Insane DeltaV burns (3-4km/s) to Match planes with Joolian Moons

[LapraLapso]

Hello. So I usually use MechJeb2 to do my manuevers as I'm still learning, and I also play in Sandbox, but I always hate going to Jool because of the INSANE DeltaV required to match planes (change inclination) with a moon such as Laythe or Pol. I really hate it. When I go into Jool, circularize, and set a moon as a target, and check my Ascending or Descending nodes, it'll ALWAYS say any number above 150°. my orbits are almost the same inclination wise to the moons but it uses so much Dv to do a hohmann transfer to them. Please dont hate on me for being dumb. I've only been playing for 54 hours.

[jhousen]

you are lining yourself up retrograde to the direction the moons are travelling(hence the 150 to 220 degree plane change), this is best solved well before entering Jool's SOI as it is quite costly once in orbit.

once you have an encounter, focus on jool and look at the track you will follow compared to the orbits of the moons, if you are far enough out a few puffs of rcs can adjust it where you want.

[MythicalHeFF]

You're getting ridiculous inclinations relative to the moons because I assume you aren't doing a mid-course correction. For me, I find about the time I cross Dres's orbit the best time to set up a maneuver node and plan and encounter that will bring you along Jool's equatorial plane. Laythe, Vall, Tylo, and Pol are all located near the equatorial plane of Jool, so you won't have insane relative inclinations with them. 

You do need to make sure you're coming in in the correct direction, if you're coming in retrograde (the wrong direction) the AN/DN will read almost 180 degrees. Also make sure your Pe isn't too out of whack with the moon you're targeting, as you will be going very fast relative to it when you get there.

Also, nice job even getting to Jool with only 54 hours! When I had 54 hours I had just done my first Minmus landing! Good work! 

[Dman979]

Guys, please keep it on topic. Arguing about who said what is unproductive and will only derail the thread.

Some posts have been removed.

Edited September 4, 2017 by Dman979
Changed wording.

[LapraLapso]

@Dman979 Thank you.

[Dman979]

11 minutes ago, LapraLapso said:

@Dman979 Thank you.

You're welcome.

[Bill Phil]

Planning ahead will help you exponentially. Use hyper edit or cheats to put a decent ship in an orbit over Jool and set up a transfer Laythe or Tylo. A little while before you enter the SOI, select the target. You'll see your flyby orbit of the target body. Do some burns along different vectors, small burns, along prograde, normal, radial and so on. Doing these small burns will change your flyby trajectory. You can do some significant changes . As mentioned before, these are mid course corrections.

Alternatively, you can capture into a high altitude (way above Tylo) orbit and adjust inclination there, where your orbital velocity is minuscule. Then just burn retrograde to fall towards the big moons.

[The Space Dino]

@LapraLapso you could watch some YouTube videos to help you, as I've been to Jool (with an unexpected Laythe encounter) but I'm pretty sure I've been playing less than 54 hours.

Well, mostly because I've been watching lots of videos for a couple of years until I got the game so yeah..

(Ugh why is my internet so slow all of a sudden)

Edited September 4, 2017 by The Space Dino

[natsirt721]

If you do have to do insane plane changes, it can sometimes be far cheaper to boost yourself into a highly elliptical orbit and change planes near apogee - just make sure your apo is near the node. The plane change dV is a function of your current velocity and at apo you're going much slower, hence less dV.  Even though the maneuver requires three burns instead of one it can (sometimes) take less total dV at the cost of increased time.

IIRC the Ulysses probe required a near polar sun orbit, so they shot it out to Jupiter and used a gravity assist to perform the plane change.

[something]

If you are in an orbit and want to change your inclination, you have to kill part of your orbital velocity and add velocity in another direction in order to remain at the same Ap/Pe.

Say you are in LKO (equatorial) with an orbital velocity of 2200m/s and you want to get in a polar orbit of 2200m/s. You need to completely kill your equatorial component (2200m/s) and add 2200m/s in your polar component if velocity. So,  if you do this as two separate burns, you need 4400m/s of dV. If you do just one burn, you get along with 3112m/s of dV  (use the Pythagorean relation: sqrt (2200**2 + 2200**2)=3112).

You could as well burn prograde, so your Ap reaches the edge of the Kerbin SOI. That's about 1000m/s dV. Then at Ap your velocity will be close to zero with reference to Kerbin. If you do your inclination change there it will be extremely cheap. Let's say 200m/s or less, depending in your actual velocity. Once you are back at Pe you do another 1000m/s circularization burn. You are done with 2200m/s - merely your initial orbital velocity.

If you have different celestial bodies around, you could as well use their assistance to change your inclination. It might be even cheaper than the SOI method, depending on your approach and your target orbit. Especially in the joovian system, there are plenty of bodies to assist. That's what @natsirt721 meant with Ulysseus. Instead of killing Earth's orbital velocity and accelerating again into a polar solar orbit, they decided it was cheaper to aim for the Jovian south pole, complete half an orbit around Jupiter and then getting the inclination change for the cost of a Jupiter transfer...

Moral/TLDR: Your Inclination change costs really depend on your strategy. As shown above, savings of a factor of two are easily possible.

[Vanamonde]

Now that we know the kind of problem OP was having, this thread belongs in Gameplay Questions. And so, moved. 

[Zhetaan]

@LapraLapso:

In the interest of giving some clearer instructions, please let me illustrate what's going on.

Here's a quick transfer orbit picture I threw together from some old paint I found somewhere.  The yellow-green bit is Kerbin, the greener bit is Jool, the blue ellipse is your transfer orbit, the hollow green circle is Jool's future location when you transfer there, and the path of travel about the orbits is indicated by the colour-matched arrows.  I know that you've got this part of the transfer down because otherwise, you wouldn't be able to circularise.

This is the Jool system.  Jool is the big green dot and the moons are all on their orbits with arrows nearby indicating the direction of travel.  Note that from this perspective (looking from celestial north, or from 'above' when considering the standard map view), the moons all orbit anticlockwise around Jool.  The white triangles off to the right side are two spacecraft.  The one on the red orbit passes around Jool in the same direction as the moons.  The one on the blue orbit passes around Jool retrograde to the moons.  They can both circularise for about the same amount of delta-V, but the one on the blue orbit will have prohibitively expensive moon transfers because the its orbit around Jool is the opposite of the moon's travel, and in order to orbit a moon, you need to be going the same way.

What's happening to you is that you're coming in on the blue orbit or its equivalent, but what you need to do is come in on the red orbit.  They're actually pretty close to one another, but the subtle differences add up when in the Jool system.

In truth, the pictures are greatly exaggerated, but I needed to illustrate the problem.

This is actually caused by MechJeb.  MechJeb is not doing anything wrong; it's just that MechJeb normally tries to give you the most efficient transfer possible, and that works out to one that passes just sun-wards of the target planet.  For a tiny prograde puff of propellant at the beginning of the transfer burn back at Kerbin (or a slightly larger puff midway to Jool), you can send your trajectory out and around the far side of Jool with respect to the sun and you won't have the problem any more.

Edited September 6, 2017 by Zhetaan

[DerekL1963]

4 hours ago, Zhetaan said:

This is actually caused by MechJeb.  MechJeb is not doing anything wrong; it's just that MechJeb normally tries to give you the most efficient transfer possible, and that works out to one that passes just sun-wards of the target planet.

Nonsense.  The advanced transfer planner is designed to generate an insertion burn that places you on a trajectory to arrive at the proper location (the red line on your diagram).   That doesn't always happen because MJ isn't magical and there is some imprecision in the game.

The solution is easy however...  You use the "fine tune closest approach to target" option in the Maneuver Planner to create a mid-course correction maneuver, this is rarely more than 1m/s and puts the vehicle back onto your red trajectory.  Or you can execute your orbit corrections and plane change maneuvers as soon as you enter Jool's SOI.  (Or, if you know in advance you're going to one of the highly inclined moons, both.)

 

[Zhetaan]

@DerekL1963:

I appreciate your input, but I'm afraid that I am not yet convinced.  My own testing consistently produces sun-ward passes, or at most direct collisions, of Jool.  If the cause is only imprecision, I would expect the variation to be more random and to see some trajectories overshoot Jool.  That is perhaps a function of my choice of transfer window (and the original poster's, as well), or some other minor-but-significant oversight:  I freely admit that my testing was less than exhaustive, so my conclusion may be premature, but I think we can agree that the issue is still in the MechJeb trajectory planner without needing to agree on precisely what the issue is--especially since the solution of a mid-course correction is the same in either case.