Hawker Siddeley Harrier AV-8As to the AV-8C configuration—the work focused mainly on extending useful service lives and improving VTOL performance. The AV-8C and the remaining AV-8A Harriers were retired by 1987. These were replaced by the Harrier II, designated as the AV-8B, which was introduced into service in 1985. The performance of the Harrier in USMC service led to calls for the United States Air Force to procure Harrier IIs in addition to the USMC's own plans, but these never resulted in Air Force orders. Since the late 1990s, the AV-8B has been slated to be replaced by the F-35B variant of the
Hawker Siddeley Harrier function had originally been added following the input of US Air Force Colonel Bill Chapman, who worked for the Mutual Weapons Development Team. Water injection was necessary in order to generate maximum thrust, if only for a limited time, and was typically used during landing, especially in high ambient temperatures. The aircraft was initially powered by the Pegasus 6 engine which was replaced by the more powerful Pegasus 11 during the Harrier GR.1 to GR.3 upgrade process. The primary focus throughout the engine's development was on achieving high performance with as little weight as possible, tempered by the amount of