Quick Orbit Question

[ping111]

Salutations.

I had an idea for a realistic Münar transmission system once we get persistence, but I need help.

Here\'s the plan:

Once you orbit Kerbin, you drop a satellite.

You do your TMI, and enter LMO (Low Mün Orbit).

You drop another sat, then land and return home.

The Mün-sat would be like a radio antenna, relaying messages to the Kerbin-sat. Then, the Kerbin-sat would relay the messages home.

Easy, right? Wrong.

Is there a way so that the Kerbin-sat will always face the Mün, or would the orbit be too big or too small, and vice versa?

Any help is appreciated.

Godspeed.

[The_Stargazer]

Is there a way so that the Kerbin-sat will always face the Mün, or would the orbit be too big or too small, and vice versa?

What do you mean by 'always face the Mun'?

If you\'re talking about the satellite always facing the Mun while orbiting Kerbin inside the Mun\'s orbit, that would need to be done by station keeping gyros or thrusters on the satellite.

If you put it in orbit of the Mun, always facing the Mun is easily done by making the satellite 'long' so gravity keeps it pointing at the Mun (Gravity-gradient stabilization).

Also the satellite would occasionally have its view of the Mun blocked by Kerbin as it orbits, unless you put it in one of the Kerbin-Mun Lagrange points (points where the gravity of Kerbin and the Mun cancel out and an object is stationary relative to the two bodies).

[trbinsc]

I you want to have a continuous signal you would need to have multiple satellites.

[NovaSilisko]

All you need is at least 3 satellites in mun orbit and a series of ground stations. Just make it so all three satellites can see each other, and the ground stations can see at least one satellite.

[ping111]

I don\'t think you guys understand my question.

The question is: How do you make two satellites never not be able to see each other.

[Kryten]

They do understand-the problem is that it\'s impossible.

[ping111]

They do understand-the problem is that it\'s impossible.

Oh, so there\'s no such thing as a reverse Geosynchronous orbit?

Or you can\'t make an orbit the same speed as the Mün?

Okay.

[trbinsc]

Well, you can, but it would need to at pretty much the exact same orbit of the Mun.

[Hypocee]

Well, both of those things are true...

but Kryten apparently forgot that inclined/polar orbits exist. So yeah.

[Kryten]

A polar orbit would still be blocked at certain times, when the plane of the orbit moves behind Kerbin relative to the Mun

[RulerOfNothing]

What about a polar orbit around the Mun?

[ping111]

What about a polar orbit around the Mun?

It would have to be massive in order to be constantly visible.

[SteevyT]

It would have to be massive in order to be constantly visible.

And even then it would be blocked occasionally at certain times during the Mun\'s orbit.

[Goddess Bhavani]

I am unsure if an L-point relay satellite is viable.

Is station keeping difficult within the L-point?

[Guest GroundHOG-2010]

Its impossible (at the moment, lets see in 200 years or so) to put 1 object into a Kerbin orbit and one in the mun, and have the two always in communication with eachother without the use of relays or other satelites.

[The_Stargazer]

I am unsure if an L-point relay satellite is viable.

Is station keeping difficult within the L-point?

Lagrange points are extremely easy to station keep.... Thats one of the things that make them special... They\'re the 'neutral' points between Kerbin and the Mun. Only adjustments you have to make are for tugs by the Kun.

[Motorsheep]

...the problem with LaGrange points being that they don\'t exist (yet) in KSP because it doesn\'t support n-body gravity. You can\'t be in both the Kerbin\'s and the mun\'s SOI at the same time.

It might be though that if you manage to hit exactly the altitude at which you leave Kerbin\'s SOI, the effect of Kerbin\'s gravity pulling you in will be so low that it\'s effectively zero.

[Iskierka]

Only L4 and L5 are easy to station-keep. L3 maybe. L2 and L1, quite difficult, you\'re probably better off with two sats in low orbit at either end, making sure at least one can communicate at all times.

[khyron42]

All you need is at least 3 satellites in mun orbit and a series of ground stations. Just make it so all three satellites can see each other, and the ground stations can see at least one satellite.

That was interesting to dig up some high school trigonometry to figure out the minimum altitude orbit for an equilateral triangle of satellites to see each other around a 600km sphere in the center. If you\'re working from a single ground station on the Mun and trying to talk back to KSP without a series of ground stations, you\'ll need three sats on each end.

If you\'re going for lowest possible three-comsat constellation, for Kerbin you need three equally spaced (120 degrees apart) in orbit at about 1500 km. For the Mun, the triangle arrangement needs to be at about 500 km altitude.

You could, if you use Keosynchronous satellites on the Kerbin side, get away with using just two of them with about 1200 km separation from each other. One could be directly over KSP and one offset just enough that it\'s always visible when the KSP one is blocked by the planet. However, then your Munar relays end up sending signals across 3000 km some times and 19000 at other times; better to relay across a more predictable 11000 km distance to a lower-orbit 3 satellite setup, I think.

You can also go with lower-altitude comsats if you add more than three, but the calculations of just how low get more complicated fast. Lastly, you could park a single Munar one 2500 km ahead of the Mun in the same orbit and use it, if your Munar base can see that point in the sky at all times.