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 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.

These systems have advantages over both alternatives. Unlike a single-state-to-orbit craft, most of the dry mass is not carried into orbit. This results in a greater fraction of the mass-to-orbit being payload, rather than "dead weight," which reduces launch cost. Simultaneously, it is less complex and has fewer separation events than a system with more stages, which reduces cost and risk of failure.

Plot of GLOW vs Structural Coefficient for LEO mission profile.
Importance of Structural coefficient and ISP for Single-Stage-to-Orbit (SSTO) and restricted stage Two-Stage-to-Orbit (TSTO) vehicles. Based on a LEO mission of Delta v = 9.1 km/s and payload mass = 4500 kg for range of propellant Isp. GLOW=Gross Lift-Off Weight
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