Front-row Guidance slide-rule folks?...

[GarrisonChisholm]

Good afternoon all. I just put a probe into orbit around Duna, and I would like to verify my plans in my very non-math, trial-and-error game before I make an uncorrectable error (ideally avoiding f9-ing).

My orbit is, just like my first Eve probe, extremely polar (celestial "north") and extremely elliptical. My Ap is at 39,000km and my Per is 100km. I have (should-have, >50% fuel remaining in stage) enough dV to leave Duna's SOI, but I want to clarify my plans.

To Escape Duna, my thrust needed at Per should be very small, as all I am doing is lifting my Ap to escape. However, once I Escape, will I be in a solar orbit at Duna's distance (therefore only needing a retrograde intercept burn with Kerbin to lower my Per and come home), or will I find my ship is falling in towards Kerbol on a very low Peri orbit due to a small exit "velocity" from Duna?

My orbital mechanics lessons continue; thank you for your continued patience...

[KerikBalm]

"However, once I Escape, will I be in a solar orbit at Duna's distance"

More or less, yes

"will I find my ship is falling in towards Kerbol on a very low Peri orbit due to a small exit "velocity" from Duna?"

Your exit velocity will be small compared to your solar velocity, so your peri may be lower, but not "very low", it will still be more or less even with Duna.

It may be higher, that depends on how you eject relative to duna's motion (since you haven't specified enough orbital parameters in your post).

Your Apopasis is over the north pole? so you'll be burning to escape over the south pole? half the Dunar year, you'd escape into a slightly higher orbit, half the dunar year you'd escape into a sightly lower orbit.

At any rate, its most efficient to do your entire burn within Duna's gravity well, and get a direct trajectory back to kerbin, without going into a solar orbit and then "needing a retrograde intercept burn with Kerbin to lower [your] Per and come home"

By the way, why did you insert into such an orbit (the extremely eliptical polar orbit)?

[Jodo42]

When you captured around Duna, you made your orbital velocity relative to the Sun essentially the same as Duna's. By burning to escape its sphere of influence, you're increasing your velocity relative to Duna to the extent that its gravity just barely can't keep you orbiting around it instead of the Sun. Your orbit will be very similar to Duna's in terms of periapsis and apoapsis magnitude.

If you're trying to get back to Kerbin you might want to perform a Hohmann transfer from Duna to Kerbin, which would be easier if you were in an equatorial orbit. You might be able to do this cheaply in terms of dv: if your argument of periapsis is right your ascending/descending node will be at a high point in your orbit allowing you to make a low dV plane change.

[GarrisonChisholm]

By the way, why did you insert into such an orbit (the extremely eliptical polar orbit)?

Guilty. Simply a lack of sophistication in my burns due to a lack of experience. I am most of the time so happy to get an intercept at all that as soon as it appears in the node I gratefully accept it "as-is". :\

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If you're trying to get back to Kerbin you might want to perform a Hohmann transfer from Duna to Kerbin, which would be easier if you were in an equatorial orbit. You might be able to do this cheaply in terms of dv: if your argument of periapsis is right your ascending/descending node will be at a high point in your orbit allowing you to make a low dV plane change.

This may be possible, but I fear the cost of the plane change as I have a great deal of uncertainty about how much of the required fuel I actually have to make it back. :\ - On my next game I will use mods, and have a greater ability to properly communicate in the true medium of exchange of this forum, dV. - And thank you for confirming that my post-escape solar orbit will be as I expected, as roughly at the orbit of Duna.

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At any rate, its most efficient to do your entire burn within Duna's gravity well, and get a direct trajectory back to kerbin, without going into a solar orbit and then "needing a retrograde intercept burn with Kerbin to lower [your] Per and come home"

I don't think I know enough to see how this is possible. If I am at a South-pole Peri, a direct to Kerbin burn would - to my mind - seem to require "forging" a whole new escape trajectory in a "sun-ward" dircetion, when if I escape celestial North it will require only a little bit of thrust to extend that trajectory beyond Duna's SOI.

Maybe I should just resign myself to trying it multiple ways, and making this a true learning experience.

[GoSlash27]

Garrison Chisolm,

You're going to be pretty much exactly following Duna's orbit about Kerbol.

Your maneuver node patched conics should reflect this.

To get back to Kerbin cheaply, you will want to do the entire burn close to Duna where your velocity is highest (yay Oberth!).

Not a problem *unless* your periapsis is positioned all wrong for the transfer.

You want to leave Duna retrograde to Duna's orbit around Kerbol, so your periapsis needs to be somewhere between Duna's dawn terminator and noon. If it's not there, you're in danger of running out of gas.

Good luck!

-Slashy

[Vanamonde]

This question about how to play the game has been moved to Gameplay Questions.

[Kryxal]

I'd actually say that the peripasis IS positioned wrong for a very good escape burn, and the orbital plane MAY be wrong for the return. The plane is easily fixed with a burn near apoapsis - you'll want to orbit such that your normal vector is directly towards or away from the sun. Positioning of periapsis isn't so cheap, though you can probably make do by executing your escape burn maybe 10-15 minutes (at a guess) before periapsis. I'd STRONGLY suggest planning your escape maneuver WELL in advance, so you can adjust your orbital period as needed to dodge Ike on the escape.