Convair's CX-4 concepts

In 1962-1963, pursuant to the CX-4 requirement issued by the US Air Force in 1962 for for a heavy-lift transport to replace the C-133, the Convair San Diego division of General Dynamics worked out a series of advanced concepts for a heavy-lift strategic airlifter. Convair's San Diego division had some previous experience with jet airlifter design, having proposed the Model 63 design for the SOR-182 competition won by the Lockheed C-141 Starlifter, and it therefore decided that participation in the CX-4 program was worth a try.

Top: Six-engine concept by Convair San Diego for CX-4
Bottom: Convair's preferred concept for CX-4 with four STF-200-D2s

Convair San Diego's resulting design studies for the CX-4 requirement investigated three turboprop and six turbofan options. One set of designs focused on a heavy-lift transport with six Pratt & Whitney T57 turboprops fitted with variable camber propellers, and the turboprop-powered iteration measured 195 feet 6 in (59.6 meters) long with a wingspan of 210 feet (64 meters), a gross weight of 560,000 lb (254,200 kg), a payload capacity of 165,000 lb (74,842 kg). However, the propellers extended over the whole span and offered very little ground clearance. The initial baseline jet-powered design for CX-4 was powered by six Pratt & Whitney TF33 turbofans and had a low-mounted horizontal stabilizer, and alternate engine options for this iteration included six Pratt & Whitney STF-200-D2s or eight turbofans housed in four paired nacelles. The preferred CX-4 design from Convair San Diego was powered by four STF-200-D2s and measured 204 feet 6 in (62.3 meters) long with a wingspan of 230 feet (71.3 meters), a gross weight of 546,000 lb (248,200 kg), and a T-tail configuration. The high- and low-mounted CX-4 designs featured a side-hinged nose positioned ahead of the cockpit and a tail ramp to allow both drive-through loading and air-dropping, and the four-engine iteration had the flight deck raised in a bulge over the cargo area.    

In addition to land-based studies, Convair looked at an amphibious CX-4 transport proposal to address the possibility that a destination airbase might not be available and not at every brushfire war location. It was powered by four STF-200-D2 turbofans and measured 204 feet 6 in (62.3 meters) long with a wingspan of 234 feet (71.3 meters) and a payload capacity of 165,000 lb (74,842 kg). The amphibious CX-4 iteration featured a pair of clamshell doors, which were designed to load cargo at a land base and deliver it to a seadrome, and the aft fuselage had a side-loading door.

In November 1963 the CX-4 program was renamed CX-HLC (Heavy Logistics Carrier), which changed its name to CX-HLS (Heavy Logistics System) in January 1964. None of Convair's designs would ever see the light of day, and Convair itself neglected to take part in the CX-HLS competition.

References:

Cox, G., and Kaston, C., 2020. American Secret Projects 3: U.S. Airlifters Since 1962. Manchester, UK: Crécy Publishing.