Following the Hawker (later British Aerospace) Kestrel operational trials (#19), the first of six production-designed Harrier developmental aircraft flew on 31 August 1966. The production GR1 Harrier entered service with the UK Royal Air Force on 1 April 1969 powered by the 19,000 lb Bristol (later Rolls-Royce) Pegasus Mk 101. US Marine Corps AV-8As were purchased in 1969, powered by the 21,500 lb thrust Pegasus 11. A navalized version, the FRS1 Sea Harrier, entered service with the Royal Navy in 1980. McDonnell Douglas (later with British Aerospace) began developing the AV-8B GR5 Harrier II in 1974, and began flight testing in 1981. With a more powerful engine, a larger, composite supercritical wing, optimized Lift Improvement Devices (LIDs) and other improvements, the Harrier II was able to double the payload and range when making short takeoffs. The F402-RR-408 Pegasus 11-61 has now reached 23,800 lb thrust. Today, the Harrier is the only operational V/STOL aircraft in the world. In addition to the USMC, the UK RAF and RN, the Harrier is also operated by Spain, Italy, India and Thailand.