Flightdeck Friday: Hypersonics – Douglas D-558-III

As the emerging theme this week is hypersonics, what with the reference to the Shenlong and the upload to the Virtual Library of a comprehensive 3-volume history of the US hypersonic effort from the X-15 through the NASP, reader and frequent correspondent Thommy Thomason joins the scene as a guest author of this week’s Flightdeck Friday.  The subject?  The D-558-III Skyrocket.  The D-558-III?  Well, we’ll let him explain.  Oh, and see the box at the end for a special notice too…

“D-558-3” is commonly used to identify the proposed Douglas hypersonic research aircraft follow-on to their successful D-558-1 Skystreak and D-558-2 Skyrocket program. Although the designation doesn’t officially exist, that hasn’t hindered its use by aviation historians and others to refer to two somewhat different Douglas designs, the Model 671 and 684. What’s more, the name “Skyflash” is sometimes used in conjunction with the D-558-3 designation but appears to have even less validity.

The original D-558 type name came from the Navy’s practice in the 1940s for identifying aircraft types that they procured for purely experimental reasons and that did not already have a military or civil type identification. In those cases, the Navy simply gave the aircraft a designation that combined the Navy’s letter for the manufacturer with the manufacturer’s internal model number for the design.

For example, the Vought skimmer demonstrator for the F5U program was the V-173 and the swept-wing P-63s that the Navy bought were referred to as L-39s, L being the Navy’s manufacturing letter for the Bell Aircraft Corporation. In the case of the D-558, the D stood for Douglas and 558 was the Douglas model number for the design originally proposed for the Navy’s high-speed research aircraft program. The –1 and –2 differentiated the two types that resulted from the first two phases of the program.

Manufacturers use model numbers for their own internal records, typically assigning each of their designs a number, not always sequential but sometimes actually begun with “1” denoting their first. Not all of these designs are built, of course, and some companies keep two different lists, one for predesign studies and another of projects that have been committed to detail design and fabrication. For marketing reasons, model numbers of committed projects are often not sequential but are based on some hoped for association, the Bell Helicopter Model 209, 309 and 409 gun ships, for example. In some cases what appears to be a Model number, e.g. Boeing 707, is not actually the model number, which was “367-80”, explaining why the prototype was referred to as the “Dash 80”. Design study numbers tend to be sequential since they are assigned by engineering as the need to document a project for future reference arises. 

Although there was a third phase in the original 1945 Navy contract with Douglas that resulted in the D-558s, it was for a full-scale mockup of an operational jet fighter, not a third research aircraft type. As it turned out, the mockup requirement was cancelled and the aircraft requirements and specifications contained in the first two phases were considerably revised. The D-558-3 designation has persisted, however, if for no other reason than the direct lineage back to the D-558-1 and –2 research aircraft programs of the follow-on work that did not result in a new contract.

The last of the three D-558-2 Skyrockets was completed in 1949. By November 1953, one of them had been flown to the highest altitude and highest speed that the type was to achieve, both records at the time but soon to be bettered during X-1 flights. The only follow-on high-speed rocket aircraft program at the time was the troubled X-2, and while it would eventually exceed 100,000 feet and Mach 3, the USAF, U.S. Navy and NACA were all interested in going much higher and faster.

In 1954, the Office of Naval Research contracted with Douglas for a predesign study of the next generation of rocket-powered research aircraft, to determine what altitudes were attainable (1,000,000 feet was desired) and recommend design and test requirements. The resulting design has been informally referred to as the D-558-3, in recognition of it being the next Douglas high-performance research aircraft, but was in fact designated by Douglas as their Model 671.

It bore a family resemblance to both D-558 types, combining a straight wing and empennage similar to the –1’s with a fuselage like the rocket powered –2’s. Relatively little detail design definition was provided for the aircraft, however, since it wasn’t required for the purpose of the study. Only enough detail was created to communicate the design concept and make approximate weight, performance and program cost projections. The report stated that configuration changes would likely result as the design evolved.

Among other things, Douglas concluded that while climbing to 1,000,000 feet was doable, pulling out of the subsequent dive was not, the profile closely resembling that of a lawn dart thrown almost vertically. They projected a maximum altitude of 700,000 feet was achievable and on a different flight profile, a maximum speed on the order of Mach 7, assuming success with evaluation of temperature-resistant materials. Other conclusions closely described the actual X-15 program activity.

The USAF and NACA were also considering manned, rocket-powered, hypersonic aircraft projects, the former a Super X-2 and the latter, what eventually became the X-15. Of the three projects, the USAF was the most modest in performance goals and the Navy, the most ambitious but focused on altitude with speed as a literal fallout. The NACA project was intent on achieving Mach 7.

In December 1954, the NACA, Air Force, and Navy agreed to undertake joint development of the proposed hypersonic research aircraft. The performance goals were 250,000 feet in altitude and 6,600 feet per second, roughly Mach 6.7. In January 1955 it received the designation X-15. That same month, the Air Force (which administered the design and construction phases of the project) held the first briefings for potential contractors. This culminated in a competition involving Bell, Douglas, North American, and Republic, their proposals being submitted on 9 May 1955.

Although the design requirements had not changed significantly from the ONR study and the same engine was used, the Douglas proposal only superficially resembled the Model 671 and was given a different Model Number, 684. It was somewhat bigger in every dimension, with the horizontal tail relocated and a ventral fin added, and much greater detail was provided than for the Model 671. The engine remained the same.

The Navy’s George Spangenberg was very impressed by the Douglas proposal for the X-15 program, particularly the innovative way that they met the airframe heating problem with a light weight structure, using magnesium instead of the expected steel. The Model 684 was either a close second, tied for first, or first, depending on how the evaluation data was added up and which agency was polled. However, North American used the preferred basic structural approach, a steel derivative, and was announced the winner on September 30, 1955.