Polar Orbit Darkness Time

[wisnoskij]

There are a load of resources for calculating darkness time for circular equatorial orbits. But What about polar?

I have a nice group of contracts right now for setting up a satellite in polar orbit, and a RT2 interplanetary relay. I think they will work good together.

The orbit I am asked to get is a roughly circular, roughly Mun sized orbit (12M by 7M), and is polar.

I have two main questions:

1) I am not sure how long a mun/minmus eclipse would last.

2) Right now the orbit looks like it would never be eclipsed by Kebin. But I suppose relative to the sun the orbit will rotate around and eventually be getting eclipses? And these will be really long as I am quite far out. So the only solution is a highly eccentric orbit? Where I am high over one pole and very low over another? Do I just not even worry about battery power then, as it going to be out of position for most of the solar system then anyways?

Lowing my P in a high orbit should be easy, I am imagining, as I am already traveling slow relative to a smaller orbit, so less V to negate.

Is it possible for an orbit to be so eccentric that while your P is above 73K, the sides of the orbit are actually closer to the planet? An orbit that is 12 million KM wide and less than 140K tall? I never thought of ever entering the atmosphere with both a P and A above it, but it should technicality be possible.

Edited June 14, 2015 by wisnoskij

[guitarxe]

I don't believe the darkness time in a polar orbit would be any different from an equatorial one? You are still blocked by the body you orbit for only half an orbit in both cases, and being eclipsed by Kerbin is no different in a polar orbit than it is in an equatorial orbit.

[MarvinKitFox]

Polar orbit maximum eclipse duration is very very slightly shorter than for an equatorial orbit of same size, worst case.

reason:

Kerbin is exactly round, so its shape does not affect eclipse duration.

By not orbiting in the same plane, same direction as Kerbin around the sun, your eclipse is not artificially lengthened by Kerbin's motion around the sun.

In addition, you will tend to encounter eclipses only for a very brief period (about 3 days, or less) twice per year, when your orbital plane is aligned with the sun.

If you *really* want to get the shortest possible eclipse period, and almost as infrequently as possible, then you want an intermediate altitude retrograde orbit, inclined about 20 degrees.

The low-ish, retrograde orbit reduces maximum darkness time by a few seconds relative to prograde orbit.

The inclined orbit reduces occurrences of eclipse to only twice per year.

Of course, if you **really** don't want eclipses, just park your satellite just outside Kerbin's SOI, and exactly ON its orbit. Due to KSP's patched conics approximation, your orbit will never get disturbed and your distance to Kerbin will remain quite constant.

Edited June 14, 2015 by MarvinKitFox

[wisnoskij]

Great information.

[Rhomphaia]

Is it possible for an orbit to be so eccentric that while your P is above 73K, the sides of the orbit are actually closer to the planet? An orbit that is 12 million KM wide and less than 140K tall? I never thought of ever entering the atmosphere with both a P and A above it, but it should technicality be possible.

No it is not possible, the center of the planet is always a focus of the elipse of your orbit, the periapsis is the point on the orbit closest to that focal point. Since in KSP the planets and the atmosphere are spherical, if your periapsiis is out of the amosphere so will be the rest of the orbit.

If your planet and atmosphere were oblate enough it could be possible with a polar orbit.

[FancyMouse]

>>1) I am not sure how long a mun/minmus eclipse would last.

Screw that. In theory such eclipse shouldn't block all the solar energy due to their size, so I'm happy with just spamming solar panels and prepare battery just for Kerbin night time for satellites around Kerbin. Note that this should apply to satellites around Mun/Minmus but you need your orbit to be high enough to make the eclipse from Kerbin non-continuous.

>>2) Right now the orbit looks like it would never be eclipsed by Kebin. But I suppose relative to the sun the orbit will rotate around and eventually be getting eclipses? And these will be really long as I am quite far out. So the only solution is a highly eccentric orbit? Where I am high over one pole and very low over another? Do I just not even worry about battery power then, as it going to be out of position for most of the solar system then anyways?

Pretty much what #3 already said. The other solution is just to have 4 of them on the same polar orbit high enough, and not preparing for the night time - in this case, at least one pair of opposite satellites will work regardless of the night time and then you'll have full coverage for relay. Personally I'm just having 2 at the moment and probably will shoot another 2 if I have time.

Of course, since the game doesn't simulate Ec change when the ship is not on focus... But it's up to you whether you want to exploit that.

And some other suggestion. You probably don't want your relay satellite to get too high, as the long range dish antenna has a really small cone angle. Say for connection to Duna when you use 88-88 (it already has the largest angle among those that can reach Kerbin), because of the closest distance between Kerbin and Duna, if you point your Duna satellite to target Kerbin (not a specific satellite), then you need your relay satellite's Ap to be less than 6Mm in order for the relay satellite to accessible from the satellite orbiting Duna. I think RT is simulating that and you'll get broken connection in those cases. Better keep that in mind.

Edited June 14, 2015 by FancyMouse

[wisnoskij]

@MarvinKitFox: Now that is an interesting idea. I would just target Kerbin and after leaving its SOI, switch to target mode and zero my relative velocity?

It just hit me, in that position, i would have constant coverage of every single equatorial object, in particular my RT2 relay. So I can use the light 88, pick one at random and point it at it. Then have full coverage of the solar system, and point any incoming connections at that sat (so absolutely no need to worry about broadcast angle)

[Kryxal]

Don't zero your relative velocity, match your orbital period with same height and near-zero vertical motion. Get maybe an hour past the SoI change first, though.

[wisnoskij]

@Kryxal: So go into target mode, zero velocity, then switch to orbit mode and burn along retrograde or prograde to lengthen or shorten period? I am not sure how I would zero vertical relative to Kerbin specifically.

[Kryxal]

You don't have to zero vertical relative to Kerbin, just do it relative to the sun with matched altitude. It's mostly a case of "get in front, make sure you stay in front" and orbital period and a circular orbit are plenty for that.

[tutike2000]

Is it possible for an orbit to be so eccentric that while your P is above 73K, the sides of the orbit are actually closer to the planet?

No, the Periapsis is always the lowest altitude in your orbit, and Apoapsis is the highest.