The ISS as fuel depot testbed
I've been wondering for some time if the ISS could be used to advance the technology required to perfect propellant depot technology. It seems that nearly all of the required hardware is already on board the station. The current ECLSS on the ISS electrolyze waste water and condensate to generate breathable oxygen, with the excess hydrogen being vented to space. If the hydrogen stream could be diverted into a Sabatier reactor, then CO2 could be scrubbed from the atmosphere and turned into methane fuel.
I've also been wondering for if a propellant depot infrastructure should be resupplied with pure water rather than the cryogenic fluids. The favorable density and handling properties of water as opposed to LO2/LH2 would appear to make it the better choice for transporting aboard a very simple tanker over potentially long time periods. When shipped as water, the "fuel" can be transported in a non-cryogenic, inert state, and then once delivered to the depot, it can be cracked into O2/H2, liquefied, and stored until it is needed.
Of course, this assumes that your depot has sufficient power and/or time to split water and keep the fuel properly chilled. I found this page which describes the Russian Elektron unit. This article cites a passage from a NASA training manual:
NASA TRAINING MANUAL ON ‘ELEKTRON’
From the “NASA Familiarization Manual on Russian Segment Crew Systems”, Published in 1997.
3.1. Oxygen Supply System
The decomposition of 1 kg (2.2 lbs) of water yields 25 L (0.88 ft3) of oxygen per hour at a pressure of 760 mmHg, which is enough to support one crew member for one day. To provide the daily amount of oxygen for 3 - 4 crew members, 3 - 4 kg (6.6 - 8.8 lbs) of water must be decomposed. Power consumption of the process is ~ 1 kW.
A kilogram per hour at one kW, seems like a very reasonable rate, at that's just for one Elektron unit (if I'm reading this correctly). If necessary, this can scaled up with additional units if sufficient power is available. It may be possible to build even more power efficient electrolysis system if the work of Dr. Nocera at MIT can be turned into a practical device that will operate in zero-G conditions.
If the depot is also crewed, then a steady supply of water would be required any way. The crewed depot could also generate methane as an additional propellant option if the ECLSS included a Sabatier reactor as well as an electrolysis unit. With the exception of the Sabatier reactor and some cryogenic storage tanks, the ISS already possesses all of the hardware. The existing U.S. electrolysis units even have hardware available for connecting to a Sabatier reactor.
So, what would be the point of having the ISS generate and store propellant? Well, first of all, it is currently our only manned research outpost in zero-G. It would therefore be expedient to take advantage of these facilities to work out the basics of cryogenic fluid transfer in micro-gravity. The data obtained from ISS experiments could go along way towards reducing the techological risks associated with propellant depot development.
The ISS also needs fuel for station-keeping. Having the ability to generate it's own propellant would be a nice capability to have. This would also simplify the ISS resupply requirements. Rather than having to deliver water and propellant, each in the individual compartments, only water would be required. The water tanks could be made larger, and therefore more mass efficient. Since propellants would no longer need to be transported (I think hypergolics are currently used), the handling of the payload for resupply missions would be much less hazardous.
Finally, with the ability to produce fuel, the ISS could support a small array of space tugs and transfer vehicles which would service the station and other objects in nearby orbits. For example, imagine that SpaceX puts up a couple of Dragon lab modules in orbits which are coplanar with the ISS. Now imagine that the ISS has a slightly modified ATV docked to it. It might then be possible for an ISS astronaut to use the ATV to rendezvous with the Dragon module and perform any necessary maintenance or repairs.
I have some additional thoughts on how to convert the ATV into a crewed orbital transfer vehicle, but I think I'll save those for another post.