New Virgin launch vehicle designed as air launched two stage rocket called LauncherOne acquired its conveyance - Boeing 747 called Cosmic Girl.
It seems that Pegasus rocket will get competitor on the market of air launched rockets. Virgin Galactic announced about choosing designated lifting plane for their new LauncherOne rocket operating similar to Orbital ATK Pegasus. In this case "similar" not necessary stays for "identical".
In 2012 after announcing LauncherOne project Virgin Galactic revealed also its mother plane which was WhiteKnightTwo jet cargo plane. WhiteKnightTwo was originally lifting plane for SpaceShipTwo space plane test flights. During further development it began to show that one WhiteKnightTwo plane will not manage to cover demands of two ambitious programs. Lately Virgin Galactic announced that lifting vehicle for LauncherOne will become Boeing 747-41R belonging to Virgin since 2001. First flight of combination Cosmic Girl with LauncherOne is planned for 2017.
LauncherOne is Virgin Galactic response for growing demand for launch system suitable for lifting small and nano satellites at the fraction of cost of conventional rockets. Concept was modeled on Orbital ATK Pegasus rocket with few differences. First was lifting plane. In case of Pegasus rocket it is special passenger plane Lockheed L-1011; at the beginning lifting plane for LauncherOne was special designed cargo plane called WhiteKnightTwo. It changed now and both systems will be utilizing conventional passenger planes with modifications letting to suspend the rocket. Next, Pegasus was designed as three stage rocket vehicle with conventional wings and tail. LauncherOne is pure rocket without any additional lift surfaces. It was designed as two stage liquid fueled rocket with payload capacity at 200 kg. Pegasus is able to lift over 400 kg of payload and consists up to four stages. Significant difference refers to the costs per launch. In case of LauncherOne it is assumed to stay under $ 10 million, with Pegasus one launch costs around $56 million (example of NASA Ionospheric Connection Explorer mission from 2015, with combined missions costs for second payload could be lower)
With change of lifting plane, LauncherOne concept slightly changed. Utilizing Boeing 747 gives opportunity to enlarge LauncherOne - longer fuel tanks were implemented, NewtonTwo engine in first stage and NewtonOne in second stage were replaced with NewtonThree for the first stage. Difference is in thrust: NewtonTwo was able to provide at 211 kN, NewtonThree will give at 335 kN. For the second stage it is planned to utilize NewtonFour engine with thrust at 22 kN. All variations of Newton engines are liquid fueled (LOX/RP-1). It is another difference between LauncherOne and Pegasus, which is solid fueled (except optional fourth stage). Additional details about whole starting procedure in case of air launch rocket on the Pegasus example please find here.