Challenges of Hypersonic Flight

Challenges of Hypersonic Flight

by: Athena Roberts

Written: April 1997

.Thankyou for inviting me over to speak with you all about Hypersonic Flight. In flight school, you might have briefly gone over this as a curiosity, so I will define hypersonic flight. We rather arbitrarily define the speed where hypersonic flight begins. This is because the effects of that realm manifest themselves over a wide range of speeds. Some occur around three times the speed of sound, or Mach 3, while others are not of any consequence until Mach 7. Designers picked Mach 5 as the beginning of hypersonic flight since that is about the average speed those effects occur. Flying at over Mach 5 presents the designer with many challenges. The three most important challenges are what materials to use, the actual design of the aircraft, and the proper engine to use. Conventional materials, designs, and engines will not work. Sorry to disappoint you, but there is no way to make a Cessna travel at hypersonic speeds, or even supersonic speeds.

What material to use is the first challenge. A hypersonic aircraft experiences very high temperatures and very high drag due to its speed. A hypersonic aircraft experiences temperatures of 3,000°F to 5,000°F on its nose. The leading edges of the wings experience 2,000°F to 3,000°F temperatures. The temperatures in the engine can reach 12,000°F (Pope-3 26:5). The high temperatures come from a combination of friction with the air and the shockwave. Aerospace engineers refer to that as "aerodynamic heating" (Reithmaier 257). We all understand how friction creates heat, so I will go on to heat generated from the shockwave. All aircraft that travel faster than sound create a shockwave. As you might have learned, a shockwave is a very rapid compression of the air on the aircraft since the air can't get out of the way fast enough. When you compress a gas, the temperature goes up. Very High drag is the next thing a hypersonic aircraft experiences. You all learned about drag and you experience it when you lower your flaps and landing gear for landing. Drag is the air's resistance to being pushed around. Combined with the intense heat, drag poses a major problem for a designer. Drag can cause an aircraft to break apart as it did when we first tried to break the "sound barrier." For a hypersonic aircraft, the drag is many thousands of pounds. Most of the materials available today would quickly melt, oxidize (burn), or fatigue under those conditions.

So, what do we build it out of? Designers responded to that challenge by coming up with new materials. Some of these materials are super alloys, refractory metals, and graphite and ceramic composites. Super alloys are usually chrome and nickel-based materials, such as Rene 41, that resist oxidation up to 2,000°F (Reithmaier 177:3). They have structural usefulness to about 1,900°F. The famous X-15 hypersonic research aircraft, which reached Mach 6.72 (Spick 99:8), was built from a chrome-nickel alloy called Iconel X nickel (Boyne 161:2). The next group of materials are refractory metals. These are metals with melting points above 4,000°F that can also retain their strength at very high temperatures (Reithmaier 177:1). We use those on parts of the aircraft that encounter temperatures of 1,900°F to 3,100°F. These are areas such as the leading edges of the wings and tail. Columbium, molybdenum, tantalum, and tungsten are refractory metals. The last group of materials is graphite and ceramic composites. Designers use these for areas that reach more than 3,100°F such as the nose and the combustion chamber of the engine. Graphite-copper and titanium aluminide with ceramic strands are two examples of that type of material (Pope-3 26).

Materials and design are so closely related that we must consider them simultaneously when designing a Cessna or a hypersonic aircraft. That is why I say design is the next challenge for building a hypersonic aircraft. This is an area with many original ideas. The most promising ideas for designs are lifting bodies, waveriders, airspikes, variable-flight-axis crafts, and neural-network control systems.

A neural network control system is one promising idea even though it is not exactly an aircraft design. It is a computer with many smaller computers (nodes) interconnected like neurons in the brain. It can process huge volumes of data at once (Pope-1 28). Each node assigns a weight, or value, to inputs from the other nodes. By changing values it can change the way it responds (Sweetman 40). That allows to teach itself to fly and to handle the many problems of hypersonic flight.

Some of these problems are squeezing of the shock layer, boundary layer separation, stability, and controllability. As an aircraft travels faster, its shock wave bends further back. At hypersonic speeds, the shock wave is more like a needle. Consequently, the space between the shock wave and the aircraft, called the shock layer, decreases. We call this "squeezing of the shock layer" (Smith 207:1). When this happens, high-energy air can mix with the boundary layer on the wing making it turbulent. As you learned in ground school, the boundary layer is the layer of air between the wing and where the air velocity reaches that of the free airstream. A turbulent boundary layer is thicker than one with laminar flow. That means higher drag, and, at hypersonic speeds, that means considerably higher drag. In extreme cases, it can cause a complete separation of the boundary layer. That is another problem of hypersonic flight. Shock layer squeezing and boundary layer separation lead to other problems like the stability and controllability of the aircraft. Avoiding disaster requires responses far faster and in greater frequency than a human pilot could handle. A neural network computer could handle those complex split-second decisions.

Lifting bodies are another promising design for hypersonic aircraft. A lifting body is an aircraft with no wings, or very small ones, that generates its lift from the shape of its body. It has several benefits. That it does not need a wing is the first benefit. For a wing to withstand the stresses of Mach 5+ requires a great deal of weight to make it strong enough. That extra weight outweighs the benefits of the aerodynamically superior wing-and-body design. No wings mean less weight. That means better performance, which is another benefit. A lifting body design can also use its body to compress the shock layer air before it enters the engine. Higher compression benefits engine performance just like with the engines in your Cessnas. Lifting bodies have other benefits: A thick center section for storage of fuel, fairly sharp leading edges for efficient aerodynamic performance, and a bottom surface bulged along the center line to force the boundary layer to the outside edges of the craft. That last one is a benefit because it limits engine ingestion of low energy air (Reithmaier 173).

Another promising idea is the waverider design. A waverider is a lifting body with a special twist. A waverider rides on its shock wave. Because of that, engineers design them to resemble the shape of the wave they will produce. When the plane catches up to its wave (which starts out detached), the wave attaches itself to the plane and drags it along (Ward 24:4). As you can see, that design would save a lot of fuel. Also, with a top speed of Mach 8, waveriders are well in the hypersonic realm .

The air spike is the next design idea to show promise. An airspike has several benefits. The first is it substitutes directed energy for the mass of a nose-cone to drive the air from the plane's path (Kandebo 66:3). It uses an electric arc plasma torch placed in front of the craft. In flight, the arc uses its concentrated energy to drive air radially from the aircraft's path like a blast wave. A low density air pocket forms behind it which reduces the heat transfer effects in the aircraft and that means less stress. It does such a good job that a vehicle traveling at Mach 25 would only experience the conditions of Mach 3 (ibid. 66:5). Mach 25 is what it takes to get to orbit. Another benefit is it alters the shock wave. It transforms the traditional strong window shattering conical bow shock wave into a weaker parabolic-shaped oblique shock wave. All that allows engineers the benefit of easier design. They can now make hypersonic vehicles that are ultralight, blunt-edged, and lens or saucer-shaped. That is an enormous improvement over even lifting body designs since they still require heavy reinforcement for the temperature and drag stresses of hypersonic flight. Additionally, the air spike concept could possibly help propel an aircraft if set up properly. It does this by compressing the air in the shock layer. Equipment then ionizes the air which allows two superconducting magnets that ring the craft to push the air past it. This set up would also end sonic booms since it eliminates the pressure changes that cause them (ibid 67:6).

Another design, promising in its originality, though it was not intended for hypersonic flight, is what I call the variable-flight-axis design. This is an aircraft with no fuselage so it is also a "flying wing." It is designed to fly two ways. For take-off and landing it flies in the conventional manner. Once it reaches a high speed, it changes its orientation to the flight path. It flies sideways. This allows it the benefit of high lift for low speed operations and low drag when flying at its design speed of Mach 1.6. The fuel savings are significant compared to the Concorde. It could carry 400 passengers from Los Angeles to Tokyo in half the time as a 747 and only burn slightly more fuel ("Fuselage-Free" 20). However, this design is also very unstable. So unstable that it requires an onboard computer to constantly adjust the rudder and wing flaps to keep the airplane from breaking apart.

The type of engine is the final most important challenge of hypersonic flight. There are four types of engines being considered for a hypersonic aircraft. They all have various advantages and limitations. For that reason, Engineers intend to use none of them as the sole means of propulsion. These engines are the turbojet, the ramjet, the scramjet, and the rocket. After I describe them, I will show how engineers are combining them to produce a viable hypersonic vehicle.

The first engine is the turbojet ("Energy Conversion"). You are probably all familiar with this engine. One of its variants, the more efficient high-bypass turbofan, propels modern passenger jets. Other types of turbojets power the F-16 and the new F-22 fighters. A turbojet works by sucking air in and compressing it with a series of blades that alternately rotate and remain stationary. We call the ones that rotate "rotor blades" and the ones that don't "stator blades." This newly compressed air reaches the combustion chamber where nozzles squirt in fuel and a burner ignites it. From here, the gas expands rapidly and moves out the back to provide thrust. Along the way, it spins a turbine that drives the compressor in the front of the engine. The hotter the gas the more thrust that is available. The temperature rating of the turbine is the only limiting factor to available thrust. If the exhaust gas temperature gets too high, it can cause the turbine to fail which could lead to a possible explosion if one of the hot shards hit a fuel tank. Current technology limits turbojets to around Mach 3. A turbojet's two primary advantages over the ramjet and scramjet is it can start from a standstill and it can fly at low speeds. That is because it can compress its own air unlike the ramjet and scramjet. Its advantage over the rocket is that it does not need to carry an oxidizer to burn the fuel since it gets its air from the atmosphere.

The next hypersonic aircraft engine candidate is the ramjet. A ramjet is also called a "flying stove pipe." It is possible to literally look through one end and see out to the other. That is because it has no compressor and no turbine. Ramjets normally power missiles and drone aircraft since they have no self starting or low speed capability. A ramjet works by the "ram effect." At high speeds, the shape of the ramjet intake slows down the supersonic airflow to subsonic speeds. This causes a rise in pressure and a rise in temperature. From here fuel is injected into the airstream as it heads to the burner. The burner ignites the fuel-air mixture and the expanding gas goes out the back to provide thrust. Ramjets pickup where turbojets leave off at Mach 2.5 to 3. They are practical up to over Mach 5. They share the same advantage with the turbojet over the rocket in that they are air breathing. The advantage they have over the turbojet is the higher speed capability and simplicity.

Scramjets are the primary engine for sustained hypersonic flight. They cover speeds of Mach 5 and beyond. A scramjet is a Supersonic Combustion Ramjet. That means instead of compressing the airflow by slowing it down to subsonic speeds before combustion it slows it down to supersonic speeds. There are a few reasons for doing that. The first is when you slow down a high speed air you get a rise in temperature along with a rise in pressure. At hypersonic velocities the air would be around 3600°F when it reached the combustion chamber. At that temperature most fuels would simply decompose ("Jet Engine"). The fuel would absorb rather than release energy and the ramjet would only produce drag. Another reason is that, at those high temperatures, the ramjet structure can fail. Also, the last reason is the excessive pressure results in pressure losses. The scramjet solves those problems by not slowing down the airflow as much. At high Mach numbers, it is still possible to achieve significant compression with this design. Scramjets are unique in that air compression and exhaust acceleration occur mostly outside of the engine inlet. The shape of the aircraft's belly and shock layer squeezing provide the needed compression. The rear of the aircraft's belly provides the proper shape to accelerate the exhaust. The result is that a scramjet is little more than a supersonic combustion chamber. A major problem with supersonic combustion is ignition. Some people have described it as being like trying to light a match in a tornado. It is very difficult to keep a flame going for ignition at those high speeds. For that reason, engineers decided to use slush hydrogen as a fuel. Hydrogen is the only fuel that can handle high temperatures and ignite in a high speed airflow. Hydrogen also has the added benefit of being able to cool the structure.

The rocket is the final engine considered for hypersonic flight. Rockets have a pretty much unlimited maximum speed. However, since they must carry their own oxygen, that adds a great deal of weight. So, they don't find much use in typical aircraft operations. Their main uses are for accelerating ramjets to a satisfactory velocity for ignition and for space flight. Rocket engines function by mixing fuel with an oxidizer and igniting it. The gas then expands violently through a nozzle. That provides the propulsion.

Because none of these engines can perform all the necessary functions for achieving hypersonic flight, engineers choose to combine them. The two primary combinations are the turboscramjet and the scramjet/rocket. The one chosen depends on the mission of the aircraft. A turboscramjet is a scramjet with a turbine engine inside. At takeoff and at low speeds, the turbojet provides power. As the speed increases to Mach 3, the engine bypasses the airflow into the scramjet portion which acts as a ramjet up to around Mach 5 or 6. Then it acts as a scramjet for the higher velocities. This switch from ramjet to scramjet is possible by changing the position that the hydrogen is injected into the airflow. By injecting the hydrogen into the airstream earlier during scramjet mode, the hydrogen still has enough time to mix and burn properly (Pope-2 47). As with normal scramjets, the aircraft design is very important in achieving a functioning turboscramjet. aircraft designed for transporting people and goods around the world would use this type of engine.

The other type, the scramjet/rocket combination, is the engine design intended for the National Aero-Space Plane (NASP). This is a vehicle designed to take off as a normal aircraft and get to orbit under its own power. It does this by first igniting the rocket for takeoff and acceleration to Mach 3. At that time, the scramjet takes over and propels the craft to Mach 18. At this point, it has gained enough altitude so that an air-breathing scramjet can no longer work. Here, the rocket reignites to launch the craft into orbit. The benefit of this engine design is that both the scramjet and the rocket can run off of hydrogen. Conversely, the turboscramjet would require kerosene to power the turbojet along with hydrogen for the scramjet.

As you can see, hypersonic flight presents certain challenges for an engineer. The three main ones being what to make it out of, what form it should take, and how to power it. Designing a hypersonic aircraft requires new way of doing things and new ways of thinking since typical designs will not work. I hope that this has been an enjoyable and educational experience for you.

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