I think you're looking for the 666 rule which I can't find a link to, but its an engineering rough estimate that mach 6 at 60 kft is 6% of the way to orbit (from an energy perspective). It comes up a lot in back of the envelope type debates about overall system / stage design and performance. Note that 6% of the way to orbit sounds pitiful for a first stage but it actually has a pretty big effect on available payload and nozzle design of the second stage and reusability of the first stage and all that. SSTO is tough, but sticking something close to a SSTO on top of a 666 booster helps a surprising amount.
On the other hand in aerospace its unusual to bolt the first prototype together then just punch it and see what happens, like a drag racer driver. Smallest reasonable steps and all that.
SST is not designed for orbital flights, nor will it used for that, so I don't know how it's that relevant to talk about how much close it is to orbital speeds.
On the other hand in aerospace its unusual to bolt the first prototype together then just punch it and see what happens, like a drag racer driver. Smallest reasonable steps and all that.