So is this thread is about hypergolic depots versus cryogenic depots rather than about Dragon or its derivatives? Or both?
adding a mission-critical rendezvous event seems overall less safe than simply designing the mission to fit within the constraints of a single launch.
A SpaceX's small depot in LEO could take the function of buffer.
I think the main advantage of hypergolic depots would be their early availability.
This is a scenario where having a depot combined with the station could arguably increase the safety of crewed missions that came to visit!
If you were thinking about using a hypergolic depot in LEO to refuel a vehicle going to L2 or points further afield there is the present situation that there are only two current vehicles which could benefit from it: Briz M and Fregat. Other users would have to be developed. This is another example of the chicken-and-egg problem that was mentioned earlier.
On the other hand, if you had a cryogenic depot in LEO you could potentially refuel the upper stages of Atlas, Delta, Ariane, HII, Long March 3 series. So in a restricted sense there is a greater potential market for cryogens.
This is a difficult trade to make, not knowing what the future demand might be. You could imagine both being developed.
Another point is that LEO is not the best place for a hypergolics depot or refuelable spacecraft using hypergolics, L1/L2 is. It is accessible with existing launchers, even for manned applications, using two launches and EOR in LEO, just like Constellation or DIRECT would have done. It is also so close to the edge of the Earth's gravity well that the relative inefficiency of hypergolics is much diminished, especially if you use advanced trajectories and SEP. In the latter case, the tables might even be turned on all-chemical even on a kg for kg basis. Not that that is the decisive criterion (cost/kg is much more relevant), but it's a good thing to know.Hypergolics are a much better fit for spacecraft than for upper stages, which is why hypergolics are being phased out for upper stages, but not for spacecraft. And if they are ever phased out for spacecraft too, they are likely to be replaced by SEP, not cryogenic chemical propellant.
Or Ariane's EPS or the Delta II upper stage, or Soyuz / Progress / ATV / HTV / Dragon / CST-100. All of these would have to be modified for refueling of course.But if you built a refuelable transfer stage that acted as its own depot (say a cross between the Orion SM and the Delta II upper stage), then you avoid the chicken-egg problem.
Something that just dawned on me as I read this: You know how we have hybrid cars? Why not extend the idea to spacecraft: SEP/hypergolic?
Delta II isn't going to be around for much longer.
Apart from EPS all the others vehicles are already part of what is already almost a hypergolic depot--the ISS itself.
So in a sense, what you are proposing has already started to happen.
Your second point can apply just as well to cryogenic vehicles.
But I think for exploration applications, in the long term, cryogenic is superior.
(Don't you get a sense of deja vu about this thread already?