How to consistently plan a Tylo>Kerbin Gravity Assist

[rudemario]

I'm sure we're all familiar with the "Tylo flyby to perform a Joolian Capture" For whatever reason, this is extremely easy to perform. I suspect this has to do with the orbital period of Tylo. Any longer and there's a solid chance we'd end up missing it sometimes (if I'm correct)

 

Now, how about the opposite? Using Tylo from Laythe to boost yourself down to a Kerbin encounter? Now, I want to preface this by saying I've accomplished this 2 times before, so I know it's possible–people have mentioned to me it wouldn't be possible before. See here for an example of a successful demonstration. And here's the resulting solar orbit. I am also able to add about 0.5m/s at Laythe and reach Eve as well, if I wanted.

 

Now, how about the opposite? Using Tylo from Laythe to boost yourself down to a Kerbin encounter? Now, I want to preface this by saying I've accomplished this 2 times before, so I know it's possible–people have mentioned to me it wouldn't be possible before.

 

Theoretically, it's completely possible, and I have proven I can do it. The issue comes from the compounding variables involved. If we don't enter a parking orbit around Jool, we're tethered to Laythes orbital period. We need to perform a minimum magnitude of a maneuver to encounter Tylo with enough energy so that when we eject solar retrograde out of the Joolian system, we end up with a low enough periapsis that we can encounter Kerbin.

 

The problem begins to compound here. We have to encounter Tylo such that our ejection angle if we were to lower our periapsis of Tylo to around 10,000 or slightly higher (for greatest affect) results in a solar retrograde ejection. In order to do this, we have to change our magnitude and maneuver time in order to adjust this. But we cannot go below a certain magnitude.

 

As you can see, this has resulted in me (over the course of a few months) spending hours and hours (sometimes 8 in a day during the summer) fiddling with maneuvers and getting what is basically chance encounters with Kerbin. I have been searching for a way to make this consistent, but have not been able to. Mainly, I end up restarting and eventually I end up choosing a good enough place and amount of orbits to start my encounter and things go smoothly. If I restart, I can't usually replicate it for a while. This maneuver will take me more than an hour to plan because of this, if I'm lucky, I might get it in half hour.

 

I thought maybe a parking orbit would make things easier, but that adds extra Delta V to the requirements of the maneuver and results in me getting less Oberth effect bonus. (But you can lower your orbital period to add more opportunities/more accuracy)

 

This is all before, by the way, lining up the maneuver duration to actually make the transfer window. This is complex and adds to the issues, but I feel this can be optimized once a consistent method for planning the maneuver setup can be achieved.

 

Some people have said to me this is too specific to even be possible. But others have casually mentioned the ease of execution of a Tylo>Kerbin. I am at a loss. I spent another 4 hours yesterday after work trying some different methods, and I found Bradley Whistance's video on Gravity Assists. He states that step 2 is to achieve the magnitude required to "place your apoapsis" by adding prograde and then moving the maneuver forward in time to continuously add energy into the maneuver. But this hits diminishing returns with Tylo (since it seems I need an apoapsis between Tylo and the next moon Bop in order to get to Kerbin) and I haven't been able to even get close with his method (he used the Kerbin system for this demonstration)

 

If anyone else can provide some help on the issue, that would be greatly appreciated. I definitely need some other opinions right now.

Edited February 5, 2021 by rudemario

[king of nowhere]

all i can say is that gravity assists are tricky, and depending on a host of other variables.

one thing to take into account is that tylo and laythe have a syncronized orbit, so they only meet at some angles. you can counter that by reaching tylo in more than one orbit; instead of launching and reaching tylo immediately, you launch from laythe to an apoapsis that missed tylo entirely, your orbit brings you back to periapsis (without encountering laythe again) and now you meet tylo on your apoapsis. it should give you more freedom to plan your manuevers. also, it's virtualy impossible to discuss this withou drawing.

as for reaching kerbin, once you have an intercept for kerbin orbit, you can burn there to change your orbital time, and get an actual kerbin encounter in a later orbit. the cost for this is quite small, if you don't mind waiting some orbits. and if you burn retrograde, it's partially offset by a lower intercept speed.

 

finally, i see your posted image has 2 additional manuevers. it is not clear if they are minor course corrections or large manuevers (burning at periapsis during a gravity assist is a common practice for some trajectories). i suggest you highlight them to show their deltaV, like i did, for example, here

[rudemario]

10 minutes ago, king of nowhere said:

all i can say is that gravity assists are tricky, and depending on a host of other variables.

one thing to take into account is that tylo and laythe have a syncronized orbit, so they only meet at some angles. you can counter that by reaching tylo in more than one orbit; instead of launching and reaching tylo immediately, you launch from laythe to an apoapsis that missed tylo entirely, your orbit brings you back to periapsis (without encountering laythe again) and now you meet tylo on your apoapsis. it should give you more freedom to plan your manuevers. also, it's virtualy impossible to discuss this withou drawing.

as for reaching kerbin, once you have an intercept for kerbin orbit, you can burn there to change your orbital time, and get an actual kerbin encounter in a later orbit. the cost for this is quite small, if you don't mind waiting some orbits. and if you burn retrograde, it's partially offset by a lower intercept speed.

 

finally, i see your posted image has 2 additional manuevers. it is not clear if they are minor course corrections or large manuevers (burning at periapsis during a gravity assist is a common practice for some trajectories). i suggest you highlight them to show their deltaV, like i did, for example, here

A note I took before says: With just 3 nodes and as little as 745 dV (80+81+579)

I had an initial transfer of 579, as shown in the image, and then I created a node halfway along to adjust radial and inclination I believe. The final one might be a burn at periapse like you mentioned. It has been a few months since I took this screenshot, and I'm currently at work, but I do have a saved quicksave with this exact maneuver I believe, so I will post what you're asking after I get off work.

I'm not sure the three nodes I used are that important, though, because yesterday I got the transfer to work with just a single node I believe. I think it only reached Kerbin just barely though, and it wasn't in the transfer window because I was just testing.

 

16 minutes ago, king of nowhere said:

one thing to take into account is that tylo and laythe have a syncronized orbit, so they only meet at some angles. you can counter that by reaching tylo in more than one orbit; instead of launching and reaching tylo immediately, you launch from laythe to an apoapsis that missed tylo entirely, your orbit brings you back to periapsis (without encountering laythe again) and now you meet tylo on your apoapsis. it should give you more freedom to plan your manuevers. also, it's virtualy impossible to discuss this withou drawing.

This is what makes me believe it's possible to get this consistent and easy to repeat. A parking orbit or even just an eccentric orbit like you mention (resulting in anywhere from 1-40 rotations around Jool or Laythe) should eventually produce more options. It's just it's hard to then line these up with the proper exit trajectory AND enough magnitude. Seriously, if someone were boot up the game and try to do what I'm doing (Parking orbit around Jool or from Laythe) and tried to do this twice—two separate instances—I would bet they would have problems replicating it and taking the same amount of time (or less) to plan the manuever.

I'm talking I either get it in half an hour, or two hours, and some days I don't even get it at all. Maybe there is something I am misunderstanding about planning gravity assists?

[Spricigo]

3 hours ago, rudemario said:

Theoretically, it's completely possible

Theoretically you just need to plan a Kerbin>Tylo>Jool Capture in reverse. In practice, you already noticed, is not so easy.

The issue what happens if you don't get the he encounter with Tylo right. Arrive in a less convenient time/position  and you still end in orbit of Jool, let it happen when leaving and you end up in Moho instead of Kerbin.

Edited February 5, 2021 by Spricigo

[rudemario]

2 hours ago, Spricigo said:

Theoretically you just need to plan a Kerbin>Tylo>Jool Capture in reverse. In practice, you already noticed, is not so easy.

The issue what happens if you don't get the he encounter with Tylo right. Arrive in a less convenient time/position  and you still end in orbit of Jool, let it happen when leaving and you end up in Moho instead of Kerbin.

Encountering Tylo right is literally the entire battle. if you could just Hohmann transfer to Tylo that would be easy. A hohmann transfer does not contain enough kinetic energy that when bent around Tylo at 10km and ejected towards solar retrograde doesn't get you a low enough solar periapse. So you burn more energy than required for a hohmann transfer. This makes your rendezvous more narrow. Everything compounds.

Someone suggested to me to use KSP Trajectory Optimizer Tool; In it is a tool called "Multi-Flyby Maneuver Sequencer. This tool only allows you to plot flybys of bodies of the same parent. There is no way to plot Laythe>Tylo>Kerbin in this tool because Kerbin is not a child of Jool.

 

Hmmmm... This isn't getting any easier.

[king of nowhere]

no, it isn't getting any easier. i have a ship in jool orbit without enough deltaV to make it back without some assists, and i've spent at least 5 hours of game time in the last three days trying to figure out the best trajectory. I haven't yet reached a conclusive answer.

in my opinion the hardest thing there is getting the hohmann transfer jool-kerbin right. getting a kick out of tylo is easy, but getting it with the right angle to get a good intercept is the problem. I am in the jool-duna transfer window (i'm going to duna instead of kerbin, and i'm starting from a 100000 km orbit, but the difficulties are the same) and a direct burn works wonderful. getting a tylo assist, on the other hand, sends me with a wrong time, so that i fail to get an intercept. oh, i can intercept duna's orbit for less deltaV, but i will find the planet on the wrong place. or once i managed to intersect the planet, but with a wrong trajectory that added a lot of intercept speed, losing any advantage.

 

I think mechjeb should have a useful functionality for planning assists in that you can choose to force an intercept with a body in a certain place. in stock game getting a "close approach" marker is always complicated, to the point that i've now gotten good at eyeballing where my close approach should be, and then create fake manuevers just to make the program see it. but mechjeb should be able to simulate where exactly the planet will be after some time, which would be useful to tweak the right trajectory. maybe, because i don't use mechjeb

[rudemario]

16 hours ago, king of nowhere said:

no, it isn't getting any easier. i have a ship in jool orbit without enough deltaV to make it back without some assists, and i've spent at least 5 hours of game time in the last three days trying to figure out the best trajectory. I haven't yet reached a conclusive answer.

in my opinion the hardest thing there is getting the hohmann transfer jool-kerbin right. getting a kick out of tylo is easy, but getting it with the right angle to get a good intercept is the problem. I am in the jool-duna transfer window (i'm going to duna instead of kerbin, and i'm starting from a 100000 km orbit, but the difficulties are the same) and a direct burn works wonderful. getting a tylo assist, on the other hand, sends me with a wrong time, so that i fail to get an intercept. oh, i can intercept duna's orbit for less deltaV, but i will find the planet on the wrong place. or once i managed to intersect the planet, but with a wrong trajectory that added a lot of intercept speed, losing any advantage.

 

I think mechjeb should have a useful functionality for planning assists in that you can choose to force an intercept with a body in a certain place. in stock game getting a "close approach" marker is always complicated, to the point that i've now gotten good at eyeballing where my close approach should be, and then create fake manuevers just to make the program see it. but mechjeb should be able to simulate where exactly the planet will be after some time, which would be useful to tweak the right trajectory. maybe, because i don't use mechjeb

Exactly this! 

Honestly. When you get enough speed to get low enough to Duna, you're not anywhere near it because you're no longer ejecting towards solar retrograde. Then, when you manage solar retrograde, you don't have enough energy. So you try again. Now you manage to get both down, and you don't end up making the window and now you can intercept. Now it's been 1 hour and 45 minutes and you've basically done nothing. You start over at an earlier date to make the transfer window plus maybe using a radial burn in deep space to make up for the difference but now you can't even get any of those lined up. Now it's been 3 hours and you've literally done nothing.

You can repeat this for however many times I've tried to develop a consistent strategy for making this work.

I still genuinely think it's possible, so that's why I keep trying.

[king of nowhere]

oh, it is possible to get those gravity assists. i just don't think there is definite algorithm to find them easily.

 

in my case, i finally managed to get the right idea: I get to tylo (which, since i was starting from a high orbit, required going down toward jool) and i get captured there. i stay in tylo in a highly elliptic orbit, periapsis at 20 km, apoapsis close to the end of SoI. Going there costed me some 300 m/s, mostly for the retrograde burn from jool orbit, with an extra bit spent for orbital plane matching.

then, 25 days later, tylo and my elliptic orbit were aligned well enough that burning prograde at periapsis would end with me leaving jool at an optimal angle. and with 200 m/s I could get to duna. but duna was in the completely wrong part of the orbit; I'd have to make a  200 m/s retrograde burn at solar periapsis to syncronize my orbit to hit duna on the next solar periapsis. form there it was 700 m/s for capture, for a total of 1600 m/s (including the cost of getting to tylo, and including an additional plane change in solar orbit), total travel time 8 years. A direct transfer from my original Jool orbit would cost 1000 m/s to reach Duna and 1000 for capture. as my ship had 1700 m/s available, it's an important reduction.

Then i found an even better solution. A 300 m/s burn at tylo periapsis would get me a kerbin intercept. kerbin is also in the wrong part of the orbit, but not too far, so it's not too hard to get there. I had to move a bit my burn; i discovered that if i burned slightly before periapsis, i would subtly change my angle of exit from jool so that i would get a higher apoapsis. this wastes a bit of energy (took me 50 m/s more for the same kerbin intercept) but it makes me lose time to find kerbin at my solar periapsis.

I use a kerbin gravity assist to lower my solar apoapsis. I do not intercept duna with this - would have been too much to ask - but i can make a small 70 m/s course correction in my next solar apoapsis to reach duna the next year, with 700 m/s intercept. It's still close to 1600 m/s, but i reach duna in 5 years instead of 8. I could also aim for a second kerbin flyby, to better match my solar apoapsis with duna, it would certainly save some fuel if done correctly, but it's probably not worth the extra time. At duna I can refuel anyway.

 

 

A few tips i can share from all this:

1) often you can get the right solar periapsis at the wrong time. in which case, if time is not a problem, you can always burn there to modify your orbit and meet your target the next time you pass there. it's more expensive than a direct intercept and insertion, but it's likely cheaper than not making a gravity assist.

2) if you reach the intercept point ahead of your target and you must lose time, you can raise your periapsis a bit. it's more expensive, but if it's a small difference then it's overall worth it over manuever 1)

3) if your angle of exit from the intermediate body is wrong, you can get captured there, park in an elliptic orbit, and leave later. it costs, but if it makes the manuever work then it's worth it

Edited February 6, 2021 by king of nowhere

[rudemario]

2 hours ago, king of nowhere said:

1) often you can get the right solar periapsis at the wrong time. in which case, if time is not a problem, you can always burn there to modify your orbit and meet your target the next time you pass there. it's more expensive than a direct intercept and insertion, but it's likely cheaper than not making a gravity assist.

See, this I already knew, but

2 hours ago, king of nowhere said:

2) if you reach the intercept point ahead of your target and you must lose time, you can raise your periapsis a bit. it's more expensive, but if it's a small difference then it's overall worth it over manuever 1)

This is interesting to keep in mind. It's something I haven't thought about before, and even though it doesn't help with this specific issue, because I use a life support mod, it does actually help a lot when time is important. You're right. If I have to lose time, this does help save time over other riskier options. Thank you for this, interesting to keep in mind.

2 hours ago, king of nowhere said:

3) if your angle of exit from the intermediate body is wrong, you can get captured there, park in an elliptic orbit, and leave later. it costs, but if it makes the manuever work then it's worth it

This is by far the coolest thing you mentioned. I never thought of something like that. A flyby where you get captured? Does that even result in the same effect? It's really interesting, because you still gain the speed from being captured, but you're storing it for a later date. This is actually really cool and something I haven't even thought of. Honestly, this just might be crazy enough to work. You still gain the speed of the secondary body when you're captured I believe, so that means you must still gain an assist when you leave it. Or maybe this is wrong, and being captured doesn't actually result in an assist.

But, if you look at the example of explaining a gravity assist with the Train method, i.e What happens to a ball thrown by a person standing next to an oncoming train? If the person throws the ball at 20 km/h, and the ball hits the front of the train and bounces off at 20km/h, what happens to the ball? Does it still go 20km/h? or does it go whatever the train was going plus 20km/h?

In that example, if we take what you mentioned, I think it would still be true. It's just that instead of the ball bouncing off at 20km/h, it sticks to the train until the ball leaves later. The problem is the ball loses it's momentum, so maybe the energy comes from the person throwing, and that would be lost (getting captured instead of flying by). Or maybe the energy comes from the train, and spending a little to leave the train (just enough to burn back and get uncaptured again) it results in postponing the "bounce off the train."

This is really cool, and something to think about. This might make the issue a little easier, I will give it a try.

3 hours ago, king of nowhere said:

Then i found an even better solution. A 300 m/s burn at tylo periapsis would get me a kerbin intercept. kerbin is also in the wrong part of the orbit, but not too far, so it's not too hard to get there. I had to move a bit my burn; i discovered that if i burned slightly before periapsis, i would subtly change my angle of exit from jool so that i would get a higher apoapsis. this wastes a bit of energy (took me 50 m/s more for the same kerbin intercept) but it makes me lose time to find kerbin at my solar periapsis.

This is neat as well, I'm also going to try burning around periapsis to change my resulting orbit and see what that does. I've tried before, but I know more now, so maybe this might result in more success.

[king of nowhere]

14 minutes ago, rudemario said:

This is by far the coolest thing you mentioned. I never thought of something like that. A flyby where you get captured? Does that even result in the same effect? It's really interesting

since you find it interesting, i thought to add pictures too

this is my starting jool orbit, with the manuever to get to tylo. first periapsis lowering, the 143 m/s on the top right corner. second, the purple node in the top middle, it's a plane change. It would certainly be possible to skip some plane changes and save some fuel, but this manuever was complicated enough as it was. trying to syncronize planes so that when i leave for kerbin i will already have a good angle is beyond me. green and red manuever nodes are not manuevers, but i had to point them to make the game see the intersection. the thing is, i knew i would eventually cross tylo in that orbit, so i forced the pc to see it. when i got a close enough encounter, i adjusted it with some small correction manuever. i think i included that in the purple node. Finally i reach tylo, with only 50 m/s to get captured. that part is important. the capture burn is all energy you're losing, so this manuever only works if this amount is small

this is actual tylo insertion, after one last correction manuever (you always need some when planning ahead so far). It ended up even cheaper than planned.

you may already know this, but you can estimate the cost of a capture burn without making a node, by looking at how much the trajectory curves around the planet. the more it curves, the less it will cost to get captured. the closer  you are to making a full circle around the planet, the closer to capture.

this is the manuever after leaving jool. first the red manuever on top, it's a plane change. a kerbin flyby at 3 years 197 days send me on the red orbit. this does not reach duna immediately, but i take care of that with the purple manuever at apoapsis (4 years 130 days) and for only 70 m/s i can reach duna at year 5 day 250. with the gravity assist i can control my new kerbol apoapsis, so it's not too difficult to set up a second kerbin flyby if i need.

here is the manuever where i leave tylo. notice how i put it slightly before periapsis to go in a bit of different direction. of course, if i overdo this, i lose all the energy i have in the elliptic orbit. that ellipse is like 800 m/s already spent to raise a tylo periapsis, but it only works if i burn prograde at periapsis.

this is actually the main limitation of this manuever: you must burn according to your ellipse, and your ellipse stays fixed. so the direction at which you enter tylo orbit makes a big difference. in my case i was lucky to find a way to enter there. if i had my tylo apoapsis pointing away from kerbol, the whole manuever would be impossible. you'd have to circularize and raise apoapsis in another direction, and that would cost 1500 m/s.

this is the full trajectory. you can see the kerbol apoapsis is slightly higher than jool, and this sets my kerbin encounter right. if i had not done it, i would have crossed kerbin orbit 50 days earlier.

 

i also have a life support mod, but i have food for at least 10 years. food and water are light, better to stock extra

 

 

[Guest]

I’ve used the capture-and-go technique to get to Moho via Eve. Never considered that it would also work here. Nice work!

[rudemario]

22 hours ago, king of nowhere said:

since you find it interesting, i thought to add pictures too

I played around with this yesterday a little, but it was getting late.

I saved your post in my notes for future, the "capture and go" technique as Brikoleur called it, so I can definitely give it a try and confirm it actually behaves as I think it does (same as flyby but delayed).

Thanks for the detailed writeup!