Van Nuys Airport opened in the late 1920s, six miles west of the Burbank Airport, and like Burbank, it became a major aircraft production center during World War II, with the main plants operated by Lockheed’s Vega division. After the war the airport continued to be a center for some aircraft companies, such as the Marquardt Corporation, which developed ramjet rockets and missiles there, and at a test site in the mountains, but mostly it transitioned to a regional civil airfield, operated by the city of Los Angeles, surrounded by diversified industries in office parks. Today it is the busiest general aviation airport in the country, with private jets serving corporate clients and the entertainment industry.