rocket is similar to a one and a half stages to orbit RLV
. In thise scheme two almost identical rockets are bolted together sideways. The rockets take off vertically, but the fuel for both engines are taken from just one of the fuel tanks- they are crossfed
. When the fuel in the first rocket is empty- it separates and flies back to the takeoff point. The other flies on its way, continuing to take fuel from its tank to do this and returns to the launch site 'the long way' around after one or more orbits.
The main advantage of this scheme is that the rockets are mostly identical which brings system simplicity- only one design is needed for both 'stages', also dropping off the first rocket reduces the amount of hardware that needs to be carried to orbit and thus can improve the payload size.
The main disadvantages are that the aerodynamic losses of two vehicles are no better than a single vehicle and coupling the vehicles and separating them is complicated due to atmospheric issues.
Still at the end of the day, it is easier to engineer than SSTO, and the technology to achieve it has been demonstrated, although on different vehicles.