Drogue sizing thoughts

Discussion in 'High Power Rocketry (HPR)' started by Not Quite Nominal, Sep 7, 2019.

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  1. Sep 7, 2019 #1

    Not Quite Nominal

    Not Quite Nominal

    Not Quite Nominal

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    I'd like to make a controversial claim:

    Drogues should be sized based on the surface area of the rocket, not the weight.

    My LPR BT-70 rocket comes down far more slowly drogueless than my 54mm FG did when I had an accidental drogueless. But of course they do. There's a similar amount of drag, and much less mass.

    A 4lb minimum diameter rocket will require a different sized drogue than a 4lb big fat light rocket.

    I see the drogue as having three purposes:
    1. Prevent the collection of parts from coming down nose-down streamlined.
    2. Provide a stabilized "Flying-V" platform to allow for an undisturbed main deployment
    3. Slow the assembly down to a speed where the main can deploy safely
    Let's start with two parts of the rocket falling separately. The rate at which they will fall (terminal velocity) is increased by weight, and decreased by area.

    rocket1.png

    Terminal velocity is determined by ballistic coefficient (mass/area), or sectional density

    Let's assume that the fin can has a higher ballistic coefficient than the payload, which is likely - motor casings are heavy and parachutes are light. The fin can will fall faster than the payload, and since they are attached by the shock cord that will pull the forward end of the fin can up.

    rocket3.png

    Not bad. Fin can comes down, since the payload falls more slowly it will pull on the shock cord and pull the nose of the fin can up. Now the fin can is falling tail first, which is unstable, so it will then turn nose down again. Pretty flat and high drag.

    Now let's look at the unpleasant, but unlikely case: Payload has higher sectional density/ballistic coefficient than the fin can. Now the payload pulls the nose of the fin can down, and they both come down streamlined:
    rocket4.png
    Not good. This could occur if the break is too far up the rocket, and the fin can section is mostly air, while the payload section is a tightly-packed parachute

    So it looks like the important factor here is not the weight of the rocket, but the ballistic coefficient. If both sides have about the same ballistic coefficient, or the fin can has slightly more, it will come in stable drogueless:

    rocket2.png
    Now let's throw the drogue out on our stable configuration:
    rocket5.png
    Same amount of mass pulling down, but now we've got more area pulling up. So the whole assembly will now fall more slowly. Since the whole assembly is falling more slowly, the fin can and payload are now below terminal velocity, so they will start falling faster/drooping and pulling on the parachute. This decreases their surface area, causing speed to pick up. If the system is designed properly, it will quickly stabilize at speed where the drogue is taking up some of the weight, but the fin can and payload are still "flying" and making significant drag.
    rocket6.png
    That balance point is the V we want.

    So here's my controversial claim: For a perfect flying V, the drogue is creating about one third to one half of the drag, leaving the fin can/payload to "fly" and support the other half of their weight.

    All that matters in sizing the drogue is the side-on surface area presented by the rocket. If you have a rocket that's 100" tall and 3" wide, that's 300 in sq of area, and a good estimate for drogue size would be 100-150 sq in, or a 10-12" chute. That will give a good balance between flying the body and keeping the line taut and give us the "flying V".

    (Here I am making the assumption that the Cd of a horizontal cylinder and a flat parachute are about the same - about 0.8. If you're using a high-Cd drogue like a Fruity or Rocketman, an even smaller drogue works. If you're using a streamer, Cd of 0.2 or so, you can get away with much more area)

    If that 3"x100" rocket is heavier, it will come down faster, and if it is lighter, it will come down more slowly, but the angle of the V is determined not by the weight of the rocket, but by the ratio between the surface area of the rocket and the surface area of the parachute

    Second controversial claim: At the weights/densities of the average HPR rocket and the strength of HPR parachutes, any rocket that's coming down slowly enough to form a good flying V will be slow enough to allow for a safe deployment of the main. At most, a slider ring will be required.
     
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  2. Sep 7, 2019 #2

    JDcluster

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    A properly designed rocket will fall horizontal without a drogue. A short payload section or a weighted NC will cause an imbalance.
     
  3. Sep 7, 2019 #3

    GaryT

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    I most definitely don't want a V, I see people putting their drogue smack dab right in the middle of their shock cord and every time the NC and booster are banging into each other and twisting up together. If I'm using a 30-40' shock cord I place my drogue 2-3' from the Av-Bay.
     
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  4. Sep 7, 2019 #4

    Not Quite Nominal

    Not Quite Nominal

    Not Quite Nominal

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    Completely agreed. Didn't think of nose weight being an issue, but it would definitely cause one. If a rocket has nose weight, you should probably put the break further down the body so that the two sections have similar ballistic coefficients.
     
  5. Sep 7, 2019 #5

    JohnCoker

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    Interesting post. I generally put the nose (lighter end) about 1/3 of the way along the bridle (for the reasons GaryT mentioned). That means that there will be a small uneven V, but probably more acute angles than you show because I tend to use long bridles (5-6 times the airframe length).
     
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  6. Sep 7, 2019 #6

    Theory

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    I agree with placing the drogue closer to the payload

    Also worth mentioning is the size of the drogue chute. I like mine on the small side.
     
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  7. Sep 7, 2019 #7

    GaryT

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    Basically what I strive for is at apogees separation it's configuration resembles an upside down check mark. The thing is you can try your very best but there are just to many variables we have no control over, the biggest one? nature! @ 10K the winds are very!! Different than @ 100' Athough it is pretty cool to watch what happens when falling from a hight flight, its almost like a slow motion ballet or acrobat.

    You could also Sim just the booster and then just the upper BT & NC together, get the CD of both separately, factor in the wight and length of your SC to get an even better idea.
     
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  8. Sep 7, 2019 #8

    mpitfield

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    +4 on what GT, Theory and John say.

    I inadvertently reaffirmed my use of a drogue when I accidentally left a bit of tape on the shroud lines of the drouge and observed an A/B scenario showing the difference in decent stability with and without a drogue. I was able to view this up close as I had a camera onboard, which captured the sections interacting with each other. The first 40ish seconds of the descent was much more erratic, and once the tape on drougue came off, it settled down. This erratic interaction is also what I typically observe from the ground with most drogueless descents.

    Having said that, while my goto is using a drogue, I will submit that there may be scenarios where it is an advantage to go drogueless, I just haven't seen it yet with my configurations.
     
  9. Sep 8, 2019 #9

    Not Quite Nominal

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    Agreed. The lengths should be different to prevent banging.

    There's another reason, which I learned the hard way, causing an unintentional drogueless flight: ease of extraction.

    The event shoves the drogue down the fincan, and then the departing avbay yanks it back out. There was a Onebadhawk diagrams for this, explaining why he uses three loops for the drogue.

    If there's a lot of line between the avbay and the drogue, the nose section will have decelerated greatly due to drag by the time it's pulled the line out, and the drogue extraction will not be as positive.

    On the other hand, if the drogue is close to the bay, the nose section is still moving briskly at the point of full line extension, and will give a positive drogue extraction.

    By "V" being desirable, I don't mean "equal length ends". I agree with you, that's undesirable. My desired result is having the ends "flying" on partly on their own drag.
     
  10. Sep 8, 2019 #10

    Not Quite Nominal

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    I've gone drogueless a few times accidentally, and lately I've been building DD LPR, where weight is critical. I decided to skip the drogue on my most recent flight not because the drogue is heavy, but because the nomex was actually quite a few grams. It thrashed a bit on the way down. Thankfully it wasn't up that high.

    My recent preference is for streamers. More surface area for visibility, while still remaining low drag.

    If my math above is correct, and the optimal drogue size is 1/3-1/2 the side-on area of the rocket, then the optimal streamer size (assuming Cd of 0.2) is just a little bigger than the side area rocket itself.
     
  11. Sep 8, 2019 #11

    GaryT

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    Here's my Intimidator 5" on an M3700BD to 11,119' Drogueless. Whether I use a drogue or not is all dependent on field and upper wind conditions. Drogue size is also determined on the field. IMO you cant have to long of a shock cord (to a point) also don't ever discount its ability to "act" as a sort of drogue in itself. For this flight I used 40' of 3/8'' tubular Kevlar, its pretty clear to see how it did just that.

    PS the M3700BD was Al Gonsalves EX Blue Diablo formula, its a beast of a formula and proved to be a bit faster than the cameraman lol.


     
  12. Sep 8, 2019 #12

    mpitfield

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    That's the last time I click a rocket video link with my nine-year-old daughter by my side. I just had to watch Roc videos for 10 min; Roc has a new nine-year-old admirer!
     
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  13. Sep 8, 2019 #13

    GaryT

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    LOLOLOL that's my boy..
     
  14. Sep 8, 2019 #14

    boatgeek

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    I think there's some good advice throughout this thread. There's one spot where I disagree with your premise in the beginning.

    I don't think I've ever seen a rocket come down drogueless with the fin can first. I would expect that's because the extra weight of the motor casing is offset by the much higher drag of the fins. Also, packed parachutes aren't all that light, so a little drag from the fin can easily leads to the nose cone being stable pointing down. It's not in every case, but the payload is pointing straight down below the fin can pretty often.
     
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  15. Sep 9, 2019 #15

    Not Quite Nominal

    Not Quite Nominal

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    Very true.

    I think I was unclear about what my hypothesis. When the fin can has the greater ballistic coefficient, it will start to fall faster than the payload. This causes the shock cord to pull the top of the fin can up, causing the fin can to now fall tail first/unstable. This causes more drag, slowing the fin can back down again. The two balance each other out with the fin can falling flat to slightly nose down, mostly level with the nose cone.

    The fins trying to point the nose down and the shock cord trying to pull the nose up may also be the cause of some the shock cord thrashing as well.

    At least that's my guess watching drogueless videos.
     

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