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why do hyper sonic aircraft have flat noses?


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The straight edge allows for much more controllable (and predictable) shockwave propagation. Since both vehicles you posted pics of use box-type scramjets, the entire forebody of the vehicle is used to generate the needed shockwaves to compress the incoming air, providing a better compression ratio overall than the engine alone could ever hope to generate.

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The straight edge allows for much more controllable (and predictable) shockwave propagation. Since both vehicles you posted pics of use box-type scramjets, the entire forebody of the vehicle is used to generate the needed shockwaves to compress the incoming air, providing a better compression ratio overall than the engine alone could ever hope to generate.
That is the purpose of of a shock cone. The spatular nose on a hypersonic vehicle is largely to increase the L/D ratio.

First, a bit of context. Waverider concepts first surfaced in the early era of hypersonics research (the 1950's). The basic idea is to design airfoils specifically to position attached shocks that trap spanwise flow, generating additional lift (called compression lift). The initial waverider concepts were delta winged with blended conical bodies and downturned wingtips, and blunt drooping leading edges. You can see a bit of this heritage in the XB-70 Valkyrie project.

naa-scramjet-small.jpg

attachment.php?attachmentid=211385&stc=1&d=1358323402

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Then in 1966, British RAE (Royal Aircraft Establishment) researchers Pike & Kuchemann came up with a novel waverider design called the Kuchemann Tau. The Kuchemann Tau's spatular nose further increases the L/D ratio for the aircraft.

kuchemanntau.jpg

Edited by architeuthis
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It's to lift the shockwave off the surface of the aircraft. Slower supersonic aircraft like fighters have pointy noses so they form a clean shock cone for less drag. As you go faster, the air temperature in the shock wave increases to the point where it will melt the craft creating it. A blunt nose places a cooler subsonic bow wave between the craft and the shockwave, helping protect the craft from the high temp shock wave.

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It's to lift the shockwave off the surface of the aircraft. Slower supersonic aircraft like fighters have pointy noses so they form a clean shock cone for less drag. As you go faster, the air temperature in the shock wave increases to the point where it will melt the craft creating it. A blunt nose places a cooler subsonic bow wave between the craft and the shockwave, helping protect the craft from the high temp shock wave.

I was going to jump in after reading previous comments, but this right here is exactly right. I'm tempted to fill in a lot of details, but it gets technical fast. For detail breakdown of theory of shock waves, best thing I can recommend is Fluid Mechanics by Landau and Lifshitz, chapter IX Shock Waves specifically. Heavy reading for people with suitable background. I'm sure wizzlebippi can recommend something that's more practical from an engineering perspective, and more directly applicable to aircraft design.

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