Polar orbit Shuttles and Soviet space planes
Were canned for several reasons, none to do with payload. The delays associated with Shuttle launches were unacceptable to the military who wanted a more reliable launcher, so lobbied for, and got, additional Titan IVs. What finally killed the project was Challenger; the Vandenburg launches would have used an even lighter SRB design which would have been even more prone to leakage and disaster. NASA canned the development of the booster.
Whether or not Uragan ever existed (there is plenty of evidence it was no more than a disinformation programme), the Soviet Union certainly flew and recovered a scale-model spaceplane called BOR-4 four times in the 1980s. The BOR project had originally been part of their Spiral spaceplane, when that was cancelled, the half-scale model was used to test materials and re-entry profiles for Buran. So it's nice to see the Americans playing catch-up ;)