44 Flying Fast – Supersonic & Hypersonic Flight

Introduction

Humankind has always strived to develop flight vehicles that can fly fast and then faster still. While a modern military will always have a “need for speed” for most of their missions, the ability of civil aircraft to transport passengers and cargo quicker from point to point has never been more acute. Aircraft are not intrinsically limited to subsonic or transonic flight speeds, but for them to fly supersonically requires the consideration of several other factors, not just limited to their aerodynamics.

After the aerodynamics, stability and control, and propulsion issues of flight near and beyond Mach 1.0 were understood by the early 1950s, it was only a short time before airplanes were routinely flying supersonically. The invention of the turbojet engine and the afterburner, in which extra fuel is burned in the exhaust, increased thrust significantly and allowed airplanes to accelerate through Mach 1 and maintain supersonic flight.

Many different types of supersonic airplanes were developed starting in the 1950s, including those with various forms of delta wings, closely-coupled canard delta wings, and wings with variable-sweep geometry. In supersonic flight conditions, kinetic heating of the airframe also became apparent, posing some new challenges in designing aircraft structures and construction materials suitable for high-speed flight. The rise and fall of supersonic transport (SST) airliners such as the Concorde and Tu-144 raises awareness that even today, technological feasibility does not always equate to commercial viability.

Objectives of this Lesson

  • Become familiar with the design features of supersonic airplanes and why such airplanes should have thin, highly swept, or delta-type wings.
  • Better understand the effects of Mach number on the formation of shock waves and the aerodynamic efficiency of wings in supersonic flight.
  • Understand the principle of vortex lift on slender delta wings at high angles of attack.
  • Appreciate some of the challenges in engineering and operating a supersonic transport airplane (SST).
  • Understand the additional technical challenges for aircraft to fly at hypersonic flight speeds.

History

In 1887, Ernst Mach and Peter Salcher presented their paper on the supersonic motion of a projectile “Photographische Fixirung der durch Projectile in der Luft eingeleiteten Vorgänge,” in the journal Sitzungsberichte der Kaiserlichen Akademie der Wissenschaften [Wien], Mathematisch-Naturwissenschaftliche,” one of their famous schlieren photos being shown below. They deduced theoretically, and then verified experimentally, the existence of a conical shock wave (now called a Mach cone or Mach wave) that would form at the nose of the projectile (or any other object) that flies supersonically.

The 1887 schlieren image of the shock waves (Mach waves) formed by a supersonic projectile. Notice also the turbulence in the wake behind the projectile.
In the early 1920s, a series of delta wing aircraft concepts were proposed, although not for supersonic airplanes but instead for unconventional “tailless” airplanes, originally pioneered by Alexander Martin Lippisch in Germany after the end of WW1. By 1930 there was much interest in supersonic flight, and the aerodynamics associated with supersonic flows were being studied in wind tunnels, with parallel theoretical studies underway. Jacob Ackert published a pioneering paper on the lift and drag of a supersonic airfoil in 1925 titled “Luftkräfte auf Flügel, die mit größerer als Schallgeschwindigkeit bewegt werden” in the journal Zeitschrift für Flugtechnik und Motorluftschiffahrt.

Lippisch began to pursue more advanced delta wing concepts intended for supersonic flight. The intended application was to military interceptor airplanes, but they were not built because they lacked a suitable engine. Lippisch designed the famous rocket-powered Messerschmitt Me 163 Komet, which had a swept and (almost) delta wing and was powered by a rocket engine. It was the first piloted airplane of any type to approach (but not exceed) the speed of sound in level flight (Mach 0.8). This airplane was followed by the Messerschmitt Me 262, which also had a swept wing, and was the World’s first operational turbojet-powered fighter airplane. The aerodynamic effects of using sweep at high speeds was first investigated in Germany as early as 1935 by Albert Betz and Adolph Busemann. However, wing sweep was not used in on the Me 262 for an aerodynamic reason.

The first airplane to fly supersonically in straight and level flight was the U.S.’s Bell X-1, a rocket-powered research airplane, which performed several test flights near the sound speed in the late 1940s. The air-dropped Bell X-1, piloted by Chuck Yeager, first reached supersonic speeds in level flight in October 1947. The British were also working on supersonic airplanes. The Miles M.52 was designed in secrecy during WW2 between 1942 and 1945, with Frank Whittle‘s Power Jets being contracted to produce a suitable turbojet engine. The M.52 design had a conical nose with thin, mildly swept wings and a tail with sharp leading edges. While the aircraft never flew, scaled models were flown supersonically in late 1947. In September 1948, the tailless swept-wing de Havilland D.H.108 Swallow, as shown in the photograph below, reached just over Mach 1.0 while in a shallow dive. However, the aircraft was to kill its designer and pilot, Geoffrey de Havilland Jr., because of high unsteady aerodynamic-induced loads caused by shock-induced flow separation and stall, which resulted in a catastrophic failure of the main wing spar and an in-flight breakup.

The British De Havilland D.H.108 Swallow was one of the first swept wing high-speed research airplanes specifically designed to investigate flight near the speed of sound.

Physics of Supersonic Flight

Interest in the theory and practice of supersonic flight continued at various research centers such as the NACA, with wind tunnel tests confirming the development of the high drag on an airplane in supersonic flight, which had been known as the “sound barrier.” Although Mach 1 is not a barrier per se, these conditions result in significant changes in the flow physics of the aircraft that can affect not only its flight performance but also its stability and control characteristics.

After the work of Betz and Busemann before WW2, Robert T. Jones at NACA Langley was the first to develop an aerodynamic theory for supersonic delta wings, his work “Wing Plan Forms for High-Speed Flight” being published as NACA TR-863 in 9947. His approach combined the theory of thin supersonic airfoils with highly swept delta wings. This wing design became the basis for the first generation of supersonic delta-wing airplanes. Subsequently, several different supersonic airplane prototypes were built and flown by American and British manufacturers. However, such airplanes did not always have good handling qualities. There were many mishaps with the first generation of supersonic airplanes, including stall/spin events and other departures from controlled flight, often resulting in fatal crashes.

As an airplane flies faster and approaches Mach 1, pressure disturbances produced by the airplane begin to merge, and a strong compression wave forms in front of the airplane. As the airplane becomes supersonic, this wave becomes a single wavefront called a shock wave. In three dimensions, this shock wave is a shock cone called a Mach cone, which extends back from the nose of the airplane, as shown in the schematic below.

As an airplane flies closer to the speed of sound, pressure waves coalesce in front of the airplane and at supersonic flight conditions then a Mach cone forms at its nose.

The images below are schlieren flow visualization about a model of a fighter airplane in a supersonic wind tunnel. Notice the build-up in the number and intensity of the almost normal shock waves as Mach 1 is approached. In supersonic flight, the Mach cone becomes increasingly swept back with increasing flight Mach numbers. The angle \mu is used to denote the Mach angle, as given by

(1)   \begin{equation*} \mu = \sin^{-1} \left( \frac{1}{M_{\infty}} \right) \end{equation*}

which is the half apex angle of the Mach cone.

Schlieren flow visualization images showing the formation of shock waves as model of a fighter aircraft in a wind tunnel transitions from subsonic to supersonic flow.

Therefore, the Mach angle is 90^{\circ} at M_{\infty} = 1, and its value decreases quickly beyond Mach 1, as shown in the figure below. Notice that even by a Mach number of 2, the Mach cone is already swept back 60^{\circ} to a half apex angle of \mu = 30^{\circ}. For hypersonic speeds, the Mach angle is so steep that it can touch the body’s surface, so a blunt or bluff body shape is often a better design solution to keep the shock waves away from the surface.

Variation in the Mach angle with Mach number, showing the the Mach angle increases rapidly as the Mach number approaches 2.

In the figure below, the signature of the shock cone can also be seen through the natural condensation of water vapor in the air in the lower pressure and slightly lower temperature regions produced over the aircraft. This effect only occurs with humid air conditions, and the lighting conditions must also be suitable to capture such an image.

Natural condensation of water vapor showing the signature of a Mach cone around an airplane flying just above the speed of sound.

The photograph below is a fascinating schlieren image of the plethora of Mach (shock) waves generated by an actual airplane during supersonic flight. The optical method for visualizing this flow is called Airborne Background Oriented Schlieren (ABOS), the light source being the sun and the image being taken from a chase aircraft. Notice the particularly strong (i.e., darker looking) Mach waves at the nose and tail of the airplane, which are those responsible for the sonic booms heard on the ground. Also apparent in this image is the turbulence from the engine exhaust.

A rather dramatic image of the Mach waves generated by an aircraft in supersonic flight as obtained using an Airborne Background Oriented Schlieren” system. The Mach angle in this case suggests that the aircraft is flying about Mach 1.4. (NASA image.)
The Mach angle is important because disturbances in a supersonic flow are confined to the region of the cone determined by the Mach angle. Unlike a subsonic flow, there is no upstream influence beyond the Mach cone in a supersonic flow; pressure disturbances are only transmitted downstream. The consequence of this fact is important because if the wing’s leading edge is swept back behind the Mach cone, then the wing experiences relatively lower drag (i.e., low wave drag), as shown in the schlieren images below. Otherwise, if the shock wave reaches the wing, then the adverse pressure gradient over the shock will cause disruptions to the flow in the boundary layer and increases drag, perhaps even producing flow separation.
Schlieren flow visualization images made in the wind tunnel showing that by sweeping the wings behind the shock wave then it eliminates adverse interactions between the shock waves and the wings. (NACA image.)
Wing sweep profoundly affects transonic and supersonic drag, as shown in the figure below. This outcome is because the use of swept-back wings reduces the strength of the shock waves and prevents the shocks from interfering with the flow over the wings, which e flow separation and drag. However, although swept wings can help delay this drag rise from compressibility effects, other aerodynamic and aeroelastic problems are associated with swept wings. Therefore, aircraft designers tend to use as little wing sweep as possible, 20 to 30 degrees being typical of many airliners.
The use of sweepback on a wing provides for a very significant reduction in its drag.

The effects of sweep angle are also summarized quantitatively in the figure below, which shows the lift-to-drag ratio of a swept wing versus an unswept wing as a function of flight Mach number. In light of the preceding, it becomes clear why some sweepback of a wing is used to minimize transonic and supersonic drag. On the one hand, as an airplane with an unswept wing approaches Mach 1, then its drag increases rapidly, and its lift-to-drag ratio plummets immediately. On the other hand, it can be seen that the highly swept delta wing has a much lower lift-to-drag ratio at low Mach numbers compared to the unswept wing, but it has a much more acceptable lift-to-drag ratio in supersonic flight. The variable-sweep wing attempts to merge the benefits of unswept and swept wings over the full Mach number envelope of the aircraft, but at the price of structural complexity, amongst other issues.

The variation in the lift-to-drag ratio versus Mach number into the supersonic regime of unswept and swept wings.

Delta Wing Supersonic Designs

For sustained supersonic flight, it is known that a delta or triangular wing is close to the optimal type of wing planform, first theoretically defined by R. T. Jones in 1947 and has been confirmed using measurements. The advantage of a delta wing is that it can have a large sweep angle and a greater wing area than if a wing just swept back. However, it has also been found that other wing platforms can give similar aerodynamic benefits of low wave drag in supersonic flight.

Many variations of the delta wing design emerged in the evolution of supersonic aircraft, as shown in the figure below. The tailless delta wing is the classic design, although experience has shown it could be better from a handling qualities perspective. To this end, tailed delta wings have been used to improve pitch control and overall handling qualities of the airplane, especially when transitioning to and from supersonic flight. In addition, compound and ogival delta wings have been found to have better low-speed and high angle of attack characteristics while still having all of the advantages of supersonic flight.

Examples of some variations of the basic delta wing planform.

The closely-coupled canard delta configuration is popular because it gives better overall flight characteristics for both supersonic and subsonic flight. The design has become common on supersonic fighter airplanes, including the Eurofighter Typhoon, as shown in the photograph below. This closely-coupled configuration has a smaller “all-flying” delta fore-plane or canard in front of and above the main delta wing. This canard delta configuration beneficially modifies the airflow over the wing at high angles of attack, keeping it more attached and giving the airplane much lower landing speeds and better low-speed handling qualities. The all-flying delta-shaped foreplane also gives significant pitching moment authority for control, trim, and maneuverability.

The Eurofighter Typhoon has a closely-coupled canard delta wing.

Several other issues emerged with pure delta-wing airplanes including:

1. Controlling the significant changes in pitching moments on the airplane relative to its center of gravity as it transitioned to and from a supersonic flight, this being difficult to trim out with the normal use of the flight controls on the wing.

2. Concerns about the airplane’s low-speed flight characteristics, the thin wings being prone to stall at low angles of attack, along with a susceptibility to develop a behavior known as wing rock. Therefore, the early delta wing airplanes all had to have very high landing speeds, raising many operational challenges.

3. Because of the need to use relatively thin wings for aerodynamic reasons, it was challenging to obtain the required structural strength and stiffness of the wing while also ensuring that it was free from aeroelastic twisting and flutter issues.

With a delta wing or one of its variants, the supersonic wave drag may be minimized further by employing Whitcomb’s Area Rule along the length of a slender fuselage, which can also be used to improve the spanwise lift distribution over the wing as it passes through the fuselage; Dietrich Küchemann had originally hypothesized these ideas during WW2. The subtle use of the Area Rule is often noted when examining the fuselage shapes of supersonic fighter airplanes, which may have carefully contoured fuselage shapes to give smooth variations in the cross-sectional shape of the aircraft. A constraint in this aerodynamic shaping process for a fighter is engine integration, and the fuselage must contain the engine, the afterburner, and all of the engine-associated systems.

The principle of the “area rule” is to maintain a smooth variation in the net cross-sectional area of the airplane to reduce compressibility effects and wave drag. Wind tunnel experiments and flight testing have both confirmed the benefits of applying the area rule during airplane design at transonic and supersonic flight Mach numbers.

Variable Sweep Supersonic Wing Designs

To widen an aircraft’s flight envelope, designers strive to meld the benefits of different aircraft shapes that are known to perform well in different flight regimes. For example, for a supersonic airplane, it is desirable to combine the benefits of efficient subsonic and supersonic flight. These benefits can be accomplished by using a variable-sweep or “swing-wing” design, as shown below. However, it is not an easy solution to engineer, in part, because of the extra mechanical complexity and weight.

Examples of military variable sweep or “swing-wing” aircraft, which are designed to give the aerodynamic benefits of both subsonic and supersonic light.

There can be several missions where a supersonic capability combined with good subsonic efficiency is a highly desirable characteristic. For example, the mission may require an efficient subsonic cruise over considerable ranges, thereby conserving fuel, followed by a shorter supersonic flight to the target and then a subsonic cruise to return to base.

The variable sweep or “swing-wing” design can also better maintain trim as the center of lift on the wing changes from subsonic to supersonic flight and back again, and there is also a center of gravity movement. On the F14, F-111, and B-1 supersonic bombers, the wings are swept forward for takeoff and landing, significantly reducing the stall speed and improving the low-speed handling qualities. For example, the lift-to-drag ratio of the F-111 airplane is shown in the figure below for both subsonic and supersonic flight. Notice the improvement in the lift-to-drag ratio as the wings are swept forward, which is an expected behavior based on the corresponding uncrease in wing aspect ratio. A lift-to-drag of between 4 and 5 in supersonic flight is typical for any swept delta-wing airplane.

Lift to drag ratio of the General Dynamics F-111 as a function of wing sweep angle at subsonic and supersonic flight Mach numbers.

A disadvantage of such a swing-wing airplane is the added structural weight and complexity of the sweep pivot joint and associated actuator mechanisms. Also, it has been found that variable geometry wings are more challenging to make stealthy, which is a big concern for the military because the design is more structurally challenging to build than a regular (fixed) supersonic wing. Another issue with a variable-sweep wing is that it is impossible to use pylon stations on the outer (swing) part of the wing. An examination of any military fighter airplane will show that wing pylons are generally always needed to carry bombs and air-launched weapons.

Engines for Supersonic Flight

Engines for supersonic airplanes have ranged from rocket engines to turbojets to turbofans. In supersonic flight, the inlet to an engine must be designed so that the intake to the compressor occurs at subsonic speeds. This goal is achieved by using a duct and/or doors or baffles to give a total pressure recovery at the end of the inlet diffuser before the compressor stage, as shown in the image below for the Roll-Royce/Snecma Olympus 593 engines used to power Concorde.

Diagram of intake ramp and nozzle configurations for Concorde’s Roll-Royce/Snecma Olympus 593 turbojet engines: (a) Takeoff, (B) Supersonic cruise, (C) Landing with reverse thrust.

The thrust from a turbojet engine tends to increase linearly with supersonic flight Mach number, which is also from the increased mass flow rate through the engine at supersonic speeds. Still, their relatively high specific fuel consumption can be an issue. While many supersonic airplanes have used turbojet engines with afterburners in the past, the low bypass turbofan (with an afterburner) can be designed to give much better propulsive efficiency over a wider range of flight Mach numbers, thereby giving the airplane good fuel consumption throughout the flight envelope.

However, a continuing challenge with any supersonic airplane is maintaining an efficient supersonic cruise for more extended flight times. Therefore, besides the engine requirements, there is always a need for low drag, so, in general, the airframes must be highly streamlined, and the wings have to be relatively thin with short wingspans.

Supersonic Transport (SST) Airplanes

There was much interest in developing supersonic transport (SST) airplanes during the 1960s, the initial being effort led by the British and French. The Anglo-French Concorde became the World’s first SST, which flew first in 1969 and went into operational service in 1976. The Concorde used a tailless ogival-shaped “slender-delta” wing, which was carefully designed to give a good lift-to-drag in supersonic flight but also good low-speed flight characteristics.

The Russians soon followed with the Tupolev Tu-144, which bore a close resemblance to the Concorde but was distinctive in its use of retractable foreplane canards. Both the Concorde and Tu-144 were designed for cruising at just over Mach 2 at 60,000 feet, which was the highest Mach number that could be obtained without excessive kinetic heating of the airframe. As a result, both the Concorde and the Tu-144 had to use special aluminum alloys for their construction and various design features to allow the airframe to expand during flight. Conventional airplanes use various types of aluminum alloys, but these materials tend to soften and lose their strength at the temperatures produced by kinetic heating at Mach 2 and above.

Only twelve production Concorde SSTs were built, and it was never to prove economically viable for broader airline service. Nevertheless, it was a very popular aircraft with both the passengers and the flight crews alike. British Airways and Air France flew the Concorde for over 25 years until it was abruptly retired in 2003 after a crash. Concorde holds many aviation records, including the fastest transatlantic airliner flight (New York JFK to London Heathrow) in 2 hours and 53 minutes. However, the Tu-144 was unsuccessful as a commercial aircraft, and after two crashes, including one on a delivery flight to Aeroflot, it was permanently retired after only 55 passenger flights.

The Anglo-French Concorde and the Russian Tu-144 were SSTs designed to cruise near Mach 2 while carrying about 120 passengers.

In the 1970s, Boeing planned to develop an SST, initially using a variable geometry or swing-wing design, the final evolution of the concept shown in the figure below. The idea of the “swing-wing” design has already been discussed, which for civil airplanes means that it will be able to fly efficiently at subsonic speeds over land and in terminal areas while also having all the advantages of an efficient supersonic cruise.

Proposed SST designs considered by Lockheed and Boeing in the early 1970s.

However, despite the potential aerodynamic benefits of a swing-wing concept, the detailed engineering done by Boeing proved too challenging from an airframe weight perspective. Boeing’s final 2707 SST design concept was a more conventional supersonic delta-shaped wing. The Boeing design was also much larger than the Concorde or the Tu-144 and was intended to cruise at Mach 3. However, compounding technical problems, high development costs, concern about sonic booms, and the lack of a passenger market for the airlines eventually led to its cancellation.

Vortex Lift & the Slender Delta Concept

Like most delta-wing fighter-type airplanes, they do not have good aerodynamic characteristics at low flight speeds and high angles of attack, such as during takeoff and landing. Takeoff and landing speeds tend to be high for such aircraft and they require long runways. Concorde was explicitly designed to use a vortex-lift concept to improve the low-speed, high-angle of attack flight. The airplane had no high lift devices, so to reduce the landing speeds, the wing was designed to promote a phenomenon called vortex lift.

In the early 1950s, Weber and Küchemann at the Royal Aircraft Establishment published a series of papers on a new type of delta-wing planform, which became known as the “slender delta” concept. The idea was that if designed appropriately, delta wings could produce strong, stable vortex flows on their upper surfaces at low airspeeds and high angles of attack, as shown in the schematic below. The presence of these vortices creates very low-pressure zones and so causes the lift on the wing to be greatly increased. All modern supersonic airplanes are designed specifically to take advantage of the benefits of vortex lift at low airspeeds.

The creation of vortex lift from a slender delta wing at high angles of attack allows supersonic airplanes to have much lower takeoff and landing airspeeds.
Küchemann is most famous for his book The Aerodynamic Design of Aircraft and this book is considered by many to be the classic text on modern aerodynamic design. After Küchemann died in 1976, Weber completed the book in 1978. In addition, Weber developed methods to predict the drag on a slender delta-winged aircraft during supersonic flight.

Weber and Küchemann determined that the lift from the leading-edge vortex could be increased by maximizing the slenderness of the wing. This approach would retain good supersonic performance but also allow for high values of maximum lift at low airspeeds, as shown in the figure below, thereby offering much lower takeoff and landing speeds.

However, to achieve this condition, Concorde had to be flown at an extremely high angle of attack, which was a distinctive (if not rather alarming) flight attitude during takeoffs and landings. Indeed, the angle of attack on takeoff and landing was so high that the airplane was designed with a “drooped-nose” feature to allow the pilots to see over the nose of the airplane. Concorde was also distinctive because of its exceptionally tall landing gear, which was needed to enable the airplane to rotate to the required angle of attack for takeoff while still on the runway.

Any Future for SSTs?

The Anglo-French Concorde remains the only successful SST so far. Although many other SST designs have been studied, it remains to be seen if any future SST concept will ever prove itself economically viable for scheduled airline service. The inherently much lower lift-to-drag ratios of airplanes flying at supersonic speeds, even if the wing and airplane as a whole are highly optimized, means that SSTs will always burn much more fuel over a given range than subsonic airplane designs. The resulting seat-mile costs are considerably higher than a subsonic airplane (perhaps 3 to 5 times or more), even though the advantage is that the airplane’s flight time is less because it can cover the same distance in less than half the time.

However, the maximum achievable range of a supersonic airliner will always be much less than a subsonic airliner because of the higher fuel burn. Like the Concorde, the trans-Atlantic routes for an Mach > 2 capable SST are undoubtedly feasible. However, for longer ranges, such as over the Pacific from the USA to Asia or Australia, tops for refueling will be needed and will likely erode any speed advantage, especially for only a Mach 1.5 to 1.7 capable SST. Longer ranges may also be required using alternative routings to minimize the effects of sonic booms over land, again eroding any potential speed advantage unless the SST capable of Mach 2 or greater.

Kinetic Heating

A serious problem facing supersonic (and hypersonic) flight vehicles is the kinetic heating of the airframe caused by the friction of the air, which for the Concorde, was a serious concern at the design stage. The graphic below illustrates just how hot the different parts of the Concorde became at its cruise speed of Mach 2.1. The skin temperature ranged from a peak of 130^{\circ}C at the nose to 93^{\circ}C at the tail, which caused the airplane to expand about 18 cm or 7 inches in length during its flight. The airframe expansion meant that some special structural design features were needed to allow the airframe to expand without setting up high structural stresses.

Temperature contours on the Concorde at its cruise Mach number of 2.1, which meant that special high-temperature aluminium alloys needed to be developed.

Traditional aluminum alloys cannot survive these high temperatures, so special materials had to be developed to build Concorde. To this end,  high-temperature aluminium alloys were used for its construction, including light alloy Hiduminium RR58, high-temperature steel, stainless honeycomb, and resin-bonded glass composites. Hiduminium RR58, which has a much higher melting point than conventional aluminum alloy, was used for most of Concorde’s structure. In addition, steel was used for sections subjected to even higher temperatures and the highest loads, and resin-bonded glass fiber for regions where the skin temperature was the highest such as at the nose.

Sonic Booms

A problem facing SSTs (as well as any other type of supersonic or hypersonic flight vehicle) is the production of sonic booms. The leading and trailing shock waves formed by a supersonic airplane (one shock at the nose called the bow shock and another at the tail) appear to an observer on the ground as a pressure pulse in the form of an overpressure and an under-pressure, as shown in the schematic below. This pressure change is heard (and felt) as by an observer on the ground as an impulsive “boom-boom” noise. Besides being annoying, if not startling, the intensity of the pressure pulse from Concorde produced noise levels over 135 dB, enough to rattle or break windows on buildings. Nevertheless, research continues into ways of mitigating sonic booms.

The problem of “sonic booms” created by supersonic (or hypersonic) aircraft flying overhead is a significant cause for concern for the general public. It remains to be seen if this problem can be mitigated in the future.

In 2017, NASA issued a draft request for proposals for developing its Quiet Supersonic Transport (QueSST) low-boom flight demonstrator. This program called for the design and flight testing of a new X-plane to support the development of future-generation SST concepts. The contract was awarded to Lockheed Martin, and its Skunk Works X-plane supersonic demonstrator, as shown in the artist’s rendering below, is expected to cruise at 55,000 feet and Mach 1.4. One purpose of the flight tests will be to help quantify community response to supersonic flight over land. As of September 2022, the aircraft was constructed and readied for its first flights in 2023.

The NASA/Lockheed Martin Skunk Works X-plane design is expected to cruise at 55,000 feet and Mach 1.4. Its shape is designed to generate a softer sonic boom.

Smaller supersonic transport airplanes in the form of supersonic business jets or SSBJs have received more recent attention than larger commercial SSTs. SSBJs concepts are being designed by several companies, the intention being to transport from as few as 10 to as many as 80 passengers. Typically, SSBJs would be about twice the size of a typical subsonic business jet but much smaller than Concorde.

However, the potential SSBJ players all face immense technical and economic challenges, including a hefty amount of public skepticism. One major issue is finding engines in a civil market that currently does not have any engine manufacturers with “off-the-shelf” engines that are suitable for supersonic flight, at least for civil applications. In addition, the cost of developing a new engine could be prohibitive for most manufacturers based on the limited potential market demand for SSBJs. Another issue is for an SSBJ to gain certification from the FAA, which currently prohibits supersonic aircraft from flying over the U.S. mainland, as in most other countries.

Several groups believe that the technical issues, economic concerns, sonic booms, emissions, certification, etc., can be solved at a smaller scale, potentially opening up a significant future market for airlines and others. It remains to be seen, however, if this comes true in the decades to come. Nevertheless, the appeal of vastly reduced traveling time by flying at supersonic speeds remains attractive. Still, the overall economics remain questionable for the airlines, even for the business jet market. It is unlikely that passengers will be flying supersonic anytime soon.

Hypersonic Flight

Hypersonic flight is defined as flight at speeds at or beyond Mach 5. However, unlike what happens as the aircraft approaches Mach 1, there are no distinctive changes in the flow characteristics as an aircraft reaches hypersonic Mach numbers. Such high Mach numbers have only been achieved by rockets, certain spacecraft, and special high-altitude research aircraft. Theodore von Kármán was one of the first to analyze the problems of hypersonic flight, stating: “At such speeds, even in rarified air, the surface will be heated to the temperatures none of the known materials can withstand. The problem of the thermal barrier is much more complicated than the problem of the sound barrier.”

Challenges

Many additional aerodynamic and other technical challenges occur when flight vehicles begin to fly at hypersonic airspeeds, a summary being shown in the figure below. The main issues include the proximity of the shock waves to the vehicle, kinetic heating, viscous interactions, chemical reactions, and erosion of the surface.

There are complex aerodynamic and other issues that occur in hypersonic flight.

In more detail, these effects at hypersonic flight Mach numbers include, but are not limited to, the following:

1. The Mach angle is small (see Eq. 1), so the shock waves bend back to steep angles and can begin to interact with the airframe, which causes disruptions to the flow in the boundary layer. Usually, the boundary layer is relatively thin. Still, at hypersonic speeds, as the shock waves touch the boundary layer, the resulting flow field becomes very complex, and flow separation is likely.

2. The shock waves are very strong at hypersonic speeds, and the air undergoes significant temperature changes that affect its chemistry. The need to include chemistry in any aerodynamic analysis of hypersonic flows significantly raises the complexity of the solution methods.

3. The proximity of shock waves to the body and the effects of skin friction causes a large amount of aerodynamic heating on vehicles flying at hypersonic speeds. Even the use of high-temperature aluminum and titanium normally used for supersonic aircraft construction will melt.

4. The flow over the vehicle can strongly interact with the propulsion system. In this regard, the propulsion system design must be tightly integrated into the entire vehicle design. Usually s hypersonic vehicle must be designed so that its shape pre-compresses the air before it enters the engine.

5. A sonic boom from Mach > 3 and hypersonic aircraft will be loud enough (over 160 dB) that it can cause severe damage to structures on the ground and permanently damage human ears. It is unlikely that a hypersonic flight vehicle could ever be allowed to fly over populated areas. Nevertheless, the technical challenge remains for the future.

History

The issues associated with hypersonic flight first became apparent in the 1950s with the development of ballistic missiles, which was initially based on German research on the V-2 rocket conducted during WW2, which was capable of flight at Mach 5. However, for such missiles to have an intercontinental range, it was necessary to launch them out of the Earth’s atmosphere into space and then re-enter the atmosphere much nearer to the target. When tests on the first of such missiles were made based on the German designs, which had very pointed nose shapes, they completely melted during the re-entry phase from the intense shock waves and aerodynamic heating at Mach 5 and higher.

The answer to the problem, which was research led by H. Julian Allen and Alfred Eggers at the NACA, was opposite to that expected based on conventional aerodynamic wisdom, which was to make the nose of the missile much more rounded, as shown in the flow visualization images below. This type of “blunt body” design created a shock wave that stood off the nose and caused a boundary layer flow over the surface of the nose cone that prevented the excessive heat of re-entry from melting the structure. In contrast, pointed nose shapes allowed the shock wave to touch the surface with too thin a boundary layer to offer much thermal protection. Tests with the modified missiles confirmed the validity of this method, although the nose still needed to be made of high-temperature materials.

NACA research showed that at hypersonic Mach numbers a rounded nose created a shock wave that stood off the surface of the body and so reduced the surface temperatures and prevented the body from melting.

The information gained about high-speed flight and hypersonics in both the wind tunnel and through flight tests contributed significantly to the development of the Mercury, Gemini, and Apollo space programs. Flight test work on hypersonics was conducted by the U.S. Air Force and NASA during the 1950s. The X-15 research airplane flew for nearly ten years from 1959 and set the world’s unofficial speed and altitude records of 4,520 mph (Mach 6.7) and 354,200 feet in a program to investigate all aspects of piloted hypersonic flight.

The X-17 was a missile program that used a three-stage rocket to get the nose section to reach conditions between Mach 10 and 20 in the lower atmosphere. The nose cones survived for a short time before vaporizing but long enough for valuable measurements to be made. Ultimately, the solution of minimizing kinetic heating effects on re-entry vehicles was also to use ablative materials to help shed the heat buildup.

During the 1980s, NASA began considering a single-stage hypersonic vehicle to replace the Space Shuttle. The National Aerospace Plane (NASP) was intended to take off from a standard runway used by airliners, and once the aircraft had reached sufficient airspeed, scramjet engines would power the aircraft into hypersonic flight. Finally, a rocket system would take the NASP into orbit. The NASP eventually became the Rockwell X-30 research vehicle. While the project did not reach the flight stage, a one-third scale demonstrator was built that became known as the X-43. Three X-43 flight vehicles were built. The first was destroyed after an in-flight malfunction, but the other two flew successfully with the scramjet operating for approximately 10 seconds.

Hypersonic Aircraft Design

A thin flat plate is known theoretically to be the most efficient aerodynamic shape for a hypersonic flight, its lift-to-drag ratio being the highest achievable. However, a plate is an unrealistic design solution for a practical flight vehicle because of the need to contain engines, fuel, systems, payload, etc. Wind tunnel measurements of candidate hypersonic shapes have shown that their maximum achievable aerodynamic efficiency in terms of lift-to-drag ratio, L/D, decreases with increasing Mach number according to

(2)   \begin{equation*} \left( \frac{L}{D} \right)_{\rm max} = \frac{4 ( M_{\infty} + 3)}{M_{\infty}} \end{equation*}

which is often referred to as Küchemann’s equation. The conclusions to be drawn are that in hypersonic flight the aerodynamic efficiency is relatively low, and that increasing the lift-to-drag ratio will be challenging.

To maximize the aerodynamic efficiency of a flight vehicle in hypersonic flight then is known that some general design rules should be followed. In general, such vehicles should be blended body shapes with integrated forebodies, propulsion systems, and afterbodies. More specifically:

  • They should have as small a frontal area as possible, with a highly streamlined overall shape to minimize surface area.
  • A short-span, low aspect ratio wing should be used, with the fuselage suitably-shaped to generate lift, i.e., as a lifting body.
  • The propulsion system must be well-integrated into the vehicle’s overall shape to prevent shock waves from one component from adversely interfering with the flow at another component.

Waveriders

waverider is another type of hypersonic wing design with an improved lift-to-drag ratio. This goal is accomplished by using the shock waves generated by its own flight as a lifting surface, a phenomenon known as compression lift. The waverider concept originated from work on winged atmospheric reentry vehicles in the 1950s by Terence Nonweiler. A waverider is a delta-wing with some anhedral and is often referred to as a caret wing. With the advent of advanced hypersonic prediction methods, optimized waverider shapes have been developed, as shown in the figure below. However, heat dissipation is an issue with such optimized designs because of the closer proximity of the shock waves to the surfaces.

A hypersonic waverider concept optimized for Mach 4 flight.

Propulsion Issues

Achieving combustion and propulsion is another major problem at hypersonic speeds, and the most likely candidate is a variation of a ramjet engine called a scramjet, i.e., a supersonic ramjet, as shown in the figure below. A scramjet engine works on the principle that the shock waves on the vehicle can be used to slow down and compress the air to support combustion inside the engine.

The principle of a scramjet relies on the motion of the vehicle ito compress the incoming flow to support combustion.

To get a scramjet to operate, however, it must be accelerated close to the required flight conditions (usually close to about Mach 4) using some other propulsion system, such as a rocket engine. However, maintaining continuous combustion in the supersonic flow within the engine has met numerous technical challenges, one issue being that the extremely short transit time of the fuel mixture inside the engine to allow combustion.

The X-51A Waverider is set to demonstrate hypersonic flight. Powered by a Pratt Whitney Rocketdyne SJY61 scramjet engine, it is designed to ride on its own shockwave and accelerate to about Mach 6.

The Boeing X-51A scramjet demonstration aircraft was a waverider concept tested from 2011 to 2013, as shown in the artist’s impression below. The X-51A was used to demonstrate engine operations at hypersonic airspeeds and other aspects of hypersonic flight. The aircraft sustained a Mach 5 flight for 200 seconds during its longest tests. Measurements made during these test flights have been used for comparisons to measurements made in hypersonic wind tunnels and to predictions made using computational fluid dynamics (CFD).

Summary & Closure

Humans have always sought to fly faster, which has led to the development of aerodynamically optimized flight vehicles and advanced propulsion systems for supersonic and hypersonic flight. These challenges require the optimization of vehicle shape to minimize drag and shock wave interactions and the development of efficient engines for supersonic and hypersonic flight. The design of hypersonic engines remains a challenge, with scramjets showing some success. Kinetic heating at high speeds produces high surface temperatures, requiring the use of special materials such as ceramics. The future of hypersonic flight is likely to involve waverider vehicles.

5-Question Self-Assessment Quickquiz

 

For Further Thought or Discussion

  • Do some background research to determine what efforts are being explored to reduce the intensity of the “sonic boom” produced by supersonic airplanes.
  • Research some of the engineering issues that might be associated with kinetic heating on a supersonic airplane that cruises between Mach 2 and Mach 3. What kinds of construction materials might be required?
  • What kinds of supersonic passenger airplanes, i.e., SSTs, might there be in the future? Explain why their success ultimately depends on the availability of an engine suitable for supersonic flight.
  • Study the image below, which is a schlieren flow visualization image of what is produced when a bullet is fired from a gun. Can you explain what is happening here and the origin of all the waves?
  • Consider the Mach angle as a function of Mach number. What happens to the Mach angle at hypersonic Mach numbers, and why is this important?

Other Useful Online Resources

For additional resources on supersonic and hypersonic flight, follow up on some of these online resources:

  • Video about the World’s fastest subsonic airliner: The Convair 990A Coronado.
  • Hypersonic Waverider – How the USAF X-51A scramjet works – see the video here.
  • The Insane Engineering of the X-15 – see the video here.
  • The Lockheed A-12 – speed matters!
  • The SR-71 Blackbird – the king of speed!
  • Project cancelled: BAC TSR2 – The British Cold War strike and reconnaissance aircraft that was cancelled.
  • Great video about the English Electric Lightning –The British supersonic fighter and interceptor aircraft.
  • Video about the World’s fastest bomber: The Mach 3 capable XB-70 Valkyrie.
  • Death of a Valkyrie: The 1966 XB-70 Midair Collision
  • Kelly Johnson and the Skunk Works. The genius that changed aviation.