In This Issue
Summer Bridge Issue on Aeronautics
June 26, 2020 Volume 50 Issue 2
The articles in this issue present the scope of progress and possibility in modern aviation. Challenges are being addressed through innovative developments that will support and enhance air travel in the decades to come.

Flying at the Edge of Space and Beyond: The Opportunities and Challenges of Hypersonic Flight

Thursday, June 25, 2020

Author: Kevin G. Bowcutt

The primary benefit of hypersonic flight is extreme speed, whether to engage a time-critical or well-defended military threat, travel between global cities in a couple of hours, or achieve Earth orbit.

Hypersonic Flight and How It Is Achieved

Although the definition of supersonic speed is unambiguous—flying faster than the speed of sound—the beginning of hypersonic flight has no clear boundary. Mach 5 is generally accepted as the transition point from supersonic to hypersonic flow, but it is less clearly delineated by a speed boundary than a thermal boundary. Air passing over a vehicle traveling at hypersonic speeds becomes hot enough from shock compression and viscous dissipation to change its thermodynamic and chemical nature. The onset of these changes depends on several variables, including vehicle geometry.

Historically, beginning with rocket flight experiments and the X-15 rocket-plane (1959), hypersonic speed was attained with rocket propulsion, having no other options. But low fuel efficiency limits the practical use of rockets to propel airplanes over long distances. Because of the large fraction of propellant rocket-powered vehicles must carry (typically about 90 percent), little mass margin remains to accommodate design features and technology needed for the level of robustness and safety required by commercial airplanes.

Unlike rockets, hypersonic air-breathing engines, such as the ramjet or supersonic combustion ramjet (scramjet), can make long-range hypersonic travel practical. The primary benefit of air-breathing engines is their relatively high fuel efficiency compared to rockets, measured in terms of a propulsion performance parameter called specific impulse (Isp), which is engine thrust divided by propellant weight flow rate.

Figure 1 

Figure 1 illustrates the fuel efficiency benefit of air-breathing engines (e.g., J58 on the SR-71) compared to rocket engines (e.g., SSME on the Space Shuttle) for hydrocarbon (kerosene-type) fuels. Lower Isp levels for rockets are the result of their having to consume carried oxidizer as well as fuel, increasing the propellant weight flow rate in the denominator of the Isp equation. Although air-breathing engines generally achieve ­higher Isp than rockets, air-breathing Isp decays with Mach number, as shown in figure 1. This trend is the result of air pressure rise from combustion heating, which is the source of thrust; as the air pressure rise decreases with speed, the thrust also decreases.

One of the greatest challenges to hypersonic flight is the availability of a propulsion system to efficiently accelerate vehicles from rest to hypersonic speed and then cruise at that speed. Due to thermal limits of materials and air itself, there is no single air-breathing engine type capable of doing this.

For example, above about Mach 3 to 4, the temperature of turbofan engine components will exceed material limits. This limit can be avoided by using a ramjet engine that has no turbo­machinery components. But ramjets cannot operate from rest. They also are thermally limited because they slow down air from supersonic to subsonic speed through the action of shock waves, heating and pressurizing the air to exponentially higher levels with increasing speed.

At speed above about Mach 5 to 6, the air temperature becomes too high for efficient combustion for chemistry reasons. The ramjet flow temperature limit can be avoided by reducing the strength of inlet shock waves to maintain supersonic flow in the combustor, creating a scramjet. Multiple engine types (e.g., rockets, turbofans, ramjets, and scramjets) must therefore be used or integrated in combined cycles (in one of myriad possible ways) to enable hypersonic flight.

Figure 2 

As depicted in figure 2, scramjets are simple devices in principle—just shaped ducts with fuel injectors—but very challenging to properly design in practice. They must efficiently slow and compress air through the action of shock waves, then mix and burn fuel in a supersonic airstream in about a millisecond at relatively low air pressure but high air temperature.

Most scramjets are actually so-called dual-mode scramjets (or dual-mode ramjets), which operate as ramjets up to about Mach 6, with combustion occurring in air traveling slower than the speed of sound (i.e., subsonic flow), and thereafter as scramjets with combustion occurring in a supersonic air stream.

The Need for Speed

With the advent and explosive growth of information technology, the speed of communications and business has dramatically accelerated over the past three decades. In contrast, except for extremely limited Mach 2 Concorde service from 1976 until 2003, the speed of transportation hasn’t risen much above eight-tenths the speed of sound since the beginning of the jet age in 1958 with the introduction of the Boeing 707. This speed-of-life disparity has produced a demand for higher-speed air travel, therefore companies such as Aerion (with Boeing ­support) and Boom are developing a supersonic business jet and a supersonic airliner, ­respectively, and Boeing, the Spaceship Company, and ­Hermeus (along with organizations in Europe and Japan) are exploring design concepts for hypersonic passenger airplanes.

Figure 3 

Figure 3 plots flight time saving as a function of cruise speed, depicted as the fraction of time a subsonic airliner takes to ­travel 5,000 nautical miles (nmi), approximately the distance from Los Angeles to Tokyo, accounting for the time and distance needed to accelerate and climb to cruise speed and altitude, and then to decelerate and descend to landing. As is evident from the plot, increased cruise speed has a dramatic impact on time saving until about Mach 5, beyond which time saving ­rapidly diminishes. One source of this asymptotic time-saving behavior is the ­longer time and distance required to accelerate and decelerate as cruise speed increases, reducing the time spent cruising. At Mach 10 cruise speed, the length of the cruise leg is almost zero. If aircraft range could be increased substantially, higher speed would produce measurable time saving.

When travel speed is fast enough, making transit time short enough, round-trip international travel can be accomplished in a single day, saving days, not just hours. There is tremendous value in this revolutionary level of time saving for some of the flying public.

Boeing is designing a Mach 5 passenger airplane to explore the market potential, and the design and technology requirements, of a future hypersonic commercial airplane. The airplane is being designed for a trans-Pacific range of 5,000 nmi and sized to carry between 10 and 200 passengers. An artist’s concept of the airplane shows some of its major design features (figure 4).

Figure 4 

With a top speed of Mach 5, too slow for a scramjet, the airplane would be powered by a turbo-ramjet engine, a turbofan engine for low-speed thrust integrated with ramjet for high-speed thrust. The engines would burn a sustainable hydrocarbon fuel derived from biomatter to mitigate carbon emissions (more on this below). The fuel would also have so-called endothermic properties, allowing it to absorb large amounts of heat to cool air flow and airplane systems. It is anticipated that such a hypersonic airliner could be ready for market in 15 to 20 years.

Technology Advances That Have Enabled
Air-Breathing Hypersonic Flight

Three technology advances over the past three decades have enabled hypersonic air-breathing flight to become a reality: maturation of dual-mode scramjet engines through ground and flight testing, enhanced high-­temperature metal and ceramic materials, and dramatically faster and higher-fidelity computational simulation capabilities for fluid dynamics (applied to aero­dynamics and propulsion), materials, structures, and multi­disciplinary system design optimization. The first two of these are described in the following sections; for a discussion of computational simulation capabilities, see Candler et al. (2015).

Maturation of Dual-Mode Scramjet Engines Through Ground and Flight Testing

The idea of an engine burning fuel in a supersonic airstream—i.e., the scramjet—was first reported by engineers at the National Advisory Committee for Aeronautics in 1958 (Weber and Mackay 1958). From the 1960s through the 1990s, significant scramjet research, design, and ground testing took place in the United States and several other countries, including the Soviet Union (Russia), France, Japan, and ­Australia. The greatest advances in scramjet technology development ­during this period were made with large ­investments from the US National Aero-Space Plane (NASP) program ­(Heppenheimer 2007; Schweikart 1998).

Also in the 1990s, initial attempts were made by Russia, at first independently but later in partnership with France and then the United States, to flight test a scramjet combustor by attaching it to the nose of a ­rocket-propelled missile. Results from these flight tests were inconclusive in terms of achieving supersonic combustion. Then in 2002, the University of Queensland in Australia flew a scramjet combustor attached to the nose of a sounding rocket and confirmed supersonic combustion. But these tests were focused exclusively on testing supersonic combustion in flight and did not attempt to measure thrust, the key requirement of a propulsion system.

Figure 5 

The most definitive initial demonstration of scramjet technology occurred in 2004, with NASA and industry partners, including Boeing, twice flying the X-43A (also called Hyper-X; figure 5)—a flight experiment spin-off from the NASP program that integrated a hydrogen-fueled scramjet on a lifting body airframe—at speeds of nearly Mach 7 and Mach 10. Because of the relatively low density of gaseous hydrogen only 10 seconds of scramjet data were collected, but this was enough to demonstrate for the first time in history that an ­airframe-integrated scramjet worked as theorized, almost 50 years after the idea of a scramjet was conceived. Not only did the scramjet successfully operate, it generated thrust greater than drag at Mach 7 and approximately equal to drag at Mach 10, very closely matching preflight engine analysis predictions and verify­ing the propulsion efficiency potential of scramjets.

The next step in the advancement of scramjet technology occurred in 2013 with the final of four flight tests of a hydrocarbon-fueled scramjet on the X-51A (also called the Scramjet Engine Demonstrator, shown in figure 5) by the US Air Force Research Laboratory with industry partners, including Boeing. After rocket boost, the X-51A operated on dual-mode scramjet ­power for 209 seconds before running out of fuel, accelerating from Mach 4.8 to 5.1. A key difference between the X-43A and X-51A was the latter’s lightweight airframe and scramjet, with fuel used to cool the scramjet, whereas the former had a heavy uncooled solid copper scramjet engine and heavy airframe, more akin to what would be tested in a wind tunnel. Since the X-43A and X-51A flights, other countries have flown scramjets too.

Enhanced High-Temperature Metal and Ceramic Materials

Vehicles that fly at the lower end of the hypersonic speed range (Mach 4 to 6) can be made primarily of high-temperature metals such as advanced titanium and nickel alloys and superalloys. These materials are structurally functional in the temperature range from about 1000°F to 2000°F (540°C to 1080°C), although nickel-based alloys are on the heavy side for airframe structure applications. Current research is focused on increasing the temperature capability of lightweight titanium alloys.

Beyond about Mach 6 is the realm of carbon- and ceramic-based composite materials, typically fibers made of carbon, silicon-carbide, alumina, or silica in a matrix of one of these materials. The temperature capability of these carbon- and ceramic-matrix-composite materials extends up to 3100°F (1700°C) for multiuse and up to 3600°F (2000°C) for single use.

Why a Hypersonic Airliner Now?

One might ask, if the supersonic Concorde was not economically successful, what is different today that would allow a hypersonic airliner to be successful? Much has changed since Concorde was developed, resulting in technical and economic factors that could make hypersonic travel practical today.

Technological Advances

Technology has advanced significantly across many fronts over the past 60 years to support the design of a hypersonic airplane:

  • Sophisticated computer-based modeling and simulation capabilities allow accurate analysis and optimization of aerodynamics, propulsion, materials, structures, and flight controls.
  • The entire airplane system can be optimized, concurrently accounting for all of these disciplines, using a process called multidisciplinary design optimization.
  • Advances in metallic, ceramic, and composite materials allow airplane and engine structures to be lighter and more durable, with higher temperature capability, as described above.
  • Also available are more efficient and lighter-weight jet engines, advanced thermal management technology, lighter and more compact electronics, and lightweight high-power electric systems such as motors and actuators.

Global Economic Growth

The world economy is growing at an exponential rate, producing a 250 percent increase in real GDP per capita globally since 1970 (Bolt et al. 2018). In direct correlation, air transport demand consistently grows with progress in real GDP per capita (Boeing 2019). It follows that the demand for premium air travel will increase in proportion to overall air travel demand.

The total cost of travel can be thought of in terms of money, time, and experience. Monetary costs are often traded against improvements in the time-based or experiential costs of travel. Think of direct travel as opposed to indirect routings with awkward connections, or the demand for comfortable business and first-class seating as opposed to economy. Hypersonic travel will not be the least expensive option in the future market, but it is a different way to address premium travel demand, with money traded for major improvements in time as opposed to money traded for comfort (i.e., for first or business class travel).

In today’s world, hypersonic travel cannot be bought at any price. In tomorrow’s world, hypersonic transport may be available at prices competitive with first-class airfares, addressing a well-established market for premium travel with a compelling and unique offering, provided that other experiential costs—safety, comfort, and greenhouse gas emissions—do not also rise.

Hypersonic Airliner Environmental Challenges and Policy Hurdles

CO2 Emissions

Lifetime CO2 emissions for a hypersonic airplane are a strong function of the amount of fuel burned and a weaker function (<1 percent) of the energy and ­materials used to fabricate, assemble, maintain, and dispose of the airplane. How the fuel is created can make a large difference as well. Fortunately, biofuel versions of hydrocarbon fuels can provide significant environmental benefits because CO2-absorbing biomass is used as feedstock to create the fuel. It is expected that this fuel would reduce lifecycle CO2 emissions by up to 70 percent.

Using current performance assumptions, calculations show that CO2 emissions per passenger mile for a hypersonic airliner will be comparable to those of a super­sonic airliner, but still much higher than for a conventional subsonic transport because of the increased fuel use needed to fly at supersonic or hypersonic speed. However, a passenger on a hypersonic airliner using 100 percent biofuel will have a comparable CO2 footprint per passenger mile to someone flying first class in a subsonic airliner that uses conventional Jet A fuel. While it is true that using 100 percent biofuel on conventional subsonic aircraft would give it a CO2 advantage, the relatively small fraction of passengers anticipated to fly on hypersonic airliners will largely mitigate the effects of this difference.

Biofuels are more expensive than traditional ­petroleum-based fuels, especially in the relatively small quantities now produced (<1 percent of current jet fuel supply). This actually presents an opportunity for supersonic and hypersonic airplanes as their passengers are less likely to be sensitive to ticket price, hence fuel price. In other words, they would be more likely to pay an additional premium to travel in a more eco-friendly manner, and in so doing spur the demand for biofuel, driving down the cost for all users. Existing aircraft, with legacy fuel and engine systems, are generally limited to 50 percent biofuel blends; new aircraft, such as a hypersonic transport with new or modified engines, can be designed to use 100 percent biofuels.

The effects of emissions on stratospheric ozone from a hypersonic airplane flying at about 100,000 ft. altitude have not been fully evaluated by the atmospheric science community. NOx emissions are expected to lead to some level of ozone depletion, but the amount will depend on cruise altitude, the quantity of NOx ­emitted from engines, and fleet fuel use. Water vapor emissions from a hypersonic airplane will accumulate in the stratosphere, potentially affecting ozone as well. Moreover, CO2 and water vapor emissions are both greenhouse gases, but the climate effects of these emissions in the stratosphere have yet to be evaluated.

Atmospheric chemistry models are starting to be used to evaluate the effects of these emissions on the ­atmosphere for airplanes cruising at altitudes up to 110,000 ft. When available, the results of these assessments can be used to guide policymakers and tech­nology developers on hypersonic airplane requirements.

Sonic Boom

As with supersonic aircraft, sonic boom is a challenge for hypersonic aircraft. One might expect it to be worse for a hypersonic aircraft because of its higher speed, but in fact several high-altitude effects may reduce sonic boom more than Mach number effects increase it.

Although hypersonic airliner flight may initially be limited to overwater and unpopulated overland routes, airplane design techniques and technology being developed in NASA’s supersonic boom reduction research program may eventually be applied to hypersonic airplane design to further mitigate sonic booms.

Trade-offs between the value of speed and the technological and environmental impacts of flying faster and higher suggest that there may be an optimum speed/­altitude combination that retains the economic benefits of speed while minimizing the technological challenges of flying faster (which tend to increase by the square of Mach number) and the environmental effects of emissions and sonic boom.

Certification and Regulation

Some aspects of certifying hypersonic airplanes will be different than for subsonic or even supersonic airplanes:

  • the hot structure and associated thermal protection and management systems needed to maintain a cool environment for the cabin and systems;
  • unique systems and approaches to prevent cabin decompression, which is not permissible at the altitudes hypersonic airplanes fly;
  • a synthetic vision system needed by pilots because the unique airplane geometry required for acceptable aerodynamic performance prevents adequate optical views; and
  • higher landing and takeoff noise that are a natural consequence of having to use lower-bypass turbofan engines to achieve acceptable engine performance at supersonic speed.

The first three will require development of certification approaches that ensure their safety; the last may require a change in noise regulations until noise reduction technology is advanced for supersonic low-bypass engines. NASA is innovating a certifiable pilot synthetic vision system for the X-59 Quiet SuperSonic Technology (QueSST) experimental aircraft being developed by Lockheed Martin, giving this system a head start for certified hypersonic airplane applications (Hernandez 2019).

An additional consideration for hypersonic airplanes will be integrating them into national airspace, as they will require special handling during airport departure and return to operate within the traffic patterns of ­slower-moving aircraft. For example, they will have to fly within specific corridors during ascent to safely accelerate to speed and slow down sufficiently far from airports on approach. During ascent and descent they will also have to manage flight trajectory to focus ­sonic booms, which are stronger during acceleration and deceleration, in acceptable directions. The integration of supersonic airplanes into subsonic air traffic patterns has been a subject of study for many years (e.g., ­Underwood 2017).

Exciting Possibilities Lie Ahead with Hypersonic Flight

Imagine in the near future being able to board a hypersonic airplane and jet across the ocean in 2 to 3 hours, returning home the same day if desired. Or, in the more distant future, hopping on a two-stage spaceplane, with a Mach 5 airplane first stage and a second stage propelled by an integrated scramjet-rocket engine, to fly safely and affordably to an orbital hub on your way to the Moon or Mars. These futures are being made possible by advances in technology for hypersonic flight that started 60 years ago and are being dramatically accelerated today.

This is an exciting time with the development of numerous aerospace innovations that will transform how people and material move around the globe. Hypersonic flight will effectively shrink the globe in terms of the time required for physical interaction, an undying element of human need and experience.

Acknowledgments

Thanks for valuable inputs from Boeing employees David Franson, Marty Bradley, Steven Baughcum, Todd Magee, David Lazzara, and Hao Shen.

References

Boeing. 2019. Commercial Market Outlook 2019–2038. Seattle.

Bolt J, Inklaar R, de Jong H, van Zanden J. 2018. ­Rebasing ‘Maddison’: New income comparisons and the shape of long-run economic development. Maddison Project Working Paper 10. Groningen Growth and Development Centre, University of Groningen. Available at https://­ourworldindata.org/economic-growth.

Candler GV, Subbaredd PK, Brock JM. 2015. Advances in computational fluid dynamics methods for hypersonic flows. Journal of Spacecraft and Rockets 52(1):17–28.

Heppenheimer TA. 2007. Facing the Heat Barrier: A ­History of Hypersonics (NASA SP-2007-4232). Washington: National Aeronautics and Space Administration.

Hernandez JC. 2019. Testing of X-59 virtual forward window successful. NASA, Aug 26.

Schweikart L. 1998. The Hypersonic Revolution: Case ­Studies in the History of Hypersonic Technology, Vol III – The Quest for the Orbital Jet: The National Aero-Space Plane Program (1983-1995). Washington: Bolling AFB.

Underwood MC. 2017. Concept of Operations for Integrating Commercial Supersonic Transport Aircraft into the National Airspace System (NASA/TP–2017-219796). Hampton VA: NASA Langley Research Center.

Weber RJ, Mackay JS. 1958. An Analysis of Ramjet Engines Using Supersonic Combustion (NACA-TN-4386). ­Cleveland: Lewis Flight Propulsion Lab.

About the Author:Kevin Bowcutt (NAE) is principal senior technical fellow and chief scientist of hypersonics for the Boeing Company.