Im new to this and I just found out what shear pins or for today.
At what point do you decide to start using them?
A wieghted nose cone?
A 60 foot harness??!!!

Scotte

First off Bruce, Keep them all the same size for each joint. don't mix sizes. It will create more issues then its worth. Also I always use a minimum of 3 that are evenly spaced. Also, the first thing you need to do before determining shear pin size and quantity is to determine the exact size of the airspace you will have in each section to pressurize. Without that knowledge first its impossible to say how big or how many you will need. So once we have that then I can recommend what I think you should use..

Im new to this and I just found out what shear pins or for today.
At what point do you decide to start using them?
A wieghted nose cone?
A 60 foot harness??!!!

Scotte

Scotte,

Ive used Shear pins in many different sized rockets. Always HPR though. Most anything that I have thats either going very fast, Very high, Both, or fairly large or even that I just want to make sure it sticks together I use shear pins. It has nothing to do with the weight of the cone or the harness length. I shear pin about everything I do in flights like I mentioned above.

Conway

I agree with what you're saying, Conway. Just looking to see what people have to say. I just wanted to find out if anyone had ever tried to use two, offsetting sizes of screws. One = failure, two = possible failure, 3-4 = good to go!

I'm very much interested in this topic as I am building my 3" diam G10 level 2 bird (dual deploy) and am struggling with the number of screws vs. the volume vs the amount of pwoder used for deployment.

From an engineering standpoint, the cross section of a 4-40 at the minimum diameter is .081".which derives an area of 5.153x10-3 as compared to a 6-32's minimum diameter of .099" which has an area of 7.698x10-3, however the double shear strength of a nylon 6-32 is 97 lbf almost twice that of a 4-40 which is 50 lbf.

In essence you need two 4-40's to equal the shear strength of a 6-32

Ref: http://www.anixtercomponents.com/images/uploads/Nylon_screws_strength_data.pdf

Its safe to say, I'll probably default to whatever the lexperts in the club tell me to use. Which is......?

Screws are great, and I have used them often. Having said that, consider this stuff http://www.evergreenscalemodels.com/Shapes.htm which I have used many, many times. The 1/16" round rod (their item #222) is terrific, and available either online or at most hobby stores.

My general rule of thumb - use polystyrene rod on projects 54mm and under, 4-40's on 3"-4" diameter projects, and 6-32's on the biggest, most massive stuff. I think you would find #6 screws to be a bit much on your 3" project.

I have used the polystyrene rod to over 20,000' and I've seen other guys take it over 30K. It works...

Merry Christmas all-

On smaller birds I use the round stock that John Wilke mentions in his post. On 3" and 4" birds I use 2-56 nylon screws although if the nose is heavier I've been known to go up to 4-40. I always use three equally spaced. On the UprOar I believe we used 6-32 nylon screws which I felt were overkill.

Warren

Like JW I to have used the Styrene rod for shear pins. Mostly in my 38mm and under rockets. But what you need to use will depend on the size of the rocket, drag and weight. Also the forces applied at say motor burn out or when the apogee deployment happens on a dual deploy. You want to have it all stay together when it needs to but come apart when it needs to as well. I also base what I use on what the actual rocket is made from.

I used 4 #4-40 nylon screws to hold my booster section on of my L3 rocket Full Throttle. I used 10 grams of powder to get the unit apart. Thats with the pins It took 9 grams to get it apart with no pins as the section is just very large 6" diameter and roughly 6 to 7 feet long. For a safety measure I added a gram and made my actual recovery with 11 grams. Better to get that apogee out for sure. But having a large rocket like that coming out of Mach as well as a large charge popping I didn't want the nose cone to come popping off either. So for that I used 4 #6-32 nylon screws. That end is fully packed and still has a little room (about 2 feet @ 6" diameter) so i ended up with a 5 gram charge to safely deploy my mains. Again the rocket is CWFG tube and much stouter then most other tubes and can handle those sizes of charges.

There is a really awesome site that I recommend called High Power Strength of Materials. That have a great chart on this of tested strength. The owner of the site is an engeneer and has some great info on alot of stuff with more new stuff comming.

Here is his chart on test done with Nylon screws and some styrene products.

http://www.rocketmaterials.org./data/cord/Shear_Pins/index.php

Now remember a lot of what should determine the size you use will be the forces applied to the rocket. What happens when you take a rocket that weighs say 50lbs and make it go Mach then slow it quite rapidly? Anyone know what the negative g forces could be coming out of Mach? It would of course vary depending on the mass and aerodynamic drag from rocket to rocket. As well as how fast and how many g forces it was attaining forward and then to how much slowing factor from all of that put negative forces again back on the model.

You could end up with a rocket thats 50lbs and when it slows due to the forces implied on it weighs closer to 200lbs (or more or less) as an example.

Anyhow, those should all be the things that one takes into mind when figuring what size of shear pins to use. Or at least thats what I do..

Have a Merry Christmas!!!

Conway

I'm very much interested in this topic as I am building my 3" diam G10 level 2 bird (dual deploy) and am struggling with the number of screws vs. the volume vs the amount of pwoder used for deployment.

I have the same rocket, and I'll either use (3) 1/16" polystyrene rods, or (3) 2-56's. First, I'll ground test it with 1/16" PS, then if I don't feel that will hold the nose, I'll just enlarge the holes for 2-56's. Although you should never use just one or two shear pins, I have used two, and it worked fine. But then again, I tend to go heavy on the charges. Which is why I, like you, went with G10 tubing. I'd like to keep my birds around for a while. 8) For the bigger rocket mentioned earlier, I'm going with (4) 4-40's for the booster, and (3) 6-32's for the nose cone. I probably could get by with (4) 4-40's on the nose, but at that power level (and speed), I just won't take any chances. It took 6 grams to blow the nose off a similar rocket at ground test, but in flight it took 7 grams. If the rocket is G10, the G10 can take a little more charge. Even though I have taken the old NAR axiom, blow it apart or blow it up perhaps too literally, it's always better to have more than needed than not enough. 😉

I had a situation where my ground tests of my 3" diameter dual deploy rocket sheared both sets of the 3ea 2-56 pins on my drogue and main sections, but in the flight the main chute section failed to separate even though the charges were 20% larger. Fortunately the rocket was well built, the drogue sufficiently large enough and the snow was soft, so there was no damage.

FYI, the drogue was in a 3" dia x 30" long section which I placed (2) 3 grams of charge in. The main was in a 3" dia x 10" long section which I place (2) 1.2 Grams in. Two altimeters were used for redundancy. They were both set at 1000 feet for the mains. (I know, I know, I should have made one go off at 800 feet and put a kick-ass charge in it just in case).

I've done a post mortem analysis of the flight and have concluded that prior to the flight, I had tightened all the the 2-56 screws to full stop on final assembly before traveling, whilst the ground test had the heads of the screws above the airframe surface. Note: Both the coupler and airframe were tapped at the same time after I drilled the minor diameter holes.

This was obviosly a rookie mistake, as I inadvertantly added the clamping force of the coupler and the airframe to the shear force of the screws. The lowly charge I had tested with was not enough to overcome the additional amount of resistance between the tube assembly. I have not seen anyone address this issue before, but this situation would not have occured if I had used straight .062" dia styrene rods. Its only a problem when using screws, as the clamping force is dependant on the amount of torque, whereas in straight shear you can get the same results almost every time.

Clamping forces could also have been avoided by following up with drilling a tight clearance hole matching the major diameter of the fastener on the inner or outer tube. I suggest drilling the slip fit clearance hole in the inside tube (coupler) to make it easier to remove the sheared screw shanks later.

In essence, leave the head of the screws sticking up into the airstream. 😳

Mike
Good to know I can learn from your mistakes. 😉 On my first pinned rocket (in the next couple of months) I plan on styrene rods now.
(ok so I already have the rods but its still good to know)

At least you know why you had your failure. Even though I used moter ejection, I had 2 matches hooked to the altimeter and neither lit. hmmm

scotte

I do NOT tap the airframe holes - only the nosecone or coupler. In addition, I usually put some talcum powder on the coupler and nose shoulder to lube things a little bit. Finally, since I generally fly altitude birds, I slice off the heads of the 2-56 or 4-40 shear pin screws. Learned all this the hard way including a flight much like yours Mike. I had my 4" carbon bird come in from over 8K on a drogue with the main hung up due to insufficient charge. Again, like yours - threaded airframe, threaded nose and the compression of the screws increased the friction to the point that the charge barely had enough to blow off the nose. When I had ground tested, I didn't tighten the screws all the way. When I flew - because I had pre-prepped the bird at home, I tightened the screws all the way thereby introducing a variable I had not accounted for.

Mike, I had assumed you certified successfully last Saturday. My sympathies on not cert'ing. I've been there - 4 times before I was successful. Just to be completely clear - if the bird flew with the intent that it dual deploy, it must successfully dual deploy with both chutes fully deployed at the respective, planned altitudes (apogee and whatever the altimeters were set for on the main) or else it is a DQ. That is clear in both Tripoli and NAR rules. The mere fact that the bird returned intact is not a clean certification flight.

Warren

Mike, I thought you had certed L2 Saturday. Warren's right, if your main didn't come out, then that should DQ you. Happened to me my first time, too. Landed flat with no damage, but because the chute stuck, I couldn't get my L2 that day. Real bummer. However, whether you certed or not is among you and the two who signed your paperwork. It is none of my business. I wasn't there or involved. Maybe you used a 3' drogue, and that could count as a main. It's just a standard rule that if you choose to use dual deploy, it must work. None of my business though.
I tapped my last bird, but was advised to NOT tap the holes to hold the shear pins. Reason being, if the head is sheared off in landing, you have to drill them out, which is what happened to me. AND the added force needed to overcome the compression of a well-tightened shear pin.

I do NOT tap the airframe holes - only the nosecone or coupler. In addition, I usually put some talcum powder on the coupler and nose shoulder to lube things a little bit. Finally, since I generally fly altitude birds, I slice off the heads of the 2-56 or 4-40 shear pin screws. Learned all this the hard way including a flight much like yours Mike. I had my 4" carbon bird come in from over 8K on a drogue with the main hung up due to insufficient charge. Again, like yours - threaded airframe, threaded nose and the compression of the screws increased the friction to the point that the charge barely had enough to blow off the nose. When I had ground tested, I didn't tighten the screws all the way. When I flew - because I had pre-prepped the bird at home, I tightened the screws all the way thereby introducing a variable I had not accounted for.

I had assumed you certified L2 last Saturday. My sympathies on not cert'ing. Been there - 4 times before I was successful.

Warren

I to do NOT tap my shear pin holes either. Just a strait drilled hole in the correct size. It should be tight enough that your pin is good and tight and wont come out easy, but you can push the pin in via hand or with the help from a tool. I ALWAYS ground test as I would fly. So if I plan to have the pin in all the way and tight I make sure I have a charge that gets the job done and then add just a hair more. If im going higher Alt. I bump the charge up a full gram. (now again My airframes dont have many issues with larger charges beings they are composites like CWFG,ect and deal with the bigger charges better) When I do high Altitude I also cut the head off and flush to the body.

One thing I like about not tapping the holes for the shear pins is the removal afterwards. just push them out with a pick.