Supercavitation in air Note: This is a question ask 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:


*

*You have access to something like HfN0.38C0.51 that you could build the body's tip from and

*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.
 A: 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).
A: 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.
