Dec 292010

An early diagram for the Boeing 707-derived AWACS.  The USAF originally specced an aircraft capable of greater loiter than the 707/KC-135 was capable of, so Boeing designed a version with 8 more efficient engines. When the USAF backed off on the loiter requirement, the existing engine arrangement was workable, and Boeing went with that.

 Posted by at 11:40 pm
  • This old cop does not understand. X amount of gas in aircraft… 8 engines
    burn more gas than 4? How does this equal more time on station without refueling on station?

  • Pat Flannery

    Maybe they could shut down four engines (one in each pod) once they were at cruise altitude and speed? Still, this seems like a really odd approach.
    I was always surprised that they didn’t replace the B-52’s eight engines with four high bypass turbofans like on the 747.

  • admin

    > 8 engines burn more gas than 4?

    Not necessarily. The GE TF-34’s planned for this design might have been smaller than the P&W JT3D’s of the 707, but it was also about ten years newer (late 60’s rather than late 50’s) and more fuel efficient. As the TF-34 has about half the thrust of the JJT3D, you’d need about twice as many of them.

    One reason why a larger number of smaller engines may be appealing is that larger engines are neccesarily “taller,” in that they obviously have a bigger diameter, and this means that you have to either have them so close to the wing that the exhaust interacts with the wing structure, or they hang really close to the runway while on the ground. If you have any notions of operating from anything but the best runways, this can be an issue.

  • Damon Hill

    Another issue is the length of runways; the additional thrust might have been needed to take off shorter runways with a maximum fuel load. I’ve heard the KC-135 on straight turbojets at takeoff and it’s mindbendingly LOUD; I shudder to think what eight engines would have been like (never been on the receiving end of a B-52 takeoff).

    There have been proposals to re-engine the B-52 over the years; overhauling the beasts to bring them into the 21st century would be an expensive task. Still, it’s been done with the C-5M to good effect on payload, range, cruise altitude, and time to altitude. And dispatch reliability.

  • Pat Flannery

    The noise on takeoff is from water injection into the engines, a technique that was used on a lot of early jets to up thrust during takeoff.
    It did add around 2,000 pounds thrust per engine on the KC-135/B-707’s on which it was used, but always led to a noise problem, despite very involved exhaust nozzle designs that tried to deal with it on the airliners:
    I don’t know if those were fitted to the early turbojet KC-135’s; they don’t appear to be in this photo:

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