Joolian system tour - moon order & intra-system dV?

[MiniMatt]

I've been Kerballing for over a year and a good few hundred hours yet, barring a single kraken ravaged satellite mission, I've never been to Jool. It's time to change that.

Working on the premise that any mission not massively over-complicated and ambitious just isn't kerbal, I've decided on a single mission to tour all moons, plant a flag & recover science from each, and return budding the kerbonaut(s) safely home.

Thus I've designed a modular lander, capable (I hope) of being a Laythe SSTO, a single stage Bop/Pol/Vall lander, and a two-stage Tylo lander dependent upon equipped modular engines/wings & fuel. For extra Kerbal-ness the Bop/Pol/Vall engines also form the acsent stage of the Tylo lander and form the main engines of the inter-system delivery vehicle. Being able to tune engine thrust mid-mission is going to be very useful.

Preliminary lander design:

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(Tylo descent stage still needs some work)

(1) Getting the placement order of all these modules right in the delivery vehicle requires knowing optimum order of visits. I'm guessing on working from the inside out - thus Laythe first, then Vall, then Tylo, Bop, and finally Pol?

(2) Assuming Laythe first - aerobrake direct into Laythe or use Jool to slow and setup a Laythe encounter?

(3) Re Laythe SSTOing - I'm quite good (now, finally) at Kerbin SSTOs - am I correct in believing that a craft that is just barely capable of Kerbin SSTO should have plenty of safety margin when Laythe SSTO-ing? Any special Laythe considerations? Noting lack of decent landing spots I've enough lift such that it can take off/land at around 40m/s on Kerbin but don't know if there are any other gotchas like needing a bit more air-hoggy compared to Kerbin.

(4) Intra-system delta-v - how much is likely necessary to transfer from Laythe to Vall, Vall to Tylo, Tylo to Bop, and Bop to Poll? Delta-v maps assuming independent transfer to each from Jool would seem to suggest at least 10,000 delta-v could be needed to perform all transfers - I'm hoping this can be greatly reduced if going inter-moon.

(5) Departing the Joolian system from Pol to return home - significantly harder or easier than from any other location? I'm working on about 2,000dv to get home.

Edited July 22, 2014 by MiniMatt
answered! very ably answered too!

[magnemoe]

1) it might be smart to visit Pol before Bop, the Bop plane change is cheaper between Pol to Bop than Tylo - Bop.

2) Laythe aerobrake is cheaper but pretty much require mechjeb, braking speed depend on where it is in orbit around Jool.

because of Laythe aerobrake you will end in a orbit with low Pe and high Ap, not a huge problem wait until you can burn 3-400 m/s to get to Val.

3) An Kerbin SSTO can land on laythe and take off, main issue is to hit flat land, a plane or mechjeb is pretty much required here for me.

If plane is unable to make it back to mothership use the Val lander as a ferry.

Leave the plane in orbit for later use.

Use the same tactic on Tylo,

4) It cost around 3-4000 m/s to get into low orbit of all the muns.

Tips. don't go into low orbit, just get inside the SOI with the mothership, this cut the cost a lot

you can get from an high Ap- low Pe orbit around Tylo to Pol, then Bop and back to Jool orbit with 3000 m/s use the Val lander for this.

5) you might want to drop your Pe close to Jool or perhaps get an Tylo gravity assist then burning home.

[Laie]

Planning my Jool-5 mission right now, I'm much interested in the same questions.

(2) Assuming Laythe first - aerobrake direct into Laythe or use Jool to slow and setup a Laythe encounter?

I plan for aerobraking at Jool, mostly because it's the easier target and offers much more atmosphere to brake in. I don't use deadly re-entry, but try not to go over the top anyway. Coming from interplanetary speeds to a low Laythe periapsis, the time spent areobraking will be quite short, but I need to shed like 1500m/s. That means high G-forces. I presume it also means that I need to aim carefully, there's probably a very small margin of error between not slowing down enough and crashing on Laythe.

I hope that aerobraking at Jool will be more forgiving (iow, requires fewer F5-F9 iterations to get it right). Besides, it offers the opportunity to gather data in Jools upper atmosphere. Also, look at this: http://i.imgur.com/HNJUf2x.png (the y axis is in 10^5m, that is, 1.15 actually means 115km). With aid like that, it should be possible to get it right on the first try.

(4) Intra-system delta-v - how much is likely necessary to transfer from Laythe to Vall, Vall to Tylo, Tylo to Bop, and Bop to Poll? Delta-v maps assuming independent transfer to each from Jool would seem to suggest at least 10,000 delta-v could be needed to perform all transfers - I'm hoping this can be greatly reduced if going inter-moon.

If I look at other Jool-5 entries, most people seem to leave the bulk of their vessel in a high orbit around the moons. Only the lander (plus, maybe, a small-scale transfer stage) actually goes down to low orbit. From what I gather, you can count on less than 500m/s for every transfer from moon to moon. Though I'm accounting for 1000m/s each.

Edited July 22, 2014 by Laie

[MiniMatt]

Many many thanks!

Bop/Pol order is really useful info and with both destinations requiring the same lander setup (ie single stage, thrust tweaked low) it shouldn't impinge upon delivery vehicle design.

Re aerobraking it looks like best plan is to see where I end up upon entering system and figure out options available. Personally use very frequent quicksaves (and more than one botched aerobrake) rather than mechjeb.

And intra-sytem delta-v notes are very welcome news, difference between a c.8-9000dv mothership and a c 14-15000dv one (+ all the lander fuel use) is going to be significant.

[Oafman]

I would do a gentle aerobrake around Jool, enough to capture with a pretty high apoapsis. Once that's done, you have plenty of options. If your first orbit doesn't hit one of the inner moons, create a random maneuver node and click it forward a few orbits, and you're bound to see an encounter with something. You never have to wait long in the Joolian system before you run into one of the inner moons, assuming your orbit is equatorial.

To the OP, what dv does that lander have in Tylo configuration? You need at least 5000m/s to get down and back up, probably slightly more to be comfortable. Tylo is very unforgiving in this regard. The logic about almost SSTO-ing on Kerbin meaning you should be good for Laythe is fairly sound.

Also, don't neglect Vall. You can pretty much ignore Pol & Bop when designing a multi-purpose lander, as they require so little dv to land and return from, but Vall is not to be ignored in the same way. From memory you need something like 1800m/s to get down to Vall and back up.

One last thing; 2km/s sounds a little tight for the return journey. You might want a little more just in case.

[MiniMatt]

To the OP, what dv does that lander have in Tylo configuration? You need at least 5000m/s to get down and back up, probably slightly more to be comfortable. Tylo is very unforgiving in this regard.

The ascent stage (also forming single stage vall/bop/pol - thrust severely tweaked down for latter two) has ~3,200dv - it looks quite compact as I've allowed myself some rather cheesy part clipping with a couple of nukes occupying the same space as the outboard FL-T400 fuel tanks - I'm normally quite down on part clipping, but fitting everything on the horizontal plane is hard enough as it is. TWR of this stage on Tylo is ~1.3 full.

The descent stage in the third picture is still something of a work in progress, mainly assessing the tipper style landing & strength/rigidity of docking ports, but currently has roughly 2,600dv. Was looking to aim for about 3km/s dv each way for Tylo.

One last thing; 2km/s sounds a little tight for the return journey. You might want a little more just in case.

Good point, I usually find myself engineering in generous safety margins for every stage, the net result being that by the final burn I've got far more dv left than I was expecting. But would be heartbreaking to have to rescue such a successful mission (we hope) at the very last stage all for the want of a few teaspoons of rocket fuel

[magnemoe]

Laythe can give an 6000 m/s aerobrake if unlucky.

http://forum.kerbalspaceprogram.com/threads/48461-Grand-tour-4-new-attempt-at-an-below-100-ton-grand-tour/page2

Here is my dV breakdown for a grand tour who includes Jool's moons.

[MiniMatt]

Laythe can give an 6000 m/s aerobrake if unlucky.

http://forum.kerbalspaceprogram.com/threads/48461-Grand-tour-4-new-attempt-at-an-below-100-ton-grand-tour/page2

Here is my dV breakdown for a grand tour who includes Jool's moons.

That's a really great thread & full of very useful info.

The note everybody has made about leaving the mothership in a very high orbit around each moon rather than taking it in low is also going to be a key one.

[NecroBones]

Lots of great answers here. I'll just add my $0.02, from a "what worked for me" perspective.

For my Jool-5, I had a mothership with two landers. One lander was for Tylo-only, and was disposable, but heavy. The other had throw-away rapiers for Laythe, then would be a generic small lander for the others, so I chose this order:

1. Tylo: Hard to miss, as the SOI is huge. Needed to get rid of all the dead weight of carrying the Tylo lander around.
   
2. Laythe: Also hard to miss. But needed to use and discard the rapier engines before using the lander anywhere else.
   
3. Vall: Orbit was close to Laythe's, easy transfer.
   
4. Pol: While the orbit is large, the plane-change was half-way between Vall (just departed), and Bop (only other remaining target).
   
5. Bop: Saved for last due to high orbital inclination.
   

Definitely a high orbit (and highly eccentric, by being low at one end, to maximize the Oberth Effect) for the mothership. I used a tug to take my landers into a lower orbit for descent. In some cases, I used the tug to do the de-orbit burn, detached it, and re-orbited it right away.

Edited July 22, 2014 by NecroBones

[MiniMatt]

Yeah the tug idea is growing on me. Going to need to figure out if approaching with a tug masses less than packing more dV into the lander such that it can handle it's own approach. Vall, Pol & Bop would all be approached on nukes & with ample dV. But Laythe will approach on 350isp 48-77s with fairly tight dV & Tylo, well, Tylo is scary.

Perhaps a Xenon tug?

[NecroBones]

Xenon is doable, but I shudder at the thought of the burn times. I did mine with nukes, and that worked pretty well. The nice thing about using a tug, is that the mass and fuel for doing the orbital maneuvers doesn't have to go down to the surface and back up again, making the lander more fuel efficient. There's a reason they did it that way in Apollo.

On the other hand, the tug and the lander having separate engines, means more mass for the mother-ship to carry. Unless the "tug" is just a bunch of docking fuel tanks, and you use the lander's engines for the orbital maneuvers.

[MiniMatt]

Good point. Back-of-cig-packet maths, with Vall, Bop & Pol the fuel saved by tugging is likely to be in the order of 1.5 tons - ie. less than the mass of another nuke engine. But Laythe & Tylo are likely going to cut it close enough that either a tug is necessary or I've got to bring the whole damn mothership down into low orbit.

Guess at least with using xenon in this instance you're not worried about missing launch windows or messing up the accuracy of your ejection angle by ejecting over multiple orbits.

Question: Am I correct in assuming that the bulk of delta-v saved by leaving the mothership up high is through not having to bring the apoapsis down? Ie. bring mothership's periapsis down to 500-1000km or so (thus getting some Oberth benefit upon departure) but leave apoapsis just under the moon's sphere of influence?

PS. That Jool-5 Challenge thread, and your mission report amongst it, is a goldmine!

[metaphor]

If you've never landed on Tylo before, it's going to take a lot of delta-v. With really good piloting you can land from low orbit for only 2300 m/s, but most people spend a few hundred extra m/s on the final landing approach. 2600 m/s might be enough on the descent stage, but it could be pretty close. I would suggest 3000 m/s for the descent if it's your first time landing on Tylo. The ascent is much easier than the descent, even with low TWR, and can pretty easily be done for about 2500 m/s. Since you have so much delta-v in your ascent stage, if you run low on your descent you could just stage close to landing and land with a little fuel from your ascent stage.

If you use gravity assists, going from moon to moon in the Jool system can take very little delta-v, maybe even lower than 1000 m/s for the whole tour. But even if you don't, it still won't take very much if you park in an elliptical orbit at each moon, maybe something like 2000-3000 m/s total.

I would suggest going either Tylo or Laythe first. At Laythe you have the advantage that you can aerobrake straight into orbit, and at Tylo you have the advantage that you'll get rid of the heavy descent stage and won't have to carry it around everywhere. Tylo is so massive that even if you're coming straight from Kerbin, you can get into orbit around Tylo for only about 500 m/s of delta-v.

A tug is a very good idea, especially for Tylo and Laythe. Parking in a high elliptical orbit saves a lot of fuel on your mothership, but it's a little harder to rendezvous. If the tug is ion powered you'll have to take into account the lower solar radiation at Jool.

[metaphor]

Question: Am I correct in assuming that the bulk of delta-v saved by leaving the mothership up high is through not having to bring the apoapsis down? Ie. bring mothership's periapsis down to 500-1000km or so (thus getting some Oberth benefit upon departure) but leave apoapsis just under the moon's sphere of influence?

Yes, although I would say bring the periapsis even lower, like just above 60 km (so that you're above the atmosphere on Laythe and you can time warp at more than 10x on Tylo). When you encounter a moon, place your periapsis around 70 km by tweaking the encounter from outside the SOI, and then burn retrograde when you get to periapsis, just enough so that you get a closed orbit just inside the SOI. When you leave the moon, burn prograde at periapsis (although it might take some time for the moon orbits to to line up right).

[Oafman]

Basically you have lots of options, and all of them can work. Parking the mothership somewhere (prob laythe so you can aerobrake it) then tugging the landers out, or parking the ship around Tylo as it's in the middle so easier to access the outer rocks, or splitting up your ship in some way. All of them can be good.

The main thing I want to point out, as it's your first time there, is that you're going to love the Jool system! It's the ultimate playground in the Kerbol system, and imo makes the rest of the planets seem dull by comparison. It has so much variety, with the challenge of Tylo, and jet fun on Laythe, and of course the gas giant with cool Joolrises, and aerobraking options, and loads of easter eggs. And they're all so close together, so you're always bumping into moons, and finding options for gravity assists.

[Laie]

Good point. Back-of-cig-packet maths, with Vall, Bop & Pol the fuel saved by tugging is likely to be in the order of 1.5 tons - ie. less than the mass of another nuke engine. But Laythe & Tylo are likely going to cut it close enough that either a tug is necessary or I've got to bring the whole damn mothership down into low orbit.

It gets better if you find more opportunities to use the tug. If the engine(s) are arranged such that it will also power your mothership to Jool and between the moons, saving a few engines on the mothership, then the tugs' propulsion system is alreay bought and paid for. Or if it allows your lander to go independent for a while, e.g. the mothership goes Tylo->Bop while lander&tug go Tylo->Pol->Bop.

This can easily double the amount of docking operations during the mission, though. Don't know if the overall fuel savings are really worth the hassle.

[MiniMatt]

The main thing I want to point out, as it's your first time there, is that you're going to love the Jool system! It's the ultimate playground in the Kerbol system, and imo makes the rest of the planets seem dull by comparison. It has so much variety, with the challenge of Tylo, and jet fun on Laythe, and of course the gas giant with cool Joolrises, and aerobraking options, and loads of easter eggs. And they're all so close together, so you're always bumping into moons, and finding options for gravity assists.

Oh I am LOVING THIS. And I haven't even left the ground yet

I've always loved the engineering aspect of Kerbal, evidenced by most of my posts on this forum being taken up by Xeldrak's BSC challenges (always a finalist, never a bride, CURSE YOU GIGGLEPLEX). But this mission has thrown up two new problems for every one I've solved - and I love it

I wanted to get rid of the slightly cheesy part clipping I mentioned earlier, but even the largest landing legs are shorter than the length of an unclipped nuke engine. Solution was to use pocket i-beam girders hanging off the fuel tanks and use those to mount tipper style landing legs and on the underside, wheels. This added about 700kg mass, but that was within my dv & twr safety margins.

Then realised that alignment of module docking was going to have to be *perfect* given horizontal alignment of lander and wish to re-use landing gear (as tug and/or mothership engines). I can dock within a degree or so of perfect rotation, but this wouldn't be enough to prevent icky torque or wing dynamics. Even double-docking not likely to result in VAB/SPH perfect alignment.

Solution realised was that only the Laythe lander module needed horizontal alignment - everything else could be classic vertical lander style - and perfect alignment here could be attained by having the Laythe module pre-attached on the mothership at launch. But.... jet engines don't have twin attachment nodes, so would have to mount ~7 tons of craft off-centre or backwards.

Solution to that was the Rapier engine, because that *does* have two attachment nodes.

So now I find myself redesigning the one part that I'd got right, that worked perfectly, that looked damn cool (because that's important), and the part I started this whole mission from - the Laythe SSTO configuration.

It's great

[NecroBones]

If you've never landed on Tylo before, it's going to take a lot of delta-v. With really good piloting you can land from low orbit for only 2300 m/s, but most people spend a few hundred extra m/s on the final landing approach. 2600 m/s might be enough on the descent stage, but it could be pretty close. I would suggest 3000 m/s for the descent if it's your first time landing on Tylo. The ascent is much easier than the descent, even with low TWR, and can pretty easily be done for about 2500 m/s. Since you have so much delta-v in your ascent stage, if you run low on your descent you could just stage close to landing and land with a little fuel from your ascent stage.

This is very true.

I've only done manned landings on Tylo twice now, but I've always budgeted over 6,000 m/s total in the landers (usually more like 6,100 or 6,200). But it's important to note, that this dV budget works best if your orbital altitude prior to descent, and after ascent, is about a 40-60km (assuming circularized), and not higher.

The first time I landed there, it was a textbook landing, using about half of the dV to get down, and half to get back up and docked at 40km or so.

For the Jool-5, I wasn't paying attention to my staging, and descended more cautiously, and ended up using about 4,000 m/s of dV to get down. The remaining 2 km/s or so of dV wasn't quite enough to get back into even a 20km circular orbit, so I had to circularize with RCS, and have the tug come down to me. This is another huge win for tugs with their own engines-- rescue scenarios.

[Empiro]

I actually recently had a mission that sounds very similar to yours -- a space plane for Laythe, a staged lander for Tylo (and the ascent stage gets reused for Vali, Bop and Pol). I also had a pair of probes I dropped into Jool and Laythe.

I used a reverse gravity assist around Laythe to get into orbit around Jool. I think the gravity brake works better because you can more easily tweak both your periapsis and apoapsis around Jool. I put myself 3 million km from Jool at periapsis, and my Apoapsis touched Tylo's orbit. This let me get low orbit science around Jool (you can't get atmospheric data without being sub-orbital anyways, I believe).

After the maneuver, I dropped off my Laythe space plane (weighs about 10.5 tons), and it was easy to get an encounter with Laythe with just a few m/s. Aerobraking around Laythe was actually the hardest part -- Laythe's atmosphere grows thick very quickly, and it has a tiny SOI compared to its gravity, so either you're on an escape trajectory, or you won't leave the atmosphere. I'm playing with both Deadly Reentry and FAR, and I had to make use of the overpowered B9 air brakes to control exactly how much I slowed down. It's pretty hard to land on Laythe --I used a B9 VTOL engine on my space plane to let me land more easily. It wasn't balanced enough vertically, but it let me land at a very slow horizontal speed.

The main ship then went to Tylo before going to Vali and then finally to Laythe to pick up the space plane (it's very light when empty). I then used a gravity assist around Tylo to put myself close to Pol's orbit. It took a long time to get the orbit to match up with Pol. After landing on Pol I waited until Pol crossed AN/DN of Bop and did both the transfer burn and inclination correction burn at once, which really saved on fuel.

In the end though, my mission turned out to be massively over-engineered, and I had tons of fuel left after leaving Jool, but it was a great experience and I thought that I did a pretty good job with the flight plan.