Rocket Materials.com Test Help - Shear Pin Test

Based on my experience with breaking a lot of shear pins, I think you are making the problem more complex than it is if the only factors you plan to consider are airframe and shear pin material and size. There is more to consider than just airframe and shear pin material. For example, assume:

- A rocket with a parachute packed in a deployment bag

- There is room for the bag to slide back and forth a few inches

- During descent the section with the parachute bounces back and forth on the tether hold the two sections (classical two-stage recovery), hammering on the nose cone and possibly breaking the shear pins.

Also:

If the nose cone does not fit right it will wobble in the airframe. If the rocket bobbles sufficiently, enough force can be produced to break the pins. I've seen it happen.

Also, successful recovery requires more than choosing the right number and size of shear pins. It is only one of several factors.

The information that you seek can already be calculated if the only factors are airframe material and number of shear pins. However, data is missing for some materials under certain circumstances. If you perform tests I recommend that you also vary the temperature rather than performing all tests at room temperature. Suggested temperatures: 40F, 70F, 90F.

Dean

Don't forget the patented Kessler Pickle Fork Tines! :-)

And I STILL can't find any in SE Alabama :-)

John

It'll take more than that to bring Kurt out of the wood work. He's looking for some of that "altered chicken." ; )

Randy

McMaster-Carr, PN 9769T21

.Kurt should sue them for patent imfringement though. :-)

cp

Does that happen when you feed them the genetically altered corn?

I'm not sure. Maybe it's the steroids / hormones. ; )

Randy

.> Kurt should sue them for patent imfringement though. :-)

Anyone around here know a good attorney?

;-)

Actually, I didn't. I was thinking about you while I was writing the post! I'll throw them in while I'm doing tests.

Problems (failures) are always more complex than what they appear. When I do a failure analysis, the first answer is always "Because it broke". Every answer is then asked "why?" I keep doing this until I get to the root of the failure.

I would think that if these low stress forces broke the shear pins, then they were too small. My goal is to help determine the proper shear pin configuration to avoid those problems.

Great idea. If I have enough materials, I'll do that.... and more. Enough tests can show the Fracture Appearance Transition Temperature (FATT) of a given shear pin material. This may be beneficial to those real high flyers.

There goes Curt, peein' on my parade... :-)>

.> > Kurt should sue them for patent imfringement though. :-)

Don't complain - at least it wasn't your foot.

You guys have it all wrong. It's forked chicken, not chicken forks. Similar to pulled pork, but tastier. As to the preparation, you are welcome to sample all you like at Casa de Kesler (along with your beverage of choice), but the recipe goes to the grave with me.

Isn't that an oxymoron? You could always search the rmr archives for a bad attorney...

Bob Kaplow NAR # 18L TRA # "Impeach the TRA BoD" >>> To reply, remove the TRABoD!

Those are not low stress forces. Two tethered airframe sections bouncing and jerking on the tether during descent can produce a good amount of force that needs to be taken into account. A recording accelerometer, like an R-DAS altimeter, can provide a lot of useful data other than max altitude. For example, assume a rocket is separated into two tethered sections at apogee by a BP charge. Assume a 10G jerk when the tether is stretched tight. Assume a 5 pound nose cone and 3 pound parachute behind the nose cone. I think that translates into an 80 pound force on the shear pins. About 30-35 pounds breaks a #2 nylon screw. Therefore two pins would not be sufficient to retain the nose cone. Three might be marginal. If four pins are used then the ejection charge needs to produce at least 140+ pound kick on the nose cone to break four (#2 nylon screw) pins.

You need flight data to know what forces occur during all phases of flight.

I did a few tests a couple of years ago and decided that #2 nylon screws were the right size for rockets made from cardboard (or phenolic/cardboard) tubes, including fiberglassed cardboard. About 35 pounds is needed to shear a #2 nylon screw. About 70 pounds to shear a #4 nylon screw. Larger nylon screws can be stronger than the airframe, meaning the airframe or nose cone would rip rather than the pins shearing. One #4 screw is not always a substitute for two #2 nylon screws.

If you put a big kick behind the nose cone to separate cone and airframe, and you keep the cone attached to the rest of the rocket, then you need to take that force into account. Although it can sometimes be a pain, I recover the nose cone on a separate parachute and therefore don't need to put much if any concern into the low altitude (main chute) charge being so large.

Dean

A 5 pound NC and a 3 pound chute would suggest at least a 6" dia rocket. Using A=(d^2x3.1416)/4 then 140 lb-f / 28.2744 in^2 = 4.951psi. That's not much at all. Most black powder charge calcs suggest using considerably more. at 10 psi we get 282 lb-f on the cone.

I'm not doubting anything you are saying at all. The numbers you have for each nylon screw is interesting. Where did you get this data? Is it pure shear strength or was there any bending moment assumed? What is the density of the nylon reported? Can you find the data for polystyrene as well? What about the various airframe materials and their compressive strength when subjected to the shearing force?

This is good stuff here. Thanks for the info.

That's a good idea and I have seen several people use this method.

Many of the BP charge size calcs are wrong because they were developed when most rockets had a much smaller diameter. A large psi is needed for small diameter rockets, but is overkill for larger diameter rockets.

I used a flex phenolic tube and coupler and added weight until the pin sheared or the tubes ripped. I don't know the density of the nylon. I used the nylon screws I purchased from McMaster-Carr.

I learned the hard way to put a locator beacon on the nose cone. I once lost it for a month in a dry swamp (7' tall grass) inhabited by 10 pound mosquitoes and

5 pound ticks. To recover the cone I purchased a mosquito netting hat and used a can of "bug be gone" on my clothes, head to foot. I reeked of DEET, but I found the cone. The sonic locator didn't add any value the time the cone landed in cow poop.

I have a video of main chute deployment, but no way to share it. Video analysis is very helpful. When the charge blows, the airframe section recoils and does a

360. The pilot chute pulling the bag off the main is clearly visible.

Dean