 Developed by the Advanced Projects Group, a June 1958 report by Avro's Engineering Department described a Space Threshold Vehicle intended to "get a man into the threshold of space and recover him, flying back through the corridor", where winged flight was possible between maximum altitude that could sustain lift from a winged vehicle and maximum tolerable structural temperature.This was estimated to be an altitude of between 150,000 and 200,000 feet.The STV would have had capability for in-flight refueling, and an expected speed of 6,000 mph (Mach 8.5+).