Two stage to orbit
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A two stage to orbit (or TSTO) launch vehicle is a spacecraft in which two distinct stages provide propulsion consecutively in order to achieve orbital velocity. It is intermediate between the more common system with more stages, and a single-stage to orbit (SSTO) launcher, which does not exist yet.
At lift off 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.
The advantage of such a system is that the entire mass of the spacecraft is not carried into orbit. This reduces the difficulty involved in reaching orbital velocity.
It is not always clear when a vehicle is a TSTO. Many designs which use a very small boost at the beginning of their flight are referred to as single stage to orbit. Some have also coined the expression 1.5STO for 'one and a half stage to orbit', e.g. the Atlas.
Reusable launch systems
With reference to a reusable launch system this approach is often proposed as an alternative to single stage to orbit (or SSTO). Its supporters argue that, since each stage may have a lower mass ratio than an SSTO launch system, such a system may be built without approaching as close to the limitations of its structural materials. It therefore should require less maintenance, less testing, experience fewer failures and have a longer working life.
Critics argue that the increased complexity of designing two separate stages that must interact, the logistics involved in returning the first stage to the launch site, and the difficulties of conducting incremental testing on a second stage will outweigh these benefits.
Airplane-like first stage
Many TSTO designs comprise an airplane-like first stage and a rocket-like second stage. The airplane elements can be wings, air-breathing engines, or both. This approach appeals because it transforms Earth's atmosphere from an obstacle into an advantage. Above a certain speed and altitude, wings and scramjets cease being effective, and the rocket is deployed to complete the trip to orbit.
SpaceShipOne
While not an orbital vehicle, the successful low-cost SpaceShipOne suborbital vehicle developed for the Ansari X Prize demonstrated that the problems of integrating a two-stage system, with a winged aircraft as the "lower half", that can reach the edge of space are not insurmountable. As of 2005, the team behind SpaceShipOne is working on a commercial sub-orbital launch system based on this technology.