Hawker Siddeley much larger firms. Out of this decision, came the "order" that all future contracts being offered had to include agreements to merge companies. In 1959, Folland Aircraft was acquired, followed by de Havilland Aircraft Company and Blackburn Aircraft in 1960. In 1963, the names of the constituent companies were dropped, with products being rebranded as ""Hawker Siddeley"" or ""HS"". In this period, the company developed the first operational, and, by far, the most successful VTOL jet aircraft, the Harrier family. This aircraft remained in production into the 1990s and remains in service. In 1948, Hawker Siddeley acquired a factory in
Harrier Jump Jet of the upgraded "Sea Harrier FA2"; this version entered operational service on 2 April 1993. The Sea Harrier was also procured by the Indian Navy, the first Indian Sea Harriers were entered squadron service during December 1983. As early as 1973, Hawker Siddeley and American aviation manufacturer McDonnell Douglas were jointly working on development of a more capable version of the Harrier. Early efforts concentrated on the development of an improved Pegasus engine, designated the "Pegasus 15", which was being tested by Bristol Siddeley. During August 1981, the program received a boost when British Aerospace (BAe) and McDonnell Douglas signed