The Future of Flight

A key figure in the development of radar and now head of Lockheed’s missile development, LOUIS RIDENOUR,in this highly readable article, reviews the record of human flight and tells what lies ahead as the engineers enable men to fly higher and faster.

THE envy of birds is very old in human history; the fable of Daedalus and Icarus has come down to us from classical antiquity. Leonardo da Vinci, perhaps the greatest innovator who ever lived, dealt with the problem of flight. Observing the flight of birds, he invented the ornithopter, but he also had a remarkably prescient appreciation of the basic aerodynamic problem. Ignoring Aristotle’s wrongheaded idea that no moving body could continue to move without the continued application of force, Leonardo correctly assumed that support by the air is possible only because the air resists motion through it. This sounds paradoxical, but it is not. Lift, which is required for sustained flight in a heavier-than-air machine, can be achieved only at the cost of incurring drag, which has classically been the main problem of such machines. Leonardo understood this, though it was not generally recognized until four centuries later.

Man first got off the ground in an aerial vehicle — actually a hot-air balloon — on November 21, 1783. This historic first flight was followed by many others, and by development of the dirigible balloon which reached its apotheosis in the German Hindenhurg. Long before the Hindenburg exploded and burned at Lakehurst on May 6, 1937, carrying thirty-six persons to their deaths, it was clear that the buoyant airship was not the best means of aerial transport.

In the history of man’s endeavors to fly, there are four chapters, of which only the first — lighterthan-air craft — antedates the Atlantic. The Wright brothers’ first flight in a heavier-than-air machine was made on December 17, 1903, at about the mid-point of the Atlantic’s first century. Supersonic flight in the air, the third chapter, began toward the end of the Atlantic’s century, and flight out of the atmosphere in ballistic missiles, which has scarcely been begun in 1957, appears sure to follow soon.

In considering the past development of human flight, the lighter-than-air machines will here be dismissed, because it now appears that heavierthan-air machines have much greater present and future significance. Intrinsically, lighter-than-air craft suffer from the fatal defect that the greatest speeds of which they are capable can be matched in surface transport. For most military and commercial applications, this is a decisive shortcoming. This judgment may be wrong; the use of beryllium as a structural material and the introduction of nuclear power sources may ultimately render lighter-than-air craft competitive for some military or commercial purposes. Still, those possibilities seem so distant and so unlikely that they can be ignored.

The history of heavier-than-air machines can be started with a variety of investigators, but perhaps it is fairest to begin it with Sir George Cayley, who in 18U9 put forward clearly the novel concept of using as a lifting device a fixed wing rather than flapping wings like those of a bird.

During the nineteenth century, development and application of the fixed-wing principle of Cayley continued. Model experiments and trials of unpowered gliders were carried out by many investigators. Otto Lilienthal of Germany was the greatest of the glider pilots; he made hundreds of successful flights before he crashed to his death in 1896.

Samuel P. Langley, secretary of the Smithsonian Institution, was interested in the development of a man-carrying aircraft. He experimented with powered models, and finally designed and built an “aerodrome” (literally, air-runner) which in 1896 flew for about half a mile over the Potomac River near Washington. A later aerodrome of Langley had as a power plant the first liquidcooled radial internal-combustion engine to have been developed. It produced 52 horsepower at a total weight of 125 pounds, and was thus well ahead of its time.

Orville and Wilbur Wright, the proprietors of a bicycle shop in Dayton, Ohio, had been interested for some lime in the problem of powered flight. With the helpful advice of Octave Chanute, who had followed with interest Lilienthal’s glider experiments, the Wrights built and flew successful unpowered gliders. There being no suitable power plant available to them, they built a 12horsepower gasoline engine to power their 1903model glider. The resulting machine founded the air age by remaining aloft for about 12 seconds on December 17, 1903, with Orville at the controls. On a later flight made the same day, the Wright airplane flew for 59 seconds and made good a distance of 852 feet.

While the Wrights had no formal engineering education, nevertheless they approached the problem of flying in a thoroughly scientific manner, making primitive wind-tunnel experiments to devise an efficient wing, and proceeding from small simple models to the full-scale machine. They introduced two new features which are still typical of modern aircraft: the use of a horizontal elevator surface for the control of the aircraft in pitch, and the control of roll by flexing the rear edge of the wings, a function that is now performed by separately movable ailerons.

When the epochal achievement at Kitty Hawk was at last appreciated and believed, there instantly appeared an impressive number of aviation pioneers. Paul Santos-Dumont, a rich Brazilian residing in France, was the first to fly in Europe; he took off in 1906. The English Channel, which had previously been crossed by Francois Blanchard in 1785 with tire help of a hydrogen-filled balloon, was flown across by Blériot in a stick-and-wire airplane in 1909. In that same year, at an aviation meet held at Reims, the American Glenn L. Curtiss set a world’s speed record of 47.8 miles per hour. The world’s altitude record was also established, at 508 feet.

As aircraft improved, people flew farther, faster, higher, and for longer times. The Atlantic Ocean was first crossed by air in February, 1919, by a Naval expedition flying the Curtiss NC-4. The first nonstop crossing of the Atlantic was made the following month by Alcock and Brown, flying a Vickers-Vimy bomber. Three Douglas bombers were flown around the world in 1924. Charles Lindbergh, as everybody knows, flew nonstop from New York to Paris in 1927.

THE First World War produced a tremendous improvement in the airplane. The war was commenced with stick-and-wire biplanes capable of top speeds in the general vicinity of 60 miles per hour and maximum ranges of some 200 miles. These unarmed machines were regarded as possibly having some utility for observation and artillery spotting. Before the end of the war, only some four years later, the stressed-skin monocoque construction had been adumbrated, if not fully invented; aerial bombing had been developed — so had the fighter aircraft intended to counter it; and the egregious Billy Mitchell had organized and commanded the first thousandplane operation of history. At the close of World War I. it was clear to all concerned that the military airplane was here to stay, and a few brave visionaries even thought that there might be a future in the commercial carriage of passengers and freight by air.

This bright vision became less and less unreasonable as aircraft were improved in the years between the wars. In the mid-1920s the monoplane was established as the dominant type. All-metal aircraft were introduced very soon thereafter. Engine supercharging became practicable about 1930, propellers of controllable pitch a couple of years later.

It was now possible to design the prototype of the modern commercial transport, and three American aircraft builders did so. The historic Ford trimotor, still flying in South America, where good short-field performance is more important than speed and overall economy, should not be passed over without mention; nevertheless, the Boeing 247, the Douglas DC-2, and the original Lockheed Electra, appearing in that order in the years 1932 to 1934, really got the airline business off the ground. It was in the year 1934 that the passenger-miles flown in U.S. domestic air transport began die exponential growth, doubling every two years, that they have maintained to the present day.

Air transport is now a very important component of the national and world-wide transport business. In 1955, throughout the world, there was on the average a take-off of a regularly scheduled commercial airplane every five seconds; there is at this moment a take-off every 2 1/2 seconds. Thirty-five hundred cities are served by scheduled air transport. For all trips over 200 miles in length, the airplane is a more important vehicle than the train, the bus, the surface ship, or the passenger automobile. In the year 1956, 6.5 million landings of scheduled commercial aircraft were made on the airports of the U.S.; there were more than 300,000 landings at Chicago Midway Airport alone.

Everyone knows that this growth has not been accomplished with DC-2s or the old Electra. The conventional piston-engine transport has been vastly improved since 1934. Turbine superchargers were introduced in the late 1930s, pressurization of cabins and crew space just before World War II. These improvements, together with a general improvement in engine performance and a trend to four engines, rather than two, have brought us from the DC-2 to the DC-7, from the old Electra to the Model 1649 Constellation Starliner.

We are now on the threshold of a qualitative jump in the performance of commercial aircraft, produced by the transition to turbine engines. Commercial airliners with turbine engines, both turboprops and turbojets, are flying now, but these are the products of foreign plane makers, mostly British. American-built turbine aircraft are to make their appearance on the airlines of the world next year. The glamour items are the big turbojets — the Douglas DC-8, the Boeing 707, and the Convair 880. The work horse is likely to be the Lockhead turboprop, the new Electra. Smaller turbojets have been talked about but at this writing are not firm. It is not yet fully clear which planes will be bought by which airlines; all that is clear is that a complete cycle of re-equipment is being undertaken by the airlines of this country, and that they will come out of it with equipment at least 100 miles an hour faster than the machines which they are now flying.

However rapid the further aeronautical progress created by continuing big military budgets, this new generation of airline equipment is likely to last until perhaps 1975. The main reason for this is economic. It will be about ten years before the airlines have paid back the bankers for their investment in the new equipment.

Thus, around 1975, it will be possible for the airlines to give thought to the next step in their progress. The next step is probably the supersonic transport, flying at Mach 2 or better. (Mach 2 means twice the local speed of sound.) The main technical problem standing in the way of this development is the relatively low efficiency of engines and airframes at such speeds.

At all events, the problems of airplane design encounter a major discontinuity at about the speed of sound, or Mach 1. This is some 763 statute miles per hour at sea level, or 600 at altitude. Mach 1 at altitude is about 1000 feet per second.

The trouble with the supersonic transport is that in its present design it burns too much fuel in getting from place to place. We do not yet know how to improve the lift-to-drag ratio of the airframe, or how to push up the engine thrust produced by a given rate of fuel consumption, so as to make this airplane competitive economically. But it is almost certain that the present difficulties will be overcome with continued development.

It must be borne in mind that a faster transport aircraft, in order to be worth introduction into airline service, need not have a per-mile operating cost as low as that of the slower machine it replaces. For one thing, the traveling public has been willing to pay a premium fare for speed. In addition, the costs of the ground organization of an airline are tied to the clock; airplanes that will generate more passenger-miles per unit time incur less total cost per passenger-mile. Thus the supersonic transport, which will probably fly at least twice and possibly four times as fast as the jet planes about to be introduced, need not be as cheap to fly, per passenger-mile. However, the operating costs of designs that we can now make are out of all economic reason, and sharp improvement, attainable only by design advances, will certainly be required.

IN THE period between the world wars, military aircraft benefited from the design improvements that have already been mentioned, and by the time of the Second World War it was appreciated by all nations that the airplane was a new military tool of great potential value. The Italians used military aircraft in Ethiopia, both sides used them in the Spanish Civil War, the Germans conquered Poland and, a year later, the Low Countries and France in an air operation. Finally, the Nazi invasion of Crete, having been frustrated as a surface operation, was carried out successfully by air. This really marked the beginning of the employment of air power in something resembling the fashion which had so long been envisioned by its enthusiasts.

The German invasion of the British Isles was to have been preceded by a neutralization of the Royal Air Force and of British air defenses. This was a good idea; fortunately for the free world, it did not work. Beginning as early as 1934, the British had vigorously sought to build an adequate system of air defense; modern radar was, in a manner of speaking, invented to order. It was proposed, essentially in Lite form in which it was first built prior to the Battle of Britain, in response to a classified circular letter sent out by an ad hoc Committee for the Scientific Survey of Air Defense, headed by Sir Henry Tizard. The author of the successful response was a Scottish physicist, now Sir Robert Watson-Watt, then at the head of the Radio Department of the British National Physical Laboratory.

In addition to radar, the British first introduced on the Allied side microwave radar, airborne radar, the Pathfinder concept (in which especially trained and equipped crews and aircraft marked out targets for throngs of following bombers), radio navigation systems of great precision, the radio and radar countermeasures war, and turbojet engines. Generally speaking, the British fully appreciated the importance of electronics to war in the air some seven years before we did.

Despite this fact, we made many capital improvements in the design of the aircraft types which were subjected to the actual test of combat. The helicopter was fully realized in the early 1940s, and so was the first successful jet engine. Pressurization of high-altitude, high-performance aircraft had been pioneered in a pre-war Boeing transport, and was first used seriously in the B-29 bomber. The provision of defensive armament on bombardment aircraft and the extension of the range of escorting fighters were major preoccupations of aircraft makers during the early years of World War II. The thousand-plane raid became commonplace.

The net result was that we overwhelmed the enemy. But now that nuclear weapons have been invented, the enemy can be overwhelmed at far less cost and with far less effort than we expended in World War II. One aircraft can carry to its target a greater amount of explosive energy than the total Allied effort brought to Germany in four years of war. The new superbombs can be carried in a trunk, in the hold of an apparently harmless merchant ship, or in the cargo pit of a commercial airline transport. While we are expending considerable effort preparing to meet and counter a mass air raid on the pattern of those so effective in the Second World War, we must at the same time ask ourselves whether, given the new explosives, such a raid is the best way of causing the destruction of the chosen targets.

IN THE twelve years since the end of World War II, it can plausibly be argued, we have been engaged in the overdevelopment of aircraft. Certainly vast amounts of money have been spent on aeronautical research and development. These sums have purchased us a substantial return. With the development of practical jet engines, afterburning, ramjets, rocket engines, and superfuels, we are now able to build manned aircraft which go so fast that the human pilot is almost reduced to the role of a spectator. Our hottest lighter, the Lockheed F-104, is able at altitude to fly formation with an artillery shell, and at full throttle to pull away from it.

The difference between the F-104 and the artillery shell is partly that the F-104 has a man riding in it. This is a nontrivial difference. Unlike the guidance system of a missile, a man requires to be maintained in an environment something like the one he experiences in sedentary life on the surface of the earth. He needs air at not much less than half sea-level pressure, enriched by oxygen even then. He requires a temperature about 15 degrees Centigrade. He is provided with a complicated mechanism, to be actuated only in the event of impending disaster, which can save him in the event of a catastrophic occurrence such as the disintegration of his aircraft.

These airborne amenities are expensive. Their cost can be roughly evaluated by comparing our present heavy bomber, the B-52. whose take-off weight is in the neighborhood of 150 tons, with the unmanned Northrop Snark, which is designed to perform the same mission at a fraction of the B-52’s gross weight.

Thus the military trend is toward unmanned missiles. Within a very few years, it is estimated that more than half the military budget for flying machines will be devoted to the development and manufacture of missiles, leaving the smaller part for conventional aircraft. Much of the missile budget will be expended in the national ballisticmissile program — the effort to develop huge rocket missiles which can span intercontinental distances by flying most of the way entirely out of the air. These ballistic missiles are, in principle, equivalent to the German V-2 of the Second World War.

As has become usual, the development of aircraft for military needs has far outstripped the development of commercial aircraft. There are already military aircraft that will fly at Mach 2. This suggests that even faster military airplanes be developed. Higher speeds are certainly practicable as we learn how to make engines of a higher thrust-to-weight ratio, but there is in view a limit for the speeds accessible to an airplane modeled on the 1903 Wright biplane.

Somewhere in the neighborhood of 3500 miles per hour, flight through the air with a fixed wing providing lift and an air-breathing engine providing thrust — that is, flight as we know it today — becomes impracticable. It is defeated because of the heating produced by slamming the airframe against the stationary air at such great speeds. The resulting temperature rise increases much faster than the speed; in fact, as the square of the speed. Airframe structural materials lose their strength and ultimately melt at elevated temperatures; aluminum alloys are probably unsuitable above 2000 miles per hour, and the best stainless steels are being stretched pretty hard above 3000. Even if the airframe can stand the heat, it is difficult to keep a man alive in the cockpit.

While vigorous efforts are being made to meet this difficulty head on by investigating ceramiccoated metals and other exotic airframe materials, by devising efficient skin-cooling means, and by other means, nevertheless it is still true that there is some speed at which any of these devices come to the end of their usefulness. For higher speeds, we must turn to the ballistic missile.

THE ballistic missile is so named because, like an artillery shell or a stone hurled by the giant slingshot called by the Romans a ballista, it is propelled only over a relatively short time and distance at the beginning of its journey. For the rest of its flight, the ballistic missile is in free fall in the gravitational field of the earth. This produces an elliptical trajectory which, for any range in excess of 100 miles or so, takes the missile well out of the earth’s sensible atmosphere. A minimum-energy trajectory to a target 5000 nautical miles away would, for example, involve a flight path whose highest point is some 900 nautical miles above the surface of the earth.

Thus the ballistic missile evades the aerodynamic heating problem in the most direct way imaginable — it is out of the air over most of its flight. Immediately after take-off, when the missile is gaining speed from the great rocket engines that propel it, it is moving relatively slowly in the dense air of the lower atmosphere; the time the missile spends in the atmosphere is short, the speed with which it travels is not high, and the problems of exit heating can be managed.

When it plunges back into the atmosphere at the end of its flight, however, a ballistic missile can encounter very serious heating problems. These problems are reduced to reasonable proportions by slowing the missile down at very high altitudes where the air is not dense, which can be done by adopting a configuration whose drag is high in comparison with its weight. If mancarrying ballistic missiles are ever designed and used, this is almost certainly how they will work: upon re-entry into the sensible atmosphere, the vehicle will spread out retractable wings, slow down at very high altitude, using lift to extend its range, and finally descend to a landing at a speed which is subsonic or only mildly supersonic. Under these conditions, the problem of aerodynamic heating can be dealt with in a fairly simple and straightforward fashion.

Unfortunately, the ballistic missile whose task it is to carry a bomb to a target cannot do this. To attain the desired accuracy in spite of unknown winds aloft at the target, and to diminish the risk of successful defensive interception, the warheadcarrying missile needs to come in fast; and fast, in this connection, quite literally means hot. The shock wave created in the atmosphere by a re-entering 5000-mile missile has a temperature in the tens of thousands of degrees Centigrade — several times the melting temperature of tungsten, one of the most refractory of metals.

The heat input to the re-entry body can be lowered by the control of flow conditions around it, and the heat actually communicated to the missile can be dealt with in any one of a variety of ways. Even so, the re-entry problem is one of the more difficult aspects of long-range ballistic missile development.

The regimen of flight conditions encountered at ballistic-missile re-entry speeds is an entirely new one. In the high-temperature air of the shock wave produced by the missile, molecular dissociation, ionization, and radiation are taking place. In studying these phenomena, the classical aerodynamicist must learn from the physicist who has some knowledge of spectroscopy. The familiar wind tunnel, in which air is blown past aircraft or missile models for minutes at a time while measurements are made, is being replaced by a device rather like an artillery piece, in which the steady flow past the model is over in a twentieth of a second or less. If this time seems short, we must remind ourselves that, at the speeds concerned, any longer flow duration would cause both the model and the walls of the test chamber to melt.

Equally unfamiliar to the conventional aircraft designer is the ballistic flight vehicle itself. It is no longer winged; if it has aerodynamic surfaces, they are vestigial. Control of the flight path is accomplished by deflection of the rocket jet. The classical problems of stability and control, which have been matters of prime importance to the aerodynamicist in the past, have now been transformed into problems of servomechanism design very closely related to the overall guidance problem.

Propulsion of a ballistic missile is accomplished by a rocket engine, since much of the propulsive effort is expended at altitudes where an airbreathing engine such as a conventional turbojet or ramjet would have no air to breathe. The merit of a rocket engine is measured in terms of what is called specific impulse; that is, the pounds of thrust produced per pound-per-second of fuel and oxidizer burned to produce that thrust. The specific impulse of the rocket engines we are using now is little better than that realized by the Nazis a decade and a half ago.

To improve the efficiency of a rocket motor, the exhaust velocity of the jet of hot gas which provides the rocket thrust must be increased. There are two principal ways of doing this. One is to raise the temperature of the jet; the velocity of the rocket jet rises with the square root of the temperature. The other is to populate the jet with lighter atoms or molecules, which go faster than heavier ones at any given temperature.

When the rocket’s energy comes from chemical combination, or combustion, as it does in the rockets that we use now, there is a limit — and one we are approaching — to the rocket efficiency we can attain. The limit of the heat produced even by the most energetic chemical reaction is such that the reaction products cannot be raised to temperatures much beyond about 5000 degrees Fahrenheit. Further, the fact that we obtained the heat by chemical combination means that the jet of hot exhaust gases is made up of molecules resulting from that combination. These molecules are heavy in comparison with the lightest atoms, and their velocity is correspondingly lower.

A better rocket engine would be one producing a jet of hydrogen or helium at a temperature of tens of thousands of degrees. No one now knows how to build such an engine, but it is apparent from freshman physics that the energy powering such a rocket cannot be derived from chemical combustion. Rather, the working fluid comprising the l ocket jet will have been heated by some other source of energy — perhaps by an electrical discharge or by a nuclear device.

Looking still farther ahead, it is possible to imagine that the rocket jet itself might consist of fully ionized light gas, perhaps hydrogen, whose positive ions and electrons have been accelerated to high speeds by electromagnetic means. There have even been suggestions of a photon engine, which is supposed to exploit the very slight recoil that occurs when light leaves its source or is reflected from a mirror. It is true that a beam of light travels at the highest speed nature seems to allow, and thus is ideal from the standpoint of exhaust-jet speed. Unfortunately, the light-pressure forces which correspond to the most brilliant light sources we presently know how to create and sustain are minuscule in comparison with the thrust a practical rocket engine needs.

With the chemical rockets that we have now, we can propel a nuclear warhead a significant part of the way around the world. We can put some pounds of instrumentation into a satellite orbit, swinging round the earth like a tiny moon; this is the purpose of the Vanguard project. To do more with chemical rockets, despite the enthusiasm of such space pioneers as Wernher von Braun, is stretching the present state of the art farther than it will reasonably go. As has been true ever since the Wright brothers originated manned flight, the capabilities of available power plants define the limits of man’s ability to fly. We need a better engine if men are to leave the earth for the planets and the stars.

There is little doubt that a continuation of today’s vigorous effort in aeronautical development will yield better engines. I would be willing to contend that man will make his first space flight, probably to the moon, before the end of aviation’s first century.

Robert A. Millikan, one of the greatest American physicists of the last generation, began his autobiography with these words:

When one takes a world outlook one cannot help realizing how extraordinarily different have been the life experiences of the men and women who have lived from 1868 to 1948 from those who have lived in any other period of comparable length in human history. For it has been the lot of all the generations of mankind up to the two generations which my life span has covered to leave the world at death very much the same kind of place they found it at birth. But this will not be true of those of us who come from the vintage of ‘68. Our ordinary life experiences bear little resemblance to those of my father and much less to those of my grandfather.

The century between Millikan’s father and now is roughly the century spanned by the Atlantic. The social change that Millikan refers to is nowhere shown more plainly than in the development of aviation, both from the point of view of its violent novelty and from that of its total social importance. Its further development will enable man to leave the earth for the first time — probably within aviation’s first century, but surely within the Atlantic’s second.