Why No Concorde-II?

Challenges in Supersonic Airliner Design

By Yogesh Kumar, Aerospace Technologist

1. Introduction

1.1 Presently all Airliners fly only in subsonic region up to say max Mach No. (Mn) of 0.85–0.88. The only commercial airliner to date was Concorde which started operation in beginning of 1976. However, it had to face restrictions due to sonic boom and it has since been withdrawn from service (in 2003). It should be appreciated that Concorde was a marvel of technology, far ahead of its time, and in a way was major game changer.

1.2 Although lot of Research and Development (R&D) is going on at NASA, and other aircraft companies; commercialization of a Supersonic Airliner is still a long way to go. There are numerous challenges in design of a safe, reliable and economical passenger aircraft that can fly at almost double the speed of sound at high altitudes. This requires a deep thought on not only building on Concorde legacy but to build a plane better than Concorde with all its so-called shortcomings addressed in the new design. This paper attempts to bring out some of these challenges at a macro level for benefit of the Project Teams working on this project based on my over fifty years of experience in Aeronautics (My CV at the end). I am sure Project Teams would have already considered all these but this is just a ready reckoner. The objective is to help in design process.

2. Challenges

I shall bring out the challenges in design and development under the

various heads as below;

• Aerodynamics
• Structure
• Power plant
• Systems
• Environmental factors
• Certification Aspects
• Economics and Business Viability

2.1 Aerodynamics

2.1.1 The biggest challenge will be the sonic boom which was the main hurdle in the operation of Concorde. Sonic beam is like a thunder caused by an airplane moving faster than the speed of sound. During sub-sonic flying, pressure disturbances or pressure waves extend in all directions. At supersonic speeds, these pressure waves combine and form shock waves which travel forward from the generation or release point. The sound heard on ground as a sonic boom is a sudden onset and release of pressure after build-up by the shock waves. Ground width of boom exposure area is approximately 1 mile per 1000 feet.

A lot of research is going on to reduce the effect of sonic boom with respect to;

• Rate of rise of sonic boom
• Noise
• Over Pressure;

through wing design, shaped signature etc. so that boom is low enough to permit supersonic cruise over land. Can we achieve a target over pressure of less than 1.0 lb./sq-ft (Concorde was around 2.0 at Mn=2.0 at 52000 ft) and noise to about 70 dbs. or lower from 105 dbs.

2.1.2 Other aerodynamic challenges will be to;

• Reduce transonic and supersonic drags. These would have direct bearing on the Power-plant thrust. It may be desirable to keep maximum supersonic Mach number (Mn) at 1.8 or so to;
• Optimize on subsonic and supersonic performance.
• Reduce the heating effect; very important for Cabin Environmental Control System (ECS).
• Keep gross take-off weight and empty weight fraction as low as possible for better aerodynamic performance at all points of flight envelope.

2.2 Structure

2.2.1 Challenge will be to use low weight high strength materials including composites. Table-I below gives maximum ram and skin temperatures at various Mach numbers at cruise altitude of 60,000 feet at ISA + 30 deg. C for reference.

Table I

2.2.2 Airframe will need to be designed to withstand these temperatures corresponding to the Mach numbers. In Concorde 125 to 130 deg. C was measured in the front and leading sections. Also, fuselage shaping and wing fuselage blending will be required to reduce the sonic boom. Use of composites in fuselage may pose challenge in certification.

2.3 Power plant

2.3.1 This could be another challenge. Concorde was quite un-economical on fuel. The aircraft of future will require an order of magnitude more efficient power plant; particularly so if it is proposed to use zero carbon fuel. Target figure of TSFC could be around 1 to 1.2 lb/hr/lb of thrust at cruise Mach number. The target Thrust to Weight (T/W) of power plant could be around 5.0 at sea-level.

2.4 Systems

2.4.1 Systems on-board aircraft may pose a few challenges; some of them which are worth a consideration are briefly described below;

(a) Environmental Control System (ECS)

This system will cater to;

• Cabin air conditioning which means cooling, heating and humidity control
• Cabin Ventilation
• Smoke and Odor removal

The system can pose challenge towards;

• Providing sufficient cooling at cruise altitude of say 60,000 ft where skin temperature can be as high as 110 to 150 deg ‘C’ depending on Mach numbers (Table I). Actually, there is a major difference between subsonic and supersonic cooling. In subsonic flying, maximum heat-loads are on ground with aircraft parked in hot sun at ISA +30 Deg.C environment. The heat-loads reduce to almost sixty percent at cruise altitude of say 36,000 ft due to lower skin temps at subsonic speeds. In supersonic flying, this is not so because of higher Mach numbers (1.8–2.0) which pose very high loads on air-conditioning systems. Table II indicates the order of heat-loads for a 90-seat aircraft for subsonic and supersonic operation.

Heat-loads for a Typical 90 Pax aircraft (Mn: 1.8, ISA+30 deg. C, at corresponding cruise altitudes; 36000ft. and 60000 ft.)

Table II

It can be seen above that heat-load goes up by almost eighty percent entirely due to kinetic heating at supersonic Mach numbers. This would require a total flow of about 250 lbs./min to the cabin; out of which at least 100–120 lbs./min will be the fresh air flow.

In case a conventional bleed air-based system is deployed, it may pose lot of penalty on the engine which is already stressed to a limit for supersonic flying. It would be better to use bleed-less ECS with dedicated air compressors, electrically driven. (Similar to 787). But here also, design of a compressor to provide sufficient air at 60,000 ft will be quite challenging; including the design of electric motor.

It is estimated that ECS alone may need power of the order of 100 KW either by way of engine bleed or electrical motors (in case of bleed less design).

(b) Cabin Pressure Control System (CPCS)

Design of this system can be quite challenging. Most likely, it will be a digital CPCS with manual overrides. Max cabin altitude may be kept at 6000 ft (It was 5000 ft in Concorde). This will lead to max differential pressure of 10.2 psi assuming cruise altitude of 60,000 ft. Structure will need to be designed for this differential which could pose a challenge. There could be some more challenges in respect of the following;

• Aircraft to achieve a safe altitude of 10,000 (approx.) ft at the earliest in the event of pressurization failure either due to equipment, or structure, or explosive decompression, or window opening etc. To ensure this, climb and descent rates have to be judiciously selected for operation.

The following features will need to be included in the CPCS;

• Reliable Oxygen System both for crew and passengers; crew may need pressure breathing also.
• Rapid descent system with additional air supply.
• Ram air ventilation below 10000 ft.
• Protection devices to ensure that cabin altitude does not exceed 14000 ft under any circumstances.
  ___________________________________________________________________My comments on ECS and CPCS are based on my personal experience while working on these systems for Concorde aircraft way back in 1973–74 when I was deputed to Normalair Garrett Ltd; UK* on behalf of Hindustan Aeronautics Ltd; HAL, Bangalore for a two years’ post-graduate design course.

(* Now called Honeywell Normalair Garret Ltd.)
___________________________________________________________________

(d) Flight Control & Hydraulic Systems

I am sure with the advent of latest technologies; aircraft will have digital fly-by-wire system with enough redundancies. The challenge would be on control actuators as they would need to cater to very high loads because of supersonic speeds. They need to be robust and compact like what we deploy in supersonic fighter aircraft.

As far as hydraulic system is concerned, it could be centralized or decentralized meaning that either there is one or multiple pumps centrally mounted on Accessories’ Gearbox/Engine Gearbox or each control surface has dedicated hydraulic system.

Hydraulic pressure will need to be upward of 4000 psi; either 5000 psi or even 6000 psi to make the components small and compact.

Issues regarding heating of hydraulic fluid, seal design and leakage through the seals will need to be addressed from the beginning.

(e) Landing Gears, Wheels and Brakes and Tires

Landing Gears may not pose much of design challenge except that they will need to cater to high pitch angle and angle of attack on take-off and landing. So, they will be longer as compared to conventional.

As far as Wheels and Brakes are concerned, Brakes will most likely use carbon fiber discs. There could be some challenge in their energy absorption within the prescribed envelope and cooling of carbon discs on ground. This may affect the aircraft turnaround time also.

Tires have to be very judiciously selected taking the legacy of Concorde. These would need to be puncture proof and designed to fragment in very small pieces should a severe damage occur.

(f) Electrical Power Generation System

It would most likely be a variable frequency system; may not pose much of a challenge as this type has been used in 787 Dreamliner; except for its capability to withstand heat generated due to supersonic speeds (kinetic heat) as well as internal heat generated by the system.

(g) Avionics and Instrumentation

Some of the technologies like Augmented or Synthetic Vision may pose some challenge. Also cooling of on-board electronic and instrumentation system will need to be addressed because these systems degrade fast at elevated temperatures. This will put extra load on ECS.

(h) Aircraft Fuel System

Design of this system could be quite challenging particularly because of very high temperatures due to kinetic heating. A temperature of about 70 deg ‘C’ will need to be ensured at the inlet of the Engine Fuel Control System to avoid high Vapor to Liquid (V/L) ratio.

This may pose challenge because of heating of fuel tanks.

2.5 Environmental Factors

They have been broadly covered in the earlier paragraphs. They may pose some technology challenges in respect of;

• Sonic boom impact
• Take-off and Landing Noise
• Minimum effect on climate change, ozone depletion etc.

2.6 Certification Aspects

Although much advancement has happened in various technologies, still certification of a Supersonic Airliner after a gap of almost five decades would be a major challenge particularly with respect to;

• Sonic boom — it may require revisit on certain clauses which came in the way of Concorde by FAA; and justification for the same.
• Integration of Powerplant with Airframe.
• Environmental and Cabin Pressure Control Systems — passenger comfort and safety, cabin de-compression etc.
• Flight Control System.
• Acceptable ride and handling qualities.
• Brake energy at aborted take-off and landing with reverse thrust.
• Certification of aircraft using high percentage of composites; particularly fuselage.
• Lightning protection because of composite materials used in structure.

These are some which immediately come to mind. There could be some more which will surface up only during testing on ground and flight development.

2.7 Economics and Business Viability

That would need to be thoughtfully worked out on the basis of realistic market forecast. While doing this, following should be factored in;

• Supersonic transport aircraft will have a much smaller fuselage cross-section and relatively long length. Therefore, they carry fewer passengers as compared to subsonic planes. That means, more numbers of these are on the runway/tarmac during fleet operation. This poses challenge for logistics and ground support equipment. This is extra cost for the Airline.
• None of the major aircraft OEMs has yet announced a program for Supersonic Transport Aircraft. Alternatively, they have been working on STOL/VTOL i.e. Short Take-off and Landing or Vertical Take-off and Landing configurations. If those become commercial success, they will pose big competition to Supersonic Transport planes as quite a big part of their speed and time saving advantage will get compromised.
• Fuel cost particularly if it is planned to use zero carbon fuel as you end up consuming more fuel per passenger. Almost forty to fifty percent of Direct Operating Cost is Fuel.

3. A Few Suggestions

The objective of this note was only to create awareness about some likely technical issues. As said earlier, I have brought these out based on my experience of over fifty years (see CV later) in the field of aeronautics mostly related to front line fighter aircraft.

I give below a few suggestions to lower the risk and reduce the development cycle;

A. Extensive testing at system level should be done before the system is installed on the aircraft. This would require test-facilities to be set up upfront by the Lead System Integrator (i.e., OEM); both at sub-system level as well as at the system level.

B. Integration of Power Plant with Airframe is very important to get maximum thrust benefit.

C. Once the systems are installed on the aircraft, System Integration assumes a very important role which means testing of each system in a standalone mode followed by testing all the systems in an integrated mode. If this is done successfully, flight development leading to certification becomes relatively smooth.

D. Address issues related to manufacturing, productionization, operationalization, maintenance etc. parallelly at each stage of development; eventually the aircraft has to join the fleet.

E. And lastly and most important of all, ensure full transparency with the Certification Authorities and the Customer; they are stake holders and partners in the Project

4. About the Author

I am an Aerospace Technologist with over fifty years of experience in the field of Aircraft and Aircraft Systems. During my career, all in Research, Design and Development at Hindustan Aeronautics Ltd; (HAL), a leading Aerospace Organization at Bangalore, India, I successfully led a number of major developments programmes, most notable was the “Light Combat Aircraft -(LCA) ‘Tejas’; a front-line fighter aircraft. Presently, I am working as Specialist Designer and Adviser to National Aerospace Laboratories (NAL) Bangalore, a leading Aerospace R&D Organization. Views in this paper are purely personal.

I wish the Project Teams all the success.

Yogesh Kumar | March 02, 2022 |Yogesh_hal@yahoo.co.in

Linked in: www.linkedin.com/in/yogeshkumarhal