How to get a stable RInc

[Goth]

I reached an orbit aligned with the Moon orbital plane. After some adjustment with the "Align Orbital Plane MFD" my RInc was 0.00° but slowly it started to increase. Since I want to put a space station block in an orbit perfectly aligned with the Moon this is not good. So how can I get an invariable RInc of 0.00°? I tried switching to linear mode burning forward and backward, but at the end of the adjustment, the RInc still increases.

 

[Urwumpe]

You can't, it will always increase.

You need to do small course corrections to stay in a specific orbit plane, unless you have about 64° inclination (equatorial).

 

[Goth]

Oh. So is it impossible to build a space station with an orbital plane aligned to the moon?

 

[Urwumpe]

Oh. So is it impossible to build a space station with an orbital plane aligned to the moon?

No. It just means you have to use a little bit of fuel every month for staying aligned.

 

[Goth]

OK, thanks.

 

[Mission_CDR]

You can't, it will always increase.

You need to do small course corrections to stay in a specific orbit plane, unless you have about 64° inclination (equatorial).

Wouldn't an equitorial incination be 0 degrees? explain that to me.

 

[Urwumpe]

Wouldn't an equitorial incination be 0 degrees? explain that to me.

No, you have two ways to measure inclination in Orbiter - equatorial frame (with the equator of Earth as reference) and ecliptic (using the orbit plane of Earth around the sun as reference)

 

[Mission_CDR]

isn't that aproximatly 23.5 degrees?

 

[Urwumpe]

No. You have to remember it is 3D. It depends also on the LAN.

A ~64° inclination orbit, measured relative to the equator, is almost free of forces which try to change the plane.

 

[Andy44]

I think you're confusing things. At an inclination of ~63.4 degrees, the rotation of the orbit ellipse about the h vector is zero, in other words the argument of the perigee does not change with time. But the orbit plane does not remain fixed in inertial space, it regresses westward around the ECI z-axis at a rate depending on the semimajor axis and eccentricity (regression of the ascending node). The 63.4 degree figure applies only to earth orbits, since every planet has its own nonspherical mass properties, of course.

In any case, the OP wants to be in the Lunar plane, which is only about 28-29 degrees. 63.4 is far away from that. At this inclination, the argument of perigee will move with respect to time due to nonspherical earth (J2), but if your orbit is near circular that's not a problem. The problem is staying in the lunar plane, which as you said, is possible only with periodic propulsive maneuvers. The reason for this is both J2 effect and third body gravity from the sun and the moon.

It should be posible to choose an orbit that will remain "moon syncronous", that is, the angle between the orbit plane and the moon's orbit plane will remain constant, but I don't think you can make it zero.

 

[Quick_Nick]

Does this problem only occur with non-spherical gravity checked? I've never had my RInc changing on its own like this, but I never use non-spherical gravity.

 

[Andy44]

Without nonspherical gravity the problem should be greatly reduced, but if you time accel long enough, the RInc will change very slowly due to solar gravity, and to a much leser extent, Jupiter and the other planets. The lower your altitude is above the Earth, the less effect you will see.

Short answer: Yes. Non-spherical gravity is the primary cause of this problem.