Photo shows test cabin and nozzles.
Mach 10 for several minutes. The Mach 14 nozzle was not used, however, because of unexpected problems with the pebble-bed heater. The heater was built like a battleship with steel plate 8 inches thick. Mach 10 was the upper limit. By mid-1975 thousands of blowdowns, during which air was heated above the melting point of steel, had taken their toll.