>>17853
>[citation needed]
I learned it from some talk by professor Ted Postol about the Oreshnik, I think it was on the Daniel Davis Deep Dive or Glenn Diesen podcast, I can't remember which. He was explaining that as two solids collide at speeds higher than the speed of sound in said solids, they can't transfer the compression or shock of the impact through them faster than the speed of sound in said medium, since the "speed of sound" in any medium is the speed at which that sort of thing transmits through that medium. Because the two solids have no place to go and can't transmit that energy elsewhere, they rapidly sublimate into gasses. It made sense to me, although I will admit I'm not a physicist.
>nothing to do with the gas surrounding it
never said or implied it did.
>"Hypersonic" is kind of a bullshit term, it sounds cool but has no real definition or threshold, anything over mach 1 is technically hypersonic.
I've heard it defined as the point where a plasma sheath forms around the projectile. Typically mach 6+.
>"speed of sound" in steel is like 15 times higher than through air.
a quick search indicates that there are different speeds of sound in solids for shearing, compression, and extensional stresses. I don't know enough to know which one prof Postol was talking about. He was also talking about the Oreshnik, which goes over mach 10, as do Iranian Fattah missiles supposedly, so we're definitely talking about the kinds of speeds approaching 15 times the speed of sound in air.
https://www.engineeringtoolbox.com/sound-speed-solids-d_713.html
>[citation needed]
I'll admit that was a guestimate
>Much more likely to behave like armor piercing sabot rounds, 5000fps is about mach 5
we're specifically talking about things going faster than that because there is a clear change in what is happening at those speeds.
>look at wildcat rounds, the 4500fps muzzle velocity is theoretically is just enough kinetic energy to vaporize a lead projectile
an interesting point. I wonder if the difference has to do with the relative speed of sound in the medium they are impacting, since speed of sound is a scalar quantity while velocity has a vector. I will repeat that I am not a physicist, although I would also observe that you don't seem to be one either.