Aircrafts operating at high altitude falls into several categories:
- Wings with high aspect ratio (Global Hawk, TR-1, Scaled Composite Proteus,...)
- High velocity aircraft (Concorde, A-12, X-51,...)
Aircraft falling into those two categories cruise between 15km and 30km high.
- Balloons (up to 50km high)
- Suborbital aircraft (X-15, Space Ship Two), performing their mission between 80 and 120km high.
- higher resides low earth orbit, and thus satellites which can hardly be qualified as aircraft.
I cannot find any aircraft designed to perform its mission at an altitude between 50km and 80km, which corresponds more or less to the mesosphere. Is there any aircraft designed to perform its mission into this 30km wide slice of atmosphere? What is the usefulness of this slice for aviation?