A (probably, spherical, likely done inflatable) storage tank of liquid hydrogen (covered by a mirror material to reduce thermal influx; to reduce the thermal influx further, place a mirror on its side, the mirror could be attached to either the tank or another component of the system.
A similar storage for liquid oxygen.
A water storage tank (water is brought from the Earth, because it is far cheaper to transfer than hydrogen; in the future water could also be brought from the Moon or even from Mars). It also can be inflatable and covered by a mirror material.
A hydrogen production engine (the best way is possibly Photoelectrochemical Water Splitting with another attached mirror to focus Sun light, but electrolysis using big solar panels should also be considered).
A hydrogen and oxygen cooling engines (working from solar panels).
"Ports" for loading water and unloading liquid hydrogen and liquid oxygen.
A propulsion engine to correct the orbit.

The tanks should be connected to the main module with two tubes, to be able to move gas or liquid through the tubes by pumps to the cooling engine inside the main module.

The curved mirror for Photoelectrochemical Water Splitting may be dangerous for this space station if by mistake focuses its light on the unintended parts of the station.

The station may be not manned (but people coming from time to time to repair if something breaks) or manned. If manned, it could also be used as a general purpose space station, such as to conduct scientific experiments.

Orange lines are light.

We decrease the volume of the liquids (and therefore fuel used by the starship) we need to transfer into the space as calculated by the following formulas: (V(H2) + V(O2)/2) / V(H2O) = (28.8 + 28.04/2) / 18 = 2.38 times. Here V(X) is the volume of a mole of the substance X.

Solar panels are optional, because we can instead use some kind (chemical electric or a combustion engine) of hydrogen engine to produce electricity in the main module.

The water should be always available and constantly cooled/heated as necessary, because otherwise we may have water tubes and the water tank frozen and our system be severely broken.

The calculations show that this scheme is sustainable. Assuming device parameter PWS at 2 watts (on the Earth 200 milliwatts of power are sustained with much less ultraviolet than in space) per square centimeter of electrode area and square 10m×10m sized electrode (in fact, we can make it smaller but focus more light on it, especially the ultraviolet one), we get 141×10^6×1000 / (2×100^2) ~ 7×10^6 sec ~ 1/4 year to produce a ton of hydrogen - here (is Si system) 141×10^6 is the per-mass energy of hydrogen as a fuel, 1000 is the mass of hydrogene in kgs, 100^2 is 10m size squared of the PWS panel, 2 is the power of the PWS. For long-term space missions it is OK. Moreover, we can have multiple fueling stations in the space (probably on different heights of orbits).

I didn't take into account the energy needed to freeze the gases and preserve them fluid. But I shown principal sustainability of the system. Well, "Theoretically, 3.23 kWh/kg of work is required to liquefy one kilogram of hydrogen gas, but in practice, 15.2 kW/kg of work is required, which is almost half of hydrogen's lower heating value." (https://www.sciencedirect.com/science/article/pii/B9780323955539000078). This is very much less than the energy of hydrogen firing, so can be not taken into account in this approximate calculation.

Need to note that (Wikipedia) "At room temperature, gaseous hydrogen is mostly in the ortho isomeric form due to thermal energy, but an ortho-enriched mixture is only metastable when liquified at low temperature. It slowly undergoes an exothermic reaction to become the para isomer, with enough energy released as heat to cause some of the liquid to boil. To prevent loss of the liquid during long-term storage, it is therefore intentionally converted to the para isomer as part of the production process, typically using a catalyst such as iron(III) oxide, activated carbon, platinized asbestos, rare earth metals, uranium compounds, chromium(III) oxide, or some nickel compounds." Definitely, this can be done in space.

The satellite is needed to be put to a high orbit like 600km, because at that height the decay happens just to 589.99km orbit in about 884 days provided the surface 10m^2, what eases compensating orbit decay and also provides for astronauts much time to repair it, if the propulsion engine breaks.

To decrease the surface of the satellite, the reservoir can be done ellipsoids/ovals instead of spherical. It would require meticulous calculations to decide the exact most effective shape of reservoirs.

A similar device (with removed propulsion module) can be installed on the Lunar surface. In this case, reservoirs (and some of the machinery, such as the cooling engine) can be put underground to protect against meteorites. This may require thermos reservoirs, because "Once you get down to 2 meters under the surface of the Moon, the temperature remains fairly constant, probably around -30 to -40 degrees C." is too hot for liquid gases and too cold for water.

As follows from the above (assuming full load of the system), in a year we produce 4000kg of hydrogen, that is process 36000kg of water what amounts around 36m^3 of water that is 1 launch of Starship, because its load is around 1000m^3 for our orbit. Alternatively we launch 4000kg (56.5m^3) of hydrogen and 7.936*4000kg = 31744kg (27.7m^3), total 84.2m^3 - also a single Starship launch. Thus my system is probably not viable on Earth orbit, because the limited amount of transferring liquids is about Starship mass capacity, not volume capacity.

It is nevertheless economically viable on the Moon surface, using water from Moon underground, because it replaces several $10M flights of Starship to the Moon by just one (transferring the device).

#space #spacetech #satellite #spacestation