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  1. #1
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    27th August 2013
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    Ducted Rocket and Pressure Gradient???

    I've recently designed a rocket with an unconventional tail end with the idea being to reduce turbulence over the fins therefore making them more effective, however, having built it i'm not so convinced with the science and was hoping there was someone I could bounce my theory off just to let me know if there is any credence to the idea. The rocket flies well, but I don't know if the tail end is related to this or not.

    The idea was to have slots cut into the body tube on either side of the fins. Enclosed by the body tube is effectively a conical boat-tail with the theory being to have fast moving air entering this cavity through the slots. As the volume increases towards the rear of this cavity the air should begin to slow down and, using the Bernoulli principle, a decrease in velocity should relate to an increase in pressure. This should mean that an area of high pressure is created at the very rear of the rocket and an area of low pressure at the front of the cavity under the slots leading to a suction effect pulling air over the fins reducing turbulence. However, it has been pointed out to me that on all rockets there is an an area of low pressure at the very rear in the form of base drag. How does this effect the system, and could this system be said to reduce base drag? Furthermore, if this system is working in the first place, does pulling air over the fins reduce turbulence or could it in fact be making the problem worse?

    This video shows a 3D drawing of the system, but the slots are not included.


    It could very well be that I am using the Bernoulli principle incorrectly, but any input as to what theoretical effect this system is having to the rockets flight (if any) would be much appreciated.

    Thanks

  2. #2
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    If you step into the reference frame of the rocket, it would be nice to have the rocket aft-end have the same velocity as the free-stream air approaching the rocket from the front. This would be the ideal situation and there would be no "net" pressure drag (at best you break even on pressure drag), although, there is still skin drag. In reality there is an aft turbulent wake and the air flow does not resume a full free flow pattern. As you have said the aft external flow is somewhat higher than the free upstream flow and by the Bernouli equation this external flow has a lower pressure. This low pressure area is imposed across the rocket base. Boat-tails tend to alleviate this adverse low pressure aft region. Of course, for most model rocket motors you then exceed the minimum motor diameter and have more frontal area. The increase in frontal area will probably wipe out the streamline benefits in the aft region. Air flow slots will probably not buy any performance gain, because they are new sources for more skin drag and wake flow.

    To a large extent only so much can be done. Nose cones are the best source of pressure drag reduction. Boat-tails probably come in second. In essence removing both pressure drag and skin friction is like trying to overcome the Second Law of Thermodynamics. If there were a perfect solution, there would be no increase in entropy and there would be isentropic flow. With isentropic flow things like perfect airfoils could glide around the world forever.
    Last edited by aerostadt; 28th August 2013 at 04:38 AM.

  3. #3
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    27th August 2013
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    Thanks for your feedback aerostadt.

    I agree that in altitude terms there cannot be a performance boost because I am increasing surface area and in turn skin drag. However, this rocket was designed to fly in UKAYRoC (British version of TARC) and for the competition consistency between flights was very important. The idea was to make the fins more effective by sucking air over them and reducing turbulence hopefully meaning they are more effective at smaller angles of attack and in turn making the rocket fly straighter and achieve more repeatable altitudes. A performance loss in altitude wasn't necessarily a bad thing as it could easily achieve 750ft anyway, any higher just lost us points.

    Is there any merit in the suction effect I'm describing actually occurring in the real world and would such an effect actually make the fins more effective? I mentioned the low pressure base drag as I am unsure as to how this could effect the proposed suction action.

    I'll add pictures later today to better illustrate the design.

  4. #4
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    I tend to think that sucking in air from the outside is probably self-defeating in terms of drag reduction. If you look at the Reynolds number of your model based on the length of the model, you will find that your model is well into the turbulent region and turbulent wakes cannot be avoided. If you want a diameter larger than the motor diameter, use a boat-tail. Look at rapidly accelerating professional or high performance model rockets for competition or setting records, they do not rely on extra channeling to direct flow for reducing turbulence. In fact these designs tend to be clean and avoid extra attachments.

    There have been proposals for airplanes to have an on-board mechanical suction to suck (boundary layer suction) in air on wings and re-laminarize the boundary layer. It is my understanding that such concepts do work but at a cost of increased weight for the air suction system. I don't know of any practical operating airplane that uses this system.

    There has been talk of sucking air in for rocket thrust augmentation. This will not work for model rockets and in fact has not been found to be practical for professional rockets, although some technical papers show that it will work. These papers are probably not correct, because assumptions were made that outside air and rocket exhaust instantly mixed together, which is wrong.

  5. #5
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    18th January 2009
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    An interesting idea

    Esconian,

    On a purely qualitative basis, I am not sure that improving the airflow over the fins (if you could even do such a thing) would give you much of a benefit, at least in regard to increasing the consistency of flight profiles.

    I think you would gain more consistency from improving the quality of your fin airfoils and especially from improving the alignment of fins with respect to the body axis. I think you would gain far more drag reduction from exposing that boattail and losing (removing) that last section of full-diam body tube. (BTW, keep the transition/corner from BT to boattail smooth and, if possible, rounded.)

    On a side note, remember that the motors you are using also tend to have a small amount of performance variation, motor to motor, due to manufacturing tolerances. You are worrying about something relatively small (fin drag variations) while seemingly accepting other relatively large variations (in motor thrust).

    Your idea to reduce fin turbulence (and drag?) might make for an interesting subject for a smoke-tunnel model; this would allow you to see whether the flow behaves the way you are hoping or whether it does something worse/completely different. My SOTP estimate of the levels of drag reduction is that even with a finely made wind tunnel model set up with pressure taps for force measurements and wake rakes to survey momentum losses behind the model, your potential drag reductions will be in the "noise level" of the measurement capabilities of your experimental test hardware.

    My two cents
    (what is that in British coin?)
    Last edited by powderburner; 29th August 2013 at 06:11 AM.
    This is the country that built the transcontinental railroads, the Hoover dam, the Panama canal, coast-to-coast interstates, and put men on the moon....but we can't build a fence.

  6. #6
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    It's even more complicated than that

    Quote Originally Posted by aerostadt View Post
    There has been talk of sucking air in for rocket thrust augmentation. This will not work for model rockets and in fact has not been found to be practical for professional rockets, although some technical papers show that it will work. These papers are probably not correct, because assumptions were made that outside air and rocket exhaust instantly mixed together, which is wrong.
    Ducted rocket exhausts intended to entrain external air mass and enhance thrust are also very quirky about finding an optimized design that balances primary exhaust mass flow and secondary airflow inlet area. This is far more difficult to achieve than the average "armchair" rocket hobbyist understands or believes. This balance is also impossible to achieve with hobby construction techniques such as are available to us, when the rocket motor thrust and mass flow varies wildly between peak thrust and sustaining thrust while the secondary airflow inlet remains fixed in size. Even if true thrust augmentation can be achieved at some moment during the thrust curve, all the other flight time will be at non-optimum conditions and will generate far more drag than the brief moment of increased thrust would pay for.
    This is the country that built the transcontinental railroads, the Hoover dam, the Panama canal, coast-to-coast interstates, and put men on the moon....but we can't build a fence.

  7. #7
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    On a side note, remember that the motors you are using also tend to have a small amount of performance variation, motor to motor, due to manufacturing tolerances. You are worrying about something relatively small (fin drag variations) while seemingly accepting other relatively large variations (in motor thrust).
    This is a very good point.

    I know doing home-made wind tunnel work would be beyond the time and energy that I would want to invest in a project. Just building a reliable wind tunnel itself is a lot of work. BTW there is a good basic article about measuring nose cone drag in the latest Apogee magazine. I would recommend reading it. It might be good to start thinking in terms of drag coefficients (CD). I believe TARC contestants and other competitors look at using a CD for their altitude predictions. The CD wraps up the pressure drag and skin drag together, so it is very convenient. I would guess that most of the drag is coming from pressure drag. I was surprised a few years ago when I was able to review data for a university team in the Student Launch Initiative as to how low their CD was. The CD for their model was on the order of about 0.30. I think RocSim might use typical values around 0.6 . For competitors I imagine a Cd=0.3 is no big deal. There use to be a nice simple altitude predictor on the internet, which allowed the user to put in his own Cd estimate. Unfortunately, the last time I checked that website is no longer available. In any case I would run that program for my 4x Orbital Transport with a whopper Cd=1.4 . The altitude predictions were pretty good. The OT has lots of ducts, strakes, attachments, etc. , and hence is very draggy. Again, this supports my belief that low drag performance probably comes from a clean model.

    (Thrust augmentation is really going off on a tangent. Nevertheless, it is an interesting tangent. The only successful rocket thrust augmenter tube, both for the model world and the professional world, that I ever heard of was the Jetex motor. They had data that showed that the thrust augmentation worked. There are several theoretical reasons why this could be so. First, the motor had a long burn time. Second, the motor had only a pin-hole throat (there was no convergent/divergent section). As a consequence, the exhaust was severely under-expanded. This allowed 2 things to happen. The exhaust plume could expand and mix with outside air in the augmenter tube. The small pin-hole throat created a large motor base area for sustaining back pressure. In a control volume analysis both the pressure integral and the momentum integral have an opportunity to show thrust augmentation.)
    Last edited by aerostadt; 29th August 2013 at 05:59 PM.

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