Douglas Model 671/684: The X-15’s Shadow

Model 684

A schematic diagram of the Douglas Model 684. It was submitted to NACA in 1954 as part of the X-15 design competition. Though evaluation suggested it would be the superior suborbital spacecraft, it lost to North American Aviation’s bid. Image from “USAF Project 1226, Douglas Model 684 High Altitude Research Airplane”. Click for a larger view.

What it was: Douglas Aircraft’s 1954-55 attempt at a suborbital spaceplane, with support from the US Navy and eventually NACA, intended for testing high Mach numbers in the atmosphere. Launched from a bomber, it would use a ballistic flight to get as high as 344 kilometers and then use the drop back down into the atmosphere to build up speed.

Details: NASA’s predecessor, the National Advisory Committee for Aerospace (NACA), was devoted to basic aerospace research programs whose results could be used by industry to make better aircraft. By the 1950s hypersonic travel was in the cards and they resolved to develop a research aircraft that could reach Mach 7, solely for the purpose of studying the aerodynamic and heating problems of moving through the atmosphere at that speed. Interestingly, they were not interested in studying spaceplanes or re-entry, as they considered manned space travel something for the 21st century, but the speeds involved were creeping up on those issues regardless of their intentions. With that in mind engineers at their Langley Research Center roughed out a basic design that is recognizably the X-15.

A lot of NACA’s work was done in conjunction with the US Air Force and Navy, partly because they were the groups most interested in cutting-edge aviation and partly because the Department of Defense had a budget roughly 150 times larger than NACA did. Accordingly, basic design in hand, NACA met with the other two organizations on June 11, 1954 to discuss where to go next.

The Air Force had been working with Bell Aircraft—builders of the X-1 and X-2—but the Navy had been working with Douglas Aircraft on two successive planes, the D-558-1 and the D-558-2. At the meeting they revealed that they were in the early stages of getting Douglas to work on what the manufacturer called the Model 671 (informally known as the D-558-3 in years since, though that name was never actually assigned to it).

Unlike the NACA idea, the Model 671 was designed for height as well as velocity. Although the work done on it was still preliminary, Douglas had already come to the conclusion that they could make it reach 1,130,000 feet—or, in more modern terms, 344 kilometers. The International Space Station is actually allowed to drop as low as this before being boosted again, so this is well into space; Douglas did admit that the pilot would probably not survive the G forces of that flight and so recommended nothing higher than 770,000 feet (237 kilometers). The plane’s downrange capability was 850 kilometers for both high and low flights, which is suborbital, but for both in height and distance this is considerably farther than Alan Shepard went in Freedom 7.

Given that NACA and the Air Force were now looking at similar programs, the Navy cancelled the Model 671 and joined up to launch a design competition. On December 30, 1954 twelve contractors were invited; only four came up with proposals, probably because of the risk involved and the minimal profits that would stem from the two airplanes that NACA wanted built. Three of the replies were from Bell Aircraft, North American Aviation, and Republic Aviation.

Douglas replied with the Model 684. Their proposed craft would hit a maximum of 7300 kilometers per hour, and reach heights of 114 kilometers—in other words, they had to tone down the Model 671 just to meet the NACA requirements. Even at that, this is still the edge of outer space: the Model 684 would have been the first suborbital spaceplane.

As it was headed for space, the pilot compartment was completely pressurized, and could carry two if the research instrumentation was removed. Anyone onboard would wear a pressure suit (the X-15 program would actually develop the space suit used by Mercury astronauts), and in case of a dire emergency the entire forward fuselage would cut loose, push away from the main body of the craft on a small jet, and drift down to Earth under a parachute.

The Model 684 would have been lifted to about 30,000 feet by a B-50 Superfortress bomber where it would be dropped. At that point it would have ignited its liquid oxygen and ammonia engine and taken off on a trajectory for either speed or height. After reaching its apogee it would glide back to Earth, eventually landing at a long conventional airstrip at about 300 kilometers per hour.

Like the other proposals this was a “hot structure” craft, which is worth explaining. The Space Shuttle’s fuselage, for example, is built mostly of aluminum. As a result it’s completely incapable of standing up to the heat of re-entry and must be kept cool. In the particular case of the Space Shuttle this was done by covering it with ceramic heat tiles, but other cold structure options include ablative coverings (which the Model 671 would have used) or cooling using some sort of liquid inside the skin that would be allowed to boil off.

A hot structure, on the other hand, approaches the problem head on: build the fuselage out of a material that holds up to high temperatures. NACA had suggested to the design competitors that they might want to look at Inconel X, a nickel-chromium alloy that doesn’t begin to soften until very high temperatures. Three of the bidders took the hint.

The Model 684 would have used HK31, an alloy of magnesium, thorium, and zirconium which is no longer in use since the three percent that is thorium makes the alloy radioactive. At the time its relatively low radioactivity was not considered much of a problem, though, and it had the advantage of being much lighter than Inconel X. This meant that the Model 684’s skin could be much thicker, which would reduce costs and would dramatically increase the heat capacity of the plane and keep it from pushing 1000 Celsius on re-entry. The leading wing edges would be made of copper, which would conduct heat away quickly into the rest of the plane.

The total estimated cost for research and development, then the production of three planes, came to US$36.4 million, with the first flight anticipated by March of 1958.

What happened to make it fail: This one actually came quite close to existing, as it was a strong second in the NACA competition to the North American Aviation ESO-7487; in the formal evaluation it actually outscored its rival 152 to 150. Essentially the decision came down to unhappiness with the choice of the HK31 alloy for its fuselage over Inconel X. As a research craft, they wanted the X-15 to be subjected to the heat of hypersonic travel. Inconel X would go up over 800 Celsius when at the heights and speeds NACA wanted; HK31’s higher heat capacity would have kept the Model 684 to about 300 Celsius during the relatively short flights the X-15 would undertake. It was a better solution if one were just making this aircraft, but not if the whole point was to study high temperatures in flight for future aircraft.

Basically it came down to what NACA was looking to build. They didn’t want a spaceplane, they wanted a regular, if extreme, aircraft. The NAA ESO-7487 may not have been able go as high as the Model 671, but that was OK. In looking to make something that would be relatively easy to develop into something the Navy would want to buy later for service, Douglas was too ambitious for their own good. The ESO-7487 would become the X-15.

What was necessary for it to succeed: North American Aviation actually asked to withdraw from the X-15 competition in October 1955, after it had informally been awarded the contract but before it was official. A slew of new design work had come their way and they no longer thought they could make the 30 month deadline for first flight that the contract would impose.

NACA, the Air Force, and the Navy mulled over two options. Either they could award the contract to the Model 684 if it was switched to an Inconel X skin, or they could give NAA an eight-month extension. They decided on the latter course, but if they hadn’t the Model 684 would have flown.


3 thoughts on “Douglas Model 671/684: The X-15’s Shadow

  1. It’s a pretty similar-looking aircraft – not surprising given the intended flight régime – but clearly the small differences modified the performance a lot. With the Inconel skin, though, would its performance really have been noticeably different from the X-15’s?

    • The major difference between the two alloys seems to have been ease of machining (in favour of HK31), though, not performance, so I imagine swapping one out for the other wouldn’t have made much difference. Answering your question with anything other than a guess bumps up against the limit of my education and what I can find in the record, though.

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