11 OUTLANDISH ATTEMPTS TO BUILD THE NEXT CONCORDE

11 Outlandish Attempts To Build The Next Concorde
By Chris Clarke, Popular Mechanics, 24 November 2015.

Every few months or so, the internet becomes abuzz with some gleaming new scheme to resurrect passenger air travel that exceeds the speed of sound. Some of these designs go viral as "the next Concorde" before disappearing into vapourware. But there are also legitimate patents from established enterprises like Airbus, and next-generation aerospace concepts from NASA and Boeing for supersonic planes we might actually see in this lifetime.

As we look back on the final Concorde flight that graced the skies 12 years ago this month, we have to question why there isn't something new and better. Why has every other attempt failed where the Concorde succeeded nearly 40 years ago?

Why Supersonic Is So Hard

Throughout aviation history, only two aircraft have entered commercial service as supersonic transportations (SST). The first was the Soviet built Tupolev Tu-144. The Tu-144 made only 55 passenger flights in only one year of service, but continued hauling cargo through 1983, totalling just 102 flights. Concorde found much more success during its 27 years of commercial service, although only 20 were ever built, at a tremendous cost and with large government subsidies. Concorde eventually became profitable for a while when it found an elite customer base that was willing to pay a premium for the novelty of traveling on the world's fastest airliner. The economic downturn in the late 1990's combined with the Concrode's only crash in 2000 and rising maintenance cost on the aging fleet, finally led to its retirement following the very last flight in November of 2003.

Here are the top four problems for companies trying to revive supersonic transport:

• Sonic Boom: One of the most obvious by-products of supersonic flight is the loud boom. The shockwave that creates it is like an invisible boat wake that sounds like a loud clap to those on the ground and can even rattle and crack window panes. This has led to strict regulations governing allowable levels of sonic boom, and in 1973, supersonic flight over land in the United States was officially prohibited. Many people have tried to invent a wing or fuselage design that creates a "quiet" sonic boom. Another tactic to reduce how much the boom is heard on the ground is to fly at much higher altitudes. Unfortunately this solution leads to the next obstacle.
  
  
• Propulsion: Most modern aircraft propulsion systems are air breathing, meaning they burn oxygen from the ambient air mixed with fuel. At the extreme altitudes required for practical supersonic flight, the air density would not be adequate to feed such an engine and still produce sufficient thrust to accelerate the aircraft beyond the speed of sound. This has lead many designers to look elsewhere for non-conventional propulsion like rocket engines and ramjets.
  
  
• Aerodynamics: Supersonic planes would need to use the same airports as other aircraft. But a supersonic jet's need to be as slippery as possible often results in poor stability at slower speeds. SSTs need very fast take-offs and landings that require an unusually long runway. This can severely limit where a supersonic plane can operate.
  
  
• Costs: This all costs an extreme amount of money not only to develop but also to operate. Fewer routes due to sonic boom restrictions and airport runway limitation reduces the potential customer base. Add the exorbitant fuel costs and increased maintenance needed due to the extreme conditions, all of which make supersonic transportation cost prohibitive.

1. Zero Emissions Hypersonic Transport

Airbus designed a hypothetical supersonic plane that some are calling the Concorde 2. The aircraft would be a long-haul commercial transport flying at altitudes of 100,000 feet. The study calls for advanced smart materials able to withstand the extreme temperature changes associated with such high altitudes and a planned maximum speed of Mach 4.

The concept is anything but simple, with a radical three-tier propulsion system. The power required for take-off would be created by a typical turbo jet engine that would accelerate the craft near the speed of sound. Rocket boosters would then take over, propelling the aircraft in excess of Mach 2, after which supersonic ramjets would continue the acceleration to Mach 4.

If that sounds crazy, just imagine a proposed fuel source made from a combination of seaweed and hydrogen/oxygen resulting in the emission of zero pollutants. Airbus recently filed patents confirming plans to develop such an aircraft, but don't expect to see anything like this rolling up to your gate at the airport anytime soon. The first take-off isn't scheduled until something like 2050.

2. Aerion AS2

Aerion Corporation's plane for a possible flying supersonic passenger aircraft has a lot going for it. Supersonic and transonic airflow across the natural laminar flow wing has been proven in wind tunnel test and has also been tested in conjunction with NASA's F-15 flying testbed. The company has partnered with Airbus Defence and Space to collaborate on certification issues. Airbus most likely would produce the major airframe components in Europe, which would be shipped to a final assembly site somewhere in the United States.

Of course, there are setbacks. The first flight has already been delayed to 2021, two years beyond the original projected date. A few key decisions have yet to be made, like where the final assembly plant will be located and which manufacturer will build the three jet engines needed to propel the craft beyond the speed of sound. Aerion expects to be the first to bring a supersonic business jet to the market with a recent announcement of 20 firm fleet orders to their launch customer Flexjet.

3. HyperMach SonicStar

Expected to travel at speed in excess of Mach 3, the SonicStar is the brainchild of Sonicblue Aerospace CEO Richard Luggs. At the heart of the outlandish business class jet are two (theoretical) SonicBlue Supersonic-Magnetic Advanced Generation Jet electric turbine hybrid supersonic engines. If all those buzzwords have your head spinning, you'll be even more befuddled when you try to understand how it's actually supposed to work.

Traditionally on a jet engine, a bypass fan at the front is connected to a free-spinning turbine that is spun by hot exhaust gasses exiting the rear. The electric hybrid design proposed for the SonicStar would use electricity generated from burning fuel to rotate the bypass fan. The fan speed could then be varied to the optimal speed for maximum efficiency.

The SonicStar would need extreme tech to reduce the high temperatures caused by friction between the air and the skin of the aircraft. The plans include carbon composite structural skins and panels with alloy leading edges wrapped around titanium structure wings to reduce weight.

None of this is cheap. And some of these technologies can't be bought because they haven't been invented yet. To eliminate the sonic boom, for instance, the SonicStar would use electromagnetic drag reduction technology (currently not in existence). If all goes according to plan, first test flights will begin in 2021.

4. Tupolev Tu-444

If any company can succeed in creating the next supersonic transport, it's Russian firm Tupolev, since they invented supersonic passenger transport. Unfortunately little information is known about the development of their supersonic business jet and official details are no longer available. This raises the question as to whether this secretive project is still in the works or has been cancelled altogether.

5. Reaction Engines A2

British aerospace firm Reaction Engines Ltd (REL), as part of a European Union program to examine advanced propulsion systems, has created a unique design study. It's intended to imagine environmentally friendly, long-range, high-capacity commercial transportation with a top speed in excess of Mach 5. The term "design study" could be taken to mean it's vapourware, but REL claims they would move toward commercial development of the LAPCAT A2 within 25 years - if and when a market for such an aircraft demanded it.

The engines on the A2 would be fuelled by liquid hydrogen, which would make it nearly twice as efficient as a typical kerosene-fuelled turbojet engine. The hydrogen would also play a secondary role by pre-cooling the highly compressed air entering the engine. The cooler air will burn more even at high speeds and allow engine construction to be made of lighter materials that would normally be vulnerable to extreme heat.

Reducing the extreme heat generated by hypersonic airflow on the surface of the aircraft presents a challenge. In order to minimize this effect, the A2 is designed without windows. Flat panel displays would be installed to show passengers scenes from outside.

6. Spike S-512

Spike Aerospace is a start-up with ambitious plans to create a supersonic business jet that will fly from New York City to London in under three hours. Few details about the design have been released other than a cruise speed of Mach 1.6 and the capacity to haul 18 passengers. Like the Reaction Engines A2, the Spike will be without passenger windows, relegating occupants to viewing the outside world through curved displays.

While this project is in its infancy, the team includes engineers experienced at Gulfstream Aerospace, Boeing, NASA, and Airbus. They're hopeful that the plane's smaller size and unique wing design will reduce the supersonic shockwave. Those on the ground should hear just a soft muffled clap as the airplane passes overhead.

7. SAI Quiet Supersonic Transport

Michael Paulson, son of Gulfstream Aerospace founder Allen Paulson, had big dreams of building a virtually boomless supersonic aircraft. His design would use a curved gull-wing, an inverted V-tail, and a curvilinear fuselage. Supersonic Aerospace International (SAI) was formed and even commissioned Lockheed Skunkworks to develop the business jet-sized QSST, but progress stalled in 2010 over patent disputes.

The first test flights were anticipated to take place in 2011 while the first passengers were to board in 2013. But as 2013 came and went without QSST taking to the skies, Paulson resurrected the idea on a grander scale. He envisioned a Boeing 737-sized transport aircraft that would carry up to 30 passengers in an all-first-class cabin. But SAI seems never to have found investors, and further progress is unlikely.

8. Boeing 2707

In response the British/French Concorde and the Russian Tu-144, the United States in 1963 raced to build an airplane to rival the European competition. The ambitious goal was to build a commercial aircraft with a capacity for 250 passengers (twice as many as Concorde), with speeds of Mach 3 and a transatlantic range of 4,000 miles.

The 2707 was originally envisioned with variable sweeping wings using design cues gleaned from the XB-70 Valkyrie and a droop nose similar to Concorde. But after three prototypes and millions of dollars in research, the government cancelled the program. Public opposition to noise pollution and potential damage to the ozone layer raised serious concerns about the plane. The government also got itself entangled in the Vietnam War and decided to cut funding.

Some good came out of the program, which lead to the High Speed Civil Transport project. As a key part in NASA's High-Speed Research program, this project ran throughout the 1990s and was supported by a team of aerospace companies from the United States.

9. Sukhoi-Gulfstream S-21

Gulfstream Aerospace got serious about supersonic research in the 1980s and 90, partnering with Russian Sukhoi Design Bureau to develop a supersonic small business jet. The S-21 would be capable of sustained cruise at speeds above Mach 2, and the team put a lot of research and development into managing the troublesome transonic effects associated with air speeds in excess of Mach 1.

But in the early 90's, market demand for a commercial supersonic transport was wavering, which resulted in Gulfstream questioning their commitment to the project. The partnership dissolved, but Sukhoi continued work on the S-21. Sukhoi sought cooperation from the French aircraft manufacturer Dassault as well as the Chinese government to build a supersonic business jet, but the design has never moved passed the research stage.

10. Next Generation Supersonic Transport

The Japanese Space Agency (JAXA) has also expressed interest in joining the supersonic aircraft development race. Their design is expected to carry three times as many passengers as Concorde at roughly the same speed, but with twice the range. The goal is to create a supersonic passenger plane with a ticket price rivalling that of ordinary jets.

A scale model was successfully flown to Mach 2 in 2005, and plans called for a full-scale prototype to fly this year. That hasn't happened yet, but development is still in progress.

11. Skreemr

A new twist on supersonic transport is the Skreemr. Much like the Airbus design, Skreemr would use multiple propulsion systems to work its way to peak altitude and, ultimately, four times the speed of sound. Unlike the Airbus design, Skreemr will not use traditional turbine jet engines as the initial source of thrust. Instead, designer Charles Bombardier has envisioned a jet that is launched from a magnetic railgun. A similar electromagnetic launch system is in development by the United States Navy as a means to catapult military airplanes from the deck of aircraft carriers.

Once airborne, the Skreemr would light liquid-fuelled rocket motors that would increase speed and altitude up to the point where scramjet engines would come online. These scramjets would use highly compressed air gathered from the aircraft's forward momentum, combined with burning hydrogen gas, to propel the airplane to theoretical speeds of Mach 10.

At this time, the Skreemr is no more than a concept, and no work has begun on construction. If feasibility studies ever determine a way to overcome the huge technological barriers to achieve a practical method of supersonic transportation, it might prove that creativity is more important than precedent.

Top image: Spike S-512 Supersonic Jet. Credit: Spike Aerospace.

[Source: Popular Mechanics. Edited. Some images and links added.]