In my talk at Space Access in April on the “Impedance Matching” panel, I raised the issue of how to completely decouple atmospheric vehicles from pure in-space ones. That is, right now, all paths to LEO seem to go through a launch pad, even coming back from some place else (e.g., the moon or deep space). This is because it’s difficult and expensive to circularize there from places less deep in the well. It’s difficult to do aerobraking safely and reliably in a single pass, and multiple passes means that the maneuver can take a long time, which can be a problem for crewed vehicles. And of course, this doesn’t even address the issue of getting into the right orbital plane. But until we can fix this, we’ll always have the ugly and inelegant situation of having to come all the way back the the earth’s surface from any beyond-LEO destination, and have to spend resources relifting crew for each trip, and make a true transportation node in LEO (i.e., one that can be reached from any destination, either from the surface or in space) impractical.
Anyway, I’d like to see what kinds of ideas get kicked around in comments here, perhaps with the hope of doing a presentation at the SSI conference in October.
Circularizing propulsively is of course an option, but it’s hard to see how it’s a cost-effective one, until propellant in space is really cheap. Assuming that one doesn’t aerobrake at all, it takes just as much delta-V to get into LEO as it does to leave it, and it would require an improbably large vehicle if the departing vehicle has to carry enough propellant to recircularize on the way back. Which, of course, again demonstrates the value of depots. With one at L-1, perhaps supplied from either the lunar surface or an asteroid, it might make sense to fuel up there for the circularization in LEO. It all comes back of course, to the point that I made in my essay last year — that reusability implies gas stations, and that it’s impractical without them. As Jon Goff demonstrated with his amusing “We don’t need no stinkin’ depots” slide at Space Access, which was a picture of his car with extra gas tanks for driving cross country, the more often you can fuel on a trip, the smaller your vehicle can be and, due to the exponential nature of the rocket equation, that goes in spades for space vehicles.