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The dragon can achieve mach 1.5 in level flight and can achieve mach 2 in a dive.

It likes to hunt its prey through use of kinetic energy like a Peregrine falcon.

My theories:

The dragon can repurpose its fire breath to come out of its rear, sort of like an afterburner.

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    $\begingroup$ Biology needed: straight-up magic. The Tsiolkovsky equation, which relates rocket velocities and fuel impulses, tells you pretty quickly that this is gonna be unfeasible unless it shoots out reaction mass (aka afterburner combustion products) really quickly $\endgroup$
    – Punintended
    Commented Feb 12, 2020 at 23:22
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    $\begingroup$ @Punintended well, if the dragon breathes fire, then magic is probably already assumed to be present. I like the twist in how they get their fire out. $\endgroup$
    – The Square-Cube Law
    Commented Feb 12, 2020 at 23:43
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    $\begingroup$ It needs to be (In the words of the inimitable Lady Sybil Ramkin) an absolute Whittle. $\endgroup$
    – Joe Bloggs
    Commented Feb 12, 2020 at 23:53
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    $\begingroup$ Second comment: Assuming an altitude of 45,000 ft. at Mach 2, this has the kinetic energy of roughly 174,000 joules/kilogram. Or, to put it in other words, roughly the same energy as a car. Per kilogram of dragon. I don't know how this thing is surviving, and the prey is going to be a fine mist after impact. $\endgroup$
    – Halfthawed
    Commented Feb 12, 2020 at 23:56
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    $\begingroup$ An animal (dragon) trying to cruise at Mach 1.5 will waste vast amounts of chemical energy that would be better used to keep it alive and well doing nothing and keep it that way for a very long time. No animal could hunt enough to provide itself enough chemical energy for this (pointless) activity IMO. $\endgroup$
    – StephenG - Help Ukraine
    Commented Feb 13, 2020 at 0:24

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The fastest bird is a peregrine falcon, which can dive at up to ~200mph (~350km/h). Your dragon will be going nearly 8 times as fast, which means 82=64 times the drag. You're going to need a far more aerodynamic body and a massive amount of thrust. All that drag also gets turned into heat, so it's a good thing dragons are fireproof.

However, dragons have a key advantage over birds: fire. If your dragon's trachea extends through its body instead of stopping at the lungs (and I can't see how evolution would provide that), then with the proper shape, it could form a crude ramjet. Climb up really high by flapping, dive straight down like a rock, open mouth and burn the air passing through.

This does require an enormous energy input, as in a material fraction of the dragon's weight for a few seconds of full thrust, but if the reason for its voracious appetite was the internal production of something like hydrazine, it might work. (I haven't run the numbers.)

However, with that kind of power available, there's simply no need for supersonic speeds; it could fly around lazily and instantly kill anything in sight just by breathing in its general direction, with far less effort. The challenge would be having enough accuracy and control to merely cook prey instead of reducing it (and everything else nearby) to ashes.

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    $\begingroup$ This is sort of what I was imagining, a biological jet engine. $\endgroup$
    – In the name of the story
    Commented Feb 13, 2020 at 2:55
  • $\begingroup$ @user72381 The key is a ramjet rather than a turbojet; the latter is obviously not biologically possible. The former can't be started on the ground, but if you can dive at high subsonic speeds... $\endgroup$
    – StephenS
    Commented Feb 13, 2020 at 7:12
  • $\begingroup$ Actually some sort of biological pulse jet may be possible. $\endgroup$
    – Thucydides
    Commented Feb 14, 2020 at 1:25

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