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Note: This is a question asking for things that might theoretically be possible in the "near future"

Project Thor is pretty much dropping 8 ton tungsten spears on things. One of the problems is that you lose a lot of kinetic energy through drag - makes the price point rather high. If you're going to spend the money putting 8 ton bullets in orbit you would want a higher bang for the buck (quite literally)

As I understand it, Supercavitating bodies eliminate drag by generating a bubble of gas surrounding as much of the body as possible. Generally the very tip of the body is the only portion generating drag.

Assuming the following:

  1. You have access to something like HfN0.38C0.51 that you could build the body's tip from and
  2. Eliminating drag implies eliminating the body melting and hence you can use whatever substance you like for payload

The question: How might you go about achieving supercavitation in air?

I believe you could do this by if it were possible to generate a vacuum or stable low pressure cavity around the body , but how might you do that using technology that is "close to possible"? Obviously you can't use a physical container.

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Supercavitation works well at sub-sonic speeds. At the hypersonic speeds you describe, you actually run across supercavitation's big brother: shock waves. These separate the supersonic flow from a sub-sonic flow near they body of the craft. You actually don't have to design to get them: you will get them simply by traveling at those speeds.

However, at those speeds, skin drag is not as big of a player as it is in a sub-sonic underwater device. Other powerful effects like wave drag play a much bigger factor.

I am rather confident that the value of supercavitation becomes unimportant as you approach the speed of sound in the medium. I don't have the equations, but it makes sense to me because that's where wave drag becomes a big issue, and every article I researched regarding "supersonic submarines" admitted that they could not use supercavitation to exceed the speed of sound in water. (presumably "supersonic" was a reference to the speed of sound in air, which makes them sound cooler).

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  • $\begingroup$ Thanks for the info. I guess I don't really understand what wave drag is. Looking at some equations it seems that the only way to increase terminal velocity [once you have a streamlined body] is to reduce the drag coefficient. I was wondering if you could somehow wrap your body in vacuum whether that might do the trick. Will have to go with bespoke subatomic surfaces or just good old unobtainium. $\endgroup$
    – Tibrogargan
    Commented Jan 12, 2018 at 22:38
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In the 80's, they looked into using an air spike for just this purpose. It was basically a plasma cutter mounted on a pole. You may have noticed that Elon Musk does this now with the Falcon 9 on its re-entry burn. It dissipates the hyper-sonic bow wave and encapsulates the rocket in relatively cold exhaust gas.

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