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Aviation History
1964
1964 - 1917.PDF
PERFORMANCE MANUAL SELECTOR^ ^SWITCHES 1060 FLIGHT International, 25 June 1964 Inlet control system (inlet in plan view, system as block diagram): A, Mach transducer; B, static-pressure transducer; C, computers; D, dual hydraulic actuators; £, electric input; H, hydraulic input B-7O... Powerplant The XB-70A is powered by six General Electric YJ93-3 turbojet engines. Sea-level static thrust for the J93 is "in the 30,0001b range"; this includes approximately 34 per cent afterburner power, since reheat is in continuous operation. The engine is 237in long, 52.5in deep and 42in across the intake. Thrust/weight ratio is 6 : 1. The compressor is of "moderate pressure ratio" variable-stator, single-rotor construction. The compressor casing is split for in- spection and maintenance, as is the casing over the two-stage turbine. The latter has air-cooled blades that permit operating temperatures "several hundred degrees higher" than on previous engines. "Special materials" are also an advertised feature. The afterburner has a variable-area converging/diverging exhaust nozzle. NAA claims that, with one engine out, the XB-70A could continue Mach 3 cruise with a loss of about 7 per cent in range. All six engines are interchangeable in what is described as a "plug-in" feature where one engine can be removed and a replace- ment installed in 25min. Engine controls and accessories are packaged in a removable pod suspended under the compressor section. The engines have automatic thrust control, since cables would present severe problems. A secondary power-generating subsystem, made by Sundstrand Aviation, Rockford, Illinois, consists of a gearbox driven by a power takeoff protruding forward from below the intake of each engine. Spaced around the gearbox are a constant-speed drive and 60kVA generator, a primary-system 95 US gal/min hydraulic pump, a speed switch, the gearbox and a 57 US gal/min hydraulic-reservoir pump for the utility system. All aircraft systems and components are powered from the six subsystem packages. The hydraulic- systems drive requirement alone totals 2,000 hydraulic horsepower. Secondary systems are discussed in Part 2 of this article. General Electric at Evendale, Ohio, had the YJ93-3 flight-rated at the time the XB-70A was first supposed to fly, more than 18 months ago. Today these engines have accumulated some 5,000hr in tests. The 68hr Preliminary Flight Rating Test actually took place in 1961, so the tests since that time have been chalked up to a "flight support programme." The original development schedule called for J93 flight time at Mach 2 in a pod slung below a B-58. This part of the programme was killed long before it got off the ground, for reasons of economy. The J93 is a Mach 3 engine, and at off-design conditions is pro- gressively inefficient. At Mach 2, for example, the combination of downgrading aerodynamic and engine performance would cost the XB-70A some 15 per cent range. As part of the engine test pro- gramme General Electric built a Mach 3 high-altitude wind tunnel. In this tunnel and at Arnold Engineering Development Center's high-speed tunnel at Tullahoma, Tennessee, the YJ93-3 accumu- lated 660hr between Mach 2 and Mach 3. The engine inlet-system test programme conducted at Tullahoma accumulated 52hr at high Mach numbers, with 154 starts, 109 engine stalls and over 200 inlet unstarts. The inlet in these tests was built at 0.577 scale with a real engine installed. The J93 is considered too small for the SST pro- gramme, but has contributed greatly to its technology. When the XB-70A is on the ground the engines are cooled by means of auxiliary doors. At Mach 3 there is ample surplus air spilled from the inlet and ducted around the engines for cooling. The infra-red signature of the XB-70A travelling at Mach 3 is suffi- cient to simplify the detection problem from an anti-aircraft missile point of view. While the structure surrounding the engine is rela- tively cool, and the cooling air flow—according to GE engineers effectively cloaks the afterburner, at maximum reheat temperature there is a great streak of hot gases pumped out of each engine for all to detect. Whether the electronic countermeasures and infra-red flares, originally proposed, could have downgraded this problem in an operational version is part of the B-70 debate. Fuel for the engines is JP-6. This is described as a highly refined JP-4 with improved heat-stability and resistance to the formation of solids, achieved by stricter quality control and additives. Those people who object to the engine noise from the present generation of commercial jets will have violent reactions to the XB-70A. While no one is positive what decibel level will be generated by a taxying aircraft, a General Electric expert guesses that it will be from 130 to 140db per engine at a distance of 200ft. The sonic boom will also be considerable; but no one is even guessing at that publicly. Structural Design The high aerodynamic temperatures encountered at Mach 3 virtually preclude the use of aluminium in the XB-70A. Such materials add up to approximately 1 per cent by weight, with stain- less steel (PHI5-7 Mo) honeycomb sandwich construction adding up to 68.9 per cent of the total. North American engineers say they designed with stainless-steel honeycomb-core sandwich struc- tures for several reasons: light-weight, high-strength structures; low-drag aerodynamic smoothness; low heat-transfer from skin at 450° to 640°F; and reliable strength at high temperatures, fatigue resistance and rigidity. Producibility and cost were also once con- sidered prime factors, although more recent studies of producing SST structures at Lockheed and elsewhere have concluded that, although brazed honeycomb sandwich is desirable, a stiffened skin structure can be built with a minimum of new capital equipment at a cost of approximately one-third that of a sandwich structure. Not that a Mach 3 SST design would not use extensive honey- comb sandwich: North American Aviation stick to the contention that the choice of brazed stainless-steel honeycomb was logical, and that the range of the XB-70A—twice that of an SST—means longer heat-soak times and hence primary structure would still have to be of sandwich construction. Total dry weight of the XB-70A airframe is estimated at 150,0001b. Three types of titanium account for 12,0001b (8 per cent of the total), mainly in the forward fuselage. Titanium 6A1- 4V, heat-treated to 160,0001b/sq in u.t.s., is used in thicknesses of 0.030in to 0.070in for the skin of the forward fuselage section, some 60ft long, of conventional skin and stringer design. This section is subjected to aerodynamic heating in the 450° to 500 F range. In- ternal titanium members are made of 4Al-3Mo-lV heat-treated to 170,0001b/sq in. The third type of titanium, 7AI-4Mo, is heat- treated to 170,0001b/sq in u.t.s. A total of 22,000 individual titan- ium parts are contained in the XB-70A, 12,000 of them in the for- ward fuselage. Of the total weight of titanium used, 50 per cent is in sheet, 25 per cent plate from 0.75in to lin thick, and the remainder in heat-treated extrusions and forgings. Titanium plate is used in the canard main box and canard flap. Titanium corrugated spars sine-welded together are used in both the canard and fins. Rene 41 is used in the engine compartment, because of the diffi- cult requirements of high strength at high temperature. The radome (in the lower nose section, housing electronic gear, is of laminated Comparison of majoritypes of structural material: the vertical scale is yield stress divided byldensity divided by 10,000 RELATIVEMATERIAL EFFICIENCY 80 60 40 20 RR-58 2024-T86 MJL 17-7 PH CUES
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