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Small wind tunnel for a model rocket

  1. Jul 9, 2007 #1
    I need to test a model rocket which will have average airspeeds around 200 mph. I am hoping to construct a plexiglass box about 9 feet long, 3 feet wide and 3 feet tall. For these dimensions and airspeed requirements it looks like I'd need a 160000 cfm blower. Is this something I can feasibly obtain? This is supposed to demonstrate a model rocket roll control system, which will be my senior design project.

    Maybe I should look into a smaller rocket, eh?

    Sorry, I'm not an ME so this might be a dumb question.
  2. jcsd
  3. Jul 10, 2007 #2


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    160,000 scfm is about 20 Lbm/sec. That's a lot of air. I would seriously look into scaling if at all possible. If you can get the scale of the rocket down and make the test section smaller, you can take advantage of the neck down to get your velocity up. Either that or find someone who already has a wind tunnel.

    Just to give you an idea of scale here, are you familiar with the blower units that janitors use to dry out carpets? Those put out, at best, about 1500 scfm. If you go to www.mcmaster.com and enter 3225k32 in the Find Products box, you'll see what I am talking about.
  4. Jul 10, 2007 #3


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    leright, this is mostly for vertical flight, right? I think I remember your thread on the control and feedback system. If the rocket is going to servo mostly vertically, can you just make the diameter of the tunnel only a few times the outer fin tip distances? Can you get it down to more like 1 foot in diameter? I don't know if that helps enough, though.
  5. Jul 10, 2007 #4


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    I recently did a study on an air handling unit in a skyscraper in Philly. The air handling unit was roughly the size of a small apartment (but taller) and had a capacity of 110,000 CFM and a 125 HP motor. The fan wheel itself was 6 feet in diameter.

    Yah, I'd consider scaling it down too...
  6. Jul 10, 2007 #5
    unfortunately, to house the electronics and servo motors/controllers the rocket 'tube' will need to be at least 6 inches in diameter I believe, and the rear fins plus tube diameter will be roughly 18 inches in diameter. I could get away with using a tube with a 20 inch diameter, I think. I could actually get away with using a 40,000 cfm blower if I were to do that. Ideally though, I would be able to get the wind speed to the maximum speed of the rocket, which is roughly 500 mph.
  7. Jul 10, 2007 #6
    lol, thanks, I will consider that. sorry, I just have no intuition or feel for these types of things.

    Maybe I will look into renting a university wind tunnel instead.
  8. Jul 10, 2007 #7
    ha, yeah, I'm going to look into using someone else's wind tunnel.

    Problem is, I will need to empirically determine the airframe dynamics of the control system (relationship between roll speed and control fin angles) and this will require many different test runs. I assume renting a wind tunnel is not a cheap thing, even at a university.

    Thanks everyone for your guidance.
  9. Jul 11, 2007 #8


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    You never know! Play the "poor college student" routine to the hilt. You may be surprised. When I did my senior project, there were a couple of companies that donated time and a few items to our cause. After hours time on a small tunnel doesn't seem like it would be too much of a donation.
  10. Jul 18, 2007 #9


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    Well just a couple of ideas that might help

    a) get a real wind tunnel,

    b) if not a, gravity might be your friend -- terminal velocity
    might be around your 200mph velocity regime, so if you
    can find somewhere to drop the streamlined thing off
    a cliff while it's running a data collection / controls
    actuation test that you can gather data from, that'll get
    you empirical data that's at least in the realm of being
    accurate for your problem. Intact recovery shouldn't be
    that infeasible depending on your test site and setup.

    c) flight test the thing under its own power
    if that's easy; if you're collecting data and have appropriate
    exercises programmed in to the actuators, I'd think you'd
    be able to gather some useful data before the velocity
    decreased too much. Start out with neutral controls and
    get increasingly adventurous as altitude increases then
    let it parachute (or whatever your design does) home
    after the high altitude / high velocity period is over.

    d) If you tethered it to a rotating arm or cable it seems
    like it shouldn't be too hard to use a conventional motor
    to spin it up to reasonably high airspeeds safely.
    Rather than using a giant propeller with high speed
    blades to push air at high speed down a wind tunnel,
    it simplifies things a bit to just use the airspeed
    at the tip of a rotating arm itself if you can stand the
    centripetal accelleration and the radius of body turn
    in the airstream is within the realm of servo control that
    you wanted to test anyway. Clearly unrealistic for 500mph,
    but clearly do-able for 100mph (45m/s), and somwhere
    between the two is your limit.

    e) you can use a nozzle (just like .... a rocket, hey!)
    to convert high pressure/temperature air to low pressure
    high velocity air... So it could conceivably be within the
    realm of engineering feasibility to generate low pressure
    200mph to 500mph air on the output of a
    static combustion chamber with a nozzle, but, of course,
    you'd need to have some reasonable balance of
    temperature vs. speed vs. pressure (which would need to
    remain above atmospheric, of course). Certainly
    nothing to attempt for a huge diameter / length of test
    chamber, but possibly something that would be very
    do-able for a very restricted size.

    f) Would it be that bad to just use computer models to
    estimate the forces involved, then to use some kind of
    self-calibrating servo algorithm using feedback sensors
    etc. to implement something like a PID loop or whatever?
    How far to turn the fin? Well start turning it a little,
    if that's not enough, turn it some more... etc.
  11. Jul 18, 2007 #10


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    g) drop it from a small weather balloon and use
    a terminal velocity fall for some calibration tests.
    That's something you could repeat several times a day
    if you had a safe test area with little cross-wind and
    launched/recovered the balloon and test payload once
    every hour or so and limited the altitude to some modest
    few thousand foot level.
  12. Jul 18, 2007 #11
    The Russian's used large compressed-air banks to test supersonic flows. Could this concept be of use at lower velocities?
  13. Jul 18, 2007 #12
    Fred Garvin; Can I get that SCFM to LBs equation from you? I've been looking for that for sometime.
  14. Jul 18, 2007 #13


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    Actually, that's a great idea for certain flow regimes!

    I suppose I didn't put it out as an idea since they'd
    been tossing around 100,000.00 CFM numbers a few
    posts back talking about the "pie in the sky" dimensions
    for an "I wish I had" wind tunnel. You'd run out of
    compressed air pretty fast at 100,000.00 CFM, but then
    that kind of flow is impractical for any simply built
    wind tunnel, you'd need a full size jet engine or something
    to get those numbers.

    But if the tunnel test could be scaled down, certainly,
    one could get a nice clean cool stream of several
    hundred MPH air out of a few SCUBA tanks exhausting
    2200 to 3300 PSI air through some nozzles into a
    chamber a very small number of cubic feet (or less)
    of volume.

    You'd have no real control or stability of flow velocity
    in that the tanks pressures would drain pretty quickly,
    but for testing automatic servo mechanisms, I'd guess
    you could learn a lot in only 5 seconds of test time.

    And if you staged 4-5 tanks and had a few people open
    the valves in a staggered arrangement e.g. start to
    open the next tank valve when the first tank is
    down to half-pressure, you could prolong the test time
    and get somewhat more stable air flow rates since as
    one tank emptied and lowered pressure you'd be adding
    the boost of another.

    And, actually, having quickly varying air-flow rates would
    be perfectly realistic of a rocket with high accelleration
    anyway since it'd be going from something like 0 to
    200MPH in just a few seconds, so constant air-flow rate
    would NEVER occur in a real world environment for his
    setup anyway.

    You can get SCUBA tanks of compressed air up to
    120 CU FT capacity (in STP CF equivalents) with a peak
    pressure in the 3200PSI range, so figure at least 60CF
    will be available at a "pretty high" pressure and it'd
    diminish in pressure for a single HP120 tank from there
    as it got more empty.

    You can get much bigger industrial gas cylinders that
    have several hundred CF capacity at ~ 2000PSI max.
    pressure, and it's not uncommon to put a few of those
    in parallel for higher capacity.

    So it's an option if you can figure out a way to use
    pretty cold fast air in the range of 50CF to 500CF total
    ambient air pressure CF capacity delivered at 2000-500
    PSI pressure decreasing fairly rapidly in time.

    You might need to use DIN valves (for SCUBA tanks)
    or some other kind of valve that lets the flow rate you
    need escape; the normal tanks have relatively narrow
    orifices so that a large fraction of the tank can't escape
    in less than several seconds.

    Of course you'd also have do be sure to do this kind of thing
    SAFELY so that you don't have any hazards from freezing
    air, accidental air pressure surges causing ruptures of any
    restricted flow areas (make sure the exit flow tubes
    can NOT get blocked so that no high pressure can
    develop), and that normal high pressure tank/gas
    handling precautions are observed.
  15. Jul 18, 2007 #14


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    It is simply [tex]\dot{m}=\rho*Q[/tex] where

    [tex]\dot{m}[/tex] = mass flow rate
    [tex]\rho[/tex] = density at reference condition
    [tex]Q[/tex] = volumetric flow rate at reference condition
  16. Jul 18, 2007 #15


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    In regards to the air farm set up...it really is an extremely large endeavor to provide ample air like that. It is a good way to do it, but the facilities required are tremendous mostly because to get a usable/beneficial amount of air, you need a very large storage system.

    For example (granted, a large example) is the air farm built for the Pluto Project that LLNL did back in the '50s:

    We air start some of our engines and I have played around a lot with scuba tanks, etc...You really don't get that much usable air out of a scuba tank. They deplete pretty quickly. Even the larger K bottles at 5000 psi were pretty small.

    The idea of the rotating arm and using the rocket's own engines is a great idea. You would have to strap a camera to the arm and the runs would be short lived, but it would definitely save you on the wind tunnel construction. It's definitely something to consider.
  17. Jul 18, 2007 #16
    I dont see how you are going to control anything thats tethered to a rotating arm. Although, it would probably be good for ripping the rocket appart. I guess the Gyros will just have to ignore the massive accelerations....

    You can't 'control' anything with that setup. What you can do, is try to fly the rocket like a control line model. In other words, you can only control up/down motion along the horizontal axis. Because the rocket has 4 fins, you can first get 2 fins working, then rotate the rocket, and get the other 2 fins working. So you will only be able to work out 2 control loops. But if it does not work, you are going to disintegrate your rocket as it flys into the ground at 100mph because it did not get enough up command. Also, when the motor runs out, it has no wing. In other words, it will fall into the ground like a brick and break off your fins anyways.
    Last edited: Jul 18, 2007
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