Editing Project Mercury (section)

===Orbital flight===
Orbital missions required use of the [[Atlas LV-3B]], a man-rated version of the [[SM-65D Atlas|Atlas D]] which was originally developed as the [[United States]]' first operational [[intercontinental ballistic missile|intercontinental ballistic missile (ICBM)]]{{sfn|Catchpole|2001|p=211}} by [[Convair]] for the Air Force during the mid-1950s.{{sfn|Alexander & al.|1966|p=22}} The Atlas was a "one-and-one-half-stage" rocket fueled by [[kerosene]] and liquid [[oxygen]] (LOX).{{sfn|Catchpole|2001|p=211}} The rocket by itself stood {{convert|67|ft}} high; total height of the Atlas-Mercury space vehicle at launch was {{convert|95|ft}}.{{sfn|Catchpole|2001|p=212}}

The Atlas first stage was a booster skirt with two engines burning liquid fuel.{{sfn|Catchpole|2001|pp=225, 250}}{{refn|group=n|At the time, the word "booster" was sometimes used for the first stage of the launch stack. Later, "booster" came to refer to additional single-stage rockets attached to the sides of the main launch vehicle, as on the Space Shuttle.}} This, together with the larger sustainer second stage, gave it sufficient power to launch a Mercury spacecraft into orbit.{{sfn|Catchpole|2001|p=211}} Both stages fired from lift-off with the thrust from the second stage sustainer engine passing through an opening in the first stage. After separation from the first stage, the sustainer stage continued alone. The sustainer also steered the rocket by thrusters guided by gyroscopes.{{sfn|Catchpole|2001|pp=458–459}} Smaller vernier rockets were added on its sides for precise control of maneuvers.{{sfn|Catchpole|2001|p=211}}