Design Reference X-15 Proposal

[mbauer]

While researching the X-15, I created a word document about the companies who submitted proposals for the NACA X-15 request.

Really interested in what others know as well, please add more to this thread. Really want to know more about this incredible aircraft!

Here is what I found:

The Competition = NACA Proposal for X-15 Rocketplane:

The following is paraphrased from a free downloadable PDF put together by NASA: X_15_Frontier_of_Flight-Ebook available here: Aeronautics e-Books | NASA

The Beginning
Hugh Dryden sent a letter on 4 May 1954, to Lieutenant General Donald L. Putt at Air Force Headquarters writing that the NACA wanted a new manned hypersonic research aircraft. The letter suggested a meeting between the NACA, Air Force Headquarters, and the Air Force Scientific Advisory Board to discuss the project. Putt responded favorably and recommended inviting the Navy as well. The general also wrote that "the Scientific Advisory Board previously did some research in this area and has formally recommended that the Air Force initiate such a program.

The Office of Naval Research (ONR) announced at the meeting that Douglas Aircraft Company had already investigated a manned vehicle capable of achieving 1,000,000 feet altitude and very high speeds.

Douglas Aircraft Company D-671 (D-558-III) Program

The "High Altitude and High Speed Study" by the El Segundo Division of the Douglas Aircraft Company, is overlooked in how insightful it was regarding many of the challenges that would be experienced by the X-15 a few years later.

D-671 Specs:
Length = 41.25 ft Take Off Weight = 22,200 lbs
Wingspan = 18 ft Wing Area = 81 ft2
Proposed Thrust = 50,000 lbf
Performance was almost equal to X-15 performance.
Looked very similar to the D-558-II Skyrocket


First Steps:
Approval for the first formal NACA research authorization was given on 21 July 1954. Covering tests of an 8-inch Langley model in the 11-inch hypersonic tunnel to obtain 6-component, low-angle-of-attack and 5-component, variable-angle-of-attack (>50 degrees) data up to 6.86 Mach.

After this and other research, NACA decided on a proposal.

NACA Proposal:
5 October 1954 NACA passed Resolution recommending the construction of a hypersonic research aircraft.

Interested companies were asked to attend a bidders' conference on 18 January 1955

Anticipated Cost? $12.2 million




The Engine Situation Was Complicated:

Power Plant Laboratory said bidders could only use the Aerojet XLR73, Bell XLR81, North American NA-5400, and Reaction Motors XLR10 as engines for the airframe.

-Aerojet XLR73-AJ-1 = single thrust chamber that used white fuming nitric acid and jet fuel as propellants. The engine developed 10,000 lb at sea level, but a new nozzle upped it to 11,750 lbf. The engine was re-startable in flight and infinitely variable between 50% and 100% thrust.

A cluster of several engines was needed to provide the thrust needed for the new research airplane. At the time the Power Plant Laboratory recommended the engine, a first flight was scheduled for April 1956.

- Bell XLR81-BA-1, (Hustler engine), was part of Project MX-1964—the Convair B-58 Hustler. The B-58 carried its nuclear weapon in a large external pod, and the XLR81 provided the pod with extra range after it was dropped from the B58. A new design based on the engine used in the GAM-63 RASCAL missile.

One thrust chamber used red, fuming nitric acid and jet fuel to produce 11,500 lbf at sea level and 15,000 lbf at 70,000 feet. Sufficient thrust for the hypersonic research airplane would come from a cluster of at least three engines.

The existing XLR81 was not throttleable or restartable in flight.

When the Power Plant Laboratory recommended the engine, a first flight was scheduled for January 1957

- NA-5400 had little to offer the program. North American basis was for component development. If they had, It developed 5,400 lbf at sea level

The turbopump assembly was thought capable of supporting engines up to 15,000 lbf, The power plant proposed for the research rocketplane consisted of three separate engines arranged as a unit.

The engine was restartable in flight using a catalyst ignition system. The propellants were hydrogen peroxide and jet fuel, with the turbopump driven by decomposed hydrogen peroxide
The Reaction Motors XLR10 Viking engine, although Reaction Motors had already abandoned further development; went with more powerful XLR30 "Super Viking" derivative.
Exists: the XLR10 produced 20,000 lbf at sea level using liquid oxygen and alcohol propellants.
The XLR30 under development produced 50,000 lbf using liquid oxygen and anhydrous ammonia. T
The Power Plant Laboratory wanted to connect two XLR10 thrust chambers to a single XLR30 turbopump, thinking this took better advantage of well-developed components = lowered the risk.

As designed, the engine was not throttleable or restartable in flight, or was man-rated

Proposals:
The airframe proposals from Bell, Douglas, North American, and Republic arrived on 9 May 1955.

Two of the bidders chose the Bell XLR81 engine, and the other two went with the Reaction Motors XLR30.

Bell D-171:
Bell decided the Bell-manufactured XLR81 was the engine; however, the XLR30 had certain advantages, so Bell proposed an alternative D-171B variant using this power plant.

The design had three XLR81s arranged in a triangular pattern with one engine mounted above the others.

A throttle lever was moved increasing engine thrust by actuating switches as the pilot shoved the lever forward in a conventional manner.

The D-171 weighed 34,140 lbs at launch (21,600 lbs of fuel). The landing weight = 12,595 pounds.
A launch at Mach 0.6 and 40,000 from a B-36
The hoped for maximum altitude during the "space leap" = 400,000 feet. At altitudes between 85,000 & 165,000 ft, the velocity was in excess of 6,600 fps, (max of 6,850 fps @ 118,000 ft).

Thrust Reaction controls used 8-hydrogen peroxide (H2O) thrusters: 1 pointed up / another down at each wing tip for roll control, 1 up and 1 down at the tail for pitch control, and 1 pointing left and 1 right at the tail for yaw control. A single control stick in the cockpit gave the pilot control of the thrusters and aerodynamic control.

Bell selected a conventional tricycle arrangement (a nose wheel and two main skids located midway on the fuselage). The nose gear and skids were retractable and covered with doors, unlike the X-15 where the rear skids do not retract inside the fuselage.

Bell did not believe the hot-structure data provided by NACA. Available materials showed that Inconel X was the best available high-temperature alloy for a conventional structure (same reached by Langley)[Hot-Structure].
Bell guessed that an Inconel X airframe would weigh 180% more than an equivalent structure of aluminum 75S-T.

Bell wanted to use what they called semi-conventional structures. In addition to absorbing the heat load, the structures would be free to warp and bend as they heated.

Bell also investigated actively cooled structures, such as the "water wall" concept. The structure weighed little more than a conventional aluminum airframe, but resulted in the concept being 200–300% heavier.

Bell looked at a structure protected by external insulation and concluded that ceramic materials would work for insulation, however; development is not well enough advanced

Bell used an unique double-wall structure using air as an insulator, permitting heat transfer by radiation in addition to conduction.

Douglas Model 684:
The Model 684 was a conceptual follow-on to the successful Skyrocket
series that Douglas built under Navy sponsorship since 1944. Model 684 benefited from the experience Douglas learned investigating the Model 671.

Douglas took a unique approach to designing the structure of the Model 684, somewhat following the hot-structure concept developed at NACA Langley, but adding several new twists. The most obvious was that instead of Inconel X, Douglas chose a magnesium alloy "of sufficient gage that the structure [sic]
temperature will not exceed 600°F." The use of copper for the leading edges permitted temperatures approaching 1,000°F. All of the proposed structure could be manufactured using conventional methods.

The Model 684 weighed = 25,300 pounds fully loaded / landing weight of 10,450
lbs, making it the lightest proposal.

One Reaction Motors XLR30 exceeded performance specifications, max 6,655 fps velocity @ 110,000 feet altitude. Douglas said it appeared "possible to explore altitudes up to 375,000 feet without exceeding the structural limits.
Douglas proposed landing gear consisted of two main wheels, a nose wheel, and a tail wheel. The nose gear was located behind the cockpit, main gear retracted into the wings. The ventral stabilizer housed the tail wheel, which was needed due to the relatively high approach attitude.

Douglas chose the wedge principle and used the shape for the vertical and horizontal stabilizers. Douglas flared the aft fuselage for more stability at high Mach numbers.
Douglas used a reaction control system with 12 hydrogen peroxide thrusters, two in each direction about each axis. A backup system provided (hence the two thrusters at each location), both systems are capable of maneuvering the aircraft.

The Model 684 was light enough that a Boeing B-50 Superfortress could carry it aloft.

Republic AP-76:
Heavyweight of the competitors, with a launch weight of 39,099
pounds. Republic expected the design to exceed the speed specification at 6,619 fps, it fell somewhat short of the altitude requirement at only 220,000 ft.

Republic used the XLR81-BA-1 engines, same choice as Bell, the AP-76 used 4 of them. Each engine produced 14,500 lbf, a total of 58,000 lbf @ 40,000 feet.

Fuel called JP-X that consisted of 40% unsymmetrical dimethylhydrazine (UDMH) and 60% jet fuel was used as propellant.
The oxidizer was red, fuming nitric acid. The combination was hypergolic, so no ignition system is needed.

Republic submerged the pilot inside the fuselage. 3-glass panels provided side vision from launch until the airplane has descended to 25,000 feet. When the AP-76 has slowed to Mach 0.7, a hatch is raised 13 degrees at its leading edge to expose a mirror system that for forward vision during approach and landing.

Republic chose this system "because of protecting the pilot from the high temperatures and, if need be, from cosmic radiation.

With the Inconel outer skin reaching up to 1,200°F, the interior titanium structure
should never exceed 300°F. (Cold Structure)

Republic did not insulate the wing structure, designing it to carry the design flight loads at high temperatures without developing high thermal stresses.

The reaction control system used six 90-lbf thrusters (1 on each wing tip and 4located at the rear of the fuselage). The thrusters are linked to the same control column that the aerodynamic controls used, and a switch in the cockpit activates them when needed.

The landing gear was two main skids and one tail skid. The main skids,mounted on the side of the fuselage bottom just before of the center of gravity, use pneumatic shock absorbers. Just before touchdown, the tail skid automatically extends when the pilot drops the ventral stabilizer.

Republic chose a Convair B-36 bomber as the carrier aircraft due to the high weight of the AP-76.

High-Speed Flight Profile:
Began with the airplane being carried aloft by a B-36. The B-36 would drop the AP-76 at a predefined release point, 540 miles from Edwards, at an altitude of 40,000 feet and a true air speed of 350 knots. The AP-76 pilot would fire all 4-rockets, pulling into a 20-degree angle climb, running out of fuel 105 seconds after engine start, near 140,000 feet.

The AP-76 would free-flight trajectory to 220,000 ft @t 69 seconds after burnout. Climbing through 100,000 ft, the pilot will turn on the switch for the reaction control system.

The rocketplane would continue descending.

Around 150,000 ft the aerodynamic controls start to regain effectiveness.

Descending through 25,000 ft @ Mach 0.7, the pilot jettisons the ventral stabilizer, and raises the hatch for forward visibility. Finally ending the testing by sliding to a stop on its' skids at Rogers Dry Lake.

North American Aviation (NAA) ESO-7487:

North American truly understood what the government was trying to do with the project. Douglas, who otherwise came closest—worked at designing an airplane that met the performance requirements.

North American, "determined that the specification performance can be obtained with very moderate structural temperatures; however, the airplane has been designed to tolerate much more severe heating in order to provide a practical temperature band within which exploration can be conducted."

Wing leading edges could reach 1,400°F during extreme conditions, well beyond the ability of Inconel. To allow this, the company proposed to use a laminated glass cloth that would "melt or burn locally during these extreme cases." Protecting the structure. Totally replaceable after each flight.

One aspect of the NAA design was the large fuselage side fairings used to carry propellant lines, control cables, and wiring outside around the inner propellant tanks.

Bell, did not believe that a hot structure was compatible with integral propellant tanks, North American chose such an arrangement from the beginning.

Engineers decided that conventional longerons and stiffeners would lead to temperature gradients causing the structure to warp or fail. Thick Inconel skins cover a simple Inconel X structure for the NAA submittal.

A feature that was the subject of some debate after the contract was awarded was the use of all-moving "rolling" horizontal stabilizers instead of conventional ailerons and elevators.

Reaction Motors XLR32-RM-2 thrusters (four 90-lbf units in the nose, and one 17-lbf thruster at each wing tip). Unlike the other competitors that used the same control stick for the aerodynamic and reaction systems, North American used a separate lever on the right console.

North American chose a B-36 mostly because the only other available aircraft—the Boeing B-50 Superfortress could not lift the X-15 above 25,000 ft.

Landing gear uses two strut-mounted skids that retracted against the outside of the fuselage beneath the wing leading edge and a 2-wheel nose gear located forward.

Following Table Shows How They Compare:


Please add other 3-Views etc if you have some. Detailed drawings would be great.

Moderators: Better place for this?

Mike