Unseen hypersonic airliners from the Los Angeles area, part 2: liquid hydrogen designs from Inglewood and Long Beach

What did Douglas and North American Aviation have in common in the 1950s and 1960s? They both built research aircraft for investigating supersonic and hypersonic flight, with the D-558-2 Skyrocket becoming the first airplane to reach Mach 2 and the North American X-15 entering the history books as the first manned aircraft to fly at hypersonic speeds. Also, the two companies conceived designs for a supersonic airliner in the early 1960s, with Douglas working on the Model 2229 and North American envisaging the NAC-60, the latter which was submitted for the Federal Aviation Administration's National Supersonic Transport (NST) competition but eventually rejected by the FAA in favor of the Boeing 2707 and Lockheed L-2000 as one of the finalist designs for the NST contest. With this rich experience in mind, Douglas and North American decided to take the unorthodox next step in high-speed aircraft development by looking at the notion of hypersonic air travel. 

Left: Cutaway artwork of a 1967 concept by North American for a Mach 10-14 hypersonic airliner
Right: Artwork from 1967 of two North American Mach 6 hypersonic airliners (cutaway view provided for one of them)

In the 1950s and 1960s North American Aviation made huge strides in developing superfast aerospace vehicles with tests of the SM-64 Navaho supersonic cruise missile in the 1950s and flight tests of the XB-70 Valkyrie prototype Mach 3 strategic bomber and X-15 hypersonic research aircraft, the latter which became the first aircraft to reach Mach 6. Fortified by prior experience with the XB-70 and X-15 in addition to having worked on the NAC-60 supersonic airliner project, in early 1967 North American unveiled two concepts for airliners capable of traveling at hypersonic speeds. One hypersonic airliner concept envisaged was a highly swept delta wing design with two outward canted vertical stabilizers on a flattened rear extension aft of the trailing edges of the delta wings, and it would have had a seating capacity for 130 passengers with the crew of two. Power came from four turboramjets fueled by liquid hydrogen or liquid methane stored in fuel tanks below the interior, and the aircraft would have had a top speed of Mach 6 and cruising altitude of 80,000-100,000 feet (24,384-30,480 meters). Another concept involved a design with a semi-conical fuselage, backswept wings which had pentagon-shaped vertical fins at the wingtips, seating capacity for 136 passengers, and a top speed of Mach 10 to 14. It was powered by four liquid hydrogen-fueled scramjet engines and had a large expansion ramp at the rear of the fuselage for the scramjet ducts, and it would cruise at altitudes of 110,000-140,000 feet (33,528-42,672 meters). In both concepts, the jet turbines would be ignited at speeds of up to Mach 3, and the air inlets for the turbo-compressors would be closed as the aircraft cruised at hypersonic speeds. Given that heat friction is generated by speeds beyond Mach 3, it seems reasonable to assume that these two proposals would have been constructed from heat-resistant alloys such as Inconel X and Beta-21S. No company designations are known for North American's hypersonic airliner concepts, but presumably those designs bore internal designations within the D-400 to D-434 numerical sequence because the company's D-435-1-4 proposal for a delta-wing modification of the X-15A-3 and D-436 and D-458 proposals for the initial study phase of the Advanced Manned Strategic Aircraft (AMSA) program were conceived shortly after the hypersonic airliner projects were unveiled.

Company artwork of the giant Douglas DC-2000 hypersonic airliner concept

In 1973, the Long Beach division of McDonnell Douglas conceived its own proposal for a hypersonic airliner for operational use by the year 2000. Designated DC-2000 by the company, it was 475 feet (144 meters) long with a wingspan of 170 feet (52 meters), a gross weight of 875,000 lb (396,893 kg), a top speed of Mach 6, and a range of more than 5,000 miles (8,047 km). The DC-2000 would carry 500 passengers, and power was provided by a combination of four liquid-hydrogen fueled turbojets and four ramjet engines, with the turbojets being used for speeds of up to Mach 3.5 and the ramjets providing thrust at speeds in the hypersonic flight regime. In addition to having a slender fuselage, it had a single vertical stabilizer and clipped delta wings with backswept trailing edges. The passenger capacity being envisaged for the DC-2000 was quite unusually high compared to that of the earlier North American hypersonic airliner design studies yet reflected the ambitions of McDonnell Douglas for advanced long-range air travel by the end of the 20th century.

Early 1970s proposal by North American Rockwell for a hypersonic airliner

As a side note, although North American's 1967 hypersonic airliner designs did not progress beyond the design phase, in late 1972, the Space Division of North American (which by then had become part of Rockwell International) worked out a design for a hypersonic airliner with a seating capacity for 200 passengers and a top speed of Mach 6. The proposal, for which no company designation is known, was 300 feet (91.4 meters) long with a wingspan of 112 feet (34 meters), and a gross weight of 481,400 lb (218,359 kg), and it had a single vertical stabilizer and delta wings with a forward swept trailing edge running along the length of the rear fuselage and extending beyond the spine of the vertical stabilizer to point ahead of the rudder. It would have a range of 4,600 miles (7,000 km) and power was provided by four 58,000 lb (260 kN) turbojets and nine 157,000 lb (698 kN) thrust scramjets situated below the rear fuselage and fueled by liquid hydrogen.

For more on North American and Rockwell International's hypersonic airliner proposals, see the following links:
https://up-ship.com/blog/?p=20863

https://www.aerospaceprojectsreview.com/blog/?p=299

https://ntrs.nasa.gov/api/citations/19730023221/downloads/19730023221.pdf

References:

Ingells, D.J., 1979. The McDonnell Douglas Story. Fallbrook, CA: Aero Publishers.

Wise, C.E., and Wood, N. (March 2, 1967). "On to Mach 12." Machine Design 39 (5):84-89.