Date: 11 Apr 98 02:16:18 From: firstname.lastname@example.org (Stephen H. Westin) Organization: Program of Computer Graphics -- Cornell University
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I know that Concorde has a MGTOW in the range of a 767 (408,000lb., according to BA's Web site), though carrying far fewer passengers a shorter maximum range. I also know that nearly half of that weight is fuel, not only to maintain a Mach 2 cruise, but just to get the plane off the ground. Much of the fuel is burned in the takeoff run, both because the engines are running inefficiently and because the wing doesn't work that well at low speeds: it has to get going a lot faster than most airliners just to get off the ground. I read, in fact, that an aborted takeoff in Concorde has to dissipate as much energy as one in a 747; mass is lower, but V1 is higher. These inefficiencies, of course, are due to compromises between high-speed performance and low-speed performance. Those pure turbojets are relatively efficient at Mach 2 and 60,000 feet, but at low speed and altitude, with afterburners engaged, the specific thrust figures are pretty low. And the supersonic delta wing is pretty draggy at low speeds. I was wondering about using a second aircraft to avoid these compromises; say, strap an SST to the back of a 747, Shuttle-style, for takeoff. After attaining a decent airspeed and altitude, the SST would start engines and unlatch from the carrier plane. Think of it as a multi-stage rocket: it's not necessary to carry the extra fuel tanks and big engines all the way to orbit, and upper stages can be optimized for their high-altitude environment. The big wing and high-bypass engines of the 747 would work better up to, say, 350 knots, where a specialized high-speed wing and engines would take over. Perhaps the carrier plane could even be unmanned, and return to land automatically. Of course, the SST would still land on its own, but I don't think that uses a lot of fuel anyway. I realize that this would complicate things a lot, and there would be big questions of economy, reliability, and safety. But I wonder: o How much could this extend the range of an SST, as its fuel tanks would still be fairly full at the start of cruise? Could Concorde become a trans-Pacific aircraft this way, for example? o Could this actually result in a net fuel savings, despite the extra weight and drag of the carrier plane? o Would the 747 be big enough for the job, or would something bigger have to be built? This might be a job for an expanded GE90, for example. o The Concorde/747 combo might be a basis for concrete estimates, but as Concorde weighs over twice what the Shuttle Orbiter does, it probably wouldn't work. What optimizations could be made with a purpose-built 2-plane combo, based on current technology? -- -Stephen H. Westin Any information or opinions in this message are mine: they do not represent the position of Cornell University or any of its sponsors.