Pitot Tubes?

Discussion in 'Competition Rocketry' started by theLabyrinthMaker, Oct 17, 2018.

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  1. Oct 17, 2018 #1

    theLabyrinthMaker

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    We're in a competition that requires us to use a pitot tube to determine the rocket's velocity. I've built a couple rockets before, but never even heard of using a pitot tube for this application. Is there any guiding wisdom here? Do I have to build one myself or is there somewhere I can by one? Do I literally bolt it on the side or is there some way to integrate it into the nosecone?
     
  2. Oct 18, 2018 #2

    Dipstick

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    I'm not a fluid expert, but I would think going through the nosecone would require a create exit flow through which would probably distort your number. I would think on the side of the airframe...
     
  3. Oct 18, 2018 #3

    dhbarr

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    I just punched pitot tube into Google Shopping, plenty of action.

    I should think, if possible, you'd want to mount it as a nose tip so as to get the cleanest possible sample as well as having the NC volume for battery & processing electronics.
     
  4. Oct 18, 2018 #4

    Nytrunner

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    Read up on the relationship of Total Pressure, Static Pressure, and Dynamic Pressure, then look up Pitot-static tubes.
    That should get you going in the right direction.

    Am I correct in assuming you're a student of some level?
     
  5. Oct 18, 2018 #5

    theLabyrinthMaker

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    That's correct, I'm an undergrad studying aerospace engineering. Theory and application of pitot tubes for airplanes has come up in my classes, so I'm familiar with how they work, I've just never heard of their use in model rocketry.
     
  6. Oct 18, 2018 #6

    Nytrunner

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    Ah! Good, then you have the theory down and just have to work out the execution.

    I haven't read of pitot-static tubes being used in rockets recently, but that's probably because our deployment electronics barometric sensors fulfill our needs these days.

    I'd think the necessary system would be a bit heavy for unclustered Estes BP motors, but if had to put in a ps-tube sensor, I'd go with F or G composite motors.

    Sounds like a fun project, let us know how it goes!
     
  7. Oct 18, 2018 #7

    aerostadt

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    Total pressure is on the very front of the nose cone. Static pressure is measured many body tube diameters back of the nose. These may not be practical, so you may need a compromise. In principle, if you had a long clear vertical tube in a vertical rocket measuring total pressure at the front end connected to the static pressure port in the back with water in this sort of U-tube manometer and you were riding in the rocket (far-fetched, yes), the difference in height would be the dynamic pressure and that measurement you could mathematically convert into a velocity. (Read up as Nytunner said.) All of this is a tall order. A shorter approach would be to make a spring device in the nose cone to measure the total pressure and convert that to a velocity.
     
  8. Oct 19, 2018 #8

    watheyak

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  9. Oct 19, 2018 #9

    Nytrunner

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    If he got/made an actual pitot-static tube that could go out the nose tip, it could be a small enough diameter to get the static ports in position.
     
  10. Oct 19, 2018 #10

    BEC

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    Watheyak beat me to it. I've not flown an Eagle Tree pitot/static tube and airspeed sensor on a rocket, though I do have one mostly built that I was intending to do that with. I have it projecting out the top of the nose cone.

    The Eagle Tree Prandtl tube is 0.156 inch in diameter and about 3 inches long from the tip to where the static and dynamic pressure lines split apart. The four static ports are about 4D back from the tip. It weighs 1/10 of an ounce. The airspeed sensor is fairly small and will run OK on a single Lipoly cell. running the static and dynamic pressure tubing from the Prandtl tube to the sensor can be a little amusing....

    I have flown a couple of RC airplanes with this setup feeding an Eagle Tree logger quite a bit and have gotten good data.
     

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    Last edited: Oct 19, 2018
  11. Oct 19, 2018 #11

    Stable1

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    I helped a collegiate team with this exact project. We used a digital differential pressure sensor interfaced to a Arduino computer to read and store the pressures during the flight. We used a thin tube in the air stream to sense the dynamic pressure and the sensor's low pressure port was open to the inside of the e-bay to sense the static pressure. When we crunched the numbers, they agreed very well with our altimeter's data.
    upload_2018-10-18_22-11-50.png upload_2018-10-18_22-15-2.png
     
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  12. Oct 19, 2018 #12

    aerostadt

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    Actually, the thin tube in the air stream is capturing the total pressure. The difference between the total pressure and static pressure is the dynamic pressure.

    (total pressure) - (static pressure) = dynamic pressure

    Then dynamic pressure = 0.5 x rho * (velocity)^2

    Then solve for air speed or velocity.
     

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