Best place to rendezvous in the Jool system?

[EndOfTheEarth]

1) Is it best done around a particular moon or Jool itself?

2) What altitude?

By "best" I mean in terms of fuel efficiency.

[Kerbol Prime]

I'm not an expert but I would suggest aerobreaking at Laythe. Or just aerobrake around Jool.

[TheSaint]

I was wondering the same thing myself. I have a mission to Jool planned that involves three vehicles (lander, Kerbin return vehicle, and fuel tanker) all traveling separately to Jool and rendezvousing there. The first target moon is Tylo. I'm going to try the following profile tonight:

1. First vehicle arrives and aerobrakes around Jool to a apoapsis above Tylo, then circularizes there.

2. Second vehicle arrives and aerobrakes to a apoapsis above the first vehicle, circularizes, then executes a Hohmann transfer to rendezvous.

3. Third vehicle does the same.

4. After the lander's transfer stage has been refueled and the personnel have transferred over, the lander executes a Hohmann transfer to Tylo to start its tour.

The alternatives would be: Do the same thing but aerobrake to a low orbit (below Laythe), but I think there I would waste a lot of fuel getting the lander back out to Tylo. Or rendezvous in Tylo orbit, but then I think I'll be wasting fuel circularizing stuff in Tylo orbit that doesn't need to be there (the KRV and the tanker). I'll try it out tonight and report back.

Edited July 29, 2013 by TheSaint

[Rhyunix]

To simply rendezvous with two space craft, I strongly suggest that you make an elliptic orbit that dips only slightly into Jool's atmosphere and then circularize at a low orbit around Jool. However I haven't done or been to Jool to do this

[metaphor]

I was wondering the same thing myself. I have a mission to Jool planned that involves three vehicles (lander, Kerbin return vehicle, and fuel tanker) all traveling separately to Jool and rendezvousing there. The first target moon is Tylo. I'm going to try the following profile tonight:

1. First vehicle arrives and aerobrakes around Jool to a apoapsis above Tylo, then circularizes there.

2. Second vehicle arrives and aerobrakes to a apoapsis above the first vehicle, circularizes, then executes a Hohmann transfer to rendezvous.

3. Third vehicle does the same.

4. After the lander's transfer stage has been refueled and the personnel have transferred over, the lander executes a Hohmann transfer to Tylo to start its tour.

The alternatives would be: Do the same thing but aerobrake to a low orbit (below Laythe), but I think there I would waste a lot of fuel getting the lander back out to Tylo. Or rendezvous in Tylo orbit, but then I think I'll be wasting fuel circularizing stuff in Tylo orbit that doesn't need to be there (the KRV and the tanker). I'll try it out tonight and report back.

The most fuel-efficient way to do it is to rendezvous in Kerbin orbit and then drop off stages as needed. Barring that, the most efficient way is to aerobrake all 3 around Jool to an apoapsis at Tylo's orbit, and rendezvous in that orbit. But you will have to be careful not to encounter any of the moons since it's an elliptical orbit. If the craft are arriving at very different times so there's a high chance of an unplanned encounter, then the way you described with circularizing would probably be easier. Aerobraking to a low orbit below Laythe is not efficient at all since you have to spend a lot of fuel getting back up the gravity well.

[SRV Ron]

Tried that once. I dipped too far into the atmosphere and ended up landing there. Poof! You will need to experiment or use one of the mods designed to predict orbital changes based upon aerobraking.

If the goal is to land on Laythe or drop a probe into Jool, then use aerobraking to save tons of fuel you may not have for orbiting first.

Easier is to set up a retrograde slingshot in relation to Jool's or one of its moon's orbit. Use thrusters to fine tune the maneuver after making the initial burn. Retrograde to Jool or one of its major moons will slow you down much further bringing you into a tighter orbit for less fuel expended. It can also be use to correct major planer misalignment with little additional cost in fuel. If you slingshot prograde to Jool's or one of its moons orbital path, you will be flung back out into solar orbit. I have visited more then one of Jool's moons doing slingshot burns with limited fuel. If you catch the slingshot maneuver just right, you can get a capture using only a small thruster burn to complete the maneuver to orbit. Try to get a capture in front of Jool's orbital direction so that it's gravity slows your solar orbital speed down. Then, it will take less fuel to get into orbit.

[TheSaint]

The most fuel-efficient way to do it is to rendezvous in Kerbin orbit and then drop off stages as needed. Barring that, the most efficient way is to aerobrake all 3 around Jool to an apoapsis at Tylo's orbit, and rendezvous in that orbit. But you will have to be careful not to encounter any of the moons since it's an elliptical orbit. If the craft are arriving at very different times so there's a high chance of an unplanned encounter, then the way you described with circularizing would probably be easier. Aerobraking to a low orbit below Laythe is not efficient at all since you have to spend a lot of fuel getting back up the gravity well.

Yeah, tried stacking everything in LKO but it's too big, can't stay together under thrust with only docking ports holding it together. It might work in 0.21, I haven't upgraded yet. But, in any case, the Jool orbit rendezvous has become part of the challenge now, so I'm forging ahead.

Rendezvous in a highly elliptical orbit makes my head ache just thinking about it. I'm trying to conserve fuel, but I'm also trying to keep it simple as well. Plus the vehicles are arriving about one week apart, so the chances of a random moon encounter are pretty high.

I was thinking the same thing about the low orbit option. You burn off all that velocity in the aerobraking, only to have to add it later to get your orbit back up to Tylo.

Tried that once. I dipped too far into the atmosphere and ended up landing there. Poof! You will need to experiment or use one of the mods designed to predict orbital changes based upon aerobraking.

If the goal is to land on Laythe or drop a probe into Jool, then use aerobraking to save tons of fuel you may not have for orbiting first.

Easier is to set up a retrograde slingshot in relation to Jool's or one of its moon's orbit. Use thrusters to fine tune the maneuver after making the initial burn. Retrograde to Jool or one of its major moons will slow you down much further bringing you into a tighter orbit for less fuel expended. It can also be use to correct major planer misalignment with little additional cost in fuel. If you slingshot prograde to Jool's or one of its moons orbital path, you will be flung back out into solar orbit. I have visited more then one of Jool's moons doing slingshot burns with limited fuel. If you catch the slingshot maneuver just right, you can get a capture using only a small thruster burn to complete the maneuver to orbit. Try to get a capture in front of Jool's orbital direction so that it's gravity slows your solar orbital speed down. Then, it will take less fuel to get into orbit.

Funny, I find aerobraking to be much easier than slingshotting. When I try slingshots it seems like I can get either the velocity I need, or the vector I need. Getting them both seems to require a PhD, or 100 retries with infinitesimal course changes, which I just don't have the patience for.

The goal is to land on Tylo, Vall, and Laythe with one lander (refueling in between, of course). Has to start on Tylo because it uses two stages for that landing. After the second stage is recovered in orbit it has parachutes to land on Laythe and enough delta-v to get back to orbit, and enough delta-v to land and liftoff from Vall. If I can conserve enough fuel I may try taking it out to Bop or Pol as well, secondary mission objectives, if you will.

Tried that rendezvous profile last night, worked pretty well. The only large burn was about 1100 m/s to circularize above Tylo, the Hohmann transfers were about 120 m/s, approach maneuvers ate up about 100 m/s. But all of the vehicles would have had to circularize no matter where they rendezvoused, so, the only slop was ~220 m/s for the transfer and approach of the fuel tanker and the KRV. Now the tanker is drained and abandoned, the KRV is sitting in high Jool orbit with Bob on board, and Jeb and Bill are in the lander orbiting Tylo awaiting their landing attempt this evening.

[ploxus]

I just started playing around in the Jool system this weekend so I'm definitely not an expert on getting to it's moons. But by far the most efficient way I've found to get in a stable orbit is to get a good equatorial trajectory from your transfer orbit, aerobrake around Jool at 119km, which should line you up to make an encounter with Laythe(or close to it), then aerobrake around Laythe at ~25km. Doing this should only cost you about 50 m/s of adjustments on top of your transfer burn. The key is having a good trajectory from your transfer orbit.

[TheSaint]

I just started playing around in the Jool system this weekend so I'm definitely not an expert on getting to it's moons. But by far the most efficient way I've found to get in a stable orbit is to get a good equatorial trajectory from your transfer orbit, aerobrake around Jool at 119km, which should line you up to make an encounter with Laythe(or close to it), then aerobrake around Laythe at ~25km. Doing this should only cost you about 50 m/s of adjustments on top of your transfer burn. The key is having a good trajectory from your transfer orbit.

Interesting idea. The lander would have to escape from Laythe and raise it's orbit to Tylo after the housekeeping, which would use more delta-v. But the KRV and the fuel tanker could make orbit around Laythe will almost full tanks, which means that the lander's transfer stage would leave Laythe with full tanks. And the lander will wind up back at Laythe anyway to finish the tour, so there's less fuel spent on rendezvous after it is finished. I wonder which profile would be more efficient? I have a save file of the three vehicles on their way to Jool, just after their mid-course correction. I can finish the tour with the first profile, restore the save file and start over and run this profile, and compare.

Edit: Just thinking about this more. With the profile I outlined above the lander has to deorbit Laythe and raise it's orbit to Tylo anyway, it just does it at the end of the tour to rendezvous with the KRV. So this profile is undoubtedly more efficient. I'm going to give it a try tonight.

Edited July 30, 2013 by TheSaint
More thoughts

[Crush]

1. First vehicle arrives and aerobrakes around Jool to a apoapsis above Tylo, then circularizes there.

2. Second vehicle arrives and aerobrakes to a apoapsis above the first vehicle, circularizes, then executes a Hohmann transfer to rendezvous.

3. Third vehicle does the same.

4. After the lander's transfer stage has been refueled and the personnel have transferred over, the lander executes a Hohmann transfer to Tylo to start its tour.

A much more reliable and elegant way to get into a Jool orbit with an apoapsis on Tylo orbit is when you do a gravity-assist around Tylo.

[ploxus]

Interesting idea. The lander would have to escape from Laythe and raise it's orbit to Tylo after the housekeeping, which would use more delta-v. But the KRV and the fuel tanker could make orbit around Laythe will almost full tanks, which means that the lander's transfer stage would leave Laythe with full tanks. And the lander will wind up back at Laythe anyway to finish the tour, so there's less fuel spent on rendezvous after it is finished. I wonder which profile would be more efficient? I have a save file of the three vehicles on their way to Jool, just after their mid-course correction. I can finish the tour with the first profile, restore the save file and start over and run this profile, and compare.

Edit: Just thinking about this more. With the profile I outlined above the lander has to deorbit Laythe and raise it's orbit to Tylo anyway, it just does it at the end of the tour to rendezvous with the KRV. So this profile is undoubtedly more efficient. I'm going to give it a try tonight.

One thing I thought about reading your reply. I think you'll end up in a backwards orbit in Laythe - so technically you're doing a gravity assist + aerobrake. That could complicate getting to Tylo in that you'll waste some dv along the way.

But all in all lots of things are possible in the Jool system. I could spend all day just dancing around the moons and flinging myself places.

[TheSaint]

One thing I thought about reading your reply. I think you'll end up in a backwards orbit in Laythe - so technically you're doing a gravity assist + aerobrake. That could complicate getting to Tylo in that you'll waste some dv along the way.

Not necessarily, just depends on how I approach Laythe out of the Jool aerobrake. If I have to burn a little to correct the approach I think I'll still come out ahead.

Didn't get a chance to try this last night, had a date night with the wife, then what little time I had was spent developing the Tylo landing approach (since I already had the lander in orbit for that save). I'll try the jool approach from scratch tonight and see what I come up with.

[TheSaint]

Well, haven't finished the whole mission yet, but I did complete the Laythe orbit rendezvous portion last night. Went off like a charm. The lander came into the system and had a Laythe encounter straight out of the Jool aerobrake. One week later the fuel tanker encountered Laythe on the first orbit. One week after that the KRV encountered Laythe after half an orbit, totally on accident. I was one day out and tweaking the aerobrake to bring it to my target altitude when I fat-fingered the RCS and overshot by about 4,000 km. After a minor curse, I looked over and there was my Laythe SOI indicator. All three went into standard eastward orbits, although the lander initially wanted to go west and needed some tweaking.

When all was said and done, the tanker was empty, the lander's transfer stage was full (as opposed to about 75% full on the old profile), and the KRV is at about 75% full instead of less than 50% on the old profile. So, this was a big success. Thanks, everyone, for your input.

[alterbaron]

If you're aerobraking at Jool, you might find my Jool Aerocapture Chart useful.

It's designed to facilitate aerocapture into encounters with Jool's moons.