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  1. #1
    Join Date
    7th April 2013
    Posts
    2

    Lightbulb Adding a round disc slightly bigger than rocket diameter to increase drag?

    First post, if I've made any forum Cardinal sins, please let me know.

    My rocket is flying higher than it's intended apogee, and because of a maximum weight requirement, I was thinking of simply adding something to the outside of the body tube to increase the drag and hopefully bring the apogee down. Simulations have shown that this could work, but we're worried about the performance of the fins (flow separation?) and the stability implications this might have.

    Has anybody implemented something like this? Alternatively, what ways have been found effective to increase profile drag without adding significant weight?


  2. #2
    Join Date
    15th December 2011
    Location
    Too far from the event horizon.
    Posts
    1,752
    Okay. My first question is: "Why would you want to do that?"

    I'm figuring this is some kind of competition to a specific altitude ceiling?

    Rather than add a weighty, draggy disc at the back of the rocket, why not simply add sandpaper to the surface of the nose cone? Should be lighter to keep you within whatever specific weight requirements you have, and disrupt the airflow enough where altitude would be negatively affected --which seems to be the goal...


    Later!

    --Coop

    "For although the nepenthe has calmed me, I know always that I am an outsider; a stranger in this century and among those who are still men." --HP Lovecraft

    Custom crafted recovery: http://paramedichutes.com

  3. #3
    Join Date
    7th April 2013
    Posts
    2
    You nailed it! Competition to an altitude ceiling. That makes sense! I guess the bigger issue here is that we need to drop QUITE a bit of altitude. Thanks for the idea! We're gonna brainstorm and figure out what we can do.

  4. #4
    Join Date
    15th December 2011
    Location
    Too far from the event horizon.
    Posts
    1,752
    Throw some 60-grit on the nosecone, see where that puts you...

    What're the specifications of the rocket? Are you limited in motor choices?

    That may help....


    Later!

    --Coop
    "For although the nepenthe has calmed me, I know always that I am an outsider; a stranger in this century and among those who are still men." --HP Lovecraft

    Custom crafted recovery: http://paramedichutes.com

  5. #5
    Join Date
    6th June 2011
    Location
    San Diego. CA.
    Posts
    6,434
    Quote Originally Posted by SSquirrels View Post
    First post, if I've made any forum Cardinal sins, please let me know.

    My rocket is flying higher than it's intended apogee, and because of a maximum weight requirement, I was thinking of simply adding something to the outside of the body tube to increase the drag and hopefully bring the apogee down. Simulations have shown that this could work, but we're worried about the performance of the fins (flow separation?) and the stability implications this might have.

    Has anybody implemented something like this? Alternatively, what ways have been found effective to increase profile drag without adding significant weight?
    Hi and welcome to TRF, Home to Squirrel feeders, Llama herders, split fin lovers and avatar collectors. Forgive me if I sound a little precocious, but this sounds a bit like a TARC question. Generally we always try to optimize our propellant use (hey we're cheep!) and this runs counter productive to our body waxing and balsa blessing ceremonies. The old "may the Mass be with You" works well for anchor manufacturers, but we try not to know any of those folks. You are on the right track for spoiling your aerodynamics and you could try adding pods and cones to the fins as outriggers to slow things down. If you are stuck using a particular motor, you can add a little nose wieght inside the nose cone to really slow things down. Try using the free program "OpenRocket" and with a small learning curve, you can change the mass and run a sim to find an optimal solution to your particular (and peculiar) problem. Just adding a layer of balsa fin stock to your existing fins will change the flight dynamics quite a bit. If they are airfoiled, sand them blunt, or throw a few layers of paint on it. It still needs to be aerodynamic for safety and you don't want something destroying the stability of the rocket off the pad. Just a few pointers as there are literally hundreds of ways to kill altitude most of us try to avoid. I'm sure my fellow brothers in smoke wil jump in with other ideas, so keep coming back and we'll help as much as possible. Just remember depleted uranium is illegal for civilian use. Good flying!
    TRA 2383
    Somebody told me I was on the watch list-I hope I get a Rolex.....
    The road to Hell is paved....you're welcome.
    I can't remember the last rocket I built, because I haven't built it yet.....

  6. #6
    Join Date
    10th September 2011
    Posts
    560
    Check out some of the indoor competition foam airplane flyers. They install drag plates behind the wings to do exactly what you want.

    It is basically a flat piece of foam mounted 90 degrees to the wing to produce drag and slow the airplane down.
    EDIT
    Found a pic, something like this could be added to fins and carefully adjusted and fine tuned to reach the altitude you need.

    Launch, check altitude, adjust size of plate or add or adjust size of holes, re-launch.

    This could almost be like in auto bracket drag racing.

    Set your projected altitude and the one closest with out going over wins.

    Actually sounds kinda fun to try.
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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    Last edited by RodRocket; 7th April 2013 at 10:33 PM. Reason: found pic

  7. #7
    Join Date
    17th May 2011
    Location
    Lancaster, NY
    Posts
    4,623
    depending on how much altitude needs to be robbed, fat, blunt fins hurt. Without ever having tried it, I'd assume gluing rough/loose cloth to the body of the rocket/fins would increase drag and not add too much weight.

  8. #8
    Join Date
    19th January 2009
    Location
    Stafford VA
    Posts
    7,038
    There are lots of things you can do to decrease altitude. The suggestions given already are very good. A flat/blunt nose cone also will reduce altitude. Just make sure the flat part is the width of the body tube or it won't have as much effect as you might think. Having the leading edges of the fins at 90 from the BT and flat also maximizes drag. Using split fins will also add drag. Large lugs and/or rail buttons will add a lot of drag.

    Any reason you can't just go to a smaller motor?

    Good luck with your contest.

    Handeman

    TRA #09903 L3 3/29/2015

    "If you don't use your head, you have to use your feet!" my Dad

    Tripoli Central Virginia #25 - BattlePark.org

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