Your questions answered: Supersonic airliners

NASA and Airbus are among the organisations that have recently revealed that they are working on concepts for supersonic passenger vehicles, but after the commercial failure of Concorde, is there really a future for ever-faster transport? And if there is, what technical challenges will engineers have to overcome to make it a reality?

We asked readers of The Engineer for their questions and put them to an expert panel, including representatives from Lockheed Martin, Rolls-Royce, Reaction Engines and Imperial College London.

• Anthony Pilon (AP), Lockheed Martin technical fellow
• Ricard Varvill (RV), technical director – Reaction Engines
• Paul Bruce (PB), senior lecturer in aeronautics, Imperial College London
• Alan Newby (AN), chief engineer, future programmes and technology, Rolls-Royce

What’s the business case for reviving supersonic flight?

RV: The business case for supersonic flight is weak. Generally speaking, the reduction in lift-to-drag ratio due to shock wave formation and more complex engineering of supersonic aircraft, tends to increase costs. This goes against the long-term trend of decreasing ticket costs and, in the near term, will exclude the majority of the travelling public, apart from the rich and businessmen. Nevertheless, rising living standards will eventually make supersonic travel affordable for long-distance travel, providing that engineering and environmental challenges can be solved.

PB: Despite recent tragic events, air travel remains the safest form of long-distance transportation and I think the public does appreciate that all aircraft manufacturers take this seriously. The excellent safety record of the aviation industry stems from rigorous testing and certification of new aircraft, exacting maintenance of existing aircraft and thorough training of personnel. I see no reason why any of these should be compromised when developing faster aircraft. The safety implications of cruising at 500mph at 30,000ft are not so different to cruising at 2,000mph at 100,000ft.

I believe there is definitely a public hunger to travel faster – whether it be supersonic or hypersonic. However, I also believe it is difficult to imagine a scenario where supersonic air travel is not significantly more expensive than today’s economy-class travel

How much of an appetite is there within industry to bring back supersonic passenger flight?

RV: The aerospace industry is sceptical about the commercial viability of a supersonic airliner. However, a small supersonic business jet is subject to less-stringent economics and may be built within a few years, providing the sonic boom operational limitations can be overcome.

PB: It is always possible to make a business case at some level for supersonic passenger flight. I firmly believe we will see the first supersonic business jets — Mach 1.6 cruise with a capacity of around 10 passengers — from companies such as Aerion, Spike Aerospace and Gulfstream appearing in the very near future. It is much harder to make a business case for a larger commercial supersonic aircraft. However, I think the recent interest we have seen from big industrial players such as Airbus reflects the fact that industry is well aware that the development of technologies for travelling fast has other potential applications beyond making a profit transporting fare-paying civilian passengers. This includes, perhaps obviously, applications with a military purpose but also more future-looking concepts such as developing vehicles that could provide more affordable access to space.

The environmental regulations and noise and emissions targets facing the aviation sector are more demanding than ever. Is it possible to design and operate a supersonic airliner that meets these targets and, if so, how will this be achieved?

AP: Yes, we believe it is possible to develop a supersonic airliner that is compatible with existing and planned future regulations governing noise and emissions. Take-off and landing noise is addressed through variable cycle engines [VCE] and advanced nozzle concepts. VCEs are optimised to provide the high specific thrust required for supersonic cruise, while lowering jet exit velocity to minimise take-off noise. Advanced fan and compressor technologies employed in VCEs, along with acoustic liners, help minimise fan noise on approach. Airframe noise is minimised by keeping the airframe as clean as possible, eliminating slats, multi-slotted flaps, fairing landing gear and so on. On the emissions side, the lower-pressure ratio inherent in supersonic engine designs minimised NOx emissions compared to current subsonic designs.

PB: There is no getting around the fact that flying faster will always burn more fuel, no matter how well designed an aircraft is. Recent developments in engine technology and aerodynamics can go some way to lessen this disadvantage but it will always be an issue.

Noise, on the other hand, is an area where we have made significant progress since the days of the Concorde. Airbus has suggested some clever tricks to reduce the impact of noise during take-off – where regulations are particularly stringent – and also to lessen the strength of the sonic boom produced during supersonic flight. Specifically, it mentions using conventional engines during take-off, minimising aircraft weight and volume, careful shaping of the fuselage, flying at very high altitude and an unconventional flight path during climb and descent.

RV: Meeting noise and emissions targets is more difficult with a supersonic airliner than a subsonic airliner, and is one of the most difficult obstacles to be overcome.
Take-off noise can possibly be solved by incorporating a high bypass fan to reduce the exhaust velocity providing the increased frontal area and supersonic wave drag are not too great. At high Mach numbers, NOx generation becomes an issue due to the high combustion temperatures, particularly since the vehicle will be flying in the ozone layer. Consequently, advances will be required in combustion chamber design. Reaction Engines has designed and tested a novel concept that promises to solve this issue.

AN: Future commercial supersonic aircraft will have to meet the same stringent airport noise – take-off and landing – standards as contemporary subsonic aircraft. Meeting such noise standards will likely require propulsion systems of bypass ratios higher than optimum for supersonic performance, compromising capability.

‘Conventional’ supersonic aircraft flying above the speed of sound create a sonic boom at ground level that is deemed unacceptable in many areas of the world, resulting in restrictions on supersonic overland flight. The market potential of supersonic aircraft will be limited unless they can be designed to mitigate the sonic boom and be certified against agreed regulations permitting supersonic overland flight. Such regulations are yet to be defined.

The challenge will be to produce an aircraft that meets the regulations on airport noise and sonic boom while delivering the supersonic performance and capability required by the market. Novel variable geometry propulsion system configurations incorporating the most advanced engine technology present the possibility of overcoming the challenge on performance. Research is ongoing into aircraft configurations that mitigate the sonic boom to a level expected to be socially acceptable and encouraging results have been widely reported.       

Furthermore, Concorde with Rolls-Royce Olympus 593 engines was designed before the era of computational fluid dynamics. This, coupled with advances in aerodynamics and flow control, mean a reduction in sound pressure on the ground is achievable today. If we just take reductions in noise on commercial subsonic aircraft the improvements over the past 40 years have been considerable. We only need to look at the advancements made in subsonic flight in recent times. Rolls-Royce understands a lot about quiet powerplant, the noise levels of the Trent 1000 are the quietest in class and represent significant reductions since earlier RB211 generations. Rolls-Royce continues to invest in the engineering and science of noise; and, with our academic partners at Southampton University, we continue to make progress with every new product both for civil and military applications.

The aerospace industry will need take a more integrated approach during early design phases if it is to achieve an acceptable commercial supersonic aircraft solution. We have to recognise that aircraft and powerplant noise have different impacts on overall noise during take-off – near airports – and supersonic cruise whether overland or oversea. These challenges will only be overcome by airframers working with powerplant manufactures to achieve an optimum solution. Reductions in noise are required from both the propulsion system and aircraft. As with many breakthrough technologies, costs in early years with small volumes can be significant, which inevitably adds to the overall cost. 

What technologies will be key to the next generation of supersonic airliners in terms of engine design, materials, and aerodynamics?

AP: Variable cycle engines [VCEs] and high temperature combustors for engines; lightweight composite materials for structure; and laminar flow for aerodynamics. Recently developed low boom design tools also hold promise for dramatically reducing the loudness of supersonic cruise flight and may open up supersonic flight to a much larger audience by allowing supersonic flight overland.

RV: Engine design and technology is the most challenging aspect of supersonic or hypersonic flight. In particular, handling the high air inlet temperature with low fuel consumption to give long range. The main challenge on the airframe is handling the high skin temperature requiring careful materials selection and thermal insulation. If very high range is required – for example, Europe to Australia – then hydrogen fuel is mandatory that can assist engine design but complicates the airframe further.

PB: • Hydrogen fuel is significantly more mass-efficient than conventional hydrocarbon fuels. New technologies for its production on the ground and storage – cryogenic fuel tanks – will be necessary to unlock its potential.

• Development of a tandem turbofan/ramjet engine will be necessary to enable flight at low speed (take-off and landing) and efficient supersonic cruise. This could be in the form of two separate engines that can be retracted/deployed or a single adaptable engine something more like the Lockheed SR-71 Blackbird engine.

• Thermal heating will necessitate extensive use of reasonably exotic materials – for example, titanium alloys – and careful design to account for thermal expansions and so on. This mainly applies to airframes, but may also be the case in engines; scramjets and ramjets have no moving parts, but may require heat-resistant liners in their combustion chambers.

• A ‘wave-rider’ wing design is likely to offer the greatest potential for aerodynamic efficiency for a cruise around Mach 4.

• The shape and surface finish of all aerodynamic surfaces should be designed to ensure the airflow remains laminar wherever possible. This is an important feature for small supersonic business jets.

AN: Through the development of revolutionary design concepts and application of advanced technology, it is possible that an environmentally acceptable and commercially viable supersonic corporate jet could be produced in the medium/long term.  Development of a large supersonic passenger aircraft that meets environmental and commercial targets is even more challenging, but the knowledge acquired and technology developed for the smaller corporate supersonic vehicles will provide a considerable stepping stone towards such an aircraft, that could be viable in the very long term.  

A supersonic corporate jet will create a new paradigm in aircraft capability, and will be the most fuel-efficient and environmentally friendly supersonic aircraft ever built. However, the penalty for that revolutionary capability in aircraft performance and productivity will be in fuel efficiency relative to a subsonic vehicle of the same technology standard.

The engineering, environmental and economic challenges to scale from corporate to large passenger aircraft should not be under-estimated; however they do offer a unique opportunity to achieve what is one of the most difficult engineering problems to resolve in the 21st century. These challenges won’t just be at aircraft and engine level but will stretch the minds of engineers and scientists in areas of material science, aerodynamics, computational design and thermodynamics to name a few. We have to consider whether there is a large enough demand for supersonic travel and whether society is prepared to pay a premium for such a service while acknowledging the trade off with environmental penalties.

If by comparison with traditional aircraft we mean will it achieve the same fuel mass per passenger-mile as an Airbus A350 powered with Trent XWBs, then the answer is no. The physics of flight hasn’t changed since Concorde’s day; clearly we can achieve a step change in efficiency compared to 1970s, but we have to be aware that subsonic turbofan engines have significantly improved fuel burn, thus the challenge is even greater if we are to make fair comparisons.

There are more elements than just fuel efficiency per passenger in the business case evaluation of a new airframe and one of them is time required to destination. It is true that this is not only a function of flight time, but airport and flight management operations also have a big impact on this, but supersonic flight can offer significant block-time reductions over widebody airliners in long-haul flights.

What are the main lessons – from a technology perspective – to be learned from Concorde?

AP: Concorde was a point design – Mach 2.0 cruise. Future supersonic airliners or business jets will need to be designed through a multidisciplinary approach so that cruise performance, sonic boom, noise, emissions, and operating costs are all addressed simultaneously. Without that, we’ll have another Concorde.

RV: Concorde was a superb aircraft but suffered from high take-off noise and low range. This excluded it from a large number of routes such as trans-Pacific, limiting potential sales and, in conjunction with its small number of passengers, giving high ticket costs. A future supersonic aircraft needs to reduce costs, increase range and meet future environmental legislation.

PB: There are simply too many to try and list. Technologically speaking, Concorde was and still is an engineering masterpiece. No other aircraft has come close to replicating its capabilities – not just in terms of ultimate performance but also in terms of its reliability and robustness to operate day-in-day-out.

As the speed of a given aircraft increases, its energy requirements increase. At what point would this law of diminishing returns set an upper limit on the speed an aircraft could travel while still being affordable?

AP: I would estimate that the limit, with today’s technology, is probably around Mach 2.0. Above that, the design space collapses because of cabin pressurisation issues – the aircraft will need to cruise above 60kft and losing pressurisation there is catastrophic and also for issues such as aerodynamic heating. There is also an inherent difficulty in designing an engine that can operate quietly and efficiently at both low and high speeds beyond about Mach 2.0. It isn’t feasible to develop a propulsion system that can provide high cruise efficiency and meet Chapter 14 airport noise regulations. In reality, the sweet spot from a multidisciplinary perspective when including all environmental constraints is probably closer to Mach 1.8.

RV: Simple Breguet range and propulsion theory shows that the achievable range of an aircraft is relatively independent of the cruise Mach number. The technical difficulty increases greatly with Mach number due to rising inlet air temperature. Practical engineering considerations and economics suggest that there is little point flying faster than Mach 4–5 since the duration of the longest flights becomes shorter than the rest of the journey to and from the airport.

PB: Although flying supersonically does incur a significant penalty in aerodynamic efficiency relative to low speed (subsonic) flight, research has shown that there is actually very little further reduction in the aerodynamic efficiency of a well-designed aircraft once the Mach number exceeds around 5. What this means is that from an aerodynamics perspective, if you are willing to accept the efficiency penalty to fly at Mach 5, you may as well be flying at Mach 10 or 20. On the other hand, designing an efficient propulsion system to travel at speeds above Mach 5 becomes a massive headache as current options, i.e. rockets, are far too inefficient to be considered for long-range travel.

What kind of changes would have to be made to existing airport infrastructure around the world to ensure that airports can support supersonic airliners?

AP: There probably would not be too many changes for the airports themselves. I would expect any airport equipped for a 787 would be able to handle a supersonic airliner; although there may be some additional infrastructure needed for cooling of the airframe and systems after flight. The real infrastructure challenge would be in air traffic control [ATC] – the benefits of a supersonic airliner fade away if ATC slows you to 250kt 300 miles from your destination. This is happening now with high-end business jets.

RV: Hypersonic or long-range airliners will require hydrogen fuel that necessitates large investments in airport infrastructure and results in a very long aircraft, due to the
low density of liquid hydrogen. This would require further investment in modified taxiways and departure gates.

PB: The short answer is: very little. Supersonic aircraft will generally be smaller than current commercial aircraft – maybe a bit longer but much thinner – so there should be no issues with accommodating them at existing gates and so on. It is likely they would take off using conventional – turbofan or turbojet – engines so should not present more of a noise problem than existing aircraft. As with Concorde, they may require higher take-off and landing speeds but this would not be a problem owing to their smaller size and relatively powerful engines.

Would it make more sense for industry to concentrate on refining the technologies required for hypersonic flight?

AP: No. There are multiple technical challenges to hypersonic flight that preclude this, for example, thermodynamics, cabin pressurisation, g forces and so on.

RV: Some engineering studies are already underway to study and develop the technologies for hypersonic flight. A notable example was the LAPCAT study part funded by the EU and managed by the European Space Agency.

PB: No. I think it makes a lot of sense to aim at supersonic travel first. Most engineers in the aerospace industry would agree that sustained hypersonic travel is probably an order of magnitude harder than supersonic travel. There is a long list of reasons for this, including the need for some very advanced materials to withstand high heating levels and advanced new engine technologies that are still a long way from successful flight testing. From a passenger point of view there is relatively little motivation to travel hypersonically in terms of reducing point-to-point transit time.