LEO and space stations optimized for Lunar missions

[redliox]

A selling point of space station concepts was that it'd be a halfway point between Earth and (in this case/thread) the Moon.  On just about every delta-V chart Low Earth Orbit literally is about halfway out of the Earth's gravity well, so at least propulsively this isn't an exaggeration.  The trouble seems to lie in meeting other needs outside of being a transportation hub.

...However, presuming a station's priority is to serve as a hub, as in a storage area and meeting point, more so for crewed missions, what characteristics would be required for an LEO station keyed for assisting human Lunar travel?  Would factors like an inclination matching the Moon's orbit be useful or detrimental?  Is there a specific orbital period that'd harmonize in a useful way for either Earth or Luna-bound spacecraft?  Any factors that would make a hypothetical LEO station an asset to Lunar missions.

[A_M_Swallow]

The space station would need communications, navigation, power and propulsion including fuel tanks for station keeping.
The human habitat would need rooms for the humans - toilets, cleaning facilities, a galley with cooking and food storage facilities, fresh water facilities, air facilities, other life support facilities, other storage facilities, entertainment facilities and a control room.
A reentry capsule to act as a life boat.
Docking ports for the life boat, main visiting reentry capsule, reusable cargo vehicle from Earth, deep space manned vehicle, deep space cargo vehicle, refuelling tanker and spare port.
Two robotic arms to transfer cargo between vehicles and to assemble transfer vehicle stacks. Also to perform repairs on both the space station and vehicles.
Equipment to transfer fuel from the refuelling tanker to the space station itself and to deep space vehicles.

[pochimax]

We actually have the ISS to do this job, and the future chinese space station.

I suppose a future commercial space station (Lunar optimized) should have less orbit inclination than current ISS has, but then the russians could not use it. We europeans could use it since we lauch from the better place on Earth, French Guiana.

So it depends on your costumers, only US costumers? international costumers?

You have to think that international costumers (EU and Japan, even India) doesn' t have very big rockets. So they could use this station for launching its (speculative) future reusable manned lunar landers.

[pochimax]

I suppose a future commercial space station (Lunar optimized) should have less orbit inclination than current ISS has

Hmmmm, not sure at all. It is too much complex than my knowledge. Forget this sentence.
It will be interesting. What coould be the best inclination of this space station in order to have good launch windows from earth to this station and from the station to the moon?

[Proponent]

I think the major factors to balance would be:

    1.  The rate of westward nodal regression, which is a function of inclination and altitude.  The higher the regression rate, the more frequent the launch windows.

    2.  The duration of a free-return trajectory (for crewed or reusable vehicles, anyway).  Free returns exist for all orbital inclinations, but they become slower and slower as the trajectory moves out of the earth-moon plane.

    3.  Safety and lifetime of orbit:  higher is better, to avoid drag and debris.

    4.  Energy efficiency of orbit:  lower is better, because then the trans-lunar injection benefits from more "Oberth effect." 

[redliox]

I think the major factors to balance would be:

    1.  The rate of westward nodal regression, which is a function of inclination and altitude.  The higher the regression rate, the more frequent the launch windows. 

Elaborate on this.  It sounds useful since it'd be rather inconvenient for passengers or cryogenic fuels. 

    2.  The duration of a free-return trajectory (for crewed or reusable vehicles, anyway).  Free returns exist for all orbital inclinations, but they become slower and slower as the trajectory moves out of the earth-moon plane.

Encouraging for safety.  What about, presuming a match with the Moon's 5 degree tilt, the fasted a trajectory can go?

    3.  Safety and lifetime of orbit:  higher is better, to avoid drag and debris.

    4.  Energy efficiency of orbit:  lower is better, because then the trans-lunar injection benefits from more "Oberth effect."

Regarding the Oberth effect, that would be more useful for spacecraft that start from a high orbit dipping to a low point.  LEO wouldn't offer much of a boost, but presuming the station orbit is something like 1,000 km high and the departing spacecraft dips to say 200 km there could be some Oberth effect to use.  This does make me wonder if a slightly elliptical orbit in LEO could be useful, if albeit tricky to match for arriving spacecraft.

[spacenut]

Studies were done several years ago, and are in the old threads from the early 2000's.  The Lagrange points, especially LL1 or LL2 would be best for a transfer station.  Sending a spacecraft to say LL1, then change spacecrafts for a lunar reusable lander.  This LL1 station would also be a fuel depot.  Fuels could be exchanged at the station.  Lunar LOX for the earth return ship, or Mars bound ship, then liquid hydrogen or liquid methane brought from earth. 

[Proponent]

I think the major factors to balance would be:

    1.  The rate of westward nodal regression, which is a function of inclination and altitude.  The higher the regression rate, the more frequent the launch windows. 

Elaborate on this.  It sounds useful since it'd be rather inconvenient for passengers or cryogenic fuels.

Leaving LEO for the moon entails injecting into a highly elliptical orbit which will place the spacecraft near the moon at apogee.  If the trip to the moon takes, say, 3.5 days, then you have a launch window every time the moon is 3.5 days from passing through the plane of the departure orbit.  If the plane of the departure orbit were fixed, then a window would occur twice per sidereal month, i.e., about every 13.6 days.

The plane of the departure orbit, however, is not fixed.  Principally because of the earth's equatorial bulge, the orbital plane rotates about the earth's polar axis.  If the orbital inclination lies between 0^o and 90^o, then the rotation will be westward (between 90^o and 180^o, i.e., retrograde, the rotation is eastward, which is what makes sun-synchronous orbits possible).  Since the moon orbits eastward around the earth, that means that the moon passes through the plane of the departure orbit more often that once every 13.6 days.

Rotation of the orbital plane is called precession of the nodes, the nodes being the points at which the orbit passes from the southern hemisphere to the northern or vice versa).  Precession (also called regression when westward) occurs at a rate that decreases with altitude (the further away you are, the more the gravitational field resembles that of a spherically symmetric body) and with inclination.  In a typical LEO, the rate is a few degrees per day.  I don't have any numbers in front of me, but from what I recall of previous discussions, it's feasible to get lunar launch windows every 7-10 days.

    2.  The duration of a free-return trajectory (for crewed or reusable vehicles, anyway).  Free returns exist for all orbital inclinations, but they become slower and slower as the trajectory moves out of the earth-moon plane.

Encouraging for safety.  What about, presuming a match with the Moon's 5 degree tilt, the fasted a trajectory can go?

Apollo trajectories were close to the earth-moon plane.  Free returns took about a week (pretty close to twice the time it took to go from LEO to the moon, in other words, not very different from the period of the highly elliptical lunar transfer orbit).

    3.  Safety and lifetime of orbit:  higher is better, to avoid drag and debris.

    4.  Energy efficiency of orbit:  lower is better, because then the trans-lunar injection benefits from more "Oberth effect."

Regarding the Oberth effect, that would be more useful for spacecraft that start from a high orbit dipping to a low point.  LEO wouldn't offer much of a boost, but presuming the station orbit is something like 1,000 km high and the departing spacecraft dips to say 200 km there could be some Oberth effect to use.  This does make me wonder if a slightly elliptical orbit in LEO could be useful, if albeit tricky to match for arriving spacecraft.

Perhaps I'm misusing the term "Oberth effect" (actually, I've never much liked the term; I was just hoping it would add clarification).  A delta-V executed deeper in a gravitational well is more efficient than one executed further out, even if the departure orbit is perfectly circular.