Jool 5 Itinerary Help

[AeroGav]

Hi all,

I'm ok at building and flying aircraft but orbital maneuvers are not my thing, so i thought i better ask.

I'm currently attempting a Jool 5 mission with a rather large, ISRU equipped, engine pod and external tank dropping spaceplane.      

The first stop on the tour was Laythe, where it refuelled with ISRU.  On the way out of Laythe, the last of the air breathers were shed, so the only propulsion on the motherplane are its NERVs. 

I have a save file for it in orbit around Laythe, with 3800dV.            In the front of the cargo section is a rover (which i recommend jettisoning immediately as it's worthless).      At the rear is a rocket for Tylo.   This rocket also contains our ISRU gear, and much of the aircraft's accommodation.  The rocket has very tight margins.    It needs to be released from LOW orbit.  It has a jettisonable tank that will be discarded during the descent to Tylo.    It will use its ISRU gear to refuel on the surface, but  will have to jettison that stuff to get back to orbit.   The propulsion stage then separates and the kerbal-containing parts re-dock with the spaceplane using RCS.

 

Now my initial plan was just to start in the middle, and work outwards.    I thought this would save fuel on the transfer burns, but actually, the smallest burn that's sufficient to break orbit from Laythe adds so much velocity to your orbit around Jool that you end up with a PE between Tylo and Bop.   I had to use a lot of fuel slowing down again at Vall so as to not overshoot, in fact i went through 3600 dV getting from Laythe orbit to the surface of Vall !

The next thing is that the spaceplane is a bit marginal on TWR and reaction wheel torque for Vall.   It can land there ok. but if i fill up completely on LF and Oxidizer (for the tylo rocket) TWR drops to 0.85, which means it cannot takeoff.

I ended up ditching the rover,   filling my LF tanks half full and leaving the OX empty, which gave me a TWR of 1.15 to 1.     I managed to take off with that, and reach orbit with 1680dV remaining, by using a rather unconventional technique.      This is the second savegame point i have to work with.

So,  I think i got two main choices

1) Continue from Vall orbit with our 1680dV.    Can this get us to the surface of Bop or Pol, where we can refuel?   If I can make it to either of these bodies, I can take off with a full load of liquid fuel and oxidizer , for the rocket, due to the low gravity.    Please note that once we do Tylo, we've crossed the rubicon - our ISRU equipment will have been left behind.

2) Continue from the earlier save in Laythe orbit.    Go from Laythe to Bop or Pol, then see Vall just before Tylo or something like that.   Or perhaps just before returning home, when we'll be light on fuel anyway, and missing the mass of the rocket's propulsion stage and IRSU gear.

Note,  you could just landed the Tylo rocket on Val, to save sending the plane down there, since  i think it could handle the trip without staging and still be usable for the dreaded T.      However,  driving the plane across the surface at high speed was a lot of fun and we got to see a good part of it, going down in the rocket lander we'd be stuck in one spot.   Also,  bear in mind that both my save files i have no oxidizer onboard for the rocket, we need to go to bop or pol first to fill up.

For anyone that wants a play, here's the files

https://www.dropbox.com/s/e8dwbp2v45lha3r/BACK IN LAYTHE ORBIT.sfs?dl=0    

above is the Laythe orbit save

https://www.dropbox.com/s/2hgvp8988zc1g9h/after 115to 1683dv.sfs?dl=0

..and this is the save from the orbit of Vall.

Please note, the craft has a "lounge" area that uses Kerbal Furniture mod parts, no wonder it's too heavy for Tylo !

[bewing]

I think you want to go back to the Laythe save and go to Tylo, as originally planned. Your ejection from Laythe can't have been right.

First, you should be able to adjust your burn to exit such that you leave Laythe's SOI with only a few m/s. So you can't possibly be stuck with an orbit that goes beyond Tylo. You want to stop burning shortly after your burn goes from being closed to open.

Second, If you burn so that you leave going prograde to Laythe's orbit, then your resulting orbit will be much higher than Laythe. If you exit retrograde to Laythe, your orbit will be much lower than Laythe. If you want something in between, then you exit with an angle in between prograde and retrograde. I suspect this is your problem. So I think you need to be a lot more vigorous moving your maneuver node around, looking for the proper spot (with the lowest dV needed) that will get you an Ap at Tylo.

 

[AeroGav]

12 minutes ago, bewing said:

I think you want to go back to the Laythe save and go to Tylo, as originally planned. Your ejection from Laythe can't have been right.

First, you should be able to adjust your burn to exit such that you leave Laythe's SOI with only a few m/s. So you can't possibly be stuck with an orbit that goes beyond Tylo. You want to stop burning shortly after your burn goes from being closed to open.

Second, If you burn so that you leave going prograde to Laythe's orbit, then your resulting orbit will be much higher than Laythe. If you exit retrograde to Laythe, your orbit will be much lower than Laythe. If you want something in between, then you exit with an angle in between prograde and retrograde. I suspect this is your problem. So I think you need to be a lot more vigorous moving your maneuver node around, looking for the proper spot (with the lowest dV needed) that will get you an Ap at Tylo.

 

I'd rather not go Laythe direct to Tylo, as landing on Tylo requires discarding my IRSU as stated in the OP.   The IRSU gear is on the tylo lander rocket, which has such slim margins , it can't take off again unless it jettisons the IRSU equipment.      So Tylo needs to be one of our last stops,   though we could probably call at one of the low grav bodies on the way out.

 

The SOI of the inner 3 moons practically overlap, if they are lined up, you pretty much go straight from Laythe SOI to Vall to Tylo SOI without being in Jool SOI more than a few seconds.

 

If the moons aren't all in line,  then literally the moment you add one more m/s to your orbit around Laythe and put yourself on an escape trajectory, you can find yourself in an elliptical orbit around Jool with an AP beyond Tylo.   It depends on your ejection angle from Laythe like you say - if it's prograde to Laythe's orbit of Jool, then you end up "far out".     

Looks like i should have gone Laythe > Bop > Vall > Pol -> Tylo    or   Laythe > Bop > Pol > Tylo > Vall

but i think i'll run with the second save, so long as  i can reach Pol the mission can proceed

[Aegolius13]

Yeah, it seems anomalous that you'd go straight from barely getting out of Laythe's SOI to an orbit way out in the boonies.  Sometimes, if you barely clear your original SOI, you can end up getting recaptured (here, by Laythe) and then flung out with an unintended gravity "assist."  That can lead to weird orbits showing up.  Or maybe some mod is causing problems?  I tried to open your save but the mods weren't compatible so I ended up with a weird asteroid near Kerbin.

Anyhow, I punched up an 80km Laythe circular orbit and created a maneuver node to get my apoapsis out to Vall.  There's no intercept here since the phase angles weren't right, but it should work fine at the right time.  I don't imagine it would more than a few hundred delta-v to capture and circularize here.

 

 Like @bewing says, it may be helpful to move your maneuver node around a bit.  Ideally, to get the cheapest transfer, you want it so your periapsis does not extend beyond Laythe's orbit.  (The flipside of that is that your apo is as far out as possible for a given delta-v burn). Perfect Hohmann, in other words.  Of course, that optimal transfer only works if you have the right launch window.  Some small corrections may be needed if the target is not in the exact right place, and for plane change issues.  

When you're only clearing SOI by a little bit, you get a lot of gravity deflection in your trajectory, so it can end up looking a bit weird.  So I find it's more reliable to look at the orbit around Jool than your ejection path from Laythe.  

Pic of Laythe to Vall transfer - ~600 m/s:

 

Close up of Laythe portion of same transfer:

 

 

 

[Gordon Fecyk]

I was wondering how you were doing. Back when you were asking for gravity assist help, I took your save and tried to fly your proposed mission.

Having Ferram Aerospace installed made the Laythe landing interesting and the takeoff impossible, but that's just me asking for the pain; if I could reach 250 m/s airspeed the rest of the ascent would work. Your craft loves to pitch up.

Vall was rather hard; without the Rapiers in closed-cycle still attached, I couldn't hand the whole ship safely. And I had no idea how you were going to take off from Vall with a full load of fuel and oxidizer without some kind of long, high jump ramp, because I sure couldn't. And there was no point in putting monopropellant in the Mk3 cockpit back on Laythe; that was silly of me to take on so much extra mass.

At one point I landed, refueled and launched your complete Tylo lander at Vall. Maybe after a few runs back and forth I could have refueled your mother ship in orbit as long as I didn't put any oxidizer in the top tank.

I never did get past Vall. That moon has surprisingly strong gravity and a more surprising orbital speed. The transfer from Laythe to Vall wasn't stupidly expensive though; no more than 1200 m/s if I recall. A Laythe-Laythe outward assist might have saved some dV.

--

[AeroGav]

14 minutes ago, Gordon Fecyk said:

I was wondering how you were doing. Back when you were asking for gravity assist help, I took your save and tried to fly your proposed mission.

Having Ferram Aerospace installed made the Laythe landing interesting and the takeoff impossible, but that's just me asking for the pain; if I could reach 250 m/s airspeed the rest of the ascent would work. Your craft loves to pitch up.

Vall was rather hard; without the Rapiers in closed-cycle still attached, I couldn't hand the whole ship safely. And I had no idea how you were going to take off from Vall with a full load of fuel and oxidizer without some kind of long, high jump ramp, because I sure couldn't. And there was no point in putting monopropellant in the Mk3 cockpit back on Laythe; that was silly of me to take on so much extra mass.

At one point I landed, refueled and launched your complete Tylo lander at Vall. Maybe after a few runs back and forth I could have refueled your mother ship in orbit as long as I didn't put any oxidizer in the top tank.

I never did get past Vall. That moon has surprisingly strong gravity and a more surprising orbital speed. The transfer from Laythe to Vall wasn't stupidly expensive though; no more than 1200 m/s if I recall. A Laythe-Laythe outward assist might have saved some dV.

--

The mission report thread is here  -

There's some pictures of the Kerbin launch, the laythe approach and landing (in the dark, on the side of a mountain, with stock visual enhancements - ie. no ambient light at all).   Far more dangerous than any of this, is some footage of a poor Kerbal trying to use it's bathroom (extremely dangerous, can spaghettify the Kerbal and destroy the ship) and also of the rover trying to kill off its passengers one by one.

With stock aero, landing on Laythe at 4km altitude took place at about 40 m/s.   I think by the time we get back to Kerbin we'll be under 30 with all the mass-shedding.  Takeoff was rather underpowered and it takes for-ever to accelerate, but does at least like to keep pointing in the same direction if you don't touch anything so it was mostly 40 minutes of pressing the screen grab button.

Departure from Laythe was with max LF and no ox.

Departure from Vall was with half LF and no Ox.    If you thought my Laythe departure was sketchy, see this -

I don't know if i can reach Pol with the delta V i have.  Taking off with any more fuel on board seems unlikely.    

I have two fallback plans

1. Get a gravity assist from Tylo to Pol.   It might make the difference
2. Go back to a save on the surface of Vall,  and find a way to separate the Tylo lander.  Fly it to orbit under its own power.  The plane will then be able to get off with a lot more LF on board - in orbit it can then re-dock with the lander, and carry it to Pol.   How do i separate them though? (drive the rover under it to force it out the cargo bay?)