Editing Atlas II (section)

=== Atlas II first stage ===
[[File:KSC-00pp0413.jpg|left|thumb|330px|Workers at Cape Canaveral Air Force Station prepare to erect the first stage of an Atlas&nbsp;IIA rocket in the launch gantry on Pad&nbsp;36A ahead of the GOES-L launch. Visible are the [[RS-56]] rocket engines.]]
The Atlas II first stage was {{cvt|3.05|m||abbr=}} in diameter and {{cvt|24.90|m||abbr=}} long. The stage was powered by 3 [[RS-56]] rocket engines (derived from the [[RS-27]] main engine of the [[Delta&nbsp;II]] rocket) burning {{cvt|156|t|lb|abbr=}} of [[RP-1]] and [[liquid oxygen]]. The two booster engines were the RS-56-OBA variants (the complete assembly of both engines and the aft skirt was referred to as the MA-5A), with high thrust but moderate efficiency. The sustainer (center) engine was the RS-56-OSA variant, featuring much less thrust but higher efficiency at high altitudes than the booster engines.

The [[vernier engines]] used on the first stage of the Atlas&nbsp;I (and all previous Atlas models) were replaced by a [[hydrazine]]-fueled roll control system on Atlas II. This system, mounted on the interstage between the first and second stages, utilized small thrusters to control the vehicle's roll.<ref name=astronautixAtlasII>{{cite web| title=Atlas II |url=http://www.astronautix.com/lvs/atlasii.htm|archive-url=https://web.archive.org/web/20021015095840/http://www.astronautix.com/lvs/atlasii.htm |url-status=dead |archive-date=October 15, 2002 |publisher=Astronautix |access-date=January 9, 2016}}</ref><ref name=ppg /><ref>{{cite web |url=https://rocket.com/sites/default/files/documents/Capabilities/PDFs/Propulsion%20System%20Data%20Sheets.pdf |title=Atlas Roll Control Module (ARCM) |publisher=Aerojet |access-date=November 5, 2022}}</ref> Compared to [[Atlas&nbsp;I]], the Atlas II&nbsp;first stage was {{cvt|2.7|m||abbr=}} taller.<ref>{{Cite web |title=Le lanceur Atlas 2 |lang=fr |url=http://www.capcomespace.net/dossiers/espace_US/lanceurs_US/atlas/atlas_2.htm |access-date=2021-02-14 |website=www.capcomespace.net}}</ref>

The Atlas II was the last Atlas rocket to use the "stage-and-a-half" technique, where it ignited all 3 [[RS-56]] engines at liftoff and then jettisoned the 2 RS-56-OBA side engines and their support structure during ascent. The two RS-56-OBA engines were integrated into a single unit called the [[MA-5A]] and shared a common [[gas generator]]. They burned for approximately 164 seconds before being jettisoned, when acceleration reached approximately 5.0–5.5&nbsp;[[g-force|''g'']]. The central sustainer engine on the first stage, an RS-56-OSA, would burn for an additional 125 seconds after their jettison. It featured better efficiency at high altitudes than the RS-56-OBAs.<ref>{{cite web |title=Atlas IIA(S) Data Sheet |url=http://www.spacelaunchreport.com/atlas2a.html |archive-url=https://archive.today/20160813221340/http://www.spacelaunchreport.com/atlas2a.html |url-status=usurped |archive-date=August 13, 2016 |publisher=Space Launch Report |access-date=January 9, 2016}}</ref><ref name=ppg />

The first stage also had the option to be fitted with 4 [[Castor&nbsp;4A]] solid rocket boosters as part of the IIAS version, each providing an additional {{cvt|478.3|kN}} of thrust for 56 seconds. The first two boosters were ignited at liftoff, and the other two were ignited after the first two burnt out. Both pairs of boosters were jettisoned shortly after their respective burns.<ref name=astronautixAtlasIIAS />