Editing Lockheed C-5 Galaxy (section)

===CX-4 and Heavy Logistics System===
[[File:210521-F-F3405-0004.jpg|thumb|One of the first C-5A models is given a final inspection before testing in the Arnold Engineering Development Complex 16-foot transonic wind tunnel at Arnold Air Force Base in the mid-1960s.]]
In 1961, several aircraft companies began studying heavy jet transport designs that would replace the [[Douglas C-133 Cargomaster]] and complement [[Lockheed C-141 Starlifter]]s. In addition to higher overall performance, the [[United States Army]] wanted a transport aircraft with a larger cargo bay than the C-141, whose interior was too small to carry a variety of their [[outsize cargo|outsized equipment]]. This need led to the CX-4 requirement of July 1962, for which Lockheed, Boeing, Convair, and Douglas proposed six-engined designs. When the US Army judged the CX-4 specification inadequate for its requirements, by late 1963 the CX-4 specification gave way to the CX-HLC requirement specified an airlifter with four engines, an equipped gross weight of {{convert|550000|lb|kg|sigfig=3}}, a maximum payload of {{convert|180000|lb|kg|sigfig=3|abbr=on}}, and a speed of Mach&nbsp;0.75 ({{convert|500|mph|km/h|sigfig=3|abbr=on|disp=or}}). The cargo compartment was {{convert|17.2|ft|m|sigfig=3|abbr=on}} wide by {{convert|13.5|ft|m|sigfig=3}} high and {{convert|100|ft|m|sigfig=3|abbr=on}} long with front and rear access doors. USAF studies showed that high-bypass [[turbofan]] engines were needed for thrust and [[fuel efficiency]] requirements.<ref>Norton 2003, p. 7.</ref>
{{Quote box |align=right |width=33% |quote=We started to build the C-5 and wanted to build the biggest thing we could&nbsp;… Quite frankly, the C-5 program was a great contribution to commercial aviation. We'll never get credit for it, but we incentivized that industry by developing [the TF39] engine.|source= General [[Duane H. Cassidy]], former MAC Commander in Chief<ref>Bakse 1995, p. 39.</ref>}}

The criteria were finalized and an official [[request for proposal]] was issued in April 1964 for the "Heavy Logistics System" (CX-HLS) (previously CX-HLC). In May 1964, proposals for aircraft were received from [[Boeing]], [[Douglas D-906|Douglas]], [[General Dynamics]], [[Lockheed Corporation|Lockheed]], and [[Martin Marietta]]. [[GE Aviation|General Electric]], [[Curtiss-Wright]], and [[Pratt & Whitney]] submitted proposals for the engines. After a downselect, Boeing, Douglas, and Lockheed were given one-year study contracts for the airframe, along with General Electric and Pratt & Whitney for the engines.<ref name="Norton_p8-9">Norton 2003, pp. 8–9.</ref> All three of the designs shared a number of features. The cockpit was placed well above the cargo area to allow for cargo loading through a nose door. The Boeing and Douglas designs used a pod on the top of the fuselage containing the cockpit, while the Lockheed design extended the cockpit profile down the length of the fuselage, giving it an egg-shaped cross section. All of the designs had [[swept wing]]s, as well as front and rear cargo doors, allowing simultaneous loading and unloading.<ref>{{cite news |title=Boeing CX-HLS proposal, artist concept |work=boeingimages.com}} {{cite web |title=Image 1 |url= http://www.boeingimages.com/Docs/BOE/Media/TR3_WATERMARKED/2/9/1/8/BI231307.jpg |archive-url= https://web.archive.org/web/20161220140410/http://www.boeingimages.com/Docs/BOE/Media/TR3_WATERMARKED/2/9/1/8/BI231307.jpg |archive-date=20 December 2016 |url-status=dead}} {{cite web |title=Image 2 |url= http://www.boeingimages.com/Docs/BOE/Media/TR6_WATERMARKED/a/7/2/f/BI231305.jpg |url-status=dead |archive-url= https://web.archive.org/web/20161220140400/http://www.boeingimages.com/Docs/BOE/Media/TR6_WATERMARKED/a/7/2/f/BI231305.jpg |archive-date=20 December 2016}}</ref> Lockheed's design featured a [[T-tail]], while the designs by Boeing and Douglas had conventional tails.<ref name=Norton_p12-3>Norton 2003, pp. 12–13.</ref><ref>{{cite web |url= http://airwaysnews.com/html/museums/boeing-archives-bellevue-washington-usa/boeing-cx-hls-model-196364/19149 |title=Boeing CX-HLS Model at Boeing Corporate Archives – 1963/64 |website=Airway News |url-status=dead |archive-url= https://web.archive.org/web/20141014202050/http://airwaysnews.com/html/museums/boeing-archives-bellevue-washington-usa/boeing-cx-hls-model-196364/19149 |archive-date=14 October 2014}}</ref><ref>{{cite web |url= http://bemil.chosun.com/nbrd/gallery/view.html?b_bbs_id=10044&num=169918 |title=B747기의 전신 – 미 공군 CX-HLS 초대형수송기 사업 보잉사 설계안 |trans-title=B747 aircraft – US Air Force CX-HLS super large transport business – Boeing company design |language=ko |date=29 August 2012 |access-date=30 May 2019}} {{cite web |url= http://bemil.chosun.com/nbrd/bbs/view.html?b_bbs_id=10050&num=4082 |title=Next page |url-status=dead |archive-url= https://web.archive.org/web/20160912215829/http://bemil.chosun.com/nbrd/bbs/view.html?b_bbs_id=10050&num=4082 |archive-date=12 September 2016}}</ref>

The Air Force considered Boeing's design to be better than that of Lockheed, but Lockheed's proposal was the lowest total-cost bid.<ref name= "Norton_p11">Norton 2003, p. 11.</ref> Lockheed was selected as the winner in September 1965, then awarded a contract in December 1965.<ref name=Norton_p12-3 /><ref name="Irving">Erving 1993, pp. 189–190.</ref> General Electric's [[General Electric TF39|TF39]] engine was selected in August 1965 to power the new transport plane.<ref name= Norton_p12-3 /> At the time, GE's engine concept was revolutionary, as all engines before had a [[bypass ratio]] less than two-to-one, while the TF39 promised and would achieve a ratio of eight-to-one, which had the benefits of increased engine thrust and lower fuel consumption.<ref>Bakse 1995, pp. 39, 74.</ref><ref>Phillips 2004, p. 127.</ref> Boeing lost the military contract but went on to develop the successful 747 civilian airliner with over 1,500 aircraft built when manufacturing ended in 2022 after 54 years of production.