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originally posted by: Barnalby
a reply to: penroc3
The Space Shuttle hit mach 21 in the upper atmosphere on reentry, by that logic we've shattered the hypersonic barrier since the early 60s with the X-15.
Like mightmight said, the hard part is doing it with an air-breathing engine.
That said, few to none of the rumors or stories about an 80s or 90s era TSTO ever involved an air-breathing second stage, and in all likelihood, if it ever existed, it would have likely resembled something like an M-21 on steroids that launched an X-15 on steroids from 80,000'+ at Mach 3+ where all that extra altitude and speed might just be enough to push your glorified Pegasus's mass fraction into somewhere that would make a manned single-stage craft able to actually hit Orbit