Propellant Depots

Rand posted a link yesterday to an interesting article about propellant depots on the New Scientist website. It was an interesting read, and good to see good ideas getting more of the attention they deserve. I have a few thoughts to add, however.

First the article only mentions the benefits of propellants derived in-space. But I think that even without lunar derived propellants, a propellant depot would be useful for the following reasons:

• Propellant depots allow you to develop reusable in-space transportation vehicles. While you could possibly refuel and reuse a reusable space tug without the benefit of a fuel depot, a depot would be a lot nicer.


• Depots allow you to decrease the size of your launch vehicles. Since propellant is often 75-90% of the mass of transfer vehicle in LEO, the ability to launch dry and fuel-up in orbit means that you can get away with much smaller vehicles flying at a higher flight rate, which means that you're likely to get a better price and higher reliability.


• Depots serve as a buffer between on-orbit propellant demand and delivered propellant supply. This way if there's a problem with one of the propellant-delivery launch vehicles, it may not actually delay your next departure from LEO.


• Depots allow for many players, and lots of flexibility. So long as the depot operator predefines some standard docking and propellant transfer interfaces, this allows anyone who can meet those standards to deliver or purchase propellants. It doesn't matter if the propellant is being used to boost an on-orbit assembled satellite into its final orbit, or if it's being used by a tug sending people/cargo towards the moon or mars, or if it's loaded onto an upper stage for a high performance deep space probe. It doesn't matter much if the propellant is delivered in 25 ton chunks by Boeing, 5-10 ton chunks by SpaceX or Kistler, or 1/2 ton chunks by some high flight-rate reusable booster.


• Depots also allow for a much more robust transportation network. If you have a propellant depot with multiple independent propellant delivery companies supplying you, launch failures and delays don't affect you as much. So what if Boeing or Lockheed have to stand down their booster for a half a year to a full year to figure out what went wrong, you still have 3-4 other companies that can (and want to) pick up the slack.


• Depots also allow for the best form of international cooperation--capitalism. I'm sure companies and agencies in Russia China, the EU and Japan would all be glad to launch propellants for hire if there was sufficient demand. OTOH, if there is sufficient supply of reasonably priced orbital propellants (and if the propellant transfer and docking specs were openly available to all interested parties), it might even encourage other countries and international corporations to start designing space missions that take advantage of those prestocked propellants.


• Once you have a depot like that established, it can serve as a nucleation point for other services and industries. Once people start switching to reusable orbital vehicles, they're going to want repair/maintenance/overhaul facilities. Those facilities will have a demand for spare parts. While many of those can be supplied from the ground, due to the higher lead times, and high costs of emergency shipments, it may make sense at some point to stock some raw materials and simple manufacturing equipment at the depot in order to make some of the simpler parts on orbit. Depot workers, maintenance/repair personel, and others will likely want other goods and services, which may eventually spur the start of some of the first small orbital settlements.

The other thing I wanted to mention regards Joe's comments about the difficulties of handling propellants in microgravity. Joe is right about those difficulties, but as I've explained elsewhere on this page, there may be ways around the problem. The basic problem Joe points out is that in pure microgravity, propellants don't settle like they do here on earth. This makes pumping them or measuring the current supply tricky because it's hard to avoid sucking ullage gas for pumping, or knowing where in the tank the propellants are. The thing that the article leaves out is that there are many potential ways around this that have been previously discussed. Here's a short list:

• Gravity Gradients--If you have an object with a high enough L/D ratio in orbit, it will naturally settle into a orientation with the long end pointing along an axis that passes through the center of the earth (ie along the gravity gradient). The velocity of the object determines the orbital altitude of the object's center of gravity. From basic orbital dynamics we learn that the closer an object is to the center of the object it is orbiting, the faster it has to go to maintain its orbital position. What this implies is that if you have a long skinny object, in a gravity gradient orientation, that the spaceward end of the object is going faster than other objects orbiting at it's altitude, while the planetside end of the object will be going slower than other objects in its orbit. In both of these cases, the result is that as you travel outward in this object from the center of gravity, the outward acceleration gets higher. Ie things will tend to settle at the two extremes. Now, this method is only likely to get you thousandths or hundredths of a G, but that's enough to cause propellants to settle gradually, and is enough that when combined with surface tension devices like screens to keep the propellant in a specific location within the tank.


• Tethers--One way to drag the center of gravity far away from the depot is by having a long tether with a counterweight on the end. In this manner it might be possible to get even higher settling forces, at the cost of added complexity.


• Artificial Gravity--Most of us have heard of people suggesting slowly spinning a space station to provide some level of artificial gravity toward the outer edges of the station. The same can be done with a depot. The problem this creates is that it is really hard to dock with such a station at any point other than along the centerline, so that might limit the number of vehicles that can visit the depot at any given time (unless you despin the inner section, but that introduces other issues). But it's still a decent option.


• Diaphragms and Free-Pistons--For storable propellants, simple elastomeric diaphragms might be a workable solution. If the tanks are cylindrical, a free-piston might work for cryogenic propellants. Sure a free-piston isn't the most mass or volume efficient way of doing things, but it's a fairly straightforward option. You might even be able to put a refrigeration unit inside of the free-piston to keep the liquid subcooled.


• Electromagnetic Methods--LOX is paramagnetic, which means that you can use magnetic fields to attract the oxygen to one side of the tank. Since LOX is likely to be the propellant used in the largest quantities (by mass), this might well work fine. Other propellants are diamagnetic, so there are some things you could do with them, but other methods might be easier.

Well, Paul "more-clever-technical-ideas-than-you-can-shake-a-stick-at" Dietz could probably rattle off several more that I haven't listed, but I think you can see that it may just be easier to use some technique to settle the propellants than to try and make a system that can pump, measure, or cool the propellants without settling them. Once the propellants are settled, the whole propellant storage/management process becomes liquid handling that is not too different from what is done every day by the hundreds of tons here on earth.

I wanted to mention at the end the Centennial Challenge mentioned in the article. It's good to see NASA putting up some money for such things, though I'm not sure what is really the best way to administrate such a prize. It'd also be nice to see if they could put some sort of a prize up for demonstrating microgravity propellant transfer of some sort. If they allowed the two containers to be pre-attached, you could likely run that competition inside of a Zero-G flight, or as a payload on a suborbital vehicle.

Anyhow, just some thoughts.