Editing SM-65 Atlas (section)

==Missile details==
{{unreferenced section|date=August 2015}}
The Atlas's complicated, unconventional design proved difficult to debug compared with rocket families such as Thor and Titan which used conventional aircraft-style structures and two stage setups and there were dozens of failed launches during the early years. After watching [[SM-65D_Atlas|Atlas Serial 7D]] [[List_of_Atlas_launches_(1957–1959)#7D|explode shortly after its nighttime launch]], Mercury astronaut [[Gus Grissom]] remarked "Are we really going to get on top of one of those things?"<ref>{{cite book |last1=Steward |first1=David |title=Seeking Success: A Memoir |url=https://www.academia.edu/32536245 |page=44}}</ref> The numerous failures led to Atlas being dubbed an "Inter County Ballistic Missile" by missile technicians, but by 1965 most of the problems had been worked out and it was a reliable launch vehicle. Nearly every component in the Atlas managed to fail at some point during test flights, from the engine combustion chambers to the tank pressurization system to the flight control system, but Convair engineers noted with some pride that there had never been a repeat of the same failure more than three times, and every component malfunction on an Atlas flight was figured out and resolved. Some of the repeat failures were also the result of rushed launch schedules and could have been avoided. The last major design hurdle to overcome was unstable engine thrust, which caused three Atlas missiles (Serial 51D and 48D in 1960 and Serial 27E in 1961) to explode on their launching stands.

===Pressure stabilized tanks===
Atlas was unusual in its use of [[balloon tank]]s for the propellants, made of very thin [[stainless steel]] with minimal or no rigid support structures, as already pioneered by the Soviet [[R-5 Pobeda|R-5]] first launched in 1953.<ref>{{Cite web |url=https://www.energia.ru/english/energia/launchers/rocket-r5.html |title=Rocket R-5 |access-date=2023-11-10 |archive-url=https://web.archive.org/web/20211023212134/https://www.energia.ru/english/energia/launchers/rocket-r5.html |archive-date=2021-10-23 |url-status=dead |publisher=S. P. Korolev RSC Energia}}</ref> Pressure in the tanks provides the structural rigidity required for flight. An Atlas rocket would collapse under its own weight if not kept pressurized, and had to have {{convert|5|psi|abbr=on}} nitrogen in the tank even when not fueled.<ref>{{cite web|author=John Pike |url=http://www.globalsecurity.org/wmd/systems/sm-65.htm |title=SM-65 Atlas – United States Nuclear Forces |publisher=Globalsecurity.org |access-date=19 July 2013}}</ref> The rocket had two small thrust chambers on the sides of the tank called [[vernier rocket]]s. These provided fine adjustment of velocity and steering after the sustainer engine shut down.

==='Stage-and-a-half'===
Atlas was informally classified as a "stage-and-a-half" rocket, with a central sustainer engine and set of two booster engines that were all started at launch, each drawing from a single set of propellant tanks.<ref name="Brugge">{{cite web |url=http://www.b14643.de/Spacerockets/Specials/Atlas_MA-drive-system/index.htm |title=Variants of the "stage and a half" drive system (MA) of the Atlas rocket |website=b14643.de |access-date=4 September 2022}}</ref><ref name="McCutcheon">{{Cite web |last1=D. McCutcheon |first1=Kimble |title=Part 5: The Atlas Missile |url=https://www.enginehistory.org/Rockets/RPE05/RPE05.shtml |url-status=live |archive-url=https://web.archive.org/web/20240828191532/https://enginehistory.org/Rockets/RPE05/RPE05.shtml |archive-date=28 August 2024 |access-date=4 September 2022 |website=U.S. Manned Rocket Propulsion Evolution }}</ref> Most [[multistage rocket]]s drop both engines and fuel tanks simultaneously before firing the next stage's engines. However, when the Atlas missile was being developed, there was doubt as to whether a rocket engine could be air-started. Therefore, the decision was made to ignite all of the Atlas' engines at launch; the booster engines would be discarded, while the sustainer continued to burn.<ref name="Brugge"/> A stage of a liquid propellant rocket normally consists of both propellant tanks and engines, so jettisoning one or more engines only is equivalent to "half a stage". At staging, the booster engines would be shut off and a series of mechanical and hydraulic mechanisms would close the plumbing lines to them. The booster section would then be released by a series of hydraulic clamps (aside from the early test model Atlas B, which used explosive bolts) and slide off the missile on two tracks. From there on, the sustainer and [[Vernier thruster|verniers]] would operate by themselves. Booster staging took place at roughly two minutes into launch, although the exact timing could vary considerably depending on the model of Atlas as well as the particular mission being flown. This "stage-and-a-half" design was made possible by the extremely light weight [[balloon tank]]s.<ref name="McCutcheon"/> The tanks made up such a small percentage of the total booster weight that the mass penalty of lifting them to orbit was less than the technical and mass penalty required to throw half of them away mid-flight. However, technology advanced quickly and not long after design work on Atlas was completed, Convair rival Martin proposed a solution to the air-starting problem. Their [[Titan I]] missile, developed as an Atlas backup, had a conventional two stage design.<ref>{{cite web|title=The Military Standard - Titan I Missile Overview |url=http://www.themilitarystandard.com/missile/titan1/overview.php|website=themilitarystandard.com|access-date=March 10, 2023}}</ref>

===Engines===
The booster engine consisted of two large thrust chambers. The Atlas A/B/C/D had a single turbopump assembly and gas generator driving both booster engines; the A/B/C had an interim engine with lower thrust while the D-series had the full-up engines delivering 303,000 pounds of thrust.<ref name="McCutcheon"/> On the Atlas E/F, each booster engine had a separate pump and gas generator. Later space launcher variants of the Atlas used the MA-5 propulsion system with twin turbopumps on each booster engine, driven by a common gas generator.<ref name="Brugge"/> The boosters were more powerful than the sustainer engine and did most of the lifting for the first two minutes of flight. In addition to pitch and yaw control, they could also perform roll control in the event of a vernier failure. The sustainer engine on all Atlas variants consisted of a single thrust chamber with its own turbopump and gas generator, which also powered two small pressure-fed vernier engines.<ref name="McCutcheon"/> The verniers provided roll control and final velocity trim. The total sea level thrust of all five thrust chambers was 360,000&nbsp;[[Pound (force)|lb<sub>''f''</sub>]] (1,600&nbsp;[[Newton (unit)|kN]]) for a standard Atlas D. Atlas E/F had 375,000 pounds of thrust. Total sea level thrust for these three-engine Atlas Es and Fs was 389,000&nbsp;lb<sub>f</sub> (1,730&nbsp;kN).<ref name="FEWarren">{{cite web|archive-url=https://web.archive.org/web/20080509071108/http://www.warrenmuseum.com/atlas.htm |archive-date=May 9, 2008|url=http://www.warrenmuseum.com/atlas.htm|title=Atlas SM-65|publisher=FE Warren Museum}}</ref> [[Launch vehicle|Launcher]] variants of the Atlas often had performance enhancements to the engines.<ref name="McCutcheon" />

===Guidance===
The Atlas missiles A through D used radio [[missile guidance|guidance]]: the missile sent information from its [[inertial guidance|inertial system]] to a ground station by radio, and received course correction information in return. The Atlas E and F had completely autonomous [[inertial guidance]] systems.

The ground based guidance computer was a key part of the missile system, until guidance computers were [[miniaturized]] enough to be installed inside the missile. [[Isaac L. Auerbach]] designed the Burroughs guidance computer for the Atlas ICBM missiles. The Burroughs guidance computer was one of the first [[transistor computer]]s. It processed [[24-bit computing|24-bit]] data using [[18-bit computing|18-bit]] instructions. A total of 17 of these ground computers were delivered. These same ground computers was later used for [[Atlas-Able]], [[Project Mercury]], and other early spacecraft.<ref>
[https://afspacemuseum.org/wp-content/uploads/displays/BurroughsComputer/Burroughs_Historical_Summary.pdf "Burroughs Guidance Computer Historical Summary"].
2012.
p. 1, 7, 8.
</ref>

===Warhead===
The warhead of the Atlas D was originally the G.E. Mk 2 "heat sink" [[re-entry vehicle]] (RV)<ref>{{cite web |title=Missile, Reentry Vehicle, Mark 2 |url=https://www.si.edu/object/missile-reentry-vehicle-mark-2%3Anasm_A19751430000 |website=si.edu}}</ref> with a [[W49]] [[thermonuclear weapon]], combined weight {{convert|3700|lb|-1|abbr=on}} and yield of 1.44 [[TNT equivalent|megatons]] (Mt). The W49 was later placed in a Mk 3 ablative RV, combined weight {{convert|2420|lb|-1|abbr=on}}. The Atlas E and F had an AVCO Mk 4 RV containing a [[W38 (nuclear warhead)|W38]] [[thermonuclear weapon|thermonuclear warhead]] with a yield of 3.75&nbsp;Mt<ref>{{cite web |title=Missile, Reentry Vehicle, Mark 4 |url=https://airandspace.si.edu/collection-objects/missile-reentry-vehicle-mark-4/nasm_A19660029000 |website=airandspace.si.edu}}</ref> which was [[fuze]]d for either air burst or contact burst. The Mk 4 RV also deployed [[penetration aid]]s in the form of [[mylar]] balloons which replicated the radar signature of the Mk 4 RV. The Mk 4 plus W-38 had a combined weight of {{convert|4050|lb|-1|abbr=on}}. All of the warheads deployed in the Atlas were over 100 times more powerful than the [[Atomic bombings of Hiroshima and Nagasaki|bomb dropped over Nagasaki]] in 1945.<ref>{{cite web|title=Fairchild had a missile squadron… Who knew?|date=June 4, 2014|author=Jim O'Connell |url=https://www.fairchild.af.mil/News/Commentaries/Display/Article/496434/fairchild-had-a-missile-squadron-who-knew/
|website=U.S. Air Force|access-date=March 11, 2023}}</ref>

===Comparison with R-7===
The [[R-7 Semyorka]] was the first Soviet ICBM and similarly started all engines before launch to avoid igniting a large liquid fuel engine at high altitudes. However, the R-7 had a central sustainer section, with four boosters attached to its sides. The large side boosters required use of an expensive launch pad and prevented launching the rocket from a silo. Like the Atlas, the use of [[cryogenic]] [[liquid oxygen]] meant that the missile could not be kept in the state of flight readiness indefinitely and was largely useless for its intended purpose (military) and was similarly developed into a space launch vehicle, initially delivering [[Sputnik 1|Sputnik]] and [[Vostok 1|Vostok]] into orbit. The [[Soyuz (rocket family)|Soyuz rocket]] is descended from the R-7 and remains in use today.<ref>{{Cite web|date=2020-04-27|script-title=ru:Коммерческий полет "Союза" на МКС планируется в 2022-2023 годах|trans-title=Soyuz commercial flight to the ISS planned for 2022-2023|url=https://ria.ru/20200427/1570605489.html|access-date=2020-06-26|website=РИА Новости|language=ru}}</ref>