Three 4-40 nylon shear pin (screws) right amount for 10 pound fiberglass rocket?

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rhildinger

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The subject pretty much says it. Are three 4-40 nylon shear pins more or less the right amount to use for a 10 pound, all fiberglass, 3" diameter rocket? Or is that overkill and I should be using the 2-56 size?

(Of course I would size the ejection charge appropriately)

Thanks!
-Robert
 
2/56 for sure..........
I had a 15 lb 3" X 8 1/2 ft rocket.........
3- 2/56 nylon shear pins worked well..........
The only negative I could site is the increase in the amount of
BP it takes for a certain, energetic separation............

With 4/40, you'll increase that beyond reason..........

Teddy
 
2-56 screws only. (2) 2-56 shear screws require a 50 pound load to shear, (3) 2-56 shear screws require 65 pound load to shear. https://www.rocketmaterials.org/datastore/cord/Shear_Pins/index.php You want to exceed the average shear load by at least 50%, and with your pressure cross-section of about 7 sq.in., you need enough BP to generate at least 50/7 x 1.5 = 11 psi to shear (2) 2-56 screws and 65/7 x 1.5 = 15 psi to shear (3) 2-56 screws.
 
Thank you all for your replies - I am very glad that I asked! It's unfortunate that I already sized my holes for 4-40 screws (including 4-40 tee nuts). Is it a safe assumption that I can still use 2-56 shear pins with the 4-40 tee nuts, with the obvious issue that the pins will be loose?

Also, I was looking at the various ejection charge calculators, and it seemed that the recommended range of internal pressure to generate the shearing force was somewhere in the 8-15 psi range. I saw that shearing 3 4-40 pins would require around 25 psi. I take it that going above 15 psi is considered a bad idea?

-Robert
 
It's fiberglass....do it right.
Knock out the mistakes, put some tape over the holes on inside, mix some 5 minute epoxy and fill them, A bit of sanding and your done.

Drill correct size where they belong, and now you are in the race!

We have all had to fix goofs, sooner or later. You'll feel better if you do it correctly.
 
It's fiberglass....just rotate a few degrees, drill and tap into the fiberglass and install the shear pin screws.
No need to fill the old holes.
No need for T-Nuts or fancy "cutter" strips.
No need for clearance holes on the outer tube if you install the screw as soon as you pull the tap out.

Drill, tap, and install....next flight, find a new spot and repeat...I had a rocket with about 30 flights on it and the NC shoulder got a little Swiss-cheesed, but held fine.
 
It's fiberglass....just rotate a few degrees, drill and tap into the fiberglass and install the shear pin screws.
No need to fill the old holes.
No need for T-Nuts or fancy "cutter" strips.
No need for clearance holes on the outer tube if you install the screw as soon as you pull the tap out.

Drill, tap, and install....next flight, find a new spot and repeat...I had a rocket with about 30 flights on it and the NC shoulder got a little Swiss-cheesed, but held fine.

With fiberglass I've alwasy been ab le to use the same holes over and over again, about 8 flights on my Apogee Level 2, 3 flites on my madcow 2.6in dd screech.
 
I use the same holes as well, over and over. I drill a depression, not all the way through at the av bay/body joint or the body/nosecone joint to use as an alignment mark to align the shear pin holes. When the two depression holes complete a full circle the holes are completely aligned. Works every time. I also do not tap the holes for the 2-56 screws just size them so the screws push in. After the flight no need to thread the screws out, they just pull out.
 
Gentlemen... the thread is nearly four years old. Pretty sure he's figured it out by now...

Later!

--Coop
 
Only responded because it showed up as a new post, honestly didn't look at the original post date, my bad:facepalm:
 
I am new to this so do not yell at me.. From what I have learned from brother in law (level 3 for many years) is a three inch rocket should not need shear pins, except if dual deploy you would want the nose cone (if set up this way) not to separate from the apogee charge and drag out the main. Not sure if you were talking about nose cone.
Bill
 
I am new to this so do not yell at me.. From what I have learned from brother in law (level 3 for many years) is a three inch rocket should not need shear pins, except if dual deploy you would want the nose cone (if set up this way) not to separate from the apogee charge and drag out the main. Not sure if you were talking about nose cone.
Bill

What!!!!!!!!!!
Brother in law !!!!!!!!!!!
Not an in law !!!!!!!!

lol...

Teddy
 
I am new to this so do not yell at me.. From what I have learned from brother in law (level 3 for many years) is a three inch rocket should not need shear pins, except if dual deploy you would want the nose cone (if set up this way) not to separate from the apogee charge and drag out the main. Not sure if you were talking about nose cone.
Bill

Sounds like an overgeneralization. The diameter of the BT has nothing to do with whether or not you need shear pins.

Think about what a sheer pin does: it holds the parts together until you are ready for them to separate. Sheer pins aren't the only way to hold parts together. The other way is to friction fit them (i.e., make them very tight). Loose tubes require some masking tape to get a tight fit, and this is a pain, because outside temperature has an effect on tube diameter (things swell and contract).

If you can hold your parts successfully together without sheer pins using friction fit, then don't complicate things with sheer pins. However, if your tubes are loose and you don't want to gunk them up with tape, sheer pins are a great option.

There are three things during the rocket's flight that might make the parts separate prematurely. They are:
  • air pressure build up in the tubes due to the changing atmospheric conditions as the rocket ascends
  • sudden drop in acceleration at motor burnout
  • overenergetic apogee ejection charge stretching the shock cord and causing a whip effect on the payload section, which jerks the nose cone out at apogee (this is not catastrophic, but results in a very long walk :wink: )
For bullet one, you can drill a pressure-relief hole in the booster and payload tubes to mitigate the problem

For bullet two, your simulation should give you an idea of your rocket's acceleration at burnout. If the change in acceleration is drastic, then assume your payload section will continue upwards via momentum while the booster starts to fall due to drag. Here is where your rocket may prematurely separate during its acceleration phase, which usually destroys the rocket. This is mitigated by good friction fit, or shear pins in the booster.

For bullet three, again, good friction fit or shear pins in the NC. Hold that NC in place until you are ready for your main deployment charge. This can be mitigated to some extent by making your apogee charge small enough to not stress your harness, but large enough to separate the booster and payload.

Hope this helps.
 
Oh, and let me add that you arrive at the right size ejection charges by ground testing. If you are launching with pins, then test with pins; likewise if going with friction fit.
 
It's fiberglass....just rotate a few degrees, drill and tap into the fiberglass and install the shear pin screws.
No need to fill the old holes.
No need for T-Nuts or fancy "cutter" strips.
No need for clearance holes on the outer tube if you install the screw as soon as you pull the tap out.

Drill, tap, and install....next flight, find a new spot and repeat...I had a rocket with about 30 flights on it and the NC shoulder got a little Swiss-cheesed, but held fine.

Interested about your "no clearance hole" statement- I had a very hard time getting a successful ground test on a 3" FG rocket until I enlarged the hole in the airframe. My hypothesis is that the tapped hole held the pieces too stationary, not enabling a build up of momentum. Once I gave it some room to get a running start, or separated cleanly each time.
 
+1 on Bat-mite's advice.

My experience pretty much mirrors that. I've only used friction fit on anything 4" and below. I started using shear pins on my 6" L3.

I've replaced the payload BT on my 3" and 4" rockets and both times I got apogee deployments on the first flight with the new tubes. That was because I didn't re-tape the nose cones right the first time.

I got some 2-56 nylon screws for use with the smaller diameter rockets, but I don't like the idea of drilling new holes each time and swiss cheesing my nose cone and BT. I'll probably use the brass shim material as a cutting surface when I do set that up. I also don't really look forward to having to align sheer pin holes every time I want to set up to fly, but I'm working on a method that will eliminate the alignment issues.
 
+1 on Bat-mite's advice.

My experience pretty much mirrors that. I've only used friction fit on anything 4" and below. I started using shear pins on my 6" L3.

I've replaced the payload BT on my 3" and 4" rockets and both times I got apogee deployments on the first flight with the new tubes. That was because I didn't re-tape the nose cones right the first time.

I got some 2-56 nylon screws for use with the smaller diameter rockets, but I don't like the idea of drilling new holes each time and swiss cheesing my nose cone and BT. I'll probably use the brass shim material as a cutting surface when I do set that up. I also don't really look forward to having to align sheer pin holes every time I want to set up to fly, but I'm working on a method that will eliminate the alignment issues.

Second or third time someone has mentioned needing new holes...is this a common practice? I am assuming this is because of cardboard tubes and the inability to reuse, but want to be sure I'm not missing some fundamental principle here. With a brass insert or FG tube, this isn't required, right?
 
Second or third time someone has mentioned needing new holes...is this a common practice? I am assuming this is because of cardboard tubes and the inability to reuse, but want to be sure I'm not missing some fundamental principle here. With a brass insert or FG tube, this isn't required, right?

There's joy and pain with every method. Multiple holes look ugly and eventually you run out of room. But it's convenient.

I tape my coupler to the booster (or NC coupler to the payload), drill straight through, tap the inner holes, and then redrill and expand the outer holes. Screw slides right into the inner hole, then screw it in. Perfect fit every time.

The downside? I have to use a pair of needle-nose to twist out the stubs after recovery, twisting and pulling them through the other side.

The third way is to drill the holes just the right size to secure push in the pins without threading them. Then you can just pull them out after recovery. The downside of this is that all the pushing and pulling eventually wears out the hole, and you have to redrill.

Take your pick. Do what makes sense to you.

But I just thought of an additional problems with drilling new holes every time. Not only is your booster looking like Swiss cheese, but also your coupler. If your coupler has extra holes in it, then after separation, you are introducing more air into your AV bay, which screws up your baro readings. Any thoughts on that?
 
Sounds like an overgeneralization. The diameter of the BT has nothing to do with whether or not you need shear pins.

I think everything you wrote was very good except the one part I quoted above. Because the area of the base of the nosecone is proportional to the square of the diameter, the diameter is very much a factor in deciding whether to friction fit or to use shear pins. Depending on how high you fly, how quickly you can vent the necessary volume of air from the enclosed body tube (whose volume also goes up with the square of the diameter) a larger tube is simply impractical to retain using friction, but easily retained using shear pins.


Steve Shannon
 
There's joy and pain with every method. Multiple holes look ugly and eventually you run out of room. But it's convenient.

I tape my coupler to the booster (or NC coupler to the payload), drill straight through, tap the inner holes, and then redrill and expand the outer holes. Screw slides right into the inner hole, then screw it in. Perfect fit every time.

The downside? I have to use a pair of needle-nose to twist out the stubs after recovery, twisting and pulling them through the other side.

The third way is to drill the holes just the right size to secure push in the pins without threading them. Then you can just pull them out after recovery. The downside of this is that all the pushing and pulling eventually wears out the hole, and you have to redrill.

Take your pick. Do what makes sense to you.

But I just thought of an additional problems with drilling new holes every time. Not only is your booster looking like Swiss cheese, but also your coupler. If your coupler has extra holes in it, then after separation, you are introducing more air into your AV bay, which screws up your baro readings. Any thoughts on that?

I have done as you do- drill and tap, and it works for me, but I only have a very small number of d/d flights under my belt. Glad to get the additional data that shows I'm not on a bad path, but acknowledge that there are other methods in play.
 
I think everything you wrote was very good except the one part I quoted above. Because the area of the base of the nosecone is proportional to the square of the diameter, the diameter is very much a factor in deciding whether to friction fit or to use shear pins. Depending on how high you fly, how quickly you can vent the necessary volume of air from the enclosed body tube (whose volume also goes up with the square of the diameter) a larger tube is simply impractical to retain using friction, but easily retained using shear pins.


Steve Shannon

Fair enough. It does come down to how much force is required to separate the tubes, and that the smaller the tubes, the smaller the force. Friction fit is nearly impossible to test as rockets get bigger. Wrap some tape around the coupler until it's good and tight, and then ... what? How do you know if it is tight enough or too tight? It's not like who can give it a shake test.

I guess my main complaint was that a statement like "3-inch and below don't need shear pins" is an overgeneralization.
 
Fair enough. It does come down to how much force is required to separate the tubes, and that the smaller the tubes, the smaller the force. Friction fit is nearly impossible to test as rockets get bigger. Wrap some tape around the coupler until it's good and tight, and then ... what? How do you know if it is tight enough or too tight? It's not like who can give it a shake test.

I guess my main complaint was that a statement like "3-inch and below don't need shear pins" is an overgeneralization.

I agree. There are many variables and although diameter is an exponentially effective one, so is the size of the vent hole. All must be considered, which was the very valid point you originally made.


Steve Shannon
 
. . .sudden drop in acceleration at motor burnout. . .
.

I know what you mean but that's really not accurate. It's not the rate of deceleration that causes drag separation; as you said, it's because there is more drag force on the booster section than on the payload section. It is that differential in force that pulls the sections apart. The deceleration is the effect, not the cause.
 
I know what you mean but that's really not accurate. It's not the rate of deceleration that causes drag separation; as you said, it's because there is more drag force on the booster section than on the payload section. It is that differential in force that pulls the sections apart. The deceleration is the effect, not the cause.

Works for me!
 
Second or third time someone has mentioned needing new holes...is this a common practice? I am assuming this is because of cardboard tubes and the inability to reuse, but want to be sure I'm not missing some fundamental principle here. With a brass insert or FG tube, this isn't required, right?

I don't have a lot of experience with shear pins in cardboard tubes, or fiberglass for that matter. I've got 5 flights on my L3 which is 1/8" thick wall glass tubes. I'm using 4-40 pins in holes drilled straight through (no threads). I've seen no wear or problems with any of the shear pin holes.
As for cardboard tubes, I can only report on what I've seen, and that is some rockets have very tore and oblong holes after only a flight or two. What I don't know is if they use brass shear shims, CAed the holes, etc.

I plan to start trying some shear pins in cardboard tubes, but I intend to embed brass shims in the cardboard tube and maybe in or on the plastic nose cones. I don't know how it will work yet, but I'm expecting it to last a long time without having to redrill.
 
Interested about your "no clearance hole" statement- I had a very hard time getting a successful ground test on a 3" FG rocket until I enlarged the hole in the airframe. My hypothesis is that the tapped hole held the pieces too stationary, not enabling a build up of momentum. Once I gave it some room to get a running start, or separated cleanly each time.

I wouldn't want to rely on momentum to break the pins. Make sure you have enough powder to generate a pressure guaranteed to exceed the fail strength of the pin combination.
 
I wouldn't want to rely on momentum to break the pins. Make sure you have enough powder to generate a pressure guaranteed to exceed the fail strength of the pin combination.

Oh, absolutely, and didn't mean to imply inertia was the only force, just that it's easier to keep an object from moving than it is to stop a moving object. I am getting clean ejections with 1.5g when before I was north of 2 and not getting 100% reliable separation. Full transparency, I use bp substitute, so ymmv.
 
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