speaking of 3d modelers, wings3d I hear is good for this sort of thing. Plus with the right addon you can export meshes for orbiter. The strange thing about it is that you can export .dae but not import .dae.
Not 100% sure about .dae specifically, but some formats are a whole lot easier to write to than read from.
Short version: You can have concepts in the other format that you don't have in Wings, so you'd have to write a complicated "translator" in order to convert those concepts into the Wings version, and it may not even be possible to do that correctly all the time. Exporting is easy, though, since everything you can do in Wings can be represented in the other format (or safely ignored).
Long, complicated, technical version:
Wings which uses a specific modelling paradigm known as "winged edge"--all edges must have exactly two adjacent faces, no more or less. This is (very) mildly restrictive, but it results in models which are more often well-formed.
This can present a problem when importing from formats that are a lot looser. This is exactly why I can't write an Orbiter importer (or at least, not a very good one)--Orbiter's model format allows you to have individual polygons floating wherever you want them, and Wings simply cannot represent those without adding additional polygons to "bridge the gap." Figuring out what exactly needs to be added (or combined) would be relatively easy for a person to do, since you could just look at the model and say "okay, this goes there," but it's a lot harder to write a plugin for that.
I'm not at all familiar with .dae, but I imagine it might be a similar case.