In short, I do not. But they are going to have different shear resistance if the are oriented like a "+" or a "x" with the direction of travel.
I was planning on doing the same by securing the top airframe to the coupler with steel screws and #2-56 shear pins in the nosecone. Are we allowed to use metal screws like that? I'm always fuzzy on how much metal we can use.
Here is an explanation of single vs double sheer https://www.engineeringarchives.com/les_mom_singledoubleshear.html
The style of plastic rivets that Madcow has are commonly known as Click-Lock Shanks.
See this for another example https://www.mcmaster.com/#standard-nylon-rivets/=13oh1p2
I searched around a bit, but could not find any specs. for their shear strength.
How big is your rocket and what material is it? I've never seen anyone use five #8-32 nylon shear pins.
A typical arrangement would be three #2-56 nylon shear pins, then three #4-40 pins once your NC gets over maybe 10lbs.
All of my 2-4" fiberglass rockets are set up with four #4-40 Steel screws at the coupler junction and three #2-56 nylon shear pins at the NC.
John, It sounds like you're concerned about the shock load transmitted to the nosecone shear pins during apogee ejection.
If you haven't already, consider minimizing that load instead or in addition to looking at different materials for retention. Using a nylon harness instead of Kevlar (or part nylon at least), lengthening the harness or bundling small segments with tape that will break away during ejection. All of these will reduce the shock load. If my understanding is correct, switching Kevlar to Nylon could reduce it by 3X or more (depends on TK/TN size / tensile strength to load ratio).
I know, I'm new to all this, but it came to mind, and if you hadn't already thought of it, it's worth considering. Sorry for telling you what you probably already know!
Sorry, I suppose it works both directions though. If you could use #4 shear screws instead of #8's you wouldn't need such a large charge, and 1/4" rivets would work for you. And you could consider using smaller shear screws if your drogue deployment shock was smaller, right?
I've used plastic rivets exclusively on all my builds including a couple of 50# (loaded) rockets, a 6" and a 7.5"flown on N motors. I've never had a failure with any of the rivets in any of my launches....Those plastic rivets that worked fine on your 3" build aren't strong enough for your 5" build....
In short, I do not. But they are going to have different shear resistance if the are oriented like a "+" or a "x" with the direction of travel.
I don't follow you. I use them to hold my payload tube to my coupler. I'm using five #8-32 nylon screws for shear pins, and I want to use enough rivets to make sure it shears at the screws and not the rivets. I was planning to go with five unless I can get some real numbers.
Maybe this is a better question: what is "tensile strength" vs. "double shear" on a 6/6 nylon data sheet?
Attached is the results of a spreadsheet calculation of the forces and velocities of a Formula 200 rocket with a 10.8 NC and a 10 psid ejection charge pressure.
View attachment 298815
A 10 psid ejection charge pressure will generate ~500 pounds of force (thrust). It will provide a ~45.5 G initial separation acceleration. Drag and shock chord length will lower this value. If your value for the shear strength of the 8-32 shear screws is correct, I strongly suggest you use only 3 of them which would hold off 27 G deceleration forces created by the shock chord.
I personally think that's too much force because the ejection charge pressure is too high. Since the diameter is large, you should be fine with 5 psid. This lowers the peak separation thrust to 250 pounds and the peak acceleration to ~22G. This would allow you to use (4) 4-40 shear screws that would require 150 pounds to shear and provide shock retention to 14 G or 150 pounds force.
The shear resistance of the 1/4 rocket rivets is going to be higher than the 8-32 shear screws probably by approximately the diameter ratio squared. If you have 1 rocket rivet per shear screw you should be fine.
View attachment 298816
YMMY
The plastic rivets I use have four fingers on the larger piece. The ones you have may have two, three or four. If the gap between the fingers is aligned with the direction of shear, the shear strength will be less than if the gaps are not aligned with the direction of shear. The gap in effect gives shearing a head start.
If you're using 5 #8-32 screws, I'd use 10 rivets. It'd be symmetrical and match the pattern of the shear pins.
I've used plastic rivets exclusively on all my builds including a couple of 50# (loaded) rockets, a 6" and a 7.5"flown on N motors. I've never had a failure with any of the rivets in any of my launches.
Your method seems very simple and I'm sure produces great results. As do plastic rivets for me.
What works, works.
Tony
Tony,
Don't they degrade over time and launches for you ??
The little side flaps that expand and hold would break off after a few launches..
Do you reuse then at all ?? Or new on each flight / expendable ??
I find the installation I described more time consuming and labor intensive of course,,
then just drilling for the rivets...
But once the tee nut is epoxied in place you can't get quicker, more convenient or solid then a screw in a nut...
Teddy
I just ordered 400 rivets from Amazon so I'd have plenty for BALLS. So the answer is I don't reuse them. My total cost shipped for 400 rivets was $20.68. Pretty cheap for an ample supply.Tony,
Don't they degrade over time and launches for you ??
The little side flaps that expand and hold would break off after a few launches..
Do you reuse then at all ?? Or new on each flight / expendable ??
Enter your email address to join: