Chute bay vent holes and shear pins, opinions.

I am looking for peoples opinions on these things, specifically for a 12in dia rocket.
I have an airframe with a 11in ID that has a 24in space between bulk plates.
The NC is 10lb(might be 20lb after nose weight) and the parachute is 10lb.
Vent holes: At an expected altitude of 6000' the pressure difference is 3PSI. This would put ~330lb of force on the nosecone(assuming no leaks) enough to break shear pins and seperate the NC. So I was thinking of adding a 1/4in vent hole. Bigger? Effect on ejection charge size?
Shear pins: I am considering 4x #4 pins. These break at about 70lb each for a total of 280lb of retention force. I would break these with a 4g -->5g charge, still need some test firings.
Thats all, just looking for opinions, good or bad.
RDH8
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Robert, I would use the seive approach for your altitude equalization vents. A couple of very small holes (1/16th) would allow pressure equalization as your rocket gains altitude. It takes time to get up there and the depressurization is a slow process. On the other hand, the ejection forces are lightening quick. The ejection charge gasses will not be able to exit the airframe through the tiny vents and will force off the nose cone with a shock wave effect.
I use 2 styrene rods in a 3" airframe at 12,000 feet. The styrene rods are smaller than toothpicks. Granted, the 11 inch airframe will have more force on it, but with venting you can get it close to zero, at least way less than 300 lbs.
I suggest tiny vents, maybe 4 or 6, and a ground tested ejection charge.
steve

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Robert DeHate wrote:

What material(s) is the airframe made from?

1/4" should be enough. The affect on charge size depends upon how the deployment system will function. More about this later.

Which is stronger: the airframe material or the pins? Since I don't know what the airframe is made from I cannot say. But you want to make sure the airframe is stronger than the pins or the pins will rip the airframe rather than shearing. Same for the nose cone. Are you using "blades" on the cone to help cut the pins?

Where does the chute sit? Does it sit in the 12"x24" space that you described? Are you expecting the nose cone to pull the chute out?
Have you considered a fall-apart chute canister that doubles as a piston? The canister fits under the nose cone. The charge is under the canister. The canister doubles as a piston and smacks into the nose cone to break the pins. After exiting the airframe the canister falls apart into two or three pieces, releasing the chute.
Does the (10 pound) nose cone remain attached to the rocket/chute? Or does it recover on its own chute? If the cone remains attached, will the rocket come apart when the line it is attached to snaps tight?
Dean
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Reply's inline

12in
3 layers of fiberglass.

ejection
deployment
of
what
airframe
help cut

The airframe is stronger than the pins, the NC is 3 layers of glass as well so will have good shear strength.

firings.
described?
The chute is in the 24" space. I am sort of expecting the nose to pull the chute out. The recovery harness will be grouped into small sections bound by one wrap of masking tape. These will sit below the chute so when the NC comes off, it pulls the recovery harness which pulls the chute out.

The
pins.
pieces,
No, but will consider it.

it
come
The NC is attached to the rest of the rocket via a 1in TN harness. This is rated at 5000lb. The NC bulk plate is 1/2in ply with kevlar reinforcement front and back with two U-bolts to distrubute the load. This bulk plate is held in place by the glass that makes up the cone, it overlaps 3in all around. There is also a piece of tubular kevlar that runs through both U-bolts and is embedded into the NC fiberglass layup.
There will be 20' of harness between the NC and main attachment then another 40' to the attachment on the E-bay. The Ebay has two plates glued together, one fits inside the coupler. Both 1/2in ply. There are two U-bolts for attachment here, the two ends of the e-bay are attached together by 4 x 3/8in threaded rods. There is an aluminum strap that the Ubolts go through and two of the threaded rods. I will use 4' of 2in wide TN that attaches to the bulk plate and is long enough to exit the tube, then the 1in TN is attached. Should hold up to deployment.
But I will consider letting the cone come down on its own, I can use my R9 for it.
RDH8

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The 330 lb force you estimate will only occur if the chute bay is airtight. It usually isn't. Try blowing into the vent hole, and you will see. Seepage will allow the pressure to equalize (or almost equalize) as the rocket rises. Remember, you probably have 10-15 seconds at least to reach your 6000 feet.
I would use two or three 1/16" holes symmetrically spaced. That should have a negligible affect on the charge size.
I don't have any experience with shear pins of that size.
-- David

plates.
to
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2 or 3 equidistant 1/8"vent holes should be fine
Go with 4 4-40's to hold that bad boy on. (You need that much nose weight? What is it a Bullpup?!) Accurately calculate the front section's volume, subtracting half the chute's anticipated volume (or don't account for the chute) and your EC should be fine.
I'm hoping this is a "good" opinion :-)>

plates.
to
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All opinions are good ;-)
The nose is 10lb. I might need nose weight due to the weight of the fincan. It's not the typical design.
Oh, I have a page that badly needs updating here: http://www.geocities.com/rdh82000/LDRS/index.htm
RDH8

12in
10lb.
ejection
of
firings.
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NICE detailed construction, Robert.
Have you thought about a 1:2 bungee:tubular nylon to dampen the nose cone projectory? I'm not a big fan of it (it takes up scads of aiframe volume and it's a consumable component) but there are BFR flyers whom swear by it. Sounds like it might be an option with a NC of this mass.
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I looked up some flight data. An example: 6" diameter, 25 pound (without motor) rocket on a K1275 experienced the following Gs:
Ascent 12G Apogee Charge 20G (separated rocket into two tethered sections)
The apogee G forces were positive and negative. Positive when the charge fired. Negative when the sections hit the end of the tether, which is when the nose cone might accidently break away.
Dean
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