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How We Might Have Scramjets Sooner than Expected

Posted by Zonk on from the zooom-zippy-fast dept.

loralai writes "Recent breakthroughs in scramjet engines could mean two-hour flights from New York to Tokyo. This technology, decades in the making, could redefine our understanding of air travel and military encounters. 'To put things in context, the world's fastest jet, the Air Force's SR-71 Blackbird spy plane, set a speed record of Mach 3.3 in 1990 when it flew from Los Angeles to Washington, D.C., in just over an hour. That's about the limit for jet engines; the fastest fighter planes barely crack Mach 1.6. Scramjets, on the other hand, can theoretically fly as fast as Mach 15--nearly 10,000 mph.'"

3 of 674 comments (clear)

  1. SR-71 Blackbird by wilder_card · · Score: 5, Informative
    "set a speed record of Mach 3.3 in 1990 when it flew from Los Angeles to Washington, D.C., in just over an hour."

    I feel compelled to point out that's the unclassified speed record. Its actual top speed is still speculative.

    1. Re:SR-71 Blackbird by Hamilton+Lovecraft · · Score: 5, Informative
      Hybrid turbojet-ramjet, according to wikipedia:

      The J58 was unique in that it was a hybrid jet engine. It could operate as a regular turbojet at low speeds, but at high speeds it became a ramjet. The engine can be thought of as a turbojet engine inside a ramjet engine. At lower speeds, the turbojet provided most of the compression and most of the energy from fuel combustion. At higher speeds, the turbojet throttled back and just sat in the middle of the engine as air bypassed around it, having been compressed by the shock cones and only burning fuel in the afterburner.
      --
      step 3: god dammit, it doesn't work
    2. Re:SR-71 Blackbird by Zebra_X · · Score: 5, Informative

      I though the same thing for years. However it appears that the POH for the Blackbird has become public record. This manual basically describes how to fly the plane. The manual is now online @ http://www.sr-71.org/blackbird/

      The manual clearly shows that the planes design speed is mach 3.2 - exceeding this speed requires authorization from command.
      The thing that not everyone realizes is that unlike other planes that can go mach 2 or 3, they cannot sustain this speed due to excessive heating and or fuel consumption constraints. The blackbird is different in that it is designed to fly for ~ 3 hours at these speeds. In fact there are several guages dedicated to external heating for the plane. http://www.sr-71.org/blackbird/manual/5/5-9.php

      So with all that said, the flat out top speed may be higher, but the operating manual usually wins out.

      The summary for the article is mostly incorrect regarding the blackbird. The engine design of the blackbird is a hybrid design. The engine is a turbojet but there is a ramjet bypass for higher speeds. Ramjets are also known to work at speeds of up to Mach 5+. Though the scramjet engine is not much different it's just that the characteristics of the shockwaves change so much that the shape of the engine needs to change to achieve the same effect. So the limitation is not its engines, it mostly has to do with heating of the aircraft surfaces. Of the many topics discussed in the manual for the blackbird, external and internal heating was a major area of attention.

      So if the Blackbird has issues with heating - you can bet that any other plane operating at that speed or higher will have the same problem. Unfortunately it is difficult to find a place to dump the excess heat. Any surface that comes into contact with the airstream causes friction, and heat buildup. You can use the fuel as a coolant, and the blackbird did. The JP-7 fuel that the blackbird used had an extremely high flashpoint. So it could be used to absorb some of the internal heat before being burned off. The blackbird is also much more like today's aircraft in construction - it was one of the first aircraft to use titanium alloys extensively in its construction.

      The bottom line is that you don't just build a scram jet powered plane. It's not just about the engine, but about the entire plane. The challenges run the entire range from thermal to mechanical. To simply throw out a number like mach 15 and think that it's feasible to obtain any lasting operation at that speed using today's technology shows a distinct lack of understanding of the subject matter.