Posted by Michael Hiteshew on August 21st, 2004 (All posts by Michael Hiteshew)
Speaking of jets, DARPA, the famous Defense Advanced Research Projects Agency, has been funding a new technology demonstration which, if successful, could revolutionize air travel. At the heart of the technology is the abilty to mathematically model, and so predict and control, the shape and intensity of shock waves as they propogate away from the leading and trailing edges of supersonically flying aircraft and expand into the atmosphere and across the ground below them. These shock (pressure) waves are translated inside our ears into sound and are what are commonly know as sonic booms. According to Aviation Week:
Northrop Grumman and Lockheed Martin have concluded that it may be possible to meet DARPA’s target for the sonic boom by changing the shape of the aircraft, and without using exotic technologies such as plasmas. The idea is not to eliminate the pressure wave (‘Ye canna change the laws of physics, Jim’) but to change the normal ‘N-wave’ profile of the boom to a smooth hump, removing the rapid pressure rises at the nose and tail of the aircraft.
Working from pure acoustic theory in the late 1960’s, Cornell University’s Dr. Albert George and his colleague, Dr. Richard Seebass, found a way to reshape the sonic boom into a soft, harmless pressure wave. The result became the Seebass-George theory and was published in 1971.
The problem was that, until this DARPA program, no-one had ever actually tested the theory. It’s extremely expensive to build planes, set up instruments for monitoring performance, analyse the results, change the aircraft, make more tests, then deal with a whole new set of test results. It’s only recently that low cost, high speed computers and the advent of advanced computational fluid dynamics (CFD) software have combined to allow engineers to model various aircraft configurations and test them against Seebass-George theory.
In order to verify its accuracy, DARPA provided funding to modify a supersonic, 1960’s era F-5 with a different nose configuration. Sonic boom pressure measurements were taken with an unmodified F-5 and compared to the modified F-5. In each test, the modified pressure waves matched the Seebass-George theory predictions both in their intensity and shape. Quite an accomplishment. This is exactly the kind of thing takes aerospace design and ‘kicks it up a notch’.
There’s still one major hurdle left though, basic engine noise and long term performance. General Electric, Rolls-Royce and Pratt & Whitney are studying the problem. Civil engines have to be quiet on takeoff and landing but supersonic military engines don’t. Also, supersonic engines run at full blast all the time, making it hard to run several thousand hours between overhauls. Pratt & Whitney is said to looking at a version of their new F-119 Supercruise engine developed for the F-22. It’s powerful, with 35,000 lbs of thrust, and fuel efficient since it does not require fuel guzzling afterburners to run supersonic.
DARPA envisions a plane with a 6,000 nautical mile unrefueled range while running at mach 2 to 2.4. The plane would weigh in at approximately 100,000 lbs, compared to the Concorde’s 400,000 lbs, and with a payload to weight ratio of around 20% (20,000 lbs of payload), compared to the Concorde’s 7% (28,000 lbs of payload).
So what does this mean for us? In the next 10-15 years we could begin to see supersonic overland travel becoming available. Imagine taking off from Boston at 8:00 AM and landing in San Diego at 11:00 AM. Now imagine taking off from New York at 8:00 AM and landing in Honolulu at 2:00 PM, nonstop. What do you think that will do tourism worldwide?