What is "two-stage-to-orbit"

A two-stage-to-orbit (TSTO or DSTO - Double/Dual-Stage-To-Orbit) launch vehicle is a spacecraft in which two distinct stages provide propulsion consecutively in order to achieve orbital velocity. It is intermediate between a three-stage-to-orbit launcher and a hypothetical single-stage-to-orbit (SSTO) launcher.

At liftoff the first stage is responsible for accelerating the vehicle. At some point the second stage detaches from the first stage and continues to orbit under its own power.

An advantage of such a system over single-stage-to-orbit is that the entire mass of the spacecraft is not carried into orbit. This reduces the cost involved in reaching orbital velocity as much of the structure and engine mass is ejected and a larger percentage of the orbited mass is payload mass.

An advantage over three or more stages is reduction in complexity and fewer separation events, each of which reduces cost and risk of failure.