Wed, Jun 22, 2011 — David Evans
The notion of supersonic airline flight keeps popping up, like a perennial weed. The latest concept comes from Airbus, which envisions of all things an eco-friendly supersonic jet that will fly 100 passengers at hypersonic speeds. The Airbus concept for such an airplane was revealed at the Paris Air Show, to great excitement among the aerospace cognoscenti. The flying public may take a different, more ho-hum view.
The grim legacy of the supersonic Anglo-French Concorde jet seems all but forgotten. Recall that Concorde was retired in late 2003, primarily because of the airplane’s range/payload limitations and its high operating cost. An Air France Concorde suffered a spectacular takeoff crash in 2000. The jet struck a piece of metal debris on the runway at Charles de Gaulle airport; the debris strike resulted in cut wires in the landing gear well and a punctured fuel tank. The airplane, on fire, crashed into a nearby hotel.
The accident was the last and grimmest of a long line of landing gear tire failures that punctured holes in fuel tanks. French investigators into the crash identified 57 incidents of Concorde experiencing deflated or blown tires. In 1979, an Air France Concorde on takeoff from Washington’s Dulles airport experienced punctured fuel tanks. The airplane’s magnesium wheels struck the runway, broke apart and hurled metal shards into the wing fuel tanks. With fuel dribbling from the punctured tanks, the airplane returned to Dulles. Passengers could see through the holes in the wing to the ground below.
With longer takeoff runs, higher speeds and stressed tires, the Concorde was 60 times more liable that the subsonic A340 jetliner to a tire burst. The comparison is apt, as both the A340 and the Concorde are four-engine airliners.
The energy from a burst tire is equivalent to approximately 4-5 sticks of dynamite. Yet, to save weight on the Concorde, electrical and hydraulic lines in the main landing gear receptacle were not shielded. Despite easily-punctured metal skin (about the thickness of a piece of cardboard backing a pad of paper), the fuel tanks were not protected with self-sealing rubber.
Why were such practices tolerated? Because Concorde was not certificated to the same standard as subsonic airliners. Every ounce of weight that could be pared from the airframe was critical if the Concorde was to haul 100 trans-Atlantic passengers and their luggage. Thus, instead of being designed to the 1-in-a-billion standard against catastrophic failure, Concorde was designed to a lesser standard. About 10 times lower, as a matter of fact. The waivers, deviations and special provisions necessary to yield an airplane with acceptable weight meant that passengers were flying in an airplane where the risks of failure were greater. The fatal crash occurred at approximately 75,000 flights – far less than a million, much less a billion flights.
Concorde was never an economic success. The airlines could not afford to buy it, so the Anglo-French consortium that built Concorde basically gave the airplanes to Air France and British Airways. To fly supersonically across the Atlantic, Concorde burned a ton of fuel per passenger. A subsonic airliner consumes about a quarter-ton of fuel for the same distance.
Sometimes, the Concorde’s need for sufficient fuel was so great that the passengers’ luggage made the trip across the Atlantic in a subsonic jet.
Aircraft manufacturers Boeing and Lockheed toyed with supersonic airliner designs, but the operating costs and the environmental and noise challenges proved insurmountable. Their designs never progressed beyond full-scale mockups.
Enter Airbus; at a briefing a day before the 20 June opening of the Paris Air Show the manufacturer’s executives presented their vision. Jean Botti, the manufacturer’s head of technology, said the project’s success depends on cost containment and whether or not buyers for the plane can be found.
On both counts, the effort seems doomed.
The Airbus concept is known by the acronym ZEHST, for Zero Emission High Supersonic Transport. The airplane is envisioned to carry 100 passengers (like Concorde) while cruising at 2,600 mils per hour (faster by 1,000 mph than Concorde) at an altitude of 100,000 feet (twice as high as Concorde).
While Concorde was powered by four turbojet engines, the ZEHST concept features three separate types of power.
The airplane would climb to approximately 40,000 feet using turbofan engines, whose fuel would be derived from seaweed or algae (thus satisfying the environmental dictates). At 40,000 feet, ramjet engines would take over, powering the airplane to approximately 100,000 feet, where yet another set of engines would propel the airplane at four times the speed of sound (Mach 4).
The airplane would be geared towards business travelers, who supposedly could afford the cost of a ticket. That cost would be first class plus a premium. The question is how many business travelers would be willing to pay two, three, four or more times the cost of a subsonic first-class ticket for the privilege of arriving a few hours earlier.
Given the highly public demise of Concorde, ZEHST will have to be built to a 1-in-a-billion standard, which means no slipping around or sidestepping certification requirements. Those standards are independent of the cruising speed of the airplane.
To borrow a somber nautical term, ZEHST seems dead in the water.