Hey Gripen, I love OZ, and Opals.
I just had a real good reply dissolve into the internet, but, sustained level flight is possible above 60,000 ft. I've been in a MIG-31E that got up to 67,000 ft without breathing hard. U-2s and Global hawks can get to 60,000 and higher. the issue I so wonderfully posed in my deleted reply was - at what altitude does atmospheric friction heating and high altitude balance out? It seems to me that a very high mach aircraft would encounter less heating from friction at higher altitudes, but at some point there would not be enough "air" to support level flight. To me this means that any super high mach "aircraft" would either have to employ "skipping" (going super high to avoid heating, and then returning to the "air" to gain thrust and lift to employ a zoom climb back to an altitude where it could cool off and coast in a relatively friction free environment until it's suborbital velocity required another "skip") or carrying oxidizer, which would seem to reduce it's range considerably. To my simple brain, Aurora would have to either leave the air that causes skin heating, or carry oxidizer to allow it to "fly" at altitudes where there is not enough air to either heat the skin, or provide enough oxygen to burn fuel. What altitude, and what speed might that be?
(My first reply was so much more succinct)
Shawn A