NASA's Contributions to Aeronautics, Volume 1 by National Aeronautics & Space Administration. - HTML preview

CASE

9

Case-9 Cover Image: The X-43A Hyper-X test vehicle drops away from its Boeing NB-52B mother ship in November 2004, beginning its flight to Mach 9.7. NASA.

In aerospace engineering, high-temperature structures and materials solve two problems. They are used in flight above Mach 2 to overcome the elevated temperatures that occur naturally at such speeds. They also are extensively used at subsonic velocities, in building high-quality turbofan engines, and for the protection of structures exposed to heating.

Aluminum loses strength when exposed to temperatures above 210 degrees Fahrenheit (°F). This is why the Concorde airliner, which was built of this material, cruised at Mach 2.1 but did not go faster.[1] Materials requirements come to the forefront at higher speeds and escalate sharply as airplanes’ speeds increase. The standard solutions have been to use titanium and nickel, and a review of history shows what this has meant.

Many people wrote about titanium during the 1950s, but to reduce it to practice was another matter. Alexander “Sasha” Kartveli, chief designer at Republic Aviation, proposed a titanium F-103 fighter, but his vision outreached his technology, and although started, it never flew. North American Aviation’s contemporaneous Navaho missile program introduced chemical milling (etching out unwanted material) for aluminum as well as for titanium and steel, and was the first to use titanium skin in an aircraft. However, the version of Navaho that was to use these processes never flew, as the program was canceled in 1957.[2]

The Lockheed A-12 Blackbird, progenitor of a family of exotic Mach 3.2 cruisers that included the SR-71, encountered temperatures as high as 1,050 °F, which required that 93 percent of its structural weight be titanium. The version selected was B-120 (Ti-13V-11Cr-3Al), which has the tensile strength of stainless steel but weighs only half as much. But titanium is not compatible with chlorine, cadmium, or fluorine, which led to difficulties. A line drawn on a sheet of titanium with a pen would eat a hole into it in a few hours. Boltheads tended to fall away from assemblies; this proved to result from tiny cadmium deposits made by tools. This brought removal of all cadmium-plated tools from toolboxes. Spot-welded panels produced during the summer tended to fail because the local water supply was heavily chlorinated to kill algae. The managers took to washing the parts in distilled water, and the problem went away.[3]

The SR-71 was a success. Its shop-floor practice with titanium at first was classified but now has entered the aerospace mainstream. Today’s commercial airliners—notably the Boeing 787 and the Airbus A-380, together with their engines—use titanium as a matter of routine. That is because this metal saves weight.

Beyond Mach 4, titanium falters and designers must turn instead to alternatives. The X-15 was built to top Mach 6 and to reach 1,200 °F. In competing for the contract, Douglas Aircraft proposed a design that was to use magnesium, whose properties were so favorable that the aircraft would only reach 600 °F. But this concept missed the point, for managers wanted a vehicle that would cope successfully with temperatures of 1,200 °F. Hence it was built of Inconel X, a nickel alloy.[4]

High-speed flight represents one application of advanced metals. Another involves turbofans for subsonic flight. This application lacks the drama of Mach-breaking speeds but is far more common. Such engines use turbine blades, with the blade itself being fabricated from a single-crystal superalloy and insulated with ceramics. Small holes in the blade promote a circulation of cooler gas that is ducted downstream from high-pressure stages of the compressor. The arrangement can readily allow turbines to run at temperatures 750 °F above the melting point of the superalloy itself.[5]

The High-Speed Environment

During World War II the whole of aeronautics used aluminum. There was no hypersonics; the very word did not exist, for it took until 1946 for the investigator Hsue-shen Tsien to introduce it. Germany’s V-2 was flying at Mach 5, but its nose cone was of mild steel, and no one expected that this simple design problem demanded a separate term for its flight regime.[6]

A decade later, aeronautics had expanded to include all flight speeds because of three new engines: the liquid-fuel rocket, the ramjet, and the variable-stator turbojet. The turbojet promised power beyond Mach 3, while the ramjet proved useful beyond Mach 4. The Mach 6 X-15 was under contract. Intermediate-range missiles were in development, with ranges of 1,200 to 1,700 miles, and people regarded intercontinental missiles as preludes to satellite launchers.

A common set of descriptions presents the flight environments within which designers must work. Well beyond Mach 3, engineers accommodate aerodynamic heating through materials substitutions. The aircraft themselves continue to accelerate and cruise much as they do at lower speeds. Beyond Mach 4, however, cruise becomes infeasible because of heating. A world airspeed record for air-breathing flight (one that lasted for nearly the next half century) was set in 1958 with the Lockheed X-7, which was made of 4130 steel, at Mach 4.31 (2,881 mph). The airplane had flown successfully at Mach 3.95, but it failed structurally in flight at Mach 4.31, and no airplane has approached such performance in the past half century.[7]

No aircraft has ever cruised at Mach 5, and an important reason involves structures and materials. “If I cruise in the atmosphere for 2 hours,” said Paul Czysz of McDonnell-Douglas, “I have a thousand times the heat load into the vehicle that the Shuttle gets on its quick transit of the atmosphere.”[8] Aircraft indeed make brief visits to such speed regimes, but they don’t stay there; the best approach is to pop out of the atmosphere and then return, the hallmark of a true trans-atmospheric vehicle.

At Mach 4, aerodynamic heating raises temperatures. At higher Mach, other effects are seen. A reentering intercontinental ballistic missile (ICBM) nose cone, at speeds above Mach 20, has enough kinetic energy to vaporize 5 times its weight in iron. Temperatures behind its bow shock reach 9,000 kelvins (K), hotter than the surface of the Sun. The research physicist Peter Rose has written that this velocity would be “large enough to dissociate all the oxygen molecules into atoms, dissociate about half of the nitrogen, and thermally ionize a considerable fraction of the air.”[9]

Aircraft thus face a simple rule: they can cruise up to Mach 4 if built with suitable materials, but they cannot cruise at higher speeds. This rule applies not only to entry into Earth’s atmosphere but also to entry into the atmosphere of Jupiter, which is far more demanding but which an entry probe of the Galileo spacecraft investigated in 1995, at Mach 50.[10]

Other speed limits become important in the field of wind tunnel simulation. The Government’s first successful hypersonic wind tunnel was John Becker’s 11-inch facility, which entered service in 1947. It approached Mach 7, with compressed air giving run times of 40 seconds.[11] A current facility, which is much larger and located at the National Aeronautics and Space Administration (NASA) Langley Research Center, is the Eight-Foot High-Temperature Tunnel—which also uses compressed air and operates near Mach 7.

The reason for such restrictions involves fundamental limitations of compressed air, which liquefies if it expands too much when seeking higher speeds. Higher speeds indeed are achievable but only by creating shock waves within an instrument for periods measured in milliseconds. Hence, the field of aerodynamics introduces an experimental speed limit of Mach 7, which describes its wind tunnels, and an operational speed limit of Mach 4, which sets a restriction within which cruising flight remains feasible. Compared with these velocities, the usual definition of hypersonics, describing flight beyond Mach 5, is seen to describe nothing in particular.

Metals, Ceramics, and Composites

Solid-state materials exist in one of these forms and may be reviewed separately. Metals and alloys, the latter being particularly common, exist usually as superalloys. These are defined as exhibiting excellent mechanical strength and creep resistance at high temperatures, good surface stability, and resistance to corrosion and oxidation. The base alloying element of a superalloy is usually nickel, cobalt, or nickel-iron. These three elements are compared in Table 1 with titanium.[12]

TABLE 1: COMPARISON OF TITANIUM WITH SELECTED SUPER ALLOYS
ELEMENT	NUMBER	MELTING POINT (K)
Titanium	22	1,941
Iron	26	1,810
Cobalt	27	1,768
Nickel	28	1,726

Superalloys generally are used at temperatures above 1,000 °F, or 810 K. They have been used in cast, rolled, extruded, forged, and powder-processed forms. Shapes produced have included sheet, bar, plate, tubing, airfoils, disks, and pressure vessels. These metals have been used in aircraft, industrial and marine gas turbines, nuclear reactors, aircraft skins, spacecraft structures, petrochemical production, and environmental-protection applications. Although developed for use at high temperatures, some are used at cryogenic temperatures. Applications continue to expand, but aerospace uses continue to predominate.

Superalloys consist of an austenitic face-centered-cubic matrix plus a number of secondary phases. The principal secondary phases are the carbides MC, M6C, M23C6, and the rare M7C3, which are found in all superalloy types, and the intermetallic compound Ni3(Al, Ti), known as gamma-prime, in nickel – and iron-nickel-base superalloys. The most important classes of iron-nickel-base and nickel-base superalloys are strengthened by precipitation of intermetallic compounds within a matrix. Cobalt-base superalloys are invariably strengthened by a combination of carbides and solid solution hardeners. No intermetallic compound possessing the same degree of utility as the gamma-prime precipitate—in nickel-base and iron-nickel-base superalloys—has been found to be operative in cobalt-base systems.

The superalloys derive their strength from solid solution hardeners and precipitating phases. In addition to those elements that promote solid solution hardening and promote the formation of carbides and intermetallics, elements including boron, zirconium, hafnium, and cerium are added to enhance mechanical or chemical properties.

TABLE 2: SELECTED ALLOYING ADDITIONS AND THEIR EFFECTS
ELEMENT	PERCENTAGES		EFFECT
	Iron-nickel- and nickel-base	Cobalt-base
Chromium	5–25	19–30	Oxidation and hot corrosion resistance; solution hardening; carbides
Molybdenum, Tungsten	0–12	0–11	Solution hardening; carbides
Aluminum	0–6	0–4.5	Precipitation hardening; oxidation resistance
Titanium	0–6	0–4	Precipitation hardening; carbides
Cobalt	0–20	N/A	Affects amount of precipitate
Nickel	N/A	0–22	Stabilizes austenite; forms hardening precipitates
Niobium	0–5	0–4	Carbides; solution hardening; precipitation hardening (nickel-, iron-nickel – base)
Tantalum	0–12	0–9	Carbides; solution hardening; oxidation resistance

Table 2 presents a selection of alloying additions, together with their effects.[13] The superalloys generally react with oxygen, oxidation being the prime environmental effect on these alloys. General oxidation is not a major problem up to about 1,600 °F, but at higher temperatures, commercial nickel-and cobalt-base superalloys are attacked by oxygen. Below about 1,800 °F, oxidation resistance depends on chromium content, with Cr2O3 forming as a protective oxide; at higher temperatures, chromium and aluminum contribute in an interactive fashion to oxidation protection, with aluminum forming the protective Al2O3. Because the level of aluminum is often insufficient to provide long-term protection, protective coatings are often applied. Cobalt-base superalloys are readily welded using gas-metal-arc (GMA) or gas-tungsten-arc (GTA) techniques. Nickel- and iron-nickel-base superalloys are considerably less weldable, for they are susceptible to hot cracking, postweld heat treatment cracking, and strain-age cracking. However, they have been successfully welded using GMA, GTA, electron-beam, laser, and plasma arc methods. Superalloys are difficult to weld when they contain more than a few percentage points of titanium and aluminum, but superalloys with limited amounts of these alloying elements are readily welded.[14]

So much for alloys. A specific type of fiber, carbon, deserves discussion in its own right because of its versatility. It extends the temperature resistance of metals by having the unparalleled melting temperature of 6,700 °F. Indeed, it actually gains strength with temperature, being up to 50 percent stronger at 3,000 °F than at room temperature. It also has density of only l.50 grams per cubic centimeter (g/cm3). These properties allowed carbon fiber to serve in two path-breaking vehicles of recent decades. The Voyager aircraft, which flew around the world in 1986 on a single load of fuel, had some 90 percent of its structure made of carbon fibers in a lightweight matrix. The Space Shuttle also relies on carbon for thermal protection of the nose and wing leading edges.[15]

These areas needed particularly capable thermal protection, and carbon was the obvious candidate. It was lighter than aluminum and could be protected against oxidation with a coating. Graphite was initially the standard form, but it had failed to enter the aerospace mainstream. It was brittle and easily damaged, and it did not lend itself to use with thin-walled structures.

The development of a better carbon began in 1958 with Vought Missiles and Space Company (later LTV Aerospace) in the forefront. The work went forward with support from the Dyna-Soar and Apollo programs and brought the advent of an all-carbon composite consisting of graphite fibers in a carbon matrix. Existing composites had names such as carbon-phenolic and graphite-epoxy; this one was carbon-carbon.

It retained the desirable properties of graphite in bulk: lightweight, temperature resistance, and resistance to oxidation when coated. It had a very low coefficient of thermal expansion, which reduced thermal stress. It also had better damage tolerance than graphite.

Carbon-carbon was a composite. As with other composites, Vought engineers fabricated parts of this material by forming them as layups. Carbon cloth gave a point of departure, being produced by oxygen-free pyrolysis of a woven organic fiber such as rayon. Sheets of this fabric, impregnated with phenolic resin, were stacked in a mold to form the layup and then cured in an autoclave. This produced a shape made of laminated carbon cloth phenolic. Further pyrolysis converted the resin to its basic carbon, yielding an all-carbon piece that was highly porous because of the loss of volatiles. It therefore needed densification, which was achieved through multiple cycles of reimpregnation under pressure with an alcohol, followed by further pyrolysis. These cycles continued until the part had its specified density and strength.

The Shuttle’s design specified carbon-carbon for the nose cap and leading edges, and developmental testing was conducted with care. Structural tests exercised their methods of attachment by simulating flight loads up to design limits, with design temperature gradients. Other tests, conducted within an arc-heated facility, determined the thermal responses and hot-gas leakage characteristics of interfaces between the carbon-carbon and the rest of the vehicle.

Additional tests used articles that represented substantial portions of the orbiter. An important test item, evaluated at NASA Johnson, reproduced a wing-leading edge and measured 5 by 8 feet. It had two leading-edge panels of carbon-carbon set side by side, a section of wing structure that included its main spars, and aluminum skin covered with thermal-protection tiles. It had insulated attachments, internal insulation, and internal seals between the carbon-carbon and the tiles. It withstood simulated air loads, launch acoustics, and mission temperature-pressure environments—not once but many times.[16]

There was no doubt that left to themselves, the panels of carbon-carbon that protected the leading edges would have continued to do so. Unfortunately, they were not left to themselves. During the ascent of the Shuttle Columbia, on January 16, 2003, a large piece of insulating foam detached itself from a strut that joined the external tank to the front of the orbiter. The vehicle at that moment was slightly more than 80 seconds into the flight, traveling at nearly Mach 2.5. This foam struck a carbon-carbon panel and delivered what proved to be a fatal wound. In words of the accident report:

Columbia re-entered Earth’s atmosphere with a preexisting breach in the leading edge of its left wing. This breach, caused by the foam strike on ascent, was of sufficient size to allow superheated air (probably exceeding 5,000 degrees Fahrenheit) to penetrate the cavity behind the RCC panel. The breach widened, destroying the insulation protecting the wing’s leading edge support structure, and the superheated air eventually melted the thin aluminum wing spar. Once in the interior, the superheated air began to destroy the left wing. Finally, over Texas, the increasing aerodynamic forces the Orbiter experienced in the denser levels of the atmosphere overcame the catastrophically damaged left wing, causing the Orbiter to fall out of control.[17]

Three years of effort succeeded in securing the foam on future flights, and the Shuttle returned to flight in July 2006 with foam that stayed put. In contrast with the high tech of the Shuttle, carbon fibers also are finding use in such low-tech applications as automobiles. As with the Voyager round-the-world aircraft, what counts is carbon’s light weight, which promotes fuel economy. The Graphite Car employs carbon fiber epoxy-matrix composites for body panels, structural members, bumpers, wheels, drive shafts, engine components, and suspension systems. A standard steel auto would weigh 4,000 pounds, but this car weighs only 2,750 pounds, for a saving in weight of nearly one-third.[18]

Superalloys thus represent the mainstream in aerospace materials, with composites such as carbon fiber extending their areas of use. There also are ceramics, but these are highly specialized. They cannot compete with the temperature resistance of carbon or with its light weight. They nevertheless come into play as insulators on turbine blades that protect the underlying superalloy. This topic will be discussed separately.

Ablative and Radiative Structures

Atmosphere entry of satellites takes place above Mach 20, only slightly faster than the speed of reentry of an ICBM nose cone. The two phenomena nevertheless are quite different. A nose cone slams back at a sharp angle, decelerating rapidly and encountering heating that is brief but very severe. Entry of a satellite is far easier, taking place over a number of minutes.

To learn more about nose cone reentry, one begins by considering the shape of a nose cone. Such a vehicle initially has high kinetic energy because of its speed. Following entry, as it approaches the ground, its kinetic energy is very low. Where has it gone? It has turned into heat, which has been transferred both into the nose cone and into the air that has been disturbed by passage of the nose cone. It is obviously of interest to transfer as much heat as possible into the surrounding air. During reentry, the nose cone interacts with this air through its bow shock. For effective heat transfer into the air, the shock must be very strong. Hence the nose cone cannot be sharp like a church steeple, for that would substantially weaken the shock. Instead, it must be blunt, as H. Julian Allen of the National Advisory Committee for Aeronautics (NACA) first recognized in 1951.[19]

Now that we have this basic shape, we can consider methods for cooling. At the outset of the Atlas ICBM program, in 1953, the simplest method of cooling was the heat sink, with a thick copper shield absorbing the heat of reentry. An alternative approach, the hot structure, called for an outer covering of heat-resistant shingles that were to radiate away the heat. A layer of insulation, inside the shingles, was to protect the primary structure. The shingles, in turn, overlapped and could expand freely.

A third approach, transpiration cooling, sought to take advantage of the light weight and high heat capacity of boiling water. The nose cone was to be filled with this liquid; strong g-forces during deceleration in the atmosphere were to press the water against the hot inner skin. The skin was to be porous, with internal steam pressure forcing the fluid through the pores and into the boundary layer. Once injected, steam was to carry away heat. It would also thicken the boundary layer, reducing its temperature gradient and hence its rate of heat transfer. In effect, the nose cone was to stay cool by sweating.

Still, each of these approaches held difficulties. Transpiration cooling was poorly understood as a topic for design. The hot-structure concept raised questions of suitably refractory metals along with the prospect of losing the entire nose cone if a shingle came off. Heat sinks appeared to promise high weight. But they seemed the most feasible way to proceed, and early Atlas designs specified use of a heat-sink nose cone.[20]

Atlas was an Air Force program. A separate set of investigations was underway within the Army, which supported hot structures but raised problems with both heat sink and transpiration. This work anticipated the independent studies of General Electric’s George Sutton, with both efforts introducing an important new method of cooling: ablation. Ablation amounted to having a nose cone lose mass by flaking off when hot. Such a heat shield could absorb energy through latent heat, when melting or evaporating, and through sensible heat, with its temperature rise. In addition, an outward flow of ablating volatiles thickened the boundary layer, which diminished the heat flow. Ablation promised all the advantages of transpiration cooling, within a system that could be considerably lighter and yet more capable, and that used no fluid.[21]

Though ablation proved to offer a key to nose cone reentry, experiments showed that little if any ablation was to be expected under the relatively mild conditions of satellite entry. But satellite entry involved high total heat input, while its prolonged duration imposed a new requirement for good materials properties as insulators. They also had to stay cool through radiation. It thus became possible to critique the usefulness of ICBM nose cone ablators for the new role of satellite entry.[22]

Heat of ablation, in British thermal units (BTU) per pound, had been a standard figure of merit. Water, for instance, absorbs nearly