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Quiet rocket motors

  1. Oct 8, 2012 #1
    I saw my first Space X launch from near Cape Canaveral. I've been watching launches since 1966, and this was by far the quietest of this size vehicle that I've ever heard.

    How do you suppose they made it so quiet?
     
  2. jcsd
  3. Oct 8, 2012 #2

    berkeman

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    I did a Google search on your thread title, and got a few hits. Apparently there was a Popular Science article in June 2008 about the subject -- the link recommends going to your local public library to read the article.

    BTW, I also got a hit on "Silent Rocket" -- do not click into that link... :blushing:
     
  4. Oct 8, 2012 #3

    berkeman

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  5. Oct 9, 2012 #4
    I had an opportunity to talk with an old jet and rocket engineer, who is still working and is very sharp. His specialty is the aerodynamic analyis. He pointed out that I already knew that in a jet engine, the high noise comes from the rapid and forceful mixing of the exhaust with ambient air along a shear boundary, and that the reason modern airplanes are so quiet is because we have learned to mix the exhaust gasses with ambient in a much smoother and gentler manner. That is why modern engines are so fat with a huge fan up front and a high bypass ratio. He pointed out that Space X did the same thing in another way by putting nine engines on the back, and that is why it is so quiet. I should have thought of that, but didn't.
     
  6. Oct 9, 2012 #5

    berkeman

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    So the nine engines are arranged to manage the boundary layer better? Interesting...
     
  7. Oct 9, 2012 #6
    I'll first state what I know, and then tell you my best guess as to how this works.

    They advertise that they can still make orbit with 2 or 3 engines shut down. They are also carrying less than 10% of full capacity. So they clearly have lots of reserve engine capacity.

    Guessing: With all engines running, they could throttle down the outboard engines for smoother mixing with ambient. That would leave one in the center to run at 100%.

    NASA never designed with so much safety margin. So their stuff had to be much more reliable, which costs massively in schedule and cost in all phases of program life. Space X saved a ton of money and will continue to save for the life of the program by carrying redundancy and not paying for high reliability for individual components and systems.
     
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