90,000 is optimistic for an air breather. The various fighter record altitude attempts typically involved shut down of the engine below 70,000 feet to reduce gyro effect over the top - since the engine would have flamed out anyway at about 75K.
The X-15 of course was not an airbreather.
I believe the highest altitude maintained by an airbreather was by an SR-71.
To reiterate an anecdote from another thread "found over the door at Beale AFB - the saying 'Yea though I fly through the valley of death, I fear no evil, for I am at 80,000 feet and climbing'"