Facing the Heat Barrier - NASA's History Office

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4 First Thoughts of Hypersonic Propulsion Three new aircraft engines emerged from World War II: the turbojet, the ramjet, and the liquid rocket. The turbojet was not suitable for hypersonic flight, but the rocket and the ramjet both gave rise to related airbreathing concepts that seemed to hold promise. Airbreathing rockets drew interest, but it was not possible to pump in outside air with a conventional compressor. Such rockets instead used liquid hydrogen fuel as a coolant, to liquefy air, with this liquid air being pumped to the engine. This arrangement wasted cooling power by also liquefying the air’s nonflammable hydrogen, and so investigators sought ways to remove this nitrogen. They wanted a flow of nearly pure liquid oxygen, taken from the air, for use as the oxidizer. Ramjets provided higher flight speeds than turbojets, but they too had limits. Antonio Ferri, one of Langley’s leading researchers, took the lead in conceiving of ramjets that appeared well suited to flight at hypersonic and perhaps even orbital speeds, at least on paper. Other investigators studied combined-cycle engines. The ejector ramjet, for one, sought to integrate a rocket with a ramjet, yielding a single compact unit that might fly from a runway to orbit. Was it possible to design a flight vehicle that in fact would do this? Ferri thought so, as did his colleague Alexander Kartveli of Republic Aviation. Air Force officials encouraged such views by sponsoring a program of feasibility studies called Aerospaceplane. Designers at several companies contributed their own ideas. These activities unfolded within a world where work with conventional rockets was advancing vigorously. 1 In particular, liquid hydrogen was entering the mainstream of rocket engineering. 2 Ramjets also won acceptance as standard military engines, powering such missiles as Navaho, Bomarc, Talos, and the X-7. With this background, for a time some people believed that even an Aerospaceplane might prove feasible. Ramjets As Military Engines The ramjet and turbojet relied on fundamentally the same thermodynamic cycle. Both achieved compression of inflowing air, heated the compressed flow by burning 91
Facing the Heat Barrier: A History of Hypersonics The X-7. (U.S. Air Force) fuel, and obtained thrust by allowing the hot airflow to expand through a nozzle. The turbojet achieved compression by using a rotating turbocompressor, which inevitably imposed a requirement to tap a considerable amount of power from its propulsive jet by placing the turbine within the flow. A ramjet dispensed with this turbomachinery, compressing the incoming flow by the simple method of processing it through a normal or oblique shock. This brought the promise of higher flight speed. However, a ramjet paid for this advantage by requiring an auxiliary boost, typically with a rocket, to accelerate it to speeds at which this shock could form. 3 The X-7 served as a testbed for development of ramjet engines as mainstream propulsion units. With an initial concept that dated to December 1946, it took shape as a three-stage flight vehicle. The first stage, a B-29 and later a B-50 bomber, played the classic role of lifting the aircraft to useful altitudes. Such bombers also served in this fashion as mother ships for the X-1 series, the X-2, and in time the X-15. For the X-7, a solid propellant rocket served as the second stage, accelerating the test aircraft to a speed high enough for sustained ramjet operation. The ramjet engine, slung beneath its fuselage, provided further acceleration along with highspeed cruise. Recovery was achieved using a parachute and a long nose spike that pierced the ground like a lance. This enabled the X-7 to remain upright, which protected the airframe and engine for possible re-use. 92 First Thoughts of Hypersonic Propulsion The X-7 craft were based at Holloman Air Force Base in New Mexico, which was an early center for missile flight test. The first flight took place in April 1951, with a ramjet of 20-inch diameter built by Wright Aeronautical. The X-7 soon took on the role of supporting developmental tests of a 28-inch engine built by the Marquardt Company and intended for use with the Bomarc missile. Flights with this 28-inch design began in December 1952 and achieved a substantial success the following April. The engine burned for some 20 seconds; the vehicle reached 59,500 feet and Mach 2.6. This exceeded the Mach 2.44 of the X-1A rocket plane in December 1953, piloted by Chuck Yeager. Significantly, although the X-7 was unpiloted, it remained aerodynamically stable during this flight. By contrast, the X- 1A lost stability and fell out of the sky, dropping 51,000 feet before Yeager brought it back under control. 4 The X-7 soon became a workhorse, running off some one hundred missions between 1955 and the end of the program in July 1960. It set a number of records, including range and flight time of 134 miles and 552 seconds, respectively. Its altitude mark of 106,000 feet, achieved with an airbreathing ramjet, compared with 126,200 feet for the rocket-powered X-2 research airplane. 5 Other achievements involved speed. The vehicle had been built of heat-treated 4130 steel, with the initial goal being Mach 3. The program achieved this—and simply kept going. On 29 August 1957 it reached Mach 3.95 with a 28-inch Marquardt engine. Following launch from a B-50 at 33,700 feet, twin solid motors mounted beneath the wings boosted the craft to Mach 2.25. These boosters fell away; the ramjet ignited, and the vehicle began to climb at a 20-degree angle before leveling out at 54,500 feet. It then went into a very shallow dive. The engine continued to operate, as it ran for a total of 91 seconds, and acceleration continued until the craft attained its peak Mach at fuel exhaustion. It was recovered through use of its parachute and nose spike, and temperature-sensitive paints showed that it had experienced heating to more than 600°F. This heating also brought internal damage to the engine. 6 Even so, the X-7 was not yet at the limit of its capabilities. Fitted with a 36-inch ramjet, again from Marquardt, it flew to Mach 4.31 on 17 April 1958. This time the drop from the B-50 came at 28,500 feet, with the engine igniting following rocket boost to Mach 1.99. It operated for 70.9 seconds, propelling the vehicle to a peak altitude of 60,000 feet. By then it was at Mach 3.5, continuing to accelerate as it transitioned to level flight. It reached its maximum Mach—and sustained a sharp drop in thrust three seconds later, apparently due to an engine tailpipe failure. Breakup of the vehicle occurred immediately afterward, with the X-7 being demolished. 7 This flight set a record for airbreathing propulsion that stands to this day. Its speed of 2,881 miles per hour (mph) compares with the record for an afterburning 93
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Facing the Heat Barrier: A History
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Facing the Heat Barrier: A History
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Table of Contents Acknowledgements
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Introduction As an approach to the
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