Gravity assists not boosting apoapsis above initial orbit? [ANSWERED]

[Ol’ Musky Boi]

I tried to solve this problem on my own, but after a few hours of attempts I've decided to come to the forums.

So my situation is that I'm trying to complete a gravity assist from Duna, around Kerbin (maybe twice) to boost my apoapsis to Jool. I've seen it done before so I figured it'd be easy enough. But for some reason, no matter which side of Kerbin I pass by (trust me, I've tried both sides) I can't actually raise my apoapsis. Best case scenario i'll graze Kerbin's SOI and my orbit won't change, worst case it lowers my periapsis to Eve. What am I doing wrong? Here are some photos of me passing on the "correct" side of Kerbin, and the resulting "incorrect" orbit.

Edited July 29, 2018 by Ol’ Musky Boi

[Superfluous J]

47 minutes ago, Ol’ Musky Boi said:

I've seen it done before so I figured it'd be easy enough.

It's one of the hardest things in the game. It's one of the very few things I haven't even bothered trying.

[Ol’ Musky Boi]

20 minutes ago, 5thHorseman said:

It's one of the hardest things in the game. It's one of the very few things I haven't even bothered trying.

Well darn, gravity assists were integral the entire mission I was doing. I've gotten pretty used to doing gravity assists around the Kerbin system, (you can save around 100m/s on a Minmus mission if you fly by the Mun, which is great for SSTOs), so I thought they'd be similar in difficulty. Still, I'd like to at least complete my mission, instead of going home empty handed.

[Pand5461]

The velocity gain in the gravity assist is directed along the vector from the periapsis to the assisting planet.

To raise the apoapsis around the Sun, the ship needs a positive velocity increase in the prograde direction, so the perikee must be behind Kerbin in its orbit around the Sun.

[Superfluous J]

There is a mod or web page that lets you find launch windows for multiple gravity assist missions, but I don't remember the name. A little time on Google may find it.

[Ol’ Musky Boi]

1 minute ago, Pand5461 said:

The velocity gain in the gravity assist is directed along the vector from the periapsis to the assisting planet.

To raise the apoapsis around the Sun, the ship needs a positive velocity increase in the prograde direction, so the perikee must be behind Kerbin in its orbit around the Sun.

Although it doesn't show up well in the picture I posted (my bad) that's exactly what I did, and it actually lowered my orbit! I feel as if i have stumbled across the one situation in which this could've happened because I honestly don't understand what's going on.

[Laie]

45 minutes ago, 5thHorseman said:

It's one of the hardest things in the game. It's one of the very few things I haven't even bothered trying.

Yup, same here. Well, not quite, I do bother sometimes, but for me it's unintuitive and hard to plan. A key aspect is that you don't simply gain or lose speed (strictly speaking you don't: if you just fly past Kerbin, your speed as you leave the SOI will be just the same as wen you came in), but that your trajectory gets bent by a few degrees (which can result in quite a noticeable difference after SOI transition). For gravity assists to work, it's not enough to do a close fly-by, but you have to come in at the right angle as well.

My usual screenshot:
That's two maneuvers ultimately leading to Duna, one of them using a munar assist for ~90m/s gain. See how I'm approaching the Mun's trajectory at nearly a 90° angle, but fly off at about 60°? Although I can't really explain the how or why, I know that this is what makes the magic happen. Flying out "more parallel" than you came in is what speeds you up.... I think. Did I mention that I haven't entirely mastered the topic myself?

On interplanetary trajectories the angles can be much less conspicuous.

For your Kerbin encounter (which I can't really see in the screenhot), it seems as if you're coming at Kerbin in a somewhat hohmann-ly fashion, flying nearly parallel at the time of encounter. That's why you lose no matter which side you pass it on.Try to make it so that you approach it at a small angle, that is, your PE is a little inside Kerbin's trajectory and you encounter it on the way out.

Edited June 24, 2018 by Laie

[Ol’ Musky Boi]

1 minute ago, Laie said:

Yup, same here. Well, not quite, I do bother sometimes, but for me it's unintuitive and hard to plan. A key aspect is that you don't simply gain or lose speed (strictly speaking you don't: if you just fly past Kerbin, your speed as you leave the SOI will be just the same as wen you came in), but that your trajectory gets bent by a few degrees (which can result in quite a noticeable difference after SOI transition). For gravity assists to work, it's not enough to do a close fly-by, but you have to come in at the right angle as well.

My usual screenshot:
That's two maneuvers ultimately leading to Duna, one of them using a munar assist for ~90m/s gain. See how I'm approaching the Mun's trajectory at nearly a 90° angle, but fly off at about 60°? Although I can't really explain the how or why, I know that this is what makes the magic happen. Flying out "more prallel" than you came in is what speeds you up.

On interplanetary trajectories the angles are much less conspicuous.

For your Kerbin encounter (which I can't really see in the screenhot), it seems as if you're coming at Kerbin in a somewhat hohmann-ly fashion, flying nearly parallel at the time of encounter. That's why you lose no matter which side you pass it on.Try to make it so that you approach it at a small angle, that is, your PE is a little inside Kerbin's trajectory and you encounter it on the way out.

That'll probably do it, thanks. I was aiming for a periapsis exactly at kerbin for minimum fuel expenditure, but clearly that did the opposite.

The counter intuitiveness  of orbital mechanics does amaze me some times. 

[Zhetaan]

@Ol’ Musky Boi:

Gravity assists are relatively easy to intuit, but very difficult to accurately execute.

Your current problem is that you are coming to Kerbin from behind (meaning Kerbin's retrograde), but far more important is that you are leaving Kerbin's sphere of influence to Kerbin's retrograde.  Without seeing the in-system trajectory, I can't do much to refine the path, but I know this to be the case from the result.

When it comes to gravity assists, the key to getting the most assist is that you must leave the assistant's sphere of influence along its prograde or retrograde line (or a line parallel to it; exact colinearity requires that you travel through the assistant).  The ideal assist also comes in along a line parallel to the prograde or retrograde, but moving in the opposite direction.  I assume that you know this much; I'm simply reiterating for thoroughness.

The issue is that sometimes, you don't want (or cannot get) all of the assist possible.  Cases where you don't want it would be, for example, when you have a specific destination in mind and don't want to overshoot it.  In the stock system, this is usually more of a problem with transfers between moons than between planets.  Cases where you cannot get it would be the Mun, for example:  the Mun can provide some assist, but it doesn't have enough mass to warp your trajectory all the way round unless you have such a low-energy trajectory that you cannot get anywhere in the first place.  You need to approach from a different vector to use the Mun; this is half of why @Laie's picture above shows the assist the way it does (the other half is that when you're starting from low Kerbin orbit, you're always going to have a radial component to your Mun encounter velocity unless it's a perfect Hohmann transfer anyway).  Other cases where you cannot get it would be high-velocity intercepts:  the higher your initial velocity, the less assist you can get because the assistant cannot warp your trajectory so much regardless of its mass.  In the picture above, the Mun is providing an assist to leave Kerbin's space on an interplanetary trip, so it's more important to leave Kerbin's sphere of influence in the correct direction for an efficient Hohmann transfer; leaving the Mun's sphere in the Mun's prograde would provide more assist, but in the wrong direction, and encountering the Mun at a point where its prograde and Kerbin's prograde align requires a trajectory that is too low-energy to make it to another planet if you're approaching the Mun from low Kerbin orbit.

As a side point, if you look closely, the Mun assist pictured above is perfectly positioned for what it is.  The angle between approach and departure is bisected by the Mun's prograde vector, so the assist gets everything it can within the rather severe limitations imposed upon it by the Mun's status as a low-gravity body and the trajectory's status as a (relatively) high-energy interplanetary departure.

In your case, you are coming in from the outer system, which limits your options for an apoapsis-boosting assist because your orbital velocity at your craft's periapsis is going to be higher than Kerbin's velocity (it has to be, or you wouldn't be able to return to the outer system).  That means that you cannot let Kerbin catch up to you, and that's a problem, because the ideal assist in this case is one where you enter and leave Kerbin's sphere from the prograde direction--and getting that direction requires that Kerbin catch up to you.  Therefore, the solution is a less-than-ideal assist where you approach Kerbin from an angle.  The preferred angle in this case is one where you meet Kerbin from sunward and use it to throw you out in its prograde direction; in other words, from Duna, you need to burn to lower your solar periapsis below Kerbin's orbit and catch Kerbin on the outbound leg.

Of course, burning to go below Kerbin's orbit wastes fuel, so you'd need to be certain that the amount of assist you get from using that fuel is greater than you'd get from a direct burn.

For my part, I think it is going to be difficult to get what you want.  Duna-Jool windows are relatively rare (transfer window frequency drops the farther from the sun you go, naturally), and Duna-Kerbin-Jool windows would need to be even rarer than that.  Given that it's a less-than-ideal assist, as well, I don't think you can get enough from Kerbin to go all the way to Jool in one pass.  There may be a D-K-K-J or maybe a D-K-D-D-(D-D-D-D?)-J assist in there somewhere, but you will need to find it.

Edited June 25, 2018 by Zhetaan

[Ol’ Musky Boi]

@Zhetaan I actually managed to get my apoapsis to Jool in 2 Kerbin gravity assists where I waited about 20 years for an encounter (no problem with time control 2.0). I then captured into a high orbit using Tylo and landed on pol to refuel ( I never actually mentioned it, but I’m doing a grand tour by SSTO).

Now that I better understand how gravity assists work I don’t find them too difficult, It’s just a lot of playing with manoeuvre nodes. I’ll make another post at some point when I’ve completed the tour of the mission patch I made and the craft file if people want it. (maybe I can give the new steam workshop a spin?)