Minuteman ICBM reaches Mach 1 at 60-ft above launcher?

Rusty B wrote:

I wonder if they really mean to say, the EXHAUST from the ICBM is SUPERSONIC
at 60-ft and blows back into the silo at those speeds, causing damage?

The exhaust is going to be supersonic within a second after ignition, It
blows a giant smoke ring out of the silo as the missile begins to
emerge: http://www.llnl.gov/str/JulAug01/gifs/Tyler3.jpg

Pat