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Fastest plane ever built: SR-71 ,Max Speed: 3.3 Mach , Development - 1950s

Fastest drone ever built: Lockheed-D21 ,Max Speed: 3.3 Mach, Development -1960s

Fastest cruise missile ever built: Brahmos , Max Speed: 3.3 Mach, Development - 2000s

Why Ram/turbo-Ram jets got stuck around 3.3 Mach? Is there any technological challenge beyond 3.3 Mach like heating or combustion instability or low thrust?

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  • $\begingroup$ There are some scramjet prototypes can reach upto Mach 8 which are still underdevelopment. My question is about why nations have not developed Cruise Missiles or fighter jets faster than 3.3 Mach like Mach 4 or Mach 5 which can be achieved by RAM jets theoretically. $\endgroup$ – SRD Nov 14 '18 at 3:58
  • $\begingroup$ While not off-topic here, you will likely get much more answers on aviation.SE $\endgroup$ – OpticalResonator Nov 14 '18 at 5:39
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    $\begingroup$ Power required, energy density & amount, cost, demand $\endgroup$ – Solar Mike Nov 14 '18 at 6:21
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    $\begingroup$ For fighters, agility is more useful than raw speed. For example vectored-thrust aircraft can avoid missiles by decelerating and dodging sideways faster than the missile can react, so it overtakes them instead of hitting them. For cruise missiles used against ground targets, saving a few minutes of flight time is not very important. The SR71 was not a very effective military asset - more than 30% of the total fleet were destroyed in accidents, even though none were destroyed by enemy action. $\endgroup$ – alephzero Nov 14 '18 at 9:19
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At those speeds, several effects become important. We consider air-breathing engines in what follows.

First of all, aerodynamic heating of the airframe becomes important. In broad terms, to prevent structural failure, the maximum temperature of the hottest portions of the airframe must be held below red heat. Going faster than mach 3.3 pushes this limit.

Second, for an air-breathing engine that propels the airframe from sea level and zero airspeed to mach 3.3 at high altitude, the temperature at the face of the compressor stage must be below the maximum temperature limit of the first stage compressor blades, which is roughly 800F. Going faster than this means the temperature at the face of the engine will exceed the temperature rating of the blades and the engine will self-destruct.

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