Unnusual fin design question.

Discussion in 'Low Power Rocketry (LPR)' started by CZ Brat, Sep 29, 2015.

Help Support The Rocketry Forum by donating:

  1. Sep 29, 2015 #1

    CZ Brat

    CZ Brat

    CZ Brat

    Well-Known Member

    Joined:
    Jan 3, 2013
    Messages:
    611
    Likes Received:
    2
    My club is having a contest where the goal is to fly a rocket 8 times in one day on each of the following twice: 1/2A6-2, A8-3, B6-4 and C6-5. If your rocket gets damaged, your DQ'd (not Dairy Queen). The other aspect of the contest, is the total cumulative flight time of all 8 flights (assuming your rocket is still alive). So you want it to go low and fall fast.

    So I was considering replacing normal fins with a disc. I have attached a OR screen shot to show what I mean (the best rendering I can do in OR). But the CP is much higher than I think it would be for real.

    Anyone have experience with this or something similar?

    For simulation purposes, the fins are 2.5" high, 3' thick, and root and tip chord of .25". Material is balsa.

    Capture.jpg
     
  2. Sep 30, 2015 #2

    Solomoriah

    Solomoriah

    Solomoriah

    Well-Known Member

    Joined:
    Feb 12, 2009
    Messages:
    816
    Likes Received:
    1
    Rockets using discs or other flat plates for stability are not stabilized in the normal way. Base drag created by pushing the plate through the air is what provides stability. Unfortunately, programs like OpenRocket (and the Barrowman Equations such programs are based on) only figure stability the normal way.

    Drag-stabilized rockets like your design become unstable pretty much the moment the engine burns out. They tend to start tumbling immediately. If you build one small enough, you can actually depend on that behavior for recovery (as it is, strictly speaking, a "tumble recovery" design).
     
  3. Sep 30, 2015 #3

    TheAviator

    TheAviator

    TheAviator

    Well-Known Member

    Joined:
    Jan 18, 2009
    Messages:
    814
    Likes Received:
    2
  4. Oct 2, 2015 #4

    GlenP

    GlenP

    GlenP

    Well-Known Member

    Joined:
    Oct 4, 2014
    Messages:
    1,666
    Likes Received:
    156
    the disc may get ripped off the tube without some additional glue seams and supports to hold it in place. try adding some conventional fins as supports, or angle brackets, to attach the tube to the disc. alternately, you might consider building a cone or larger diameter saucer for the tail.
     
  5. Oct 2, 2015 #5

    Solomoriah

    Solomoriah

    Solomoriah

    Well-Known Member

    Joined:
    Feb 12, 2009
    Messages:
    816
    Likes Received:
    1
    Glen, I've never seen a failure like that. I suppose it would depend on the disc material, but even a C engine wouldn't hit all that hard.
     
  6. Oct 4, 2015 #6

    BABAR

    BABAR

    BABAR

    Builds Rockets for NASA TRF Lifetime Supporter TRF Supporter

    Joined:
    Aug 27, 2011
    Messages:
    5,082
    Likes Received:
    765
    Monocopters would work as well.
    I'm assuming given your
    "The other aspect of the contest, is the total cumulative flight time of all 8 flights (assuming your rocket is still alive). So you want it to go low and fall fast."
    that this is like golf, the LOWEST score wins?
     
  7. Oct 4, 2015 #7

    Charles_McG

    Charles_McG

    Charles_McG

    Ciderwright

    Joined:
    Sep 12, 2013
    Messages:
    2,406
    Likes Received:
    497
    Location:
    SE Wisconsin
    18mm scale of Estes blender. Handles a C nicely.
     
  8. Oct 5, 2015 #8

    cbrarick

    cbrarick

    cbrarick

    Wildman CT

    Joined:
    Jan 23, 2009
    Messages:
    2,373
    Likes Received:
    179
    one of Art Applewhite's steath's
     
  9. Oct 5, 2015 #9

    GlenP

    GlenP

    GlenP

    Well-Known Member

    Joined:
    Oct 4, 2014
    Messages:
    1,666
    Likes Received:
    156
    I have never seen a rocket with fins like that, so I defer to your experience.

    If they are balsa, some thought should be given to the grain direction at least, making the disc from wedges with the grain oriented radially outward, or possibly laminating two sheets together with grain at 90deg, if not too heavy. The extra thickness would help with the body tube to disc glue joint as well for strength.
     
  10. Oct 5, 2015 #10

    AlfaBrewer

    AlfaBrewer

    AlfaBrewer

    Well-Known Member

    Joined:
    Mar 18, 2011
    Messages:
    845
    Likes Received:
    3
  11. Oct 5, 2015 #11

    GlenP

    GlenP

    GlenP

    Well-Known Member

    Joined:
    Oct 4, 2014
    Messages:
    1,666
    Likes Received:
    156
  12. Oct 6, 2015 #12

    Solomoriah

    Solomoriah

    Solomoriah

    Well-Known Member

    Joined:
    Feb 12, 2009
    Messages:
    816
    Likes Received:
    1
    AH.

    You are confused, I'm afraid. He's talking about a disc at the rear (like half a spool rocket). The design he shows is as close as he could figure out in OpenRocket; the OP expected that OpenRocket could give him guidance on the stability of the design, but it can't (as I pointed out) because discs like that are drag stabilized.
     
  13. Oct 6, 2015 #13

    GlenP

    GlenP

    GlenP

    Well-Known Member

    Joined:
    Oct 4, 2014
    Messages:
    1,666
    Likes Received:
    156
    What if you put a really large body tube with a short transition at the end of the rocket, could that work in open rocket, since it would not be using a fin model, but a body drag model. There may be some equivalent body transition size for a disc of a given size?
     
  14. Oct 6, 2015 #14

    Solomoriah

    Solomoriah

    Solomoriah

    Well-Known Member

    Joined:
    Feb 12, 2009
    Messages:
    816
    Likes Received:
    1
    You don't understand. OpenRocket does not know how to figure stability when you are using drag. It will consider a cone that flares out as improving the stability only based on the Barrowman effects, but will not take base drag into account. There is no way to simulate your design with any simulator I know of.
     
  15. Oct 6, 2015 #15

    Solomoriah

    Solomoriah

    Solomoriah

    Well-Known Member

    Joined:
    Feb 12, 2009
    Messages:
    816
    Likes Received:
    1
    (In other words, you've got to build it and test it to find out.)
     
  16. Oct 9, 2015 #16

    GlenP

    GlenP

    GlenP

    Well-Known Member

    Joined:
    Oct 4, 2014
    Messages:
    1,666
    Likes Received:
    156
    There was an interesting article in an old Peak of Flight newsletter. Not sure how valid this approach is for general disc fins. The example shows how to make a correction for base drag calibrated for a known stable rocket, the FatBoy, to match an essentially known stability margin using RockSim. Not sure if that same scheme would work for an unknown rocket. But a clever idea. Maybe not applicable in OpenRocket.
    https://www.apogeerockets.com/education/downloads/Newsletter154.pdf

    An older article about base drag in RockSim, not sure if this is taken into account in the stability analysis:
    https://www.apogeerockets.com/education/downloads/Newsletter103_sm.pdf
    "RockSim does calculate base drag. You can find it by going to the Cd Analysis screen to see what it comes up with."
    the base drag might only come into consideration in trajectory/altitude sims, but not for stability, perhaps?

    Maybe a good question for the authors of those articles. It does sound like there is some experimentation required in any case.

    from: https://www.apogeerockets.com/Newsletter/newsletter_archive
     
  17. Oct 10, 2015 #17

    bill_s

    bill_s

    bill_s

    Well-Known Member

    Joined:
    Jul 29, 2012
    Messages:
    913
    Likes Received:
    6
    ^^-- It's not really base drag, it's a drag object at the base end of the rocket. However simulating it as a cone like that, in this case larger than the airframe diameter, would be one way to do it. In other words, use a cone fin which it can sim. It might even be appropriate to move it back like that, or extend the rocket for sim purposes, depending on what is believed about flat drag surface characteristics, although actually OR's sim of cone fins can be wildly optimistic as it is. Consider also a cone fin, they are also quite draggy, diameter without much weight, a good structural shape too. On my 5.5", 5 oz. rocket you can see every dip and bump in the thrust curve during flight.

    I don't think OR can put a nose can in the back, but you can use transitions.

    The simplest and roughest calculation would be the old cardboard cutout method, turning the drag plate sideways.
     
    Last edited: Oct 11, 2015
  18. Oct 11, 2015 #18

    Solomoriah

    Solomoriah

    Solomoriah

    Well-Known Member

    Joined:
    Feb 12, 2009
    Messages:
    816
    Likes Received:
    1
    As I understand it, OpenRocket uses a version of the Barrowman equations to figure stability. Drag is considered only as it affects altitude, not as it affects stability. I might be wrong, but I don't think either OpenRocket or Rocksim can properly indicate the stability of a rocket stabilized by drag.
     
  19. Oct 11, 2015 #19

    bill_s

    bill_s

    bill_s

    Well-Known Member

    Joined:
    Jul 29, 2012
    Messages:
    913
    Likes Received:
    6
    ^^-- I don't know much about drag plate or disc characteristics or how much is known -- it may be anyone's calculation is going to be rough. Overconservatism and/or test may be needed. It is still more simulatable than when the whole rocket is a disc such as a saucer -- in that case the "virtual" CP of the drag shape being behind the shape itself becomes essential. However you should be able to calculate something. OR requires something visible in the side view to go on for the disc to even try, such as a cone. Be sure to check CP at 90 degree angle of attack as well, since that is often a worst case.

    I was told that OR can't simulate a finless rocket and that if you tried it would never show it as stable. So I tried it for a design and the only problem was OR was overoptimistic. The 90 degree AOA CP was usable though. I used it as a tool and in that sense it was helpful, but in the end I didn't let it trump wind testing results and flight seemed to confirm that was a good thing. I have another design I derate to 4 fins for CP calculation due to configuration but use all 6 for altitude and CG. You can't always use a design for sim that actually looks like your rocket.

    Just noticed in the original post: 3' thick?!? that would be another way, model the disc as much thicker, but 3 feet would be overkill ;)
     
    Last edited: Oct 11, 2015
  20. Oct 11, 2015 #20

    Micromeister

    Micromeister

    Micromeister

    Micro Craftman/ClusterNut TRF Lifetime Supporter TRF Supporter

    Joined:
    Jan 19, 2009
    Messages:
    15,074
    Likes Received:
    29
    Gender:
    Male
    Location:
    Washington DC
    Actually guys Its a combination of Drag and Base Drag that makes this kind of model stable.

    There are any number of different combinations using "Flat Plate drag stability on all kinds of models. CZ Brat your design is fine, if it were me I'd close the drag fin/body seam hole and use a decent fillet top and bottom of the seam.

    Below are just a few of the various Drag Plate flying models I have currently in my fleet. Hope the variety and different mounting methods help with your project:)

    019-sm_UFO (flying Saucer)_03-10-89.jpg

    378-A-uc12_MM MachineScrew,Nut&Washer Odd-Roc_05-17-11.jpg

    378-B-uc12_MM Stud, Nuts&Washer Odd-Roc_05-17-11.jpg

    634b-d4c-sm_2-Stage NIght UFO lit (lights off)_08-04-07.jpg

    651o1a-sm_Turkey Trottin Wings On_10-29-06.jpg

    654a-sm_CD Spool Rocket (18mm)_11-12-06.jpg

    MM 312p1e-sm_MM 3X ThumbTack Odd-roc 4pic pg 96dpi_08-14-05.jpg

    LEM-d3-sm_all 4 2pic complete(86dpi)_04-16-03..jpg
     

Share This Page

Group Builder