Question orbits of orbital transfer stations

[jedidia]

I just realised with some surprise that we don't have a sub-forum for speculative spaceflight discussions that are not directly math- or astronomy related, so I'm posting this here.

I'm wondering about transfer stations. They are often mentioned whenever the talk is about "space ports" or a transport network between earth and the moon, but somehow it's never too closely discussed, and I can't find anything that actually talks about the topic in some depth.

So what properties make a certain orbit advantageous for a transfer station? You can reach the moon from any inclination, and adjusting your target plane on the way is really not that big of an issue. Much less than changing the plane when near the moon or the earth, so wouldn't it be more efficient to have a station at zero inclination that you can more comfortably launch to (launch is the hardest part, after all), and leave the plane change as an exercise to the vessel doing the long leg of the trip?

In case of transfer stations to other planets, that would hold true even more, it seems.

Now, transfer stations that are built to be more easily reached from various (not necessarily equatorial) launch sites on earth would seem to make a lot more sense to me, but unless I understood most things wrong about the topic, that's not what is usually talked about.

So, how exactly is this envisioned to work?
(P.s: I know there's discussion about whether or not a transfer station is a sensible concept at all, that's not really what I'm after here. I'm more interested in how exactly they would be beneficial according to their proponents).

 

[Urwumpe]

well, depends on what you mean with the term. A cycler station network like the Aldrin cycler?

Or a lagrange point station? A DRO station that allows reaching most of the lunar surface and can be reached from almost everywhere?

 

[jedidia]

well, depends on what you mean with the term. A cycler station network like the Aldrin cycler?

No, more like "jumping boards" by which transport infrastructure can be divided into launch vehicles and transport vehicles that never have to touch atmosphere.

Or a lagrange point station?

That's too far out. Whenever I encountered the concept, it was usually LEO.

A DRO station

What's a DRO station? When I google for it, the only thing I can come up with is the daryaogonj railway station... :lol: Though a "railway station" probably wouldn't be the worst analog for the concept.

 

[Urwumpe]

What's a DRO station? When I google for it, the only thing I can come up with is the daryaogonj railway station... :lol: Though a "railway station" probably wouldn't be the worst analog for the concept.

I meant the distant retrograde orbit. A special kind of lunar orbit, which gets its high stability by the interaction with two lagrange points.



Its a bit further away than low orbit, but requires very little station keeping and is pretty much in the middle between the moon and its parent planet.

I don't think a classic LEO station fits into the job description. While it might be easier to access from Earth, it is pretty much too far into the gravity well too be useful for inbound traffic. Its just not the worst staging point for outbound traffic.

A LEO station makes more sense for assembling spacecraft, when lots of material has to come from Earth.

 

[Ajaja]

so wouldn't it be more efficient to have a station at zero inclination that you can more comfortably launch to (launch is the hardest part, after all), and leave the plane change as an exercise to the vessel doing the long leg of the trip?

I don't think so. Try to imagine an inverse problem - targeting a station at zero inclination on your way back from Mars, for examlple.

 

[jedidia]

Going on the last two posts... the use is more for *incoming* traffic, not for outgoing. Ok, that makes sense, I guess I never quite got that. Also, it's a long time ago since I did any return flights from a planet (somehow going *there* has just always been more fun), so I'm really not sure about this next point: Wouldn't the ideal inclination for arriving craft depend a lot on where Mars was in its orbit? Or is it just meant for an ideal launch window? Which would make sense in any case, I guess... Can't really get back from Mars in any useful amount of time if earth's not in the right place.

In that case, does anybody know what good inclinations for planets in the inner system would be (Mercury, Venus, Mars)?

 

[Ajaja]

Incoming and outgoing trajectories generally are the same. In most cases your have to use tons of extra fuel to get into/from an interplanetary trajectory from/into an equatorial (or any arbitrary) LEO.
I doubt that an ideal LEO for the transfer station exists.
Your can aproach/departure a planet from various directions and each time have different and limited set of parameters for fuel-efficient parking LEOs.
And if we speak about near-Hohmann interplanetary transfers you really can not choose between variety of such departure/aproach directions.

 

[Urwumpe]

Going on the last two posts... the use is more for *incoming* traffic, not for outgoing. Ok, that makes sense, I guess I never quite got that. Also, it's a long time ago since I did any return flights from a planet (somehow going *there* has just always been more fun), so I'm really not sure about this next point: Wouldn't the ideal inclination for arriving craft depend a lot on where Mars was in its orbit? Or is it just meant for an ideal launch window? Which would make sense in any case, I guess... Can't really get back from Mars in any useful amount of time if earth's not in the right place.

In that case, does anybody know what good inclinations for planets in the inner system would be (Mercury, Venus, Mars)?

It has to be useful for both, that is the important deal - if you need a massive braking maneuver to reach this station, it might be easier to just land on Earth.



If it is too far outside, it takes too much effort to be reached from Earth.



The ideal orbit is a compromise between many conflicting factors. Far enough outside, that you can conserve energy. Close enough for economic utility. If no direct economic connections to Earth are needed at all, it even makes sense to park into a nearby solar orbit or L4/L5 and avoid any work against gravity.


Similar to other dimensions. An equatorial station requires dogleg maneuvers to be reached from most places on Earth. A polar station can be troublesome as well, somewhere in the middle is equally bad for all, but a 63° inclination can bring some additional stability for Earths gravity field, similar factors exist for other planets.