The Republic Proposal

Republic also seemed at a disadvantage in the X-15 competition, for many of the same reasons North American was. However, the company was working on a Mach 3+ interceptor, the XF-103, and had developed the first supersonic combat-type aircraft, the experimental XF-91. With the XF-91, the company had gained experience in integrating a liquid-fueled rocket engine into a manned aircraft. The XF-103 was providing a wealth of experience (most of it unhappy), including information concerning the effects of high-speed heating on aircraft structures. In addition, Republic had Alexander Kartveli, one of the most innovative aircraft designers in the world.11271

The Republic AP-76 was the heavyweight of the competitors, with a launch weight of 39,099 pounds. Nevertheless, Republic expected the design to exceed very slightly the speed specification at 6,619 feet per second, although it fell somewhat short of the altitude requirement at only 220,000 feet.11281

Like Bell, Republic opted for XLR81-BA-1 engines, although the heavyweight AP-76 used four of them. Each of the engines produced 14,500 lbf, so a total of 58,000 lbf was available at 40,000 feet. Republic justified their choice by noting that "a sacrifice in weight was made in order to use these four units in place of a single thrust chamber engine. The increased safety of numbers as well as the increased reliability of starting one or more units influenced this choice." The engines used a fuel called JP-X that consisted of 40% unsymmetrical dimethylhydrazine (UDMH) and 60% jet fuel. The oxidizer was red, fuming nitric acid. The combination was hypergolic, so no ignition system was required. The thrust line of each engine chamber passed through the center of gravity of the airplane, eliminating any directional component of single – or multiple-chamber operations.12^

A switch panel at the normal throttle location on the left console controlled the engines, based on experience gained on the XF-91 interceptor. The XF-91 had both switches and a conventional throttle quadrant, but the pilots preferred using the switches. A fixed handgrip next to the switches ensured that the pilot’s hand would be near the switches at all times. There were nine two-position switches on the panel: a "master arm" switch, four individual "arm" switches, and four "on" switches. Igniting varying numbers of the engines varied the thrust, just as it had on the X-1 and D-558. Republic did not seem to incorporate the ability to use the "half-thrust" feature of the XLR81.130

Much like the XF-103, Republic eliminated the conventional canopy enclosure and submerged the pilot inside the fuselage. Three glass panels on each side of the fuselage provided side vision from launch until the airplane had descended to approximately 25,000 feet. Once the AP-76 had slowed to Mach 0.7, a hatch on the upper surface of the cockpit raised 13 degrees at its leading edge to expose a mirror system that provided forward vision during approach and landing. The system used two mirrors—one in the front of the hatch reflected an image downward to a second mirror on top of the instrument panel. The pilot looked at the second image. This system was similar to the one that had been developed for the XF-103 and had received favorable comments from the pilots during simulations. Surprisingly, the system offered good depth perception and minimal loss of brightness. Republic chose this unique system "because the problem of protecting the pilot from the high temperatures and, if need be, from cosmic radiation in a [conventional] canopy arrangement seem almost impossible." The cockpit and forward instrument compartment used gaseous nitrogen to maintain 40-100°F at a 5-psi differential, while the aft compartment had a 2.5-psi differential.131

The Republic AP-76 was large, heavy, and although Republic indicated it could slightly exceed the velocity requirements, it fell about 15 percent short of the desired altitude capability. In reality, very few believed it could attain the performance numbers generated by Republic, especially given the weight gains that seem to occur during any development exercise. The Republic entry placed last in the evaluation. (Republic Aviation)

To assist the pilot in flying the predetermined trajectory, Republic proposed installing a "flight program indicator." This display presented the pilot with a second-by-second trace that showed the proper speed, altitude, angle of attack, and path angle during powered flight. The pilot simply guided the airplane to match the cues on the display. It would have been a useful tool.-1132!

Normal Air Force fighter standards (+7.33/-3.00 g at burnout weight, but a great deal lower at full gross weight) provided the structural requirements for the AP-76, in contrast to the other competitors that only stressed their designs for +5 g. To accomplish this, and to withstand the expected heating environment, Republic proposed a novel structure for the fuselage. The main structure consisted of longitudinal titanium "Z" stringers. The structural titanium skin attached to the inner leg of the stringers, and the outer leg held a series of 0.020-inch-thick corrugated Inconel X shingles that formed a heat shield. The corrugations were very mild, with a 0.08 depth – to-length ratio, and permitted circumferential growth resulting from high transient temperatures. In between the heat shield and inner skin were 0.5-inch-thick blocks of Marinite insulation made by the Johns Manville Company. The 2-foot-wide Inconel outer skin sections stretched over three frames and used elongated attachment holes that allowed the sheets to expand and contract without warping. With the Inconel outer skin at its full 1,200°F, the interior titanium structure would never exceed 300°F.[133]

of the propellant tanks, and storage bottles were located below the wing carry-through structure. To the rear of the second nitric acid tank was the JP-X tank. The titanium oxidizer and fuel tanks were an integral part of the fuselage, but because nitric acid reacts with titanium at elevated temperatures, the acid tanks had removable aluminum liners.[134]

The trapezoidal wing used a slightly rounded leading edge with a flat airfoil between the 20% and 80% chord lines and a blunt trailing edge. Unlike the fuselage, Republic did not attempt to insulate the wing structure, and designed it to carry the design loads at elevated temperatures without developing high thermal stresses. The wing used three main sections: 1) the main wing structure, 2) the leading edge, and 3) the trailing edge, which consisted of a conventional single-slotted landing flap and a conventional aileron. The primary load-carrying structure was a tapered multi­cell box that ran from tip to tip and attached to the fuselage at four points (two per side). Intermediate spars were located on 5.5-inch centers with 15 spars at the root and four at the tip. The Inconel X skins were on average 0.10 inch thick. The leading edges were made of kentanium (a titanium carbide alloy) castings segmented into six parts per wing.[135]

The vertical and horizontal stabilizers were "of conventional size made possible by the use of double wedge type sections with rounded leading edges." The included angles were 10 and 12 degrees, respectively. The horizontal surfaces were all moving, but the airplane used conventional ailerons instead of the differentially moving horizontals found on the North American design. The vertical surfaces consisted of a dorsal stabilizer and a jettisonable ventral stabilizer. Wind-tunnel data from the XF-103 provided data for the rudder design, although the overall shape was different. The rudder consisted of the upper 46% of the surface and the entire trailing edge aft of the 70% chord line. Spilt flaps, consisting of the trailing 30% and 35% of the vertical and horizontal stabilizers, respectively, opened through a maximum angle of 50 degrees to increase drag and reduce the speed of the aircraft during reentry. Like the ailerons, these split flaps were each divided into three sections to permit operation while under thermal stresses. The stabilizers were generally of the same construction as the wings, and, like the wing, the leading edges of the empennage were made of cast kentanium.[136]

The landing gear consisted of two main skids and one tail skid. The 48-by-5-inch main skids, installed externally on the side of the fuselage bottom just ahead of the center of gravity, extended 18.5 inches using pneumatic shock absorbers. Just before landing, the tail skid automatically extended when the pilot jettisoned the ventral stabilizer. The landing gear could accommodate descent velocities of only 6 feet per second, considerably less than the 9 fps that tactical aircraft were design to absorb. The rationale was that "highly experienced pilots only are expected to fly this airplane." In fairness to Republic, the NACA had conducted an analysis of earlier research airplane landings and found that the majority were well below the 6-fps figure.-113^

Two hydrogen peroxide auxiliary power units each drove an alternator and hydraulic pump. A 60- gallon supply of the monopropellant could drive the power units for 30 minutes and operate the reaction control system for 3 continuous minutes. The reaction control system used six 90-lbf thrusters (one on each wing tip and four at the rear of the fuselage). Republic linked the thrusters to the same control column that the aerodynamic controls used, and a switch in the cockpit activated them when necessary. At the time of the proposal, the thrusters were throttleable, but Republic noted that "studies of a ‘bang-bang’ system, that is ‘full-on’ or ‘full-off…appear very promising."[138]

AP-76. "Consideration was given to the use of a pilot’s escape capsule in the AP-76. It was found to be extremely difficult to design a capsule which would have the necessary stability characteristics in the low density air of the high altitudes attained by the AP-76." Similarly, Republic found it was almost impossible to provide drag devices that would retard the capsule’s descent to the degree necessary to prevent excessive skin temperatures. In its place was an escape seat with leg (but no arm) restraints; the pilot would rely on his partial-pressure suit for protection during ejection.-1139!

Not surprisingly, given the weight of the AP-76, Republic chose a Convair B-36 bomber as the carrier aircraft. Republic had some experience in using the B-36 since the company manufactured the RF-84K parasite fighter used in the FICON project. The AP-76 was sufficiently large that it took up the majority of all four B-36 bomb bays. The lifting frame and main attach points were mounted on the B-36 wing box and attached to the AP-76 on top of the fuselage over its wing. It was necessary to modify two main bomb bay frames on the B-36 to clear the research airplane, and to add sway braces to "suitable strong points on the lower longerons of the bomb bay truss." A fairing with a soft gasket sealed the bomb bay when the B-36 was carrying the AP-76.!140!

Unfortunately, Republic appears to have misread the intentions of NACA and the Air Force, and its proposal stated that "the achievement of the speed [6,600 fps] is paramount whereas flight at very high altitudes has a secondary role." Because of this, Republic concentrated on designing an aircraft that would be capable of meeting the velocity requirement, while ignoring the altitude requirement to some degree. Although the proposal listed 220,000 feet as the maximum altitude of the aircraft, other data submitted with the proposal indicated that the company believed the aircraft could achieve almost 300,000 feet if necessary.-141!

The typical high-speed flight profile for the AP-76 began with the airplane being carried aloft by a B-36H142 carrier from Edwards AFB. The research airplane pilot would be riding in the comparative comfort of the pressurized compartment of the bomber. The B-36 would carry the AP-76 to a predefined release point approximately 540 miles from Edwards and launch the airplane at an altitude of 40,000 feet and a true air speed of 350 knots. After the AP-76 dropped clear of the B-36, the pilot would ignite all four rocket engines and pull into a 20-degree climb before running out of propellants after 105 seconds at approximately 140,000 feet. The AP-76 would then continue a free-flight trajectory to a peak altitude of 220,000 feet about 69 seconds after burnout. During the climb through 100,000 feet, the pilot would activate the switch that armed the reaction control system; thereafter, the movement of the control column and/or rudder pedals would activate the thrusters in addition to the now-useless aerodynamic controls.

The airplane would continue on a ballistic trajectory until it reached an altitude of 150,000 feet, where the aerodynamic controls would regain effectiveness. The airplane would go through a series of pull-ups and glides while the pilot maintained the angle of attack at a constant 6 degrees. The speed brakes on the horizontal and vertical stabilizers would open as needed. When the descent reached 25,000 feet and the speed reduced to Mach 0.7, the pilot would jettison the ventral stabilizer since it was no longer required for directional stability, and raise the hatch to expose the mirror system to provide forward visibility. Finally, the airplane would glide to a landing on its skids on Rogers Dry Lake.143

The Republic approach to the required two-seat engineering study was a little different from and decidedly more useful than the other proposals. All of the other competitors had simply deleted all of the research instrumentation and installed accommodations for an observer, although North American, at least, had provided a proper canopy arrangement. Republic, however, stretched the constant-section of the fuselage just ahead of the forward propellant tank by 29 inches. On the single-seat aircraft, two compartments held the research instrumentation (550 pounds ahead of the pilot and 250 pounds behind the pilot). For the two-seat airplane the 250 pounds in the rear compartment were deleted, and, combined with the 29-inch extension, this provided a full-size cockpit for the observer. The airplane could still carry the other 550 pounds of instrumentation – in fact, it was the only proposed two-seat aircraft that could carry any. The empty weight of the airplane increased 380 pounds and the launch weight increased 610 pounds, resulting in a degradation of performance of 170 fps.-1144

Comparison of Physical Characteristics
	Bell	Douglas	NAA	Republic
D-171	Model 684	ESO-7487	AP-76
Fuselage:
Length (feet):	44.42	46.75	49.33	52.58
Frontal area (square feet):	25.00	21.00	?	?
Maximum diameter (feet):	5.15	5.16	4.50	5.00
Fineness ratio:	8.62	9.06	?	10.5
Wing:
Airfoil:	biconvex (mod)	Clark Y (mod)	66005 (mod)	hexagonal
Span (feet):	25.67	19.50	22.36	27.66
Root section (percent):	5.0	7.0	5.0	5.0
Tip section (percent):	6.0	4.5	1.0	7.5
Root chord (feet):	13.16	10.40	10.80	16.00

Tip chord (feet):	3.86	2.75	3.00	2.25
Area (square feet):	220.0	150.3	200.0	254.0
Flap area (square feet):	15.25	14.44	?	28.80
Aileron area (square feet):	16.00	9.88	n/a	15.80
Angle of incidence (degrees):	0	0	0	0
Dihedral (degrees):	0	0	0	0
Aspect ratio:	3.00	2.53	2.50	3.00
Taper ratio:	0.30	0.22	?	0.14
Aileron deflection (degrees):	15	20	n/a	+ 17/-12
Flap deflection (degrees):	-45	-45	-40	-38
Leading-edge sweep (degrees):	37.0	40.0	25.0	38.4
MAC (inches):	112.50	105.26	123.23	130.87
Horizontal Stabilizer:
Airfoil:	biconvex (mod)	5° wedge	66005 (mod)	10° wedge
Span (feet):	13.75	11.83	17.64	15.70
Root chord (feet):	7.05	7.66	7.02	7.08
Tip chord (feet):	2.11	1.66	2.10	1.83

Area (square feet):	63.00	55.20	51.76	69.70
Aspect ratio:	3.00	2.54	2.81	3.48
Taper ratio:	0.30	0.22	0.22	0.26
Leading-edge sweep (degrees):	35.5	40.0	45.0	22.3
Deflection (degrees):	+ 10/-20	+ 5/-20	+ 15/-45	+ 7/-20
	Bell	Douglas	NAA	Republic
D-171	Model 684	ESO-7487	AP-76
Dorsal Stabilizer:
Airfoil:	biconvex (mod)	diamond (mod)	10° wedge	12° wedge
Area (square feet):	45.30	39.25	38.14	47.60
Rudder area (square feet):	13.5	7.85	?	32.0
Aspect ratio:	0.8	1.277	1.25	1.6
Leading-edge sweep (degrees):	45.0	40.0	52.0	27.9
Rudder deflection (degrees):	20	30	45	20
Ventral Stabilizer:

Airfoil:	10° diamond	7° edge	15° wedge	10° wedge
Area (square feet):	22.70	12.08	11.42	12.30
Leading-edge sweep (degrees):	45.0	60.0	52.0	45.0
Weights:
Launch (pounds):	34,140	25,300	27,722	39,099
Burnout (pounds):	12,942	10,600	10,433	15,300
Landing (pounds):	12,595	10,450	10,200	14,800
Empty (pounds):	11,964	9,208	9,959	14,388
Propellants (pounds):	21,600	14,700	16,410	23,660
Propulsion:
Number of engines:	3	1	1	4
Engine type:	XLR81	XLR30	XLR30	XLR81
Total thrust (lbf):	43,500	57,000	57,000	58,000
Fuel type:	JP-X	NH3	NH3	JP-X
Fuel quantity (gallons):	704	1,142	1,239	710
Oxidizer type:	RFNA	LOX	LOX	RFNA
Oxidizer quantity (gallons):	1,358	816	907	1,430

Jl________________ II________________ II_____________ II Performance (estimated): Maximum speed (fps): 6,850 6,655 6,950 6,619 Maximum altitude (feet): 400,000 375,000 800,000 220,000 Cost and Schedule: R&D plus three aircraft (millions): $36.3 $36.4 $56.1 $47.0 Estimated First flight: Jan. 59 Mar. 58 Nov. 57 Feb. 58