Your Captain Speaking
The next time you find yourself flying through the Austin Executive Airport, take a moment to stop in the terminal. In the middle of the lobby there is an intricate and masterful object that could be considered a work of art, a turbojet engine, or both. It is known officially as the Rolls-Royce/Snecma Olympus 593 and chances are you’ve never heard of it. It’s a good bet, however, that you are familiar with the plane it powered — Concorde.
Concorde, the supersonic airliner, one of only two in the annals of commercial aviation, is among the most iconic planes in history with its droop-able nose, ‘delta’ wing shape, under-wing turbojet (as opposed to turbofan) engines, and lack of a (horizontal) tail. Cruising at more than Mach 2.0 (twice the speed of sound – about 1,350 miles/hr.), at an altitude of roughly 56,000 feet, Concorde could fly from New York to Paris in 3.5 hours. A typical jet requires 8 hours on the same route at about 40,000 feet and a speed of Mach 0.8 (about 530 miles/hr.).
The list of technical innovations Concorde spurred is as impressive as its performance. For example, supersonic flight regime mandated the delta-wing design, but delta wings create very little lift when a plane is level and slow. During takeoff and landing, to produce sufficient lift, a plane with a delta wing must be flown at much higher air speeds and greater angles-of-attack (AOA) than conventionally configured planes. In order to make this possible the ‘droop-nose’ was developed to allow the pilots an unobstructed view during these critical phases of flight. The wing root (distance between wing leading edge to wing trailing edge) also extended almost the entire length of the fuselage, quite in contrast to “typical” wing designs on jet planes. The deceptively-simple appearance of Concorde’s delta wing belies the extensive testing and research that went into designing a wing that would perform reliably through such an extensive range of flight regimes.
Digital (electronic) controls replaced the mechanical linkages to the flight controls (called ‘fly-by-wire’); in the same fashion, engine controls ‘thrust-by-wire’ and braking system ‘brake-by-wire. The digital engine control system evolved into the now-well known and used full authority digital engine control (FADEC) computers, whereby all of the various inputs that control the engines, e.g., ignition, fuel, warning systems etc. are computer controlled.
Concorde’s landing gear was unusually tall, to allow for the high AOA, requiring it to collapse in, on itself, in a ‘telescoping’ system while being retracted. Its brakes and landing gear, to accommodate speed, also needed to be stronger than conventional design.
One of the lesser-known innovations is what the Concorde didn’t have: “trim tabs”. Trim tabs reduce pressure on the flight controls and make the plane easier to fly. They also add drag. Drag is directly related to the square of velocity. So, increasing drag just a little, at such a high speed, would have a massive impact. An ingenious workaround was devised: By transferring a measured amount of fuel, the plane’s center-of-gravity moved resulting in the same control pressure reduction as provided by trim tabs.
The Concorde’s high speed also complicated structural design requirements. As a plane climbs, the outside surface, or skin, cools. When flying supersonic a plane’s skin heats up. The process would reverse as Concorde slowed and descended. This heating and cooling resulted in Concorde flexing up to a foot during flight. The flexing was noticeable to passengers and crew alike.
Everything in aeronautical design is a give-and-take; and while high speeds are attainable, for their greatest inefficiencies they require very high altitudes. This increases the importance of cabin pressurization. The outside air pressure at 60,000 feet is low enough to present problems that don’t appear at 40,000 feet. To address this, Concorde was designed with smaller than normal windows to ensure a stronger fuselage and a reserve air supply system to augment cabin air. These precautions, as well as the employ of independent and redundant systems, would allow sufficient time for the pilots to perform a rapid descent to a safe altitude in the event of a rapid decompression.
Concorde made its first flight back in 1969 though it had been in some stage or another of conceptual design since the 1950s. It makes one wonder, with the retirement of Concorde in 2003, why isn’t a supersonic jetliner gracing the skies almost 50 years later?
First, it’s important to understand the less-technical objections to supersonic travel. There are ecological objections, concerns about radiation exposure in the upper atmosphere, and a limited market for such a high-end product. Additionally, it is illegal to fly at supersonic speeds over the United States due to the resulting sonic boom.
Lockheed Martin Skunk Works signed a $25 million contract with Supersonic Aerospace International (SAI) in 2001 to create a “Quiet Supersonic Transport (QSST)” with, theoretically, only 1% of the sonic boom Concorde produced. Projections for customer delivery were 2018 but the project stalled in 2010. SAI is still hopeful to make progress in this direction. On February 29th of this year, NASA announced Lockheed Martin’s team as the winner of a $20 million contract for 17 months of “preliminary design” for quiet supersonic technology.
It took only 60 years to make it from the Wright Flyer to Concorde. A good portion of the technology for supersonic flight has been proven. When a universal definition of an ‘acceptable’ sonic boom is agreed upon, and with a little more R&D (Research and Development) and a lot more investment, it seems that a return to supersonic transportation is only a matter of time.
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