How should I set up my Jool Encounter?

[JoeSchmuckatelli]

Having stumbled and been helped along the way into an upcoming Jool Encounter - I'm looking for tips to 'do it right the next time.'

@Snark suggested this thread: Looking to send men and machines back into the fray - but with less risk / frustration.

 

EDIT: This question relates to a discussion found in this thread: http://forum.kerbalspaceprogram.com/index.php?/topic/129741-nerv-rocket-use-stupid-from-low-kerbin-orbit-jool/ .  We decided to post this as a unique thread to help me (and others) understand how to use orbital mechanics for an elegant insert that saves fuel - rather than the brute-force 5 minute retrograde burn I had planned.

Edited January 14, 2016 by JoeSchmuckatelli

[Snark]

1 hour ago, JoeSchmuckatelli said:

Having stumbled and been helped along the way into an upcoming Jool Encounter - I'm looking for tips to 'do it right the next time.'

For context, the thread that spawned this one is here.  Areas of particular interest for the OP are "how to set up a reverse gravity assist to help capture to Jool" and "anything else to know or do."  He's arriving in a nuke-powered ship with very low TWR.

[GarrisonChisholm]

I am very interested in this conversation, and I think some of the advice given from my Failed Laythe Orbital thread (or, whatever I called it...) would be useful, to save typing for Snark & others. 

The following is the "key" post of that thread, by Snark himself, and is useful for Jool planning;

"You can't use a gravity assist from a body to help with your orbital velocity for other things in that body's SoI.  For example:  You can use a Jool reverse gravity assist if you're going to some other planet beyond Jool (none in stock, but hey, maybe you're running OPM), but not to go to a moon of Jool.  Once you're inside Jool's SoI, whether you're on Jool's prograde or retrograde side only matters to things outside of Jool's SoI.

If you want to get a gravity assist in the Jool system to capture to the Jool system, you need your gravity assist from one of the moons (generally Laythe or Tylo, because of their size)-- not from Jool itself.

The way this would work is that you do a flyby of Laythe or Tylo, which doesn't capture you to the moon but does slow your Jool-relative velocity down to the point where you're captured to Jool.  Then you can loop around and do more gravitational billiards on a subsequent pass, if needed.

For example, if you want to use a gravity assist from Laythe to slow down, there are two types of path that will work.

One is when you are still "descending" -- that is, you haven't reached Jool periapsis yet, your Jool altitude is still decreasing.  In this case, you want your trajectory to cut right in front of Laythe (on its prograde side), so that it will slow you down.

The other is when you're ascending-- that is, you enter the Jool system, go to some Jool periapsis that's lower than Laythe's orbit, pass periapsis, then you encounter Laythe when you're on your way back "up" again.  Here, again, you want to cut in front of Laythe (on its prograde side), again so it will slow you down."

Edited January 13, 2016 by GarrisonChisholm

[Plusck]

To repeat what I said in the other thread:

"Tweak your insertion when you're about three-quarters of the way there (or elsewhere if you have to do a plane change) then maybe again if you don't get something very neat to start with. Without a plane-change, it'll cost you a few tens of m/s at most.

What you want to do is play with radial in/out and prograde/retrograde, in pairs. By doing this, you can vary the time that you arrive over dozens of days.

So you tweak until you almost touch the outside of Tylo's orbit, and you tweak a bit more to make sure that Tylo is there when you get there. Place another manouvre node by Tylo and you should see yourself in Jool orbit without spending a drop of fuel.

You could do much the same thing with Laythe, but that brings you straight down deep into Jool's gravity well, which might limit your possibilities for further exploration."

 

As it happens, I have a ship coming in right now, aiming for Bop. So this is the map after tweaking about halfway there (actually I think it was closer, about 150 days out):

Spoiler

Since the node is inside after the SOI change, it gives the final orbit (I'm using the default conics value, which wouldn't show anything after the Tylo encounter if the manouvre node was still in interplanetary space). By making my flyby of Tylo tighter (the 2.3 m/s burn shown here) I could make a big mistake and end up far, far down in Jool's gravity well:

 

That isn't reeally what I want to do (dv cost of raising my orbit will be huge) so a couple of tweaks shows me what might be more useful:

Spoiler

On the following pic, virtually all of the manouvre nodes are at 0 m/s - they are just placed there so that I can see what happens next. The exception is near the top of the orbit where it changes from red to blue: that is a 200 m/s burn to raise my Pe. The problem here is that I have a number of moon encounters taking me all the way back to a Tylo encounter (going the wrong way, so it'll probably throw me out of the system), so the burn would be for nothing.
 And the final problem with this is that it is all so jittery - when I placed each of these nodes I was encountering Vall then Tylo. The simple fact of placing a node changes the calculation and shows only two Tylo encounters.

So I'll just wait until I'm in Jool's SOI to finalise all this. I certainly won't be following this particular path.

 

 

Edited January 13, 2016 by Plusck

[Warzouz]

I don't understand what you want to achieve. Do you want to do a simple Jool fly by or staying and exploring the Jool system/moons ? Do you want to recover your ship in the end ? Or will you simply transmit it science ?

If you're not to easy with low TWR, you can always add several engines (LVN) with a bi/tri/quadri coupler. Jool is quite easy to reach. Its SOI is so large you can't really miss it.

If you have a long burn time and you're not too easy with multi burns, you can go to a higher orbit where you can do longer burns (even you loose dV)

I've recently done a Jool moons flyby/orbits to get 65XP for a very quick and simple mission. I managed to get it done with less than 5000m/s and 3 hours gameplay including ship design, even if I messed up reentry becaus I was to eager to return home...

Here is the topic : http://forum.kerbalspaceprogram.com/index.php?/topic/129302-school-bus-a-level-up-mission-design/#comment-2349170

Here is my flight plan

• Escape burn 1925m/s (from 200km - 10 min)
• Jool plane change 210m/s (DN was very near Kerbin) but I got a Tylo encounter
• Tuning in Kerbol SOI : 1.2m/s (got a vall encounter)
• Tuning before Vall : 3.7m/s (got a Laythe encounter)
• Tuning before Laythe : 9.1m/s (got a very far DN with Bop)
• Bop plane change : 250m/s (Ap is on Bop Dn, cool)
• Random node tweaking 200m/s : Bop encounter ...
• Avoid crashing into Bop = 12m/s
• Circularizing at Bop = 80m/s
• Escape + partial plane change = 247m/s (a very dirty interplanetary transfer...)
• Final plane change 50m/s with Pol
• Random node tweaking 40m/s : Pol encounter
• Circularizing at Pol = 65m/s
• Dirty escape from Pol/Jool and partial plane change with Kerbin : 1280m/s (I missed the window and don't want to wait)
• Final plane change and Kerbin encounter = 150m/s

When entering Kerbin SOI, I've 1650m/s left.

The ship was a around 68tons at LKO (iirc) for 16 kerbals powered by 3 LVN. The TWR was around 0.3, if I'm not mistaken.

 

[Streetwind]

2 hours ago, Warzouz said:

I don't understand what you want to achieve. Do you want to do a simple Jool fly by or staying and exploring the Jool system/moons ? Do you want to recover your ship in the end ? Or will you simply transmit it science ?

For starters, he just wants to arrive. I.e. step 2 in your long list. He wants details how to make exactly that burn.

[JoeSchmuckatelli]

2 hours ago, Warzouz said:

I don't understand what you want to achieve.

...

 

I did a fabulous job of not explaining the context when I posted this.  Snark subsequently posted the context; which was to create a unique thread (out of a discussion originating elsewhere) - for the benefit of myself and others - on how to use orbital mechanics rather than a ton of fuel to do an elegant insert into Jool.  In my defense, it was late and whiskey was involved.  (Grin!)

 

Here's the link to that discussion: http://forum.kerbalspaceprogram.com/index.php?/topic/129741-nerv-rocket-use-stupid-from-low-kerbin-orbit-jool/

 

17 hours ago, Plusck said:

To repeat what I said in the other thread:

"Tweak your insertion when you're about three-quarters of the way there (or elsewhere if you have to do a plane change) then maybe again if you don't get something very neat to start with. Without a plane-change, it'll cost you a few tens of m/s at most.

What you want to do is play with radial in/out and prograde/retrograde, in pairs. By doing this, you can vary the time that you arrive over dozens of days.

So you tweak until you almost touch the outside of Tylo's orbit, and you tweak a bit more to make sure that Tylo is there when you get there. Place another manouvre node by Tylo and you should see yourself in Jool orbit without spending a drop of fuel.

You could do much the same thing with Laythe, but that brings you straight down deep into Jool's gravity well, which might limit your possibilities for further exploration."

 

As it happens, I have a ship coming in right now, aiming for Bop. So this is the map after tweaking about halfway there (actually I think it was closer, about 150 days out):

  Reveal hidden contents

Since the node is inside after the SOI change, it gives the final orbit (I'm using the default conics value, which wouldn't show anything after the Tylo encounter if the manouvre node was still in interplanetary space). By making my flyby of Tylo tighter (the 2.3 m/s burn shown here) I could make a big mistake and end up far, far down in Jool's gravity well:

 

That isn't reeally what I want to do (dv cost of raising my orbit will be huge) so a couple of tweaks shows me what might be more useful:

  Reveal hidden contents

On the following pic, virtually all of the manouvre nodes are at 0 m/s - they are just placed there so that I can see what happens next. The exception is near the top of the orbit where it changes from red to blue: that is a 200 m/s burn to raise my Pe. The problem here is that I have a number of moon encounters taking me all the way back to a Tylo encounter (going the wrong way, so it'll probably throw me out of the system), so the burn would be for nothing.
 And the final problem with this is that it is all so jittery - when I placed each of these nodes I was encountering Vall then Tylo. The simple fact of placing a node changes the calculation and shows only two Tylo encounters.

So I'll just wait until I'm in Jool's SOI to finalise all this. I certainly won't be following this particular path.

 

 

I found myself with a similar problem when I first started playing with maneuver nodes to attempt what you guys suggested in the other thread - specifically maneuver nodes bouncing from place to place and the entire thing being very jumpy; it was disconcerting to try to figure out the right way to do it with that happening... but I got the gist.

 

What I'm a little unclear on is where I want my line in relation to the moons.  @Snark wrote, "One is when you are still "descending" -- that is, you haven't reached Jool periapsis yet, your Jool altitude is still decreasing.  In this case, you want your trajectory to cut right in front of Laythe (on its prograde side), so that it will slow you down." (emphasis mine)

In one scenario that has popped up as I played with the maneuver nodes - the Laythe encounter had me coming 'inside the circle' of Laythe's orbit during my flyby, but it went past and crossed Laythe's orbital plane on the prograde side - is that 'cutting right in front of Laythe'?  Or is 'cutting right in front of Laythe' done as Plusck shows in his first image where the line goes outside the orbital path and cuts back across the orbital path in front of the moon?  Does it matter?

[Plusck]

56 minutes ago, JoeSchmuckatelli said:

What I'm a little unclear on is where I want my line in relation to the moons.  @Snark wrote, "One is when you are still "descending" -- that is, you haven't reached Jool periapsis yet, your Jool altitude is still decreasing.  In this case, you want your trajectory to cut right in front of Laythe (on its prograde side), so that it will slow you down." (emphasis mine)

 

In one scenario that has popped up as I played with the maneuver nodes - the Laythe encounter had me coming 'inside the circle' of Laythe's orbit during my flyby, but it went past and crossed Laythe's orbital plane on the prograde side - is that 'cutting right in front of Laythe'?  Or is 'cutting right in front of Laythe' done as Plusck shows in his first image where the line goes outside the orbital path and cuts back across the orbital path in front of the moon?  Does it matter?

 

Well, sort of.

Actually for the "crossing Laythe's orbital plan on the prograde side" yes, both situations cover the same thing. The opposite would be if you passed Laythe on the retrograde side, in which case you'd be accelerated out of the Jool system.

Consider this situation:

You arrive at A going significantly faster than the moon in question. At this stage your orbital velocity, at that altitude above the planet, is enough to put you on an escape trajectory. You enter the moon's SOI with a reduced speed at B (because the moon is itself moving, so within the moon's SOI your velocity is lower).

You therefore cut in front of the moon on its orbital path at C, going faster as you reach Pe by the moon.

On returning to the edge of the moon's SOI at D, you are going slower.

When you return to the planet's SOI, your speed vector is added to the SOI's own speed vector as you end up at E.

 

Now, IRL there is no such thing as an SOI (or rather, you are within the SOI of a huge number of bodies at the same time, and the importance of one or the other is merely relative). In real life, the moon's orbital energy would be ever-so-slightly increased forwards and outwards, and your energy reduced downwards and backwards, as you get dragged around by gravity, but you'd also be accelerated downwards by the planet and your final orbit around the planet would be highly eliptic.

In KSP much the same happens, but you also get what I would call an "SOI-assist". This is because you get taken out of the planet's SOI at a higher altitude at a high horizontal (compared to planet) speed, and then plonked back into it at a lower altitude and much more of a vertical (compared to planet) component to your speed. During this time the planet's gravity isn't affecting you at all.

So now, rather than being on an escape trajectory, you are heading towards a new planet-relative Pe on an eliptical orbit around the planet. And the SOI effect is to make this much less eliptic than it would be in real life. The SOI effect therefore gives you much more margin for error, while still getting a useful change in your orbit to assist in capture.

Edited January 14, 2016 by Plusck

[Snark]

42 minutes ago, JoeSchmuckatelli said:

In one scenario that has popped up as I played with the maneuver nodes - the Laythe encounter had me coming 'inside the circle' of Laythe's orbit during my flyby, but it went past and crossed Laythe's orbital plane on the prograde side - is that 'cutting right in front of Laythe'?  Or is 'cutting right in front of Laythe' done as Plusck shows in his first image where the line goes outside the orbital path and cuts back across the orbital path in front of the moon?  Does it matter?

As long as your path cuts in front of the moon (on the moon's prograde side) while in the moon's SoI, then it will work for reducing your velocity relative to Jool.  It'll do that regardless of whether you're climbing up from lower Jool altitude, or descending from higher Jool altitude.  Either one works.

(Actually, the above assertion is true only with the assumption that you're orbiting Jool prograde.  Technically, if you've gone certifiably cuckoo and are orbiting Jool retrograde, then you'd want to pass on the retrograde side of the moon to reduce your orbital energy around Jool.  But I'll assume you're not doing that.)

 

[JoeSchmuckatelli]

2 hours ago, Snark said:

 

(Actually, the above assertion is true only with the assumption that you're orbiting Jool prograde.  Technically, if you've gone certifiably cuckoo and are orbiting Jool retrograde, then you'd want to pass on the retrograde side of the moon to reduce your orbital energy around Jool.  But I'll assume you're not doing that.)

 

At this point, assuming anything but that I'm 'certifiably cuckoo' is a questionable assumption at best.  - but yes, I'll be orbiting prograde (whew!).

 

Thanks for the answers guys!

2 hours ago, Plusck said:

 

 

You arrive at A going significantly faster than the moon in question. At this stage your orbital velocity, at that altitude above the planet, is enough to put you on an escape trajectory. You enter the moon's SOI with a reduced speed at B (because the moon is itself moving, so within the moon's SOI your velocity is lower).

You therefore cut in front of the moon on its orbital path at C, going faster as you reach Pe by the moon.

On returning to the edge of the moon's SOI at D, you are going slower.

When you return to the planet's SOI, your speed vector is added to the SOI's own speed vector as you end up at E.

 

Now, IRL there is no such thing as an SOI (or rather, you are within the SOI of a huge number of bodies at the same time, and the importance of one or the other is merely relative). In real life, the moon's orbital energy would be ever-so-slightly increased forwards and outwards, and your energy reduced downwards and backwards, as you get dragged around by gravity, but you'd also be accelerated downwards by the planet and your final orbit around the planet would be highly eliptic.

 

Hmmm.  I 'get' it as far as the game goes - but have to ask; why does the ship's speed not even out?  I.e. as the ship enters the moon's SOI, retrograde, shouldn't it be accelerated by the moon's gravity (as it falls toward the moon from the retrograde side) by the same or a similar amount as it reduces the ship's velocity by decelerating it as the ship climbs out of the moon's well on the prograde side?  I know you've given me the correct answer - just idly wondering why the net effect isn't zero.

 

I'm guessing that this has to do with the velocity transfer at Pe, but don't quite understand how it works.

Edited January 14, 2016 by JoeSchmuckatelli

[Plusck]

52 minutes ago, JoeSchmuckatelli said:

At this point, assuming anything but that I'm 'certifiably cuckoo' is a questionable assumption at best.

 

Thanks for the answers guys!

Hmmm.  I 'get' it as far as the game goes - but have to ask; why does the ship's speed not even out?  I.e. as the ship enters the moon's SOI, retrograde, shouldn't it be accelerated by the moon's gravity (as it falls toward the moon from the retrograde side) by the same or a similar amount as it reduces the ship's velocity by decelerating it as the ship climbs out of the moon's well on the prograde side?  I know you've given me the correct answer - just idly wondering why the net effect isn't zero.

Oh but it does exactly that. The difference is that when you leave the SOI, you are not going in the same direction as when you entered it.

If the moon were big enough to drag you right round, you'd leave the SOI retrograde, not far from where you entered it. Your speed relative to the moon at B is your speed at A minus the orbital velocity of the moon. If D was very close to B, retograde, your orbital velocity on leaving the SOI at E would be your speed at D minus the orbital velocity of the moon once again. In other words, a perfect gravity assist either subtracts 2x the difference between the moon's orbital velocity and your own (if you catch up with it retrograde and pass in front of it before being ejected retrograde) or adds 2x that difference in orbital velocity (if it catches up with you, you pass behind it and then are ejected prograde).

So say your velocity relative to the planet is 700 m/s, and the moon's is 300 m/s, and you enter its SOI almost directly retrograde. From the moon's perspective, you're doing 400 m/s. If the moon is heavy enough to turn you  around nearly 180°, then you exit retrograde at 400 m/s (from the moon's perspective) but from the planet's perspective, you're now doing 100 m/s the other way. That is, minus 2x the difference between your velocity and the moon's velocity.

There is no moon in KSP that can do this at interplanetary speeds, but once you're going at intra-system speeds (like from one Jool moon to another, or from LKO to the Mun) there are many that can. And if you get your Eve or Jool encounter right you can do something similar to this with planets with respect to Kerbol itself. I haven't yet sent a manned mission to Moho but an Eve gravity assist looks like a must to cut down the cost of getting all the way down there.

However, Tylo (in my example) is big enough to give you a quarter turn or thereabouts. Assuming the same numbers just for clarity, that means you exit Tylo's SOI doing 400m/s towards the planet, plus 300 m/s along the moon's trajectory. Total velocity along your new orbital path around the planet is now 500 m/s (guess why I chose these numbers...).

Edited January 14, 2016 by Plusck

[MagicFireCaster]

This is really going to help me when i send my 109.9 ton shuttle to Jool, i will use 5 landers (4 probes, 1 manned all of the will be left over landed or in orbit) reading @Warzouz plan, maybe i'm going to be able to make it (also i will refuel from duna), with all of the payload i have 2286 m/s Of Delta V, I wonder if i will be able to do it, trying with this orbital maneuvers of course . 

[JoeSchmuckatelli]

8 hours ago, Plusck said:

...

However, Tylo (in my example) is big enough to give you a quarter turn or thereabouts. Assuming the same numbers just for clarity, that means you exit Tylo's SOI doing 400m/s towards the planet, plus 300 m/s along the moon's trajectory. Total velocity along your new orbital path around the planet is now 500 m/s (guess why I chose these numbers...).

I just literally had a moment of stunned realization.  An epiphany.  And not because I am any good at math - but because I am a carpenter.  And I would have never known my love of carpentry would help me understand anything in physics.

 

I had to read your post about 3 times to get it - but, holy frijoles, Batman!  It's the Pythagorean theorem in action - and I had only a gist of what you were describing at first read - but now I'm getting it.

 

(Carpenter's use a 'cheat' to know if something is square: we measure 3 feet in one direction from a corner and mark it, four feet in the other direction and mark it - and if the building is square... the space between the two marks should be 5 feet.)

 

To think that my angular momentum after a gravity assist would end up being C²... I am stunned.

 

Thank you!

Edited January 15, 2016 by JoeSchmuckatelli

[Warzouz]

12 hours ago, MagicFireCaster said:

This is really going to help me when i send my 109.9 ton shuttle to Jool, i will use 5 landers (4 probes, 1 manned all of the will be left over landed or in orbit) reading @Warzouz plan, maybe i'm going to be able to make it (also i will refuel from duna), with all of the payload i have 2286 m/s Of Delta V, I wonder if i will be able to do it, trying with this orbital maneuvers of course . 

It's not the first time I go to Jool system. It's a VERY nice place. You can see all 3 inner moons nearly what ever you are and navigating from one to the other is very easy (you don't have to wait months to get a window. But it's the first time I'm using gravity assist. As other said, You can't be sure to get proper encounter with Tylo or Laythe to get proper gravity assist,

My point is : pack more fuel than 4800m/s. My initial mission was 3 flyby and 2 orbits for 6500m/s

Have fun.

Edited January 15, 2016 by Warzouz