Will this work? Or potential ejection issue?

Discussion in 'Techniques' started by billdz, Jul 11, 2019 at 11:03 PM.

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  1. Jul 11, 2019 at 11:03 PM #1

    billdz

    billdz

    billdz

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    I built the below-diagramed rocket here in Europe from stock Klima components but am a bit afraid to fly it, I'm concerned whether the chute will properly eject. The rocket is powered by a cluster of 4 Klima D9-5 18mm motors. The 0.5m bottom body tube has a Klima baffle on top. When I ordered the baffle, I assumed it would be inserted into the airframe and glued near the bottom, just above the motors, but the instructions said to mount half of it inside the top of the airframe, so it acts as a coupler. The shock cord (shown in green) runs from a wooden eyehole on top of the baffle to an eyebolt on the coupler at the top of another 0.5m airframe, which contains the chute. At ejection, the second airframe is supposed to pop off the baffle and the chute is supposed to come out. So the ejection charge has to be strong enough to pop the second airframe with sufficient force to pull out the chute. I suppose that with 4 motors the force will be adequate, but I've never seen a design like this, and the chute is further from the motor than it is in the usual rocket.

    Any thoughts?
    Thanks,
    Bill
    Klima custom.jpg
     
  2. Jul 11, 2019 at 11:05 PM #2

    billdz

    billdz

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    Here are the instructions for the baffle. Klima baffle.jpg
     
  3. Jul 12, 2019 at 1:02 AM #3

    BABAR

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    Okay, I want to make sure I understand the diagram.
    The baffle is inserted as a coupler, so it is glued not only to the posterior/engine section but also to the MIDsection.
    The ejection gases are going to be passed through the baffle (presumably cooled), go to mid section and PUSH the chute forward, with the rocket separating between the midsection (what you call the “Upper airframe” and the payload section.

    If the above is correct, it should work great.

    If the planned separation is between the “Upper airframe” and the “Lower airframe” I don’t like it.

    Caveats:
    Make sure you have good retention on all motors. If one blows out the back, potentially there goes all your ejection force from the other motors.

    Make sure your payload section and nose cone are solidly fixed as a single unit, or nose cone may fall out.

    Even WITH baffles, I like to put a sheet of wadding between the chute and the baffle, call me insecure,

    Hope you have a good 12 volt ignition system and some good igniters, I have had mixed success with clusters and the Estes starters. No experience with Klima motors or igniters.

    Have a good flight
     
  4. Jul 12, 2019 at 1:37 AM #4

    kuririn

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    If you don't want to test it in flight then do a ground test.
     
  5. Jul 12, 2019 at 2:05 AM #5

    billdz

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    Yes, this is the separation point, the top part of the baffle/coupler is not glued to the Upper airframe. I think I agree with you, the top of the baffle/coupler should be glued to the Upper airframe and separation should be at the top of the Upper airframe.

    A ground test may well be in order, although I'd have to build some sort of holder.
     
  6. Jul 12, 2019 at 3:18 AM #6

    BABAR

    BABAR

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    If separation is AT the baffle, the ejection gases are going to punch the chute further into the “Upper airframe” as the pieces separate. That may get the panties in such a wad they don’t wanna come out.

    If the packing is loose enough, chute will probably slide back out, but seems less optimal than pushing the laundry out forward and separating at the Upper airframe/Payload junction.

    Given however they sell this thing, I am guessing it is a proven design.

    Of course, so was the Estes Cosmos Mariner

    https://www.rocketreviews.com/estes-industries-cosmos-mariner--by-bruce-canino.html
     
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  7. Jul 12, 2019 at 9:23 PM #7

    billdz

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    Thanks, I agree with you, separation at the baffle is not the best. I think I'm going to reassemble it, although to do this I'll have to cut out the top coupler and bulkhead, which have already been glued in. I've had a bad feeling ever since I built it, for the reasons you have mentioned. Can't really blame Klima, this is not a kit although the parts are all stock Klima, not a proven design. The baffle instructions, shown above in post 2, do not mention gluing the top half of the baffle to the airframe, so I interpreted this as meaning this is where separation should occur, but you have convinced me that just cannot be correct.
     
  8. Jul 12, 2019 at 9:40 PM #8

    Nytrunner

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    I disagree with the disagreement.

    The ejection charges should pop the upper airframe off vigorously and the shock cord should pull out the chute. Tie the chute to the line closer to the aft section and tuck the shroud lines inside the chute so it's a compact springy bundle.

    Unless you just wad and jam the chute into the tube (something an L3 flier knows better than to do), a carefully folded parachute with the cord wrapped properly will pull the chute clear with no trouble.
     
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  9. Jul 12, 2019 at 10:35 PM #9

    Sabrina

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    Separation at the baffle - formally known as "zipperless design" - is a well known and very useful construction technique. Many years ago after Super Science Bertha experienced a rocket-destroying full-length zipper, I switched to a zipperless design and it has been awesome.

    During liftoff, G-force presses the laundry down against the top of the baffle. At ejection, the expanding gas pressurizes the top portion of the rocket and separates the two sections. The laundry is attached to the shock cord within a foot of the baffle. The laundry has NEVER failed to be PULLED out of the upper section.

    An added bonus - I have NEVER had another zipper - regardless of early/late ejection. I learned about zipperless design by reading "Modern High Power Rocketry 2".

    My Super Science Bertha uses a permanent bulkhead midway up the top section. The shock cord is anchored to the bulkhead, and the section above becomes a payload bay. It is very similar to the diagram in post #1 above.




    2019-07-12 16.02.10.jpg
     
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  10. Jul 13, 2019 at 12:46 AM #10

    billdz

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    Thanks much, Sabrina and Nytrunner, very interesting. I was nervous because I had never seen this type of design. Good to know it has been successfully used. No zippers, no wadding, sounds even better than the usual design, wonder why it is not more common.
     
  11. Jul 13, 2019 at 8:49 PM #11

    SDramstad

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    I have had mixed results with that type of setup. If the chute bundle is tight in the tube chances are it wont come out. If its loose it probably will. Maybe attach a small drogue chute directly to the eye bolt on the coupler. May provide enough drag to pull things apart.
    Steve
     

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