AKA: Model 671. Status: Operational 1954. Thrust: 222.60 kN (50,042 lbf). Gross mass: 10,000 kg (22,000 lb). Unfuelled mass: 3,200 kg (7,000 lb).
Unfortunately a Rear Admiral Hatcher decided to save the $24 million development cost and let the Air Force develop the less-capable X-15 instead...
Douglas Chief Designer Ed Heinemann recalled the design:
We pressed on at the drafting boards and soon learned we couldn't quite make a million feet but settled on 700,.000. After a year and an expenditure of one million dollars, we produced the report. The aircraft itself was referred to as the Model 671.
An opening statement read, "The altitude performance of the aircraft is very clearly limited by the ultimate ability of the pilot and his associated equipment to withstand the loads required for a safe pullout." From that point on we defined our concept.
The biggest problem we faced was the drastically high temperatures anticipated during re-entry into the atmosphere. If the aircraft's skin was not insulated or cooled properly, it would reach 1,400 degrees F., clearly unacceptable. Structural materials suffer loss of strength with increasing temperature, and 1,400 degrees was beyond the practical limit of known, high-temperature material then available.
Aluminum alloys lost too much strength when overheated; stainless steel was too heavy, and ablative materials were not well-enough known. Finally we decided on a heavy skin of magnesium, three-quarters of an inch thick, which would act as a heat sink and not overheat during the dive.
There was a catch, however. The leading edge would get so hot the magnesium couldn't take it. After an exhaustive study, our engineers came up with a solid copper leading edge. The copper would actually glow in the dark, but prevented the magnesium from reaching a critical temperature limit by the time the dive was completed.
Voila! We were sure we had found the answer to this tough problem.
The plane would be launched from a mother ship traveling 40,000 feet high at Mach .75. Once it had fallen free, the pilot would start the aircraft's rocket engine and pull into a thirty-eight degree climb. The rocket would burn for seventy-five seconds, after which the aircraft would go ballistic into the ionosphere reaching about 700,000 feet. On the way down it would reach hypersonic speeds. possibly up to nine times the speed of sound.
An unswept wing of moderate taper and aspect ratio and conventional plan- form was selected. The plane was forty-seven feet long, had an eighteen-foot span. and measured thirteen feet from the ground to the top of the vertical stabilizer. Its maximum takeoff weight was 22,000 pounds, and the fuel load consisted of 15,000 pounds.
A Reaction Motors Model XLR-30-RM-2 rocket unit with 50,000 pounds of static thrust was in development at the time and was chosen as the power plant. Ammonia, we felt, was the best fuel for the project, with liquid oxygen serving as an oxidizer. Tanks for these were installed internally in the fuselage.
For the pilot's compartment, in addition to the normal material, we planned a two-inch-thick blanket of insulation to protect him from the scorching heat. The windshield enclosure consisted of double layers of one-half-inch-thick transparent quartz, spaced about one-quarter-inch apart.
Conventional aerodynamic surfaces provided stability and control within the atmosphere. Recovery to level flight on re-entry, which should be achieved at ~0,000 feet, was of vital concern. The source of the stability problem at high supersonic speed lay in the fact that the fuselage produced practically constant unstable moments throughout the high Mach number range, whereas the stabilizing moments of wing and tail surfaces decreased steadily as the Mach number increased.
As a result, any given configuration tended toward instability as the Mach. number rose. So the use of automatic stabilization by means of an autopilot with control surfaces of conventional proportions was programmed. Power-boosted controls would be required for the high-speed phase because of the enormous dynamic pressures involved. At the same time we wanted the safety of manual controls for the landing evolution.
Outside earth's atmosphere aerodynamic controls would be useless; we had to devise a method for the D-558-3 to adjust its attitude while traveling through the "airless" ionosphere. On re-entry, for example, the plane would have to penetrate the atmosphere in a relatively clean, or nose-first, fashion. An uncontrolled broad- side or inverted entry would be disastrous from the standpoint of unforeseen aerodynamic loads imposed on the aircraft. The pilot had to have a system for correcting any residual movements resulting from the powered ascent. He also had to have proper orientation of the aircraft during the flight through space.
Since the amount of control needed was small, we decided to use hydrogen peroxide, which would flow under pressure through strategically positioned jets in the tail and wing sections. Controlled by the pilot for very brief periods, the pressure of the hydrogen peroxide would clip a wing or raise the nose accordingly. As it turned out, this control system and the wing-cooling design were two of the key developments to result from the study and to be applied in some form in later years.
Takeoff and landing sites had to be carefully chosen, and the Muroc dry lake area was the most suitable area because of its lengthy runways and the wide latitude it offered in the choice of landing direction. On a typical flight the plane would travel 500 nautical miles horizontally. Since most of this distance would be covered in the ballistic trajectory portion of the flight path, there would be little opportunity to control or alter either the range or the heading by an appreciable amount after burnout. During the seventy-five seconds of powered flight, the pilot would have an opportunity to make necessary corrections to his route through the sky. Admittedly, these corrections had to be small due to the rapid acceleration involved.
Assuming the pilot elected to slow down to subsonic speed at a constant altitude of 40,000 feet (luring the re-entry phase, it would be possible to make no more than six spiral turns before reaching sea level. The maximum allowable miss distance between pullout and landing site was a function of the respective orientation of the two positions and the maximum gliding distance of the aircraft. Miss distance would be much shorter if the airplane overshot the base, because of the necessity of turning around. A misalignment of five degrees azimuth at burnout, for example, could result in a lateral miss of forty nautical miles. A two-and-one- half percent error in fuel consumption, or burning time, might mean a sixty-mile decrement.
It was also essential that the mother ship be accurately positioned and on an exact heading at the time of launch. Obviously, the D-558-3 demanded some very b precise actions by those involved in its actual operation.
Cosmic radiation and special shielding of the pilot against it were discussed at length. Since exposure for short time intervals was not considered a danger and because the amount of materials for effective protection was substantial, we decided against protective measures. Instead, limiting the number of monthly or yearly flights per pilot proved to be the most effective solution to the radiation problem.
The chances of injury to flyers and damage to aircraft due to meteor collisions were also examined. Statistical analysis showed that there was no greater danger in this regard than that faced by ordinary citizens driving an automobile. For a projected area of 225 square feet, the chances of being hit by a meteor capable of penetrating more than .08 inches of aluminum were about one in 450,000 on any one flight.
Ordinary bailout procedures were impossible for the D-558-3 pilots. Escape chutes and ejection seats would not suffice because of the hypersonic speeds involved. The study therefore indicated that there would be no provision for pilot escape. It added, however, that the development of an ejectable cockpit capsule could provide escape, at least over a part of the aircraft's flight route. Initial separation of the capsule would be accomplished by a JATO, or rocket-power, unit The capsule would be streamlined with extendable fins and thus be capable or stable flight. A parachute would deploy at lower altitudes to help complete the escape evolution.