Complex sentence: The first stage that accelerated the missile at over 100 gee, reaching Mach 10 in a few seconds, travelling so fast that its outer layer became hotter than an oxy-acetylene welding torch due to air friction.
Split into simpler sentences: The first stage accelerated the missile at over 100 gee, reaching Mach 10 in a few seconds. At these speeds, the airframe's outer layer became hotter than an oxy-acetylene welding torch due to air friction.