Another consideration is that using the ISS as a gateway station, as it was originally intended, actually helps with these issues. If you launch your crew first and your EDS second, you mitigate boil-off issues (pre-depot) and launch window issues (depot). The crew can stay safely inside the ISS while it awaits the EDS and its launch window.

ISS also helps commercial space by providing an anchor customer for manned flights and perhaps by sharing costs if tourists are allowed on the ISS, say in an attached Bigelow module.

Thirdly, having the ISS around hurts SDLV because it eats up money. This cuts both ways of course. We will probably lose the battle to kill SDLV now, but that doesn’t mean we can’t stop it from ever entering service. It also means that even though ISS may be safe for now, it isn’t safe beyond 2020 and it may require action soon if we want to do something about that.

Most of the time you cannot launch straight into the moon’s orbital plane from KSC anyway. There’s something called the Saros cycle which causes the inclination of the moon’s orbit to vary with a ~18 year period. For a lot of that time the inclination is below 28deg.

The competitors to depot centric architectures (aka SDLV architectures) suffer from the same problem because they have to do rendez-vous with an EDS that is launched first and whose orbit precesses too. Launching straight to L1/L2 would obviate that problem, but our SDLV loving friends won’t go there. I think it is for two reasons: 1) L1/L2 are too easy to reach for competitors and 2) AVUS/JUS are so damned heavy that you’d be hauling a bunch of useless metal to L1/L2, which hurts performance a lot. You could avoid this by using a Delta upper stage, but they won’t do this because this invites EELV Phase 1 which will prove their launcher superfluous. Not that they will ever admit any of this of course.

Unfortunately, we don’t have any launch facilities far enough south to take advantage of these low inclination orbits. Cape Canaveral is at about 28.6 degrees north latitude and this is currently the only launch site from which the US can launch crewed missions. SpaceX has been able to launch from the Kwajalein atoll at about 9.1 degrees north latitude, but as far as I can tell, they are currently not able to handle anything larger than a Falcon 1. Probably the most favorable site is the ESA’s launch site in French Guiana at about 5 degrees north latitude. This site can launch the Arianne 5 and is supposed to be able to handle Soyuz vehicles sometime in the near future.

Getting into orbits with an inclination higher than the launch latitude is fairly trivial. The only penalty you pay is for how much of the Earth’s rotational velocity you ignore by not launching due east. To get into a lower inclination orbit, however, you still have to do a plane change. Hence the discussion above.

With our present rocket technology, we can just barely get crews into orbit. Once the inclination of that orbit is established (at the launch latitude or higher) it is fairly expensive to make any significant plane changes. Thus if the depots are established in a LEO that can be easily reached from the surface, then perhaps there would be sufficient fuel reserves to make the required plane changes, however, you are now restricted by the phasing (timing) constraints dictated by orbital mechanics.

~Jon

“but it also needs to not take so long that you’d be better off just waiting a few days for another launch window.”

I can see why that would be so if you had to go to the moon in a hurry. What application are you thinking of?

Martijn

If the added time is only 12-24 hrs, then it really buys you something to be able to leave whenever you want. But if it adds 4 days to your trip, you haven’t gained anything. Your payload arrives at the moon at about the same time both ways, but in one way you have a simple two-burn trajectory, and in the other you have a complex trajectory with more delta-V requirements…

So my point is any “launch any time from an LEO station” approach has to not only cost little extra delta-V, but it also needs to not take so long that you’d be better off just waiting a few days for another launch window.

~Jon

For transport beyond LEO, small players would need to refuel at a depot. For cargo the resulting delay should not be a problem. For passengers it might be. But assuming that most of the traffic would be tourism, a delay might not be a problem, as long as you can stay a relatively comfortable space station. Think of it as a cruise. After the initial excitement of a launch, you get to enjoy the view from LEO for a couple of days. Then you move on to a Lagrange station, then you fly around the moon a couple of times before heading back to a Lagrange point and then back to Earth, possibly via the LEO station. All of these destinations have different touristic qualities. Sounds exciting!