all the info, sorry I haven't replied at all, but I decided this was all a bit too much for me and I cancelled the operation. I took runes advice and am currently doing missions around Minmus and the Mun. Whatever this might mean to you guys, I've done 3 missions to mars, all of them sloppily done. So I don't think I'm ready for journey to Laythe yet.

There's alot of good advise in this thread, and to be honest, as long as you can build something with enough dV to get to Jool, its no harder a target than Duna.

Minmus is definitely very good practice, especially when dealing with plane changes, albeit a rather subtle one in Minmus' case, and it should get you used to small SoIs.

With regards to the thrust vectors and maneuver nodes, I'd advise breaking your departure burn into two stages. As long as you don't have a tight dV budget you don't have to be too picky about the timing of these. The first should take you to just under kerbin's escape velocity, an orbit with an apoapse just under Minmus's orbit, or shorter still if you're underpowered. Then when you swing around your periapsis again you can make the final burn to eject you towards Jool.

Midcourse corrections can easily iron out targeting errors and get you a periapse around Jool of your choosing. If you're going with aerocapture, make sure you fine tune the periapse before you enter the Jool SoI, and this goes double for Laythe. Considering the thrust you described in earlier posts, I'd strongly recommend aerocapture around Jool.

I should point out that the above are meerly suggestions based on my own experience, and as this thread has shown, there's more than one way to tackle the problem. That said, I don't think you should give up on Laythe so quickly. If you can make it to Duna, even sloppily, Laythe is definitely doable, and a very rewarding challenge.

[Luis]

Duna is actually harder to reach than Jool, in some ways, because its SOI is much smaller. An encounter with Jool is more tolerant of sloppiness during your transfer burn.

[njd80]

I can vouch for the method that Matrix Aran suggested. A two stage burn is good for both time and thrust vectors. I tend to pick the prograde/retrograde vector - enter in all the required d/v - confirm I get a encounter with the target, then drop the maneuver d/v and burn to beyond Mun orbit. Set a 2nd node when you come back to periapsis for the remainder of the initial d/v.

As for getting to Laythe - aerobraking at Jool with periapsis of ~114km will put you approximately at the right Apoapsis. You may need some adjustments to actually encounter Laythe, but you will have saved enough fuel to do them.