Virgin Orbit achieved another major milestone on Sunday, with LauncherOne taken on a captive carry test on Sunday. Carried under the wing of Cosmic Girl, a “flying launch pad” as described by the company, the duo enjoyed a successful first flight together, paving the way for the first launch that is set to take place in early 2019.
Cosmic Girl, a Boeing 747-400 (747-41R) series aircraft under then-registration number G-VWOW, undertook its first flight on 29 September 2001 and was delivered to Virgin Atlantic Airways on 31 October 2001.
Cosmic Girl – as she was named at the time of her delivery – spent 14 years in service with Virgin Atlantic Airways primarily servicing the company’s London to San Francisco via New York City route until 29 October 2015.
I'm not sure if they're still putting this in 747-8s, but earlier 747 variants have a fifth structural "hardpoint" between the inner left engine and the fuselage that can mount a spare engine. This was designed in due to the fact that, at the time of the launch of the 747, there were no cargo aircraft available to civilians that could airlift a whole high-bypass engine the size of the JT9 or the CF6. The hardpoint allowed a sister 747 to ferry in a replacement engine in the event of a failure while far from home, without having to wait weeks for one to arrive via ship.
Oddly enough, the prototype 747 found at the Museum of Flight in Seattle doesn't seem to have any external signs of this ever having been used during testing...