Are there any aircraft designed to operate between an altitude of 50-80km?

Question

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?

Answer 1

Is there any aircraft designed to perform its mission into this 30km wide slice of atmosphere?

(between 50km and 80km) - apart from some experimental rocket planes and so on, I can't think of any. The main problem is there's so little air - very little opportunity to generate lift, and very little air to run a jet engine.

In theory it should be possible for a winged aircraft to fly up there (it is below the Kármán line) but the engineering difficulties would seem to make this impractical. For more detail I'd recommend the top answer for this question: What determines the maximum altitude a plane can reach?

What is the usefulness of this slice for aviation?

If it were possible, it would be useful for futuristic or military things like:

• spy planes like the SR-71 but higher and faster
• super-fast suborbital transport
• passing through on getting into orbit - single stage to orbit vehicles for example.

(All of these examples are pipe dreams at the moment of course.)

Answer 2

Thank you for your question, I never "realised" that even most "exotic" airplane have a maximum ceiling height of not more than 30 km (stratosphere). Why should that be?

Starting from very first principle, a force shall balance gravity, in "conventional" aircraft, lift equal weight:

$$ W = \frac{1}{2} \rho S V^2 Cl $$

Let's have a look at what happens to density ($\rho$) for height above 30 km, it goes almost to zero! So above this "threshold" it is more convenient to use other principles to contrast gravity, buoyancy, like baloons, or a force directed vertically, similarly to missiles and thurst!