Well that didn't go as planned...

Discussion in 'Low Power Rocketry (LPR)' started by FlyingMonkey, Nov 28, 2019.

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  1. Nov 28, 2019 #1

    FlyingMonkey

    FlyingMonkey

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    The Bertha has been our favorite rocket so far. So I started with that but tried my hand at a few new things. Probably too many at one time.
    TTW basswood to a 29mm mount
    Clear payload section
    Apogee strap on boosters
    At the last minute I threw on a nose cone that matched the boosters.
    Nomex chute protector.

    I Sim'd this thing with a Bertha nose cone when I had a working pc about a year ago. The rest I just winged.

    First launch was on a C6-5. That's a great motor for a standard Bertha but it turns out I added way more weight than I anticipated per the sim. It launched ok but the delay was too long. It lawn darted.
    I quickly cut the visibly damaged section off (about 1.5"), removed the payload section, and strapped the boosters on.
    C6-5, A8, A8.

    Jumped off the rod quick! But 1 booster out performed. The rocket took a turn, boosters peeled away correctly, but rocket was once again too low for a proper recovery. The ejection charge clearly lit but it didn't pop the nose cone. This time it landed like a plane. With FAR more damage.
    No wind on either launch.

    I should have started with more motor in the main. I probably should have larger fins for stability in case of uneven power distribution. I can't help but wonder if the nomex was used wrong and caused the lack of ejection. I should have weighed it.

    Love to hear your thoughts. Feel free to tell me what else I did wrong and what I should change next time around.

    Sim is currently not an option. 20191103_150359.jpg 20191116_141036.jpg 20191116_141105.jpg
     
  2. Nov 28, 2019 #2

    FlyingMonkey

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    I think there is enough good tube to cut and couple, saving the fins and mount. My hands don't want to cut that basswood again lol.
     
  3. Nov 30, 2019 #3

    Lugnut56

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    Can you download and use OpenRocket? I use Rocksim, but from reading the posts on the TRF, a lot of folks use and have good results using OR. I was going to get a set of the strap-on boosters on my last order from Apogee, but they were out-of-stock at the time.
     
  4. Nov 30, 2019 #4

    Rex R

    Rex R

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    I believe that you're going to need a bigger motor, something like a D12.
    Rex
     
  5. Nov 30, 2019 #5

    FlyingMonkey

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    With the exception of final built weight I did sim it. It sim'd great. Either the sim is wrong in regards to weight or drag because it didn't fly the same.

    The uneven propulsion is a risk with this type of cluster. If the rocket didn't turn off the rod it may have got high enough for good recovery. Not sure though. I ordered a scale and I'm going to weigh the pieces.

    Also I found video I'll try to post.
     
    Last edited: Nov 30, 2019
  6. Dec 1, 2019 #6

    John Taylor

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    Way bigger/ more powerful motor, give it more altitude to recover stability and deployment early enough for recovery.
     
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  7. Dec 1, 2019 #7

    FlyingMonkey

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    Ok so my thought about more motor seems valid. What about more fin?
    The RSO suggested that I lose the swept back look and move that fin area out away from the rocket and pods. It will definitely shift the weight but will the fins themselves offer any more stability?
     
  8. Dec 1, 2019 #8

    David Schwantz

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    How did you line up the grain of the basswood? Basswood is NOT strong if the grain runs straight along the air frame tube. On the second flight, with more power, could you have had flutter to cause the drift in the vertical line of flight? Also,if it landed like an airplane, on its belly, then when you cut off the bad section you may have changed the CG enough to cause erratic flight. With no ejection, she should have come down nose first.
     
  9. Dec 1, 2019 #9

    FlyingMonkey

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    Wood grain is probably close to 90°.
    I cant load the video yet, gonna try again today. But it came off the rod and turned. It's flight was very horizontal and low altitude. I think if it was up higher it would have lawn darted like it did the first time. I'll try to get the vid up as it will probably help
     
  10. Dec 1, 2019 #10

    FlyingMonkey

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    Ok let's try again.

    First flight is c6-5 empty payload tube.
    2nd flight is sans payload tube and 1.5" of body tube that was damaged.
    Boosters in place with A8's
     
  11. Dec 1, 2019 #11

    neil_w

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    Top 2 mistakes:
    1) Adding a bunch of stuff to a rocket and not weighing it before flight, to confirm motor has sufficient thrust to lift it safely. That's a big no-no.
    2) Not checking center of gravity on modified rocket to confirm stability. You added a lot of weight to the rear of that rocket, and a more powerful motor to add even more. If you don't sim it, you should at least swing-test it.
     
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  12. Dec 1, 2019 #12

    FlyingMonkey

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    I did sim it. The apogee boosters have a file so I simply added them. I selected basswood, 29mm (paper rings), etc, etc.

    I even played around with different payload weights. In the sim it flew fine. Now, I understand that the sim can only guess how heavy I applied glue and paint but without a scale I hoped it would be close. Guess it wasn't close enough.

    I can't sim anymore but I did order a scale so in the future I can at least use old school methods.
     
  13. Dec 1, 2019 #13

    FlyingMonkey

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    20191201_121437.jpg 20191130_121832.jpg BTW: the fin area survived. Once I get ahold of some BT60 I'll rebuild it and try again.

    Scale arrived...
    Rocket (including payload section) is 186g w/recovery gear
    Boosters are 50g w/recovery gear
    That's 236 grams combined.

    A8 is 13 grams each x2
    C65 is 24g
    Combined that's 50 grams

    286 grams all together.
     
    Last edited: Dec 1, 2019
  14. Dec 1, 2019 #14

    FlyingMonkey

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    I can't find max lift weight for A8 but if 2 A's make a B, they list the B6 and the c6-5 at 113g. Giving about 226 grams of max lift. Clearly I was under powered.

    By the chart I should have used a D12 in the rocket and the boosters could have been A,B, or C.
     
  15. Dec 1, 2019 #15

    Steve Shannon

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    If you didn’t run the simulation with the final loaded weight and with the correct center of gravity, you really didn’t sim it.
     
  16. Dec 1, 2019 #16

    FlyingMonkey

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    Fair enough, but I didn't have that capability until today. When the simulator had weights available for these added items I hoped that whoever made the files were accurate. The string test looked fine.
     
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