On orbit inclinations during lunar transferr | Orbits and burns | Forum

Here's a detail we have brushed over in our planning so far.

The Earth-moon orbital plane is not the same as the Earth's equatorial plane.  The difference between the two plans varies from 18.29° and 28.58° as the Earth wobbles.

We have three broad options:

• Launch our CM into a parking orbit aligned with the Earth's equatorial plane (taking full advantage of the Earth's rotation) and then perform an orbital inclination change maneuver, which is expensive, to put the CM into a second parking orbit aligned with the Earth-moon orbital plane.  Then we can perform TLI to get to the moon.
• Launch our CM into a parking orbit aligned with the Earth-moon orbital plane (which would involve "fighting" against the Earth's rotation a bit).  Then we can perform TLI to get to the moon.
• Launch our CM into a parking orbit aligned with the Earth's equatorial plane (taking full advantage of the Earth's rotation) and then performing TLI in that plane with very careful timing so that the CM meets the moon during one of the two points on the moon's orbit where the moon intersects the Earth's equatorial plane.

The third option is the most fuel efficient (we do not have to perform an expensive orbit inclination change and we do not have to fight the Earth's rotation) but the least flexible with regards to timing – we would only have 2 launch opportunities in each month (corresponding to the two intersection points) and an unexpected delay of only an hour or so (maybe, we'd have to crunch the numbers) might result in a launch scrub.

I don't know which of the first two options would be the easiest / most fuel efficient.  I suspect the first would be most efficient, but this could be wrong.

In addition to affecting the size and frequency of our launch windows on Earth, our decision on this matter will affect our choice of landing site.  We don't want to have to waste fuel once in lunar orbit doing an inclination change so we will want to land somewhere roughly directly underneath the orbit we find ourselves in after the lunar capture burn.  Depending on whether or not we end up orbiting the Earth on the Earth-equatorial plane or the Earth-moon plane, we will have a different area to scout for potential landing sites.