Ok, now that I’m home watching little Jonny, I think that I have two more subtopics relevant to the Lunar Transportation discussion that I wanted to post about. The first involves propellant depots, and the second revolves around how to get people to and from the lunar surface.
To Depot or not to Depot (or Better Yet, When to Depot)
One of the most common complaints about anyone who has a lunar transportation strategy that doesn’t involve spending billions on an uber-sized rocket built by the
steely-eyed missle men at BloMart and ATK, is that you’ll then have to build a huge propellant depot before you can implement your plan. Quite often comparisons to ISS are made, with the obvious implication “well, your plan requires so much infrastructure to be done first that we’ll never be able to pull it off. Better settle for what worked in the past.” But the interesting question that goes unasked (even if it is begged) is do you really even need a propellant depot in order to do on-orbit refueling? And if so, then when does it really make the most sense to build
To get to the bottom of that question, one needs to ask what having a propellant depot does for you. I can think of a few benefits:
- The bigger a cryogenic tank is, the slower boiloff occurs, due to the square-cube law. So having a station with big storage tanks in it will reduce the amount of natural boiloff during storage.
- Active propellant cooling systems are much easier to use if you don’t have to pay the propulsion penalty of shipping them around each time you fly. That allows for even more efficient storage of propellants
- A sufficient sized propellant storage facility can be designed such that its natural gravity gradient provides sufficient milli-gees to settle propellants, thus vasly simplifying the plumbing and pumping.
- A propellant depot serves as a buffer, kind of like a capacitor or an UPS for a computer. This allows you to have frequent smaller propellant shipments that then get transfered on a less frequent basis to cis-lunar and other tugs.
- A propellant depot allows you to have multiple providers and multiple customers. The group running the depot doesn’t even need to be one of the end users or suppliers. This makes both the demand and supply side more stable, since problems with a tug or problems with one company’s delivery service are less likely to throw things off.
I could probably think of other benefits, but you get the general idea. However, while it is pretty obvious that a propellant depot does at some point become desirable, it turns out that it really isn’t near term critical.
For instance, even though it simplifies propellant settling, there are ways to do that settling that don’t require a propellant depot. I discussed several ways in a post last year, but in brief review, you could deploy a tether and a mass to create your own gravity gradient, you could dock the two vehicles then spin them just enough to create a couple milli-gees worth of settling acceleration, you could use paramagnetics on the LOX side to attract the LOX to one side, or diamagnets on the fuels, for storable propellants you could use elastomeric diaphragms in the tanks, and the list goes on.
While not having active cooling systems and the square-cube law on your side will increase your boil-off loss problems, there are options there too. The first one is to not use hydrogen. It might very well make sense to use a hydrocarbon fuel at first just to avoid the worst of the boil-off issues. There’s no rule that says “Thou shalt use Hydrogen for Lunar Transfer Vehicles”. Another option would be to just eat the losses. For instance, since you’re likely going to be launching LOX first, and the LOX tanks only weigh about 1% of the weight of propellant in them, oversize them by enough that even with boiloff losses over the course of the fueling process that you’ll have enough left over at the end. If you have too much left, you can either vent down a little, or take it with you. In a well insulated tank on all but the smallest of vehicles, boiloff losses should be in the single digit percentages per month for hydrogen, so if you have a system that can handle a lunar flight per month, handling the boiloff issue shouldn’t be a big issue. Having a depot would be nice, but isn’t critical.
The one area where not having a depot-type facility would hurt is with vehicle maintenance between flights. Lander maintenance could be done on the lunar surface fairly easy, but without some sort of “dry-dock” in LEO or at L1, maintaining and repairing a lunar tug will be more difficult.
The other benefits of a depot station aren’t show-stoppers, but they definitely show why such a thing is long-term desirable.
So, in summary I think a good case can be made that you can at least start refueling operations even without a depot in place already. It isn’t as convenient, and makes at least maintenance and repair substantially more difficult, but there aren’t any total show-stoppers. You’d still want a depot eventually, but once you already have refueling traffic going on, it’ll be a lot easier to raise money for such a depot.
How to Travel
Which leaves the last thought, which was actually spurred by one of Henry’s earliest comments in the original discussion. Once you have reusable tugs, low-cost and probably reusable earth-to-orbit transportation, and a reusable lander, does it still make sense to have people carted around in a huge reentry module like the CEV? Does that really make sense when you’re trying to develop a transportation system that’s cheap enough to be long-term sustainable at robust levels of lunar flight demand?
I don’t really think so. Many of the subsystems wanted in the CEV already are needed for the other components. If you can return the habitat portion safely to LEO after the flight, why not ride down on whatever style of craft you rode up in? Why do you even need a capsule at all? Direct return to earth may be the simplest route, but it’s unlikely to be the cheapest. If you can come down from orbit in a reusable vehicle for instance, then you don’t need to ship the habitat module up and down with every flight. If that hab module is meant for in-space use only, it will also likely be designed in a substantially different manner from a capsule that has to survive reentry, steer, land on land or at sea, be watertight, have components that won’t corrode with sea-water, have bear-proof latches on the doors…etc.
But I’m still chewing on this particular thought, and was wondering if any of you had comments?
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