"the shuttle, travelling at 18,000 miles per minute"

.........
"The shuttle must attain an eventual speed of 0.7*(orbital velocity) or roughly 4.5 km/sec. at a height of about 226 Km above the surface of the Earth.
After a two-minute burn, the SRBs (Solid Rocket Boosters) are discarded, and the remaining climb to 7.75 Km/sec at 250 Km is done by hand using the Shuttle engines"

7.75 km/sec x 60 sec/min x 1.6 km/ mile = 744 miles per minute