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.

[Afternoon update]

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.


59 thoughts on “Decoupling”

  1. Dennis, any ideas for a reusable single pass aerobrake?

    Hmmm, the reusable ones tend to be too heavy. An interesting idea would be to use ISRU derived vacuum deposited iridium over nickel. Iridium is one of the highest temperature melting metals, though it is heavy as hell 22x the density of iron.

    This of course implies a robust ISRU on the Moon but you could have no shield on the way out and then pick up a new one at L1, delivered from the Moon. It would save literally tons of fuel to not have to lug anything all the way out there.

    I am really starting to be sold in the L1 gateway idea due to the very low cost for plane changes for Earth orbit.

    A SEP tug after a few passes, is going to have a severely degraded solar array. It could be retired as a gateway fuel depot at L1 and be the basis for a large station at that orbit. It is only 36 m/s/yr for station keeping and Xenon ion would be great for that.

  2. Iridium is one of the highest temperature melting metals, though it is heavy as hell 22x the density of iron.

    Whoa! It’s heavy, but not that heavy. I think Dennis meant to say 22x the density of water. Iron’s density is 7.874 g/cm^3 (i.e., about eight times that of water), and iridium’s is 22.56 g/cm^3.

    And it’s far more abundant in meteorites than it is in terrestrial rocks.

    Beautiful idea.

  3. Hmmm, the reusable ones tend to be too heavy.

    How about evaporative cooling with water? Should be easy to replensih in orbit.

  4. Are we solving the wrong problem? I think making a small and light capsule return from near-moon (eml2?) really fast (a couple of days?) directly to the earth’s surface could be a solution – you leave the habs or living modules there and don’t bring them back.

    Or then it really doesn’t work out.

    And it’s also a hardware problem. You have to send a new living volume every time.

    Or then those could return slowly with high isp propulsion or by aerobraking in a very large amount of passes.

    Decoupling tended and untended flight – fast directly to the surface and slow staying in space.

    On the other hand, this doesn’t work for longer flights very well since you still need sizable living modules for the transits.

  5. Of course, when returning from Mars or a NEO, you could leave the transit living module and service module to slowly multi-pass aerobrake untended in LEO and separate with a small capsule a couple of days before arrival.

  6. Why don’t you look up the papers entitled “The Use of Atmospheric and Extraterrestrial Resources In Space Propulsion Systems, Parts I – IV” ca 1963 (ARS & Natl. Arch. #1250057, …58 etc also McGraw-Hill Yearbook Sci. & Tech.)? Solar Power now almost as good as Nuclear; “refueling” can also be done by swooping down from orbit further reducing launch mass.

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