Ike --> Kerbin

[MPDerksen]

I totally get how to set up both the angle between Duna/Ike and Kerbin, as well as WHEN in the orbit to do my burn.  I can get to Duna pretty close with the amount of dV that the maps indicate.  However, when I return, I struggle to get the encounter without a huge amount of mid-course correction.  When I get my proximity arrows, I'm like 2-3Mk from Kerbin.  1 of 2 things is true (or both):  I'm not at the correct relation to Kerbin in the first place.  Or, I'm burning at the wrong time in my orbit.  I then spend about twice as much fuel to use the Radial/Prograde combination instead of doing it right the first time.

I assume there is a mod to tell me the first one.  I think I recall I'm looking for about 77degrees.  I'm close, but every little bit creates a big difference at the other end.  if so, which mod should I add?  As far as my "151degree ejection burn", I've just been eye-balling, and dragging my maneuver node around.  Is there a better way?

Last question:  my mission involves orbiting Ike, and returning.  Plan is to aerobrake at Duna, circularize Ike, then burn straight back from there.  No reason to leave from Duna, right?  Leaving from Ike is just as good?

[Adenosine Triphospate]

I think the problem is, assuming its not the transfer window, that you are burning directly from Ike, which could screw up an encounter in many ways:

1) Ike could be travelling in the wrong direction when you burn (prograde relative to the sun), adding to delta v to lower you orbit.

2) You could be burning at the wrong point in Ike's orbit, essentially shifting your orbit around the sun.

3) Ike could be at an inclination around Duna orbit (although your ships orbit is more likely to make a difference), which leads to a costly correction in deep space to match Kerbin orbit.

4) Burning from Ike is actually less efficient burning than at an eccentric Duna orbit! I can't explain here why (its the Oberth Effect), but the following will:

I'd recommend Matt Lowne's Duna Tutorial where he goes from Ike to Kerbin, which is from 30 minutes into the video. It covers the manoeuvre, where to burn (not at Ike) etc, it will become clear in the video. It can explain visually much more easily than any worded explanation, so its definitely worth checking it out!

 

BTW, I wouldn't recommend mods to overcome learning how to get an encounter, as it will likely just make the problem worse. Once you've done your return, go ahead, but I'd avoid mods until your comfortable with getting encounters

 

Edited May 9, 2019 by Adenosine Triphospate

[MPDerksen]

So wasn't looking for a mod to do the transfer FOR me.  I saw one in a video (Orb8ter?) where it would just give you an output on your angle instead of holding a protractor to the screen.  I'll try my mission tonight and post a picture if I get stuck.  

[Daniel Prates]

Over such huge distances, a few dozen deltaV wrong can make a big difference. I think it is just pushing your luck to execute a single burn directly from Ike's orbit, hoping that at the end you will be already all set to return to Kerbin, because in the in-between step of Ike-to-duna orbit, something might have been miscalculated by the game and your end orbit might not turn out as you expected. Besides you may be executing it a little differently from the planned maneuver, accounting for those small changes that, over such a long distance to Kerbin, end up creating huge errors.

Its more precise, I think, to exit Ike orbit first, then recalculate it all over again foe the second step (duna to kerbin).

[Superfluous J]

Another trick is to hack something into low Duna orbit. It can be anything that can make a maneuver node, like a simple antenna+solar probe.

Make a maneuver node with that probe, and then make your Ike ship's maneuver node so it makes a trajectory as parallel as possible to the probe's. If you target the probe with your ship, you'll see its maneuver node.

Timing is important. You want to time leaving Ike when it is heading as close to the probe's ejection direction as possible, and when your ship is also heading in that same direction around Ike. That way, Ike is throwing you that way and you're getting a further boost from your orbital speed. These are more important than getting the transfer window right, but you still want to be as close to it as possible.

After you have the maneuver node set correctly, delete the probe. Consider its cost to be the fee you paid the consultant to get that trajectory

[FloppyRocket]

The PreciseNode mod is good for making small changes (in time or delta-v in all of the maneuver directions) to get exactly the intended departure angle.

I don't know if the new Maneuver Node features in the latest KSP version have made PN obsolete.

Quote

When I get my proximity arrows, I'm like 2-3Mk from Kerbin

Depending on how far out from Kerbin you make the mid-course correction, you can fix that magnitude of error with very little delta-v.

 

Edited May 10, 2019 by FloppyRocket

[MPDerksen]

30 minutes ago, FloppyRocket said:

 

Depending on how far out from Kerbin you make the mid-course correction, you can fix that magnitude of error with very little delta-v.

 

Perhaps this is the true nature of my question, then.  But the original question was not how to mod the game so I didn't have to do it right.  My question is if there is a way to tell me when I'm at 75.19degrees, and where is 121.06degrees?  

 I have to get some ore from Minmus back to Kerbin, and then I'll be doing my Ike/Duna mission and will post a picture.  

For now, consider the previous time as an example:  I set up a "nearly" correct burn, using about the 610m/s burn that is called for in the guide.  I ended up with proximity arrows showing me about 2-3million K from Kerbin, with Kerbin ahead of me.  If I burn retrograde, I lose the arrows completely.  Okay, so I mess with the radial/anti-radial to get them back.  Marginally closer, so i keep trying.  I end up needing another 800m/s to get an encounter (which in this case I don't have).  I was setting this node very close to Duna, not half way along, and my Inclination nodes were pretty close (maybe 0.2?).

So when you say i can fix this, can you propose how?

[FloppyRocket]

27 minutes ago, MPDerksen said:

Perhaps this is the true nature of my question, then.  But the original question was not how to mod the game so I didn't have to do it right. 

PreciseNode doesn't do anything for you. It isn't about "not doing it right". It's about being able to make very precise adjustments to what you're doing.

[MPDerksen]

Just now, FloppyRocket said:

PreciseNode doesn't do anything for you. It isn't about "not doing it right". It's about being able to make very precise adjustments to what you're doing.

sounds exactly like what I want.  Looking for information, not an auto-pilot... lol

[Superfluous J]

10 minutes ago, MPDerksen said:

sounds exactly like what I want.  Looking for information, not an auto-pilot... lol

Then I wholly recommend PreciseNode and Kerbal Engineer. The stock maneuver node editor does a pretty good job these days, but PN has them beat on ejection information for sure.

[MPDerksen]

44 minutes ago, 5thHorseman said:

Then I wholly recommend PreciseNode and Kerbal Engineer. The stock maneuver node editor does a pretty good job these days, but PN has them beat on ejection information for sure.

I see "Kerbal Engineer Redux".  Is that the correct one?  I'm playing version 1.7.  Will that be compatible?

(PreciseNode works perfectly, and exactly what I wanted.  Thanks for that)

Edited May 10, 2019 by MPDerksen

[Kryxal]

It's tough to measure those angles, most people go with the practical "ejection burn that is directly Duna-retrograde at SoI".  Once you have about the right delta-v and exit direction, you can fine-tune with PreciseNode and knowing the changes you want:

Increase delta-v to lower PE / decrease to raise

Adjust burn position in the orbit to change ejection angle - going outward from the star gives Kerbin more time to catch up, inward gives less.

You can also change by full orbits, but that could mess with your ejection from Ike, and if you add enough orbits (a day or so worth) you'll probably want to fine-tune your Duna ejection angle

With these you can fine-tune everything but plane-change.  For that, you'll want to set a node about 90 degrees before arrival, make your plane-change there and fine-tune your altitude.