Re: Gotta wonder
It doesn't matter where you add the mass: the moments of inertia will change, but having it on the side will make repointing a lot easier than if it's on the end jammed into a thruster nozzle as was done with the Intelsat (extra mass has less effect on the moment the closer it is to the centre of mass)
There have been a huge number of proposals to actually refuel satellites on-orbit and they all run into major issues, not just with the fuel connectors but the very real possibility of having something leak during the process - a satellite or probe outside of LEO limits(*) sits in a cloud of its own outgassing "stuff" anyway (which is one of the reasons why cleanrooms are used and everything is baked to hell as part of flight preparation). Having that stuff being hydrazines or other corrosive and sticky shit would be a VERY BAD THING. The current filling procedures on the ground have the filling ports not only capped after filling, but then very thoroughly encased to ensure that they can't be disturbed in flight for this reason.
The MEV system was conceived because it was realised that refilling a comms sat or science probe on-orbit would not only be impractical but the resulting contamination cloud had a very high likllihood of ruining the spacecraft anyway - even if there were grappling and fill ports available.
Refilling a MEV away from the bird to be rescued is a different matter. The MEV is designed to be flown around and can "leave its contamination cloud behind" for the most part. Additionally, anything that settles on the MEV is not going to interfere with the mission of the item to be rescued so it can be manouvered to a parking spot for a while to ensure that it's not leaking, etc before setting off on its next mission - but being a "flying fuel tank" it can be given enough fuel for a 40 year mission in the first place, so why bother?
(*) Satellites low enough to be affected by the atmosphere (anything below about 600 miles) will also have that atmosphere(**) strip away any cloud of contamination that may be flying with them, so that's less of an issue. The tradeoff is needing regular reboosting, or coming out of orbit relatively quickly.
(**) At these altitudes it's still there but the vacuum is still better than anything we can make on the ground. The friction of colliding with a few scattered molecules when travelling at orbital velocities is enough....