Two-stage-to-orbit

A two-stage-to-orbit (TSTO) or two-stage rocket is a launch vehicle in which two distinct stages provide propulsion consecutively in order to achieve orbital velocity.

It is argued that a two-stage design should require less maintenance, less testing, experience fewer failures and have a longer working life.

In the case of airplane-like lower stages they also argue how difficult and expensive high speed aircraft (like the SR-71) are to develop and operate, and question performance claims.

Many 'mini-shuttle' designs that use transport aircraft as first stages also face similar problems with ice/foam as the Space Shuttle due to the requirement they also carry a large external tank for their fuel.

Other designs like the DH-1 concept take it a step further and use a 'pop-up/pop-down' approach, which delivers the orbiting stage to a point about 60 km above the Earth's surface, before dropping down to the launch pad again.

While not an orbital vehicle, the successful private SpaceShipOne suborbital spacecraft developed for the Ansari X Prize demonstrated that a two-stage system with a winged aircraft as the "lower half" can reach the edge of space.