Could the SR-71 Blackbird be used for nearspace tourism?
Significant issues with the concept: the SR-71 lacks RCS, so you'd lose attitude control if you got significantly above normal operating height.  I originally wrote that it doesn't handle in-flight restarts well, but apparently it has the opposite problem -- it needs them all too frequently in normal operation.  It's also not an aircraft built for near-vertical climbs or high-G maneuvers, so a zoom climb is outside its normal operational envelope.
So, you can't demonstrate long (&gt; 1 minute) periods of microgravity without the likelihood of loss of aircraft, crew, and paying passenger.  Beyond that, however, there's virtually zero visibility from the rear seat.  Who'd pay $100,000 for a &quot;near space experience&quot; with no zero G and no view of the black sky and curved horizon?
Never mind the fact that the SR-71 doesn't run on standard jet fuel (JP-1 or Jet-A); the engines could be retuned to do so, but then wouldn't continue to operate at 80,000 feet ASL or push the aircraft to around Mach 3 at that height -- in other words, it wouldn't do what an SR-71 is famous for doing.  And the special fuel the SR-71 requires hasn't been manufactured since the aircraft were retired -- it would probably cost more to restart production of that fuel than you'd make back in a reasonable twenty-year &quot;space tourism&quot; operating life (never mind cost of refurbishing an aircraft, pulling still-classified equipment out of at least the passenger seat, training new pilots -- oh, right, you'd have to refurb one of the trainers, that would at least solve the passenger visibility issue)...
Bottom line: no chance.  You'll get a better experience with the Russians; at least they can do a zoom climb and have RCS to point the nose back down so they don't wind up in a flat spin like the one Yeager had back in the day.