New to DD: Questions

Discussion in 'Recovery' started by Serac, Dec 2, 2019.

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  1. Dec 2, 2019 #1

    Serac

    Serac

    Serac

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    I am in the process of building a DD rocket. Now that I am actually in the construction process, I am coming up with questions whose answers I'm not finding in my HPR book or on forum searches. Maybe they're just basic?

    1. For electronic deployment of the drogue, the charge canister is facing down. Is this correct? If so, is there concern about the acceleration of the rocket boost emptying the charge canister? What is done to mitigate this?

    2. I haven't seen much on packing the charge canister. The canister has a hole in the bottom for the igniter, black powder is placed in the canister around the igniter, and then some sort of wadding is placed in the canister above the black powder. How much wadding? What is the wadding material? How tightly is it packed? I have some experience in muzzle loading cannons. For cannons at least, the amount and density of the wadding makes a very significant difference. Charge canisters look like mini cannons to me - a better description of wadding technique seems important. Is it?

    3. For the main chute, I see that some people deploy the nose cone, and others deploy the body tube between the nose cone and the altimeter bay (and keep the nose cone undeployed). Is one approach better? My first inclination would be the latter: this way I can install shear pins and some cutting metal in the altimeter bay and keep all the complexity in one spot. Is one approach more prone to a zipper?

    Thanks!
     
  2. Dec 2, 2019 #2

    spigalau

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    My 2 kiwi pesos...

    1) The Charge canisters are packed & taped - I personally use a pinch of dog barf on the bottom, compact it in, then add the black powder & ignitor seated in the BP, then add another ball of dog barf and compact this down on top. I then tape (paper) over the top of the well and put a wrap around the edge to hold it all in place.

    2) The hole on the base of the canister is probably for the mounting screw, not for charge lead.
    IMG_4081-600x400.png


    3) For us, the payload tube is secured to the upper section of the AV bay with push pins. We have shear pins at the nose cone end and during separation the chute is expelled forwards.
     
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  3. Dec 2, 2019 #3

    timbucktoo

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    You will get many responses here.
    1. Most people stuff any air space with dog barf and seal the charge wells with masking or electrical tape.
    2. Probably. Not sure which canister you are using.
    3. Most separate at the NC but both ways work. Personal preference.
     
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  4. Dec 2, 2019 #4

    richP

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    I can only answer with what I like to do, and has been effective:
    1. After adding the appropriate amount of powder, I fill the remaining space with dogbarf and then cover with 3 layers of masking tape.
    2. Dogbarf is cellulose insulation. Pack it in tight enough to keep the powder from moving around. 4F powder is so fine that packing it super tight is not necessary.
    3.The classic DD would eject the main from the payload tube. With the nosecone being the separation point, the parachute is literally shot-out of the payload tube. I think the biggest problem you'd have with the second scenario would be the main being pushed tightly into the payload tube during separation and subsequently not being pulled into open air.
    Are you talking about head-end deployment? Different animal altogether.
     
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  5. Dec 2, 2019 #5

    Jmhepworth

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    1. And 2. Always make sure the charge is contained. If you use a charge well, put the powder in first, then the ematch, then dog barf, and then tape. The dog barf is cellulose insulation. A bag lasts forever. And longer. I use green masking tape because it’s stickier. No acceleration will make the powder fall out if it is contained.

    3. I deploy the drogue from the motor. That lets me use motor ejection as backup. The one time I didn’t, I took a nice core sample of the salt flats when the drogue didn’t come out and the main shredded. Then I like having the main come out at the nose cone so the force from the ejection charge pushes everything out. That’s not as big a deal with the drogue because it’s smaller so things aren’t as tight. Make sure you use something to attach the payload bay to the avionics bay or you could have the payload bay separate and the chute stay in the payload bay. Sheer pins are your friend so you don’t get the main coming out at apogee. Deploying the drogue can create enough force to shake the main out. Ground test both charges and if you have room, use redundant altimeters. If you don’t have room, motor ejection backup is a must.
     
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  6. Dec 3, 2019 #6

    Serac

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    Thanks, all for the detailed responses. This helps a lot. My understanding:

    1. Fill the void of the charge tube with dogbarf, and tape the opening with masking tape. This seems simple enough.

    2. Nobody said anything like "tamp the dogbarf tightly" or anything, so it doesn't sound like the dogbarf needs to be highly compressed.

    3. Ejection of the nosecone seems to be the favored approach because it forces the main chute out. Makes sense.

    Thank you!
     
  7. Dec 3, 2019 #7

    Serac

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    One more question:

    4. There are two shock chords: one between the booster and the avionics bay for the drogue chute, and another between the top end of the avionics bay and the nosecone for the main chute. Approximately where are the drogue and main chutes placed? In the middle of each chord? Somewhere else? Do you want the drogue closer to the avionics bay so that the main chute is not close to the booster upon main chute deployment?
     
  8. Dec 3, 2019 #8

    Zeus-cat

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    Drogue placement is close to the bottom of the av bay. That way you should get good separation between the two parts of the rocket as they descend under drogue. The main should be close to the nose for the same reason. You don't want stuff banging into each other as they come down.
     
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  9. Dec 3, 2019 #9

    Serac

    Serac

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    Gotcha. Thanks!
     
  10. Dec 3, 2019 #10

    Nytrunner

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    If you are using 4F blackpowder for the charge, your statement above is true.

    If you are using coarser blackpowder of BP substitutes like pyrodex, containment and packing is much more important.
     
  11. Dec 3, 2019 #11

    Serac

    Serac

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    Understood. The plan is to use 4f black powder. Nitrocellulose is completely different than black powder, and much more complex.
     
  12. Dec 4, 2019 at 3:58 AM #12

    Zeus-cat

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    As I always tell people who are new to rocketry and putting in their first dog barf, "It ain't a muzzle loader, you don't want to ram it in!"
     
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