Drag separation/Drag difference

Discussion in 'High Power Rocketry (HPR)' started by TimothyG, Aug 18, 2019.

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  1. Aug 18, 2019 #1

    TimothyG

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    How does one calculate the difference in drag of two assemblies to quantify the drag difference force that must be overcome? I ask this after having lost a rocket recently to drag separation shortly after motor burn out and would like to prevent this in the future. Reason I want to place a value on this force is to help me make the decision in the future of what size shear pins to use.
     
  2. Aug 19, 2019 #2

    Kelly

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    Don't know the answer, but it seems this would be a great addition to OpenRocket, as it already calculates (I'm guessing) the drag on each component. Would be nice if you could specify one (or more) separation points, and have it report the drag on each component separately. Combine this with inertial forces (which the software already knows about, as well) and you'd have separation force.
     
  3. Aug 19, 2019 #3

    KennB

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    Unless you have a hugely massive (heavy) upper section, the addition of most any set of shear pins would probably hold it together. The calculation would them become the charge needed to pressurize the body to shear the pins. This is a much better documented calculation.

    If you're really keen on doing a calculation, try momentum and figure the force you'd need to overcome from that.
     
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  4. Aug 19, 2019 #4

    jderimig

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  5. Aug 19, 2019 #5

    Rob702Martinez

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    Shear pins, ejection calculator and ground testing. Depends on the size and material of your airframe IMO. What are you working with?
     
  6. Aug 19, 2019 #6

    TimothyG

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    Airframe was a high drag profile fin can decelerating from Mach 1.3 after motor burn out with a head end deployment arrangement. It had 2-56 shear pins. I’m planning on trying again but stepping up shear pin size next time but wanted to have more information and data to justify and support my design alterations.
     
  7. Aug 19, 2019 #7

    TimothyG

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  8. Aug 20, 2019 #8

    vcp

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    Relative drag forces would also be nice to know in situations where you might want drag separation, e.g., staging.
     
  9. Aug 20, 2019 #9

    mikec

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    FWIW: Not saying it's the case here, but in my experience drag separation is frequently blamed for problems just after burnout but not always the real cause. It's harder than one might expect to get drag separation unless the forward section is heavy and the aft section draggy, as John's equations will tell you. I've never managed to get it to happen when I wanted it to (staging.)

    In a lot of cases what was thought to be drag separation was actually a motor-related problem like early motor ejection, electronics glitch, etc. Obviously if you weren't using motor ejection that's not a possibility.
     
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  10. Aug 21, 2019 #10

    Steve Shannon

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    I think this should work: Calculate the drag on the assembled rocket against velocity using OR or RockSim.
    Then run the sim again flying just the sustainer. The difference at any velocity is the additional drag caused by the booster.
     
  11. Aug 21, 2019 #11

    TimothyG

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    This was a single stage drag separation not a two stage. But I should still be able to duplicate the method for the forward section and the fincan if I can make it stable.
     
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  12. Aug 21, 2019 #12

    jderimig

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    OR under component analysis gives you the CD of each component on you rocket. Use this information and the masses and plug into the drag sep equation mentioned earlier.
     
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  13. Aug 21, 2019 #13

    cerving

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    Are you saying that you had shear pins and it still separated? Something doesn't sound right... either your nose was very heavy and the momentum broke the shear pins, or your shear pins were very loose and ineffective. What was the deceleration at burnout? If you multiply that by the mass of the NC/AV bay assembly, you'll get the force that the shear pins "saw", then you can use a shear pin force calculator ( http://www.rimworld.com/nassarocketry/tools/chargecalc/index.html , it's for BP charge sizing but you can use it to back into the force) to determine if the separation was indeed due to the mass of the nose assembly.
     
  14. Aug 21, 2019 #14

    mdnehez

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    What was the body tube made from? Was it vented? I've had flights where my rocket rose so quickly, it over pressurized, and spit the nosecone, shearing the pins before any deployment event.

    -Mike
     
  15. Aug 22, 2019 #15

    jderimig

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    ^ This and what Mikec posted. It would be unusual to drag separate shear pins. Are absolutely positively sure you didn't get a premature apogee charge? Did you see it? Was the separate AT motor burnout or a little after?
     
  16. Aug 22, 2019 #16

    TimothyG

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    It was just under one second after motor burnout. Two 2-56 shearpins holding the nosecone in place.
     
  17. Aug 22, 2019 #17

    jderimig

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    What was the weight of the rocket (at burnout) and what was the weight of nosecone?
     
  18. Aug 22, 2019 #18

    TimothyG

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    Weight of rocket after burnout is 12lb weight of nosecone is not exactly known but I'd guess 2-3lb. The nosecone and Ebay assembly however is close to 5lbs total. This was Head end deployment arrangement. I believe based on the destruction of the drogue assembly that the nosecone detached as the main survived and the electronics were safely recovered thanks to this. Drogue was in the nosecone. Other things of note that I believe contributed to this is an excessively hot day on the salt. My kydex knife sheath deformed sitting in the sun so it was definitely hot enough to weaken the two shear pins. Another things of note that I've kicked myself for is that the dog barf was packed just tight enough to create some resistance during closing. Normally there's no resistance. The arrangement was flown immediately before this (3 hours earlier) to verify the drogue arrangement on a less aggressive K motor. In addition to the other mentioned contributors My ventilation was awkward as I had it vented from the nosecone into the Ebay and from there to atmosphere. If I had packed the dog barf tightly enough to cause resistance it could have easily blocked the passages to the Ebay enough to restrict the flow combined with a much more aggressive flight profile and the weakened shear pins has lead me to believe the drag separation occurred. In the future for 4" the plan right now is to replicate the same flight using larger shear pins and new deployment testing and change nothing else. I have a good video of the flight I can show but I have to figure out how to upload to Youtube as I haven't done that in a while.
     
  19. Aug 22, 2019 #19

    TimothyG

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    IMG_20190802_145952.jpg Here's the rocket so you get an idea of the profile. Not an ideal fincan for supersonic but it worked until after motor burn out. All fins where attached after the impact and bounce so I don't suspect the fins. And I imagine fin damage would occur during the most intense portion of the flight under acceleration not during a lesser moment after burnout. The comments in the video are from another member who was there when talking about suspected areas of failure before the flight.
     
  20. Aug 22, 2019 #20

    jderimig

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    That rocket is very similar to my Balls rocket. Using your numbers and drag ratio of 0.1 and if the deceleration was 5g your separation force would be ~88N or ~20lbs. If you have a better estimate of deceleration at burnout you can scale that number.

    But... but a 5g deceleration is pretty high, my 3" Mach 2.4 flight sims to only a 3g deceleration after burnout.

    Do you have any data from the altimeter?
     
    Last edited: Aug 22, 2019
  21. Aug 22, 2019 #21

    TimothyG

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    I ran a pair of RRC2+ altimeters that have been flown over mach multiple times and had several dozen flights on each of them before this. So I trust them very much. However they only give basic data points. Reported apogee is 6800 ft which looked about right and times well with the fall time for the booster.
     

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