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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
    Commented Nov 14, 2018 at 3:58
  • $\begingroup$ While not off-topic here, you will likely get much more answers on aviation.SE $\endgroup$
    – OpticalResonator
    Commented Nov 14, 2018 at 5:39
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    $\begingroup$ Power required, energy density & amount, cost, demand $\endgroup$
    – Solar Mike
    Commented Nov 14, 2018 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
    Commented Nov 14, 2018 at 9:19

2 Answers 2

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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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Several reasons....

  1. Max Q or "Maximum dynamic pressure" is a condition by which maximum speed a shape in flight or air flow can go before ripping itself to pieces due to friction and center of mass distribution. Drag equation states an object traveling thru air; double the speed you QUADRUPLE the aerodynamic drag and Octuple the power it takes to overcome the drag. This minimizes low altitude missiles to altitudes at Mach 4 or less. The Evolved Sea Sparrow follows a high altitude climb, and descent travel to fly that fast. enter image description here
  2. Fuel Economy: Basic principal of faster speeds is the drag equation, because the energy required to overcome drag increases geometrically, fighters with limited fuel supplies usually stay subsonic in combat. The Automobile industry battles this same problem (for 20+ years the fastest car in the world was the McLaren F1; with 647 horsepower and a top speed exceeding 220 mph, Its successor the Bugatti Veyron required 1,000 horsepower to exceed 250 mph. And It's Successor the Bugatti Chiron needs 1,500 horsepower to exceed 270 mph)

3: Engines: Jet engines require specific air speed to breathe before they flood with air and exhaust themselves. Ramjet or the Stovepipe engine rocket exceeds this by pinching the airflow at high speeds to slow down the flow temporarily only to be exhausted later.

4: thermal management: The drag equation also produces the effect of thermal energy from skin friction against the air. Anything faster than Mach 2.5 usually requires alloys capable of withstanding heat for prolonged periods of time.

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