Accumulate Fuel at Space Station?

I would like to know why we don't travel to Mars by first lifting many
loads of fuel to the space station. The actual spaceship would be
assembled in space, at the space station, from parts that are lifted
there the same as the fuel is lifted, by conventional rockets. Perhaps
20 to 50 trips would be necessary to finally have a suitable ship with
lots of fuel. However, the payload to fuel ratio of this ship would be
far greater than a ship launched from earth, since it has already
escaped most of earths gravity. Also, the thrust required might be only
1/20 of what it would be if launched from earth, so a small propulsive
system will do the job. I'm assuming a human crew; they would be taken
to the space station at appropriate times by conventional rockets or the
space shuttle.

This is not my idea; It has probably already been studied, but since I
never hear about it I assume there must be some catch to this idea.

If anyone knows something about the status of this idea I would very
much like to read about it.

Mitchell Timin

--
"Many are stubborn in pursuit of the path they have chosen, few in
pursuit of the goal." - Friedrich Nietzsche

http://annevolve.sourceforge.net is what I'm into nowadays.
Humans may write to me at this address: zenguy at shaw dot ca