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.

An advantage of such a system over single-stage-to-orbit is that most of the dry mass of the vehicle 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.[1]

An advantage over three or more stages is a reduction in complexity and fewer separation events, which reduces cost and risk of failure.[2]

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
  1. ^ Blanco, Philip (2022). "Learning about rockets, in stages". Physics Education. 57 (4): 045035. Bibcode:2022PhyEd..57d5035B. doi:10.1088/1361-6552/ac6928. S2CID 249535749. Retrieved 17 June 2022.
  2. ^ "Falcon 1 - Stage Separation Reliability". SpaceX. Archived from the original on 30 April 2013. Retrieved 8 January 2011.