Current/Future Eccentricity

[Popestar]

So I'm going through the tutorials in the game for getting to the Mun, primarily because that's my next career milestone.  I'm so looking forward to doing this, and because I need to learn I wanted to go through the tutorials so I can get a good idea of what to do and how and such.  But something came up in the tutorial that bothers me upon reflection, so I'm asking the community for a bit of understanding.

If you take a look at the screenshot below, you'll see a Current Eccentricity of 6.37, with a target of 0.8.  Now, this is in the tutorial, and I was able to plan my maneuver and set things so that the Eccentricity would be at the target of 0.8 after the burn.  However, while the tutorial shows this in the box with Von Kerman in it, I don't see anywhere in the screenshot that would show me this once I'm not doing this in the tutorial.  I have a couple of different mods installed (as you can see), and it's possible that this information is buried in one of them and I just have to activate the appropriate screen or keypress or display.  But I'm not sure which one, or where to find it.  For posterity, the mods I have installed are:

• Click Through Blocker 0.1.10.13
• Community Category Kit 5.1.0.0
• Community Resource Pack 1.4.1.0
• EVS 2.1
• KAS 1.7
• KER 1.1.7.2
• KIS 1.26
• Kerbal Planetary Base 1.6.12
• MechJeb2 2 2.9.2.0
• Module Manager 4.1.4
• SCANsat 20.4
• Toolbar 1.8.0.5
• Toolbar Controller 0.1.9.4
• Zero MiniAVC 1.1.0.1

I'm sure some of this is add-ons necessary to run a mod, but I wanted to list them all out in the event they are important.

Anyhow, how do I see the current and target eccentricity for planned maneuvers?  Is that possible, or do I just have to eyeball the maneuver in order to get into a path where I can then maneuver into orbit for a body whose SOI I've entered?

Any help here to gain a better understanding of this would be hot!

[eatU4myT]

KER at least (and MechJeb too I am almost certain, though I've never used it) will be able to show you the eccentricity of your current orbit, for KER it should be in the Orbit infobox.

As for the eccentricity after a planned manoeuvre, I must say I've never found it that important to know. If the proposed trajectory after the manoeuvre shows you will get into orbit around Mun (rather than encounter/escape flyby) then you know eccentricity is less than 1. If the orbit is going to be perfectly circular then eccentricity is 0.

I think in the tutorial it is probably using the criterion of eccentricity = 0.8 to ensure that your Apoapsis is low enough (does the previous step in the tutorial require you to get an encounter with a Periapsis below a certain altitude?) In practice, it is probably more helpful to look at Apoapsis/Periapsis altitudes, and with an eye on how they compare to a body's SoI, space high/low boundary, atmosphere boundary if it has one, lowest safe orbit etc.

So, for Mun, once you have an encounter you need to burn at Periapsis enough that your planned trajectory changes from a flyby to an orbit, and then you want to keep burning to bring your Apoapsis down to a more useful altitude.  Want to orbit without accidentally escaping when you use RCS thrusters? Don't need to bring the Ap down very much at all. Want to orbit so you are always in space low? Bring Ap down below 60000. Want to land? Bring Ap down to 10000. (If your Ap is high when you bring Pe down to surface level to land, you will be hurtling towards the ground very fast. If Ap is around 10000 then your speed at Pe is much lower, and your Ap is still high enough that you won't crash into any Mun mountains!) Note that in any case when you are burning to bring your Ap down, if it gets lower than your current Pe then they flip, and you start reducing your Pe instead. That's fine, reduce you Pe to whatever is useful and then go round half an orbit to start amending your Ap again.

Edited October 15, 2020 by eatU4myT

[Popestar]

That all makes sense.  Mostly.  The tutorial had me do one maneuver to get the eccentricity to 0.8, and then another to get into orbit.  With this being my first foray into Mun stuff, I just didn't think to do 1 maneuver at Pe to get into orbit.

[Fraktal]

The stock game already shows your eccentricity relative to the ecliptic in the orbital information in the lower left; you need to switch to the second tab (above the ORBIT text). You can also see your eccentricity relative to a celestial body by opening the map view, setting the body as target and mousing over the ascending/descending nodes (the Mun is on the ecliptic, so doing this with the Mun will show your absolute eccentricity).

For eccentricity after a maneuver, KER can display that - and it always displays the eccentricity you'll have in the same SoI as the maneuver, meaning that you can create a maneuver node to see what your eccentricity will be after an SoI change before you actually get there. This is a good thing because even a 3° inclination around Kerbin can get you into an 81° around the Mun to give you a much wider choice of biomes to land in without having to spend dV manually changing your inclination at the Mun (though you'll do need to expend dV to lower your Ap to an altitude from where you can start a not-excessively-vertical landing approach).

[eatU4myT]

On 10/16/2020 at 3:00 AM, Fraktal said:

eccentricity relative to a celestial body

Think this is confusing eccentricity with inclination? Eccentricity is always "relative to a circle", and doesn't depend on celestial bodies?

[Superfluous J]

30 minutes ago, eatU4myT said:

Think this is confusing eccentricity with inclination? Eccentricity is always "relative to a circle", and doesn't depend on celestial bodies?

"Relative to Mun" means "around Mun," basically.

[Fraktal]

Okay, I seem to have done goofed. My mistake.

[Zhetaan]

On 10/15/2020 at 10:00 PM, Fraktal said:

The stock game already shows your eccentricity relative to the ecliptic

48 minutes ago, eatU4myT said:

Think this is confusing eccentricity with inclination? Eccentricity is always "relative to a circle", and doesn't depend on celestial bodies?

@Fraktal:

@eatU4myT has the first part correct, and what you describe appears to be inclination.

@eatU4myT:  You're correct about the first part, but not quite the second part.  It is probably best to consider eccentricity as an intrinsic property of conic sections; because of the way it is calculated, it is at least equally correct to say that it is relative to a parabola.  This is because a conic section in the geometric sense is a construction related to a point called the focus and a line called the directrix.  Ellipses and hyperbolae have two foci and two directrices, and a parabola has one of each (though an argument could be made that it has a second focus at infinity).  Eccentricity is the relationship between the distance from a point on the section to the focus and the distance from that same point to the directrix; in other words, when the distance from a point to the focus is divided by the distance from that point to the directrix, the resulting proportion is a constant for every point on the curve, and that constant is equal to the eccentricity.

The geometric definition of a parabola is that it is the set of points exactly equally distant from the focus and the directrix, which, since these distances are divided to get the eccentricity, of course defines its eccentricity to be exactly 1.  For an ellipse, the directrix is always farther away than the focus, and for a hyperbola, the directrix is always closer than the focus.  A circle is not just a special case of an ellipse; under this definition, it's the limiting case in that the circle's directrix is at infinity.  Since eccentricity is (distance to focus) / (distance to directrix), a directrix at infinite distance implies an eccentricity of zero.

On 10/15/2020 at 3:21 PM, Popestar said:

Anyhow, how do I see the current and target eccentricity for planned maneuvers?  Is that possible, or do I just have to eyeball the maneuver in order to get into a path where I can then maneuver into orbit for a body whose SOI I've entered?

Any help here to gain a better understanding of this would be hot!

While Kerbal Engineer and MechJeb give you information about your current orbit, I don't know whether they give you detailed information about orbits post-sphere-change.  Once you're in the new sphere, I can't speak to the other mods, but I know a dirty back-of-the-envelope method that will get the information you want, albeit not the way that you'd prefer it.  The manoeuvre planner in MechJeb has an option to plan a burn to set the eccentricity to a value you define, so if you put in .8, it will give you that burn.  Of course, that is not the same as examining your current burn and telling you whether it will work.

However, if you are amenable to using a pencil and paper, there's nothing keeping you from looking at your periapsis (I assume that you want to burn at the periapsis) and calculating which apoapsis gives the target eccentricity; then it would be a matter of establishing the required burn.  KSP won't give you the equations but the other values needed, such as the Mun's gravitational parameter, are available in-game.  It's the same general idea as using MechJeb's planner, but you do the calculations yourself, which, since you seek greater understanding, is one way to get it.

Another way is to compare and contrast orbital energy and run through those calculations; you'll get not only the exact manoeuvre required to close the orbit to the desired eccentricity, but also will be able to account for the Oberth effect and all of the other little gotchas of orbital mechanics.

Any way you want to do it, good luck!

[eatU4myT]

47 minutes ago, Zhetaan said:

It is probably best to consider eccentricity as an intrinsic property of conic sections

Naah, it's definitely not best to consider anything as an intrinsic property of a conic section. Might be right, but definitely not best!