Space

In my original comment I mentioned the two main reasons why nothing ever came of it - the nuclear test ban treaty, and the destructive EMP and charged particle environment that would result from setting off those nuclear detonations in Earth orbit. Neither of these are technical limitations on the vehicle itself, though. If the treaty didn't exist and we didn't care about frying satellites we could build and launch one with the tech we already have today. But since those two limitations do exist nobody's exploring it further right now.