Plastic rivet strength?

4regt4

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I had a nose cone separate from the payload bay on my Bounty Hunter, which is BT-80/2.6" diameter. Had about 1oz of ballast attached to it. The fit was quite tight with layers of scotch tape. Anyway, I need to step up to about 3oz nose weight to fly bigger motors.

I'm thinking of attaching the nose cone with these:


Any idea as to whether perhaps 4 of them would hold the nose cone on? I'm also going to rethink the shock cord, adding in an additional length of elastic.

Hans.
 

Rob Campbell

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I recommend 2-56 plastic screws. Mark where you want them and then drill through the payload tube and nosecone. Tap the shoulder of the nose cone. don't forget a witness mark for future alignment.
 

techrat

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I recently completed a LOC Sandhawk "Park Flier" which is a 1.6 diameter rocket. It has a payload bay, and for safety, I attached a second shock cord to the nosecone to inside the payload bay. The nosecone is a nice tight fit and should never pop out on it's own, as the ejection charge pops the entire payload bay. Regardless, I figured I didn't want to lose the nosecone by accident, so I used a second shock cord to secure the payload bay and nosecone together.
 

rocketgeek101

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Those will work fine. I use them on some of my DD rockets to keep the avionics bay attached to the upper section. 4 of them would be excessive though. Just 1 rivet should be sufficient for your setup.
 

mikec

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Any idea as to whether perhaps 4 of them would hold the nose cone on?
I've used two of these to hold a payload section weighing about 5 pounds on my L3 rocket, so I think 4 should be massive overkill for your application.
 

4regt4

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Interesting.

Any idea at what point during the flight the nose cone detached from the payload bay?

Does the rocket have any vent holes?
The payload bay has vent holes, as the only thing riding in it is an altimeter.

The nose cone separated at ejection. I am assuming that when the payload bay fully extended the shock cord, momentum pulled off the weighted nose cone.

The supplied shock cord for the Bounty Hunter is a belt-and-suspenders approach, as it is 5 feet of elastic with 10 feet of Kevlar in parallel with it. So when the elastic extends to twice it's length, the Kevlar takes over. The parachute is attached to the end of this elastic/Kevlar cord, and another 3 feet of Kevlar is attached to the payload bay. One of the changes I'm going to make is to extend the shock cord. I think it's too short, and that likely contributed to the nose cone parting ways.

When I built it, I was in a big hurry to get it done before a club launch. I built it as per instructions, didn't think through if I should deviate in any way like I normally do.

Hans.
 
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4regt4

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I recommend 2-56 plastic screws. Mark where you want them and then drill through the payload tube and nosecone. Tap the shoulder of the nose cone. don't forget a witness mark for future alignment.
Aren't those screws usually used as shear pins? Wouldn't they do just that - shear?

I thought about it, and I also orders some at the same time I ordered the rivets.

Hans.
 

Rob Campbell

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Aren't those screws usually used as shear pins? Wouldn't they do just that - shear?

I thought about it, and I also orders some at the same time I ordered the rivets.

Hans.
No. Since they are securing the nose cone to the payload tube. The tube has a bulkhead to keep ejection gases out, so it won't be pressurized.
 

mikec

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If you're going to use screws, don't use plastic screws. Even though they will probably be OK, there's no reason not to use much stronger and probably cheaper metal screws.
 

LithosphereRocketry

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Anything wrong with tape? I hate putting more bumps on the outside of the airframe. Or maybe a twist-lock setup contained inside the rocket?
What I usually do for MPR is a wrap of masking tape around the outside of the seam - it's not pretty, but it's never failed me. I've had friction fit shake loose a couple times even when it felt "tight enough".
 

4regt4

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Screws: The nose cone has to come off before flight to turn on the altimeter. I'm thinking rivets might be easier to un-attach, then re-attach. Nylon screws concern me a bit, as the nose cone will need about 5 oz of weight. Inertia might shear them.

On my second flight of the day - after spending an hour looking for the missing nose cone - I did tape the nose cone on after switching on the altimeter. And then post flight I removed the tape and also much of the nose cone paint!

Hans.
 

lakeroadster

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The payload bay has vent holes, as the only thing riding in it is an altimeter.

The nose cone separated at ejection. I am assuming that when the payload bay fully extended the shock cord, momentum pulled off the weighted nose cone.

The supplied shock cord for the Bounty Hunter is a belt-and-suspenders approach, as it is 5 feet of elastic with 10 feet of Kevlar in parallel with it. So when the elastic extends to twice it's length, the Kevlar takes over. The parachute is attached to the end of this elastic/Kevlar cord, and another 3 feet of Kevlar is attached to the payload bay. One of the changes I'm going to make is to extend the shock cord. I think it's too short, and that likely contributed to the nose cone parting ways.

When I built it, I was in a big hurry to get it done before a club launch. I built it as per instructions, didn't think through if I should deviate in any way like I normally do.

Hans.

Makes sense... thanks for taking the time to explain that.

I doubt I would have thought about that possibility either. You would think the instructions would have mentioned this.

If the nose cone was balsa, some simple wood screws would be perfect.

If it were mine, I'd just use some very small sheet metal screws as @mikec suggested above.
 

waltr

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2 of those rivets will work perfectly. That is what I use to keep NC's of payload bays filled with electronics and keeping the upper BT attached to a DD ebay.
 

G_T

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Each 2-56 nylon screw is good to about 30# shear load. Pop rivets go a fair bit higher and can deal with non-fiberglass tubes better most likely.
 

heada

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Plastic rivets are MUCH easier to install/remove than plastic/nylon 2-56 or M2 screws. They also are stronger so you don't need as many. They also last much longer and have less chance of damaging the tubes. If you're not screwing into a wood ring, I would always go with the rivets for removable connections. I use 2 of them in 2.56" LOC tubes to hold my swappable MMTs around. 2 withstands 3x E12 ejection charges going through a baffle without damaging the tubes or shifting the MMT anywhere.
 

T-Rex

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I use plastic rivets. I use the same number of rivet as fins and in line for aesthetics, though I'm sure fewer would do the job.
 

teepot

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I used to use plastic rivets. They were a pain in the finger and it was a slow process. Now I use #4,6 or 8 by 1/2" screws. I bring a drill with me and can take the screws out quick and easy. Taking a av-bay off and on is much easier. The cap screw top is no bigger that a rivet head.
 

jqavins

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Screws: The nose cone has to come off before flight to turn on the altimeter. I'm thinking rivets might be easier to un-attach, then re-attach.
Probably a matter of taste and finger nail length.
Nylon screws concern me a bit, as the nose cone will need about 5 oz of weight. Inertia might shear them.
I'm pretty sure you're overthinking that. Why? Because I thought the same thing, and I'm always overthinking stuff like this.


I used to use plastic rivets. They were a pain in the finger and it was a slow process. Now I use #4,6 or 8 by 1/2" screws. I bring a drill with me and can take the screws out quick and easy. Taking a av-bay off and on is much easier. The cap screw top is no bigger that a rivet head.
I guess you're using metal screws? With a drill I imagine tearing up the heads of nylon screws in nothing flat.

If I were to use screws, I'd use machine screws with nuts epoxied in place on the inside, rather than relying on the threads of sheet metal screws grabbing the tubes. But I'm probably overthinking that. Did I mention I have a tendency to overthink stuff?
 

jmasterj

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Screws: The nose cone has to come off before flight to turn on the altimeter. I'm thinking rivets might be easier to un-attach, then re-attach.
My experience is that the rivets are kind of a pain to remove, though easy to insert.
I used to use plastic rivets. They were a pain in the finger and it was a slow process. Now I use #4,6 or 8 by 1/2" screws.
Agreed.
If I were to use screws, I'd use machine screws with nuts epoxied in place on the inside, rather than relying on the threads of sheet metal screws grabbing the tubes. But I'm probably overthinking that. Did I mention I have a tendency to overthink stuff?
That's exactly what I do. Not too difficult.
 

Off Grid Gecko

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I used some Toyota body rivets on my L2. Prolly a little oversized, even on a 4" tube, but they work for securing the forward tube to the electronics bay. There's a number of fasteners that would likely do the trick.

There was a question about vent holes and I saw an answer talking about the breathing hole for the electronics bay... I didn't see another answer unless I missed something. Vents are in the tubes outside of the e-bay so they can bleed pressure with altitude a prevent a pressure difference between inside the tube and outside, which can force a nose cone right off, possibly during ascent, and especially on larger tubes with more volume inside and a bigger bulkhead to "push against."

Those little sheer screws work fine. I use 3 to hold on a 4" nose cone with near a pound of added weight inside. Test your charges on the ground to make sure they will sheer when you need them to. Smaller tube you could prolly do one or two instead as you will be working with lower forces in your ejection charges.
 

jqavins

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If I understood right, the issue was the nose cone popping off the e-bay, so the one vent hole serves both functions. These aren't shear pins, they're meant to hold throughout the flight and only come out for access to the electronics. If I've understood it correctly.
 

4regt4

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If I understood right, the issue was the nose cone popping off the e-bay, so the one vent hole serves both functions. These aren't shear pins, they're meant to hold throughout the flight and only come out for access to the electronics. If I've understood it correctly.
The nose cone is on the payload bay, which has 4 1/8" vent holes for the altimeter. And a solid bulkhead on the bottom. The nose cone needs to come off to power up the altimeter. It's the nose cone that apparently had a "momentum" separation that I wish to remain attached.

I'm going to try a pair of rivets. Should know soon.

Hans.
 
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