Nylon shear pin testing

[AllDigital]

I've had a few launches where the shear pins snapped prematurely due to airframe whiplash. Recently we have been testing different drogue-main deployment methods and we have a backup chute in a third compartment held in with shear pins that is remote pyro deployed. It is a challenge to use shear screws on that compartment, because any main deploy whiplash of the (heavy) airframe can cause premature shear and unwanted separation. This happened last weekend, so today my son and I tested a bunch of nylon shear pins to find out what they are actually capable of holding.

To test the shear pins we built a rig using six inch coupler and airframe. We secured the airframe to the ground and then pulled vertical on a bulkhead below the coupler using a pulley in the ceiling. We used a digital crane scale and a 240fps slo mo camera to record the snap force. Each shear pin was drilled and inserted like it would be on a real flight (no nut, head flush).

The results were interesting. I have an old "Micro Plastics Mechanical Test Data" chart that I've used as my guide for nylon shear pin strength. Our tests would say that the 2-56 screws were about accurate (18lbs avg vs 19 lb spec), the 4-40's were 36% higher - not bad (56 lbs vs. 41 lb spec), and the 6-32 screws were far stronger than spec (116 lbs vs. 69 lb spec).

Also noteworthy, the range for the 2-56 screws was somewhat concerning (albeit a small sample size). The 2-56 screws ranged from 7.5 lbs to 31 lbs of breaking force. Not very consistent.

Overall, the test validate the fact that we were not under-sized and it gave us more confidence about "moving up" in gauge and what that would mean to required pyro charges.

I also suspect that all nylon screws are not created equally. The "Micro Plastics" chart below references a very specific 6/6 MIL spec.

Sharing here for future shear pin travelers...

Mike

 

[lakeroadster]

Any photos of the test fixture, and photos or videos of the testing in progress?

 

[rocketgeek101]

I am surprised at how inconsistent some of the test results were. I've always heard that it is best to use at least two pins to prevent the possibility of the two sections from binding. I wonder if you only did the test with one pin if that could be the reason for the variability you are seeing?

 

[Banzai88]

I am surprised at how inconsistent some of the test results were. I've always heard that it is best to use at least two pins to prevent the possibility of the two sections from binding. I wonder if you only did the test with one pin if that could be the reason for the variability you are seeing?

If the pieces are sufficiently tight, there will be a clean shearing and no binding. At this point, I'm of the mind that 'one pin will bind' is one of those apocryphal stories from the old days. I have more than a few one-pin rockets.....NEVER had one even act like it bound up.

 

[waltr]

Nice experiment and results. Variability is expected.

I also have a number of rockets that use only one 2-56 shear pin. Many flights and always works.
Al these are LOC cardboard tubes with plastic nose cones. Simply drilled and taped threads then thin CA on the cardboard.

 

[StreuB1]

Something to bear in mind that many who do this testing seem to miss. Nylon is a ductile material and as such, its point of yield will vary greatly with a function of time. As a ductile material, it will always cold-flow and again, this is by time. The longer you apply a force, the more time you give it to cold-flow (distort) and then reach its ultimate yield.

When used for shear pins, you need a test rig that is capable of applying impulse forces and a means to measure the resulting load and unload of the test rig with sufficient resolution to capture the flow to deformation and then the yield. This is actually no small feat to be honest.

Instron happens to be a sister company of the company I work for and they will be doing some testing for me for my Balls 2023 project and have contemplated having them test shear pin materials on one of their test machines.

Talking about the cold flow of the material. One of the other huge factors is the fitment of the coupler to the airframe. Or more generally, the shear gap that the pin is presented with. The smaller the gap, the less plastic deformation you get where the material is drawn into the gap before it yields, and the more clean and consistent the break. The shear materials also play a huge role. The harder the material, the more consistent the break as its less apt to deform in response to the pin and the shearing force. Thats where people generally implant sections of pop can into the cardboard to act as a shear plate to shear the pin and the cardboard is just the carrier of the shear plate.

Overall fitment tolerance and the ability of the sleeved parts to wobble will also affect the consistency of the shear. That is why I also shim up the coupler with tape to assure that I get no camming of the sleeved materials when presented with the job of shearing the pin. Don't worry about if its a little tight. The impulse provided by the ejection charge will easily exceed the breakaway force of the tape to airframe intersection and begin transferring the load to the pin.

The big takeaway is:
The higher the impulse, the faster you will translate from ductility to ductile fracture. Ductile fracture is what you want to achieve in a shear pin. The faster you translate to ductile fracture, the less the fitment and materials will matter, to a point, of course.

 

[AllDigital]

Any photos of the test fixture, and photos or videos of the testing in progress?

We didn't take photos of the rig, but we have dozens of 240fps videos of the scale (not very exciting). Below is a rough sketch of the rig. This was a quick hack -- nothing too scientific.

On the topic of binding, I get the concern, but I think if your coupler and airframe sleeve together well then the risk is very low. I've had plenty of binding with fiberglass without any shear pins with pieces that were not well sanded.

@StreuB1 raises a lot of valid points. This is why we tried to replicate our specific airframe, drill holes, and force direction for our testing. We flip flopped on what type of force to apply. Should it be slow and increasing or "impulse" force? On the one hand, we wanted to know how much "slow and increasing" force the pins would hold to keep the weight of the rocket together. On the other hand, we also wanted to know how much force was required to pop them with pyros. So, there is an argument for both. Ultimately, we decided to give it an impulse force, exerting breaking force within 500ms.

While there was variability in our test, the average of the results for the 2-56 and the 4-40 pins were in the range of expected. I suspect the 6-32 nylon pins we have are not 6/6 nylon, so they rated higher.

 

[StreuB1]

To achieve a higher level of accuracy and a more precise analog for the ways we use shear pins. I would suggest a pendulum impact tester, where your test article is mounted horizontally and the impact mass of the pendulum impacts the test article when the pendulum mass is vertically aligned with the pendulums axis of rotation.

This way you can control the force applied to the test article and the velocity of the impactor (impulse) via changing the pendulum mass and the length of the pendulum arm. This is an extremely accurate means of testing things and the test rig is extremely simple. The result will be did the pin shear? And, which I know you will have graduations on the test article to measure deformation distance, how far the pins deformed in the event that they did not completely shear. Doing this will help you determine, over 10 tests where the pins only deform, the consistency of deformation. If you have consistent deformation across 10 tests, that tells you that the pins will be nice and repeatable in plastic deformation. Then you want to test in ductile fracture across a range of impulses.

The big question though is, what does the impulse (pressure vs time) curve look like for a "typical" BP charge in a HP rocket look like? And, can this be replicated in a mechanical fashion? Likely this is pointless as what is a "typical" BP charge and what are all the other variables that one is presented with across the thousands of people who prepare their rockets every year?

This is why in aerospace, they don't use slip fits, they use taper joints, locking bands, ball detents, springs, etc. Things that react in a reliable, predictable, procedural manner.

 

[jahall4]

Do you have some special challenges mitigating the “whiplash” in this rocket?

 

[Handeman]

What exactly is "whiplash" and how is it different then the shock loads when hitting the ends of the shock cords at apogee ejection?

 

[rharshberger]

What exactly is "whiplash" and how is it different then the shock loads when hitting the ends of the shock cords at apogee ejection?

my guess is that "whiplash" is the laundry slamming forward at deceleration or ejection of the drogue, shearing the pins prematurely (its probably the same thing you are referring to).

 

[AllDigital]

ok, "whiplash" wasn't a very technical term. In this case, I've got a six inch 50 lb rocket that I am using to test an experimental HED deployment. It has a drogue in the nose that is released at apogee and a main under a small avbay that is held in with a tender descender. When the TD fires it allows the drogue to pull the main out via the apex of the chute. All of that works well, but when the TD fires to release the nose avbay the slack in the drogue line allows the vehicle to momentarily reorient to a ballistic ("fins up") position. As the main catches in the air, the slack is removed and the rocket gets "whiplashed" back to a vertical ("fins down") position. The force of that whiplash is about 42 gees.

Although this isn't ideal, on our production "big" rocket this wouldn't be a huge deal, but on our fiberglass test rocket I've got a backup chute below the upper airframe that is held in with shear screws and remote pyros (below a mid-rocket avbay). In the last test, the 42 gee "whiplash" reorienting of the rocket snapped four 4-40 shear screws holding in the backup chute and we had an unplanned parachute party. Four 4-40's is over 200 lbs of retention, so I figured it would be enough on the backup compartment, but the bulk of the weight is in the fin can.

In a traditional dual deploy this can also happen when the drogue is deployed, but the nose and upper airframe (with the main) is usually the lighter end. Good shock cord management (z-fold, tape/bands, etc.) can minimize the snap force. For my next test I will be doing more to tape the lines above and below the main to minimize the reorientation/rotation/shock and I'll be increasing two of my four shear screws to 6-32.

here is a crude illustration of the "whiplash" issue:

 

[rharshberger]

ok, "whiplash" wasn't a very technical term. In this case, I've got a six inch 50 lb rocket that I am using to test an experimental HED deployment. It has a drogue in the nose that is released at apogee and a main under a small avbay that is held in with a tender descender. When the TD fires it allows the drogue to pull the main out via the apex of the chute. All of that works well, but when the TD fires to release the nose avbay the slack in the drogue line allows the vehicle to momentarily reorient to a ballistic ("fins up") position. As the main catches in the air, the slack is removed and the rocket gets "whiplashed" back to a vertical ("fins down") position. The force of that whiplash is about 42 gees.

Although this isn't ideal, on our production "big" rocket this wouldn't be a huge deal, but on our fiberglass test rocket I've got a backup chute below the upper airframe that is held in with shear screws and remote pyros (below a mid-rocket avbay). In the last test, the 42 gee "whiplash" reorienting of the rocket snapped four 4-40 shear screws holding in the backup chute and we had an unplanned parachute party. Four 4-40's is over 200 lbs of retention, so I figured it would be enough on the backup compartment, but the bulk of the weight is in the fin can.

In a traditional dual deploy this can also happen when the drogue is deployed, but the nose and upper airframe (with the main) is usually the lighter end. Good shock cord management (z-fold, tape/bands, etc.) can minimize the snap force. For my next test I will be doing more to tape the lines above and below the main to minimize the reorientation/rotation/shock and I'll be increasing two of my four shear screws to 6-32.

here is a crude illustration of the "whiplash" issue:

View attachment 548012

Shock loading is the term you are looking for, and the shock load for each section has to be considered separately. The nose cone once it separates from the airframe has its own inertia based on its weight and velocity, same for the fin can.

 

[StreuB1]

Why are you blowing the NC first for a drogue and then the main body for the main chute?

There is a reason why HEDD/DD contains the main in the NC/Payload and you separate the upper section from the lower section at apogee. You also separate the airframe at apogee to sufficiently spoil the aerodynamics and the CP/CG relationship of each section sufficiently so that they come in relatively flat and not airstream.

Most people who choose to deploy in the way you are, use a tender or a cable cutter to keep the chute either inside the airframe, or keep it reefed, so that the tender is managing the cutaway of whatever lanyard you have chosen to keep the chute inside the airframe, or the reefing mechanism keeping the chute reefed/spoiled.

Shear pins are not intended to be used as a means of load carrying as there is no way you could accurately predict shock loading. As well, if you could account for worse case shock loading on the pins. The BP charge required to actually shear the pins would[could] be so violent that you could cause unintended damage to the assemblies.


I can tell you that on HEDD, the chance of the rocket streamlining in is almost a guarantee as the CP/CG relationship has not been sufficiently spoiled to prevent this from happening.

 

[AllDigital]

Why are you blowing the NC first for a drogue and then the main body for the main chute?

Nope. This is HED. Everything goes out the top. I am not blowing the main body tube (at least not intending). I probably wasn't explaining it well.
The nose blows first exposing the drogue. The drogue is anchored to an "upper avbay" that sits immediately below the nose. That avBay is the nose coupler. That contains all the electronics. The main sits below that avbay. The upper avbay is held in by a tender descender and a nylon cambuckle, so it has a solid anchor to the center of the vehicle. When the TD fires it is released and the upper avbay and the main are set free. That is the "production configuration".

In addition, on this test rocket, I have a third parachute. It is an emergency parachute that sits in the fincan and is held to the upper end of the rocket with shear screws. That is the break point I was solving for. I don't want those shear pins to pop prematurely, but I also don't want to rig up another tender descender holding the lower end of the rocket together for the emergency chute. At this point, the emergency chute is not likely needed, so I will go stronger on the shear screws and increase my emergency pyros hoping not to use them. If test #2 works well for the HED solution I'll just put PEM screws in and forego the emergency chute.

Here are some diagrams...

 

[waltr]

whiplash:

Shock loading is the term you are looking for, and the shock load for each section has to be considered separately. The nose cone once it separates from the airframe has its own inertia based on its weight and velocity, same for the fin can.

Yes, Shock loading is bad and can break things.

I and many others either Braid the cord or do Taped bundle on the cord to reduce the 'shock load'. The braid or bundles absorb energy and slow to pieces on the cord ends when they pull apart.
This includes getting the booster section turned around slower and without a zipper.

 

[jderimig]

One other option besides re-engineering shear pins and balancing pyro charges to support a 42g shock load is to work on shock load reduction.

Some options to do this:
1. Free bag technique (which was discussed in one of your earlier posts).
2. Dissipate the some of the kinetic energy from the main inflating with energy absorbing harness practices, like daisybraiding the harness or create loops with making tape that creates the come to Jesus energy shock over a longer time interval which reduces the amplitude of the impulse force.

Then after doing this the shear pin problem is a lot less complicated.

 

[AllDigital]

One other option besides re-engineering shear pins and balancing pyro charges to support a 42g shock load is to work on shock load reduction.

Some options to do this:
1. Free bag technique (which was discussed in one of your earlier posts).
2. Dissipate the some of the kinetic energy from the main inflating with energy absorbing harness practices,

Yep. I had bundled and masked the drogue line to dissipate the drogue shock load, but not the main line in the last test. I had expected the main to go straight up without any shock. I did not anticipate that the rocket would rotate 180 degrees when the main released. For the next test I'll loop and mask the main lines and shrouds.

I considered the free bag approach, but it isn't without some risks and I'd really like to keep the nose with the rocket if possible. So far, testing with the "apex pull" is working well, but I do have 2-3 back-up designs that use some variation of a free bag design. We are looking for a design that is highly reliable and very repeatable (25+ rockets) that university students can't mess up.

 

[jderimig]

Or you can place another small drogue at the harness a little upstream from the main shrouds to destabilize the lower airframe to keep it from getting to the ballistic orientation.

 

[StreuB1]

Or you can place another small drogue at the harness a little upstream from the main shrouds to destabilize the lower airframe to keep it from getting to the ballistic orientation.

I have transitioned over to larger streamers for this task. I use an 8" x 70" streamer from Preston (Top Flight) on my Demon 5 and a 7" x 60" steamer on my 5" Super Jart. The streamer has provided just enough drag to keep the NC/AVBay (true HEDD) above the rocket, but not be as draggy as a drogue so as to prevent it from slowing down too much, thus allowing the booster to become aerodynamic. Its a bit counter intuitive I have found that in some situations, keeping velocity UP actually causes the booster to fall into and out of terminal velocity and having its CP/CG spoiled. It seems to drift in and out of these two attitudes so it comes down fast, but it comes down straight and keeps the laundry above where it needs to be.

Now on the other hand, where you are doing DD with an upper payload section and as such, are breaking the airframe further down. The CG/CP of the lower and the upper section become disjointed and are both spoiled so that they fall flat. When the booster falls flat, it typically will begin to windmill and due to its centripetal force, acts like a gyroscope and no longer wants to changes its attitude and remains flat. This happened to my Ex Wildman at Mini MWP this spring when I had an anomaly at apogee that rendered ALL my electronics inoperable. It flew on an M to nearly 14k and fell that entire distance. It was unscathed after falling nearly 3 miles. Entirely due to the CP/CG relationship of the COMPONENTS allowing the rocket to stay flat the entire way down. Obviously the terminal velocity of a horizontally opposed body falling is far less than it coming in vertically aligned! lol

SIM your rocket without the upper section and w/o the NC. See what the stability looks like. Then do the same with your payload section.

My points:
*The best backup is not needing a backup. More so, not needing a backup for your backup. In my case, a collision at apogee rendered my TRIPLE dedundant recovery system (2 x RRC3 + Raven3) useless. The final line was a design that comes in flat.
*The best recovery system is the most simple recovery system for the job at hand.

KISS Principle.....

 

[AllDigital]

Or you can place another small drogue at the harness a little upstream from the main shrouds to destabilize the lower airframe to keep it from getting to the ballistic orientation.

I really like this suggestion. It also helps de-risk the possibility that the drogue/nose/avbay descends faster than the main w/rocket. So far, in testing we've been able to keep the drogue/nose above the main, but without some type of free bag configuration (where the nose floats down by itself) this is a risk.

*The best backup is not needing a backup. More so, not needing a backup for your backup. In my case, a collision at apogee rendered my TRIPLE dedundant recovery system (2 x RRC3 + Raven3) useless. The final line was a design that comes in flat.
*The best recovery system is the most simple recovery system for the job at hand.

KISS Principle.....

The rocket I'm using referenced above was purpose built to be a test platform for *new* HEDD designs. By definition, we are not testing the best recovery system, instead we are testing experimental "never been done" HED solutions. This rocket is not the actual rocket the solution is going on -- that is a 175 pound LOX/Alcohol bi-propellant rocket and very much won't have a "backup" chute. I've gone through a lot of iteration with different HED designs from others that have included spring loaded, latch systems, cut aways, tethered bags, etc. This version is securing the upper airframe using a cambuckle and tender descender (promising so far, but still novel). If there was a HEDD "gold standard" that will support a 200 pound rocket and is super simple, highly reliable, and highly repeatable then we wouldn't need all the testing (better suggestions?). The radio controlled back-up chute has saved us three times and we've never lost a rocket during testing -- so far. So in this case, the best solution for experimenting with new HED systems is to have a lot of empty desert and a back-up recovery system.

 

[Handeman]

View attachment 548012

If the booster section is going ballistic, then your drogue isn't big enough. It means the drogue is coming down faster than the booster section when it's sideways, which means the booster is coming down uncontrolled. You don't have a controlled decent of the whole rocket. You need to redesign the recovery system before the point the main is deployed.

 

[StreuB1]

In light of the new information supplied. I might suggest treating this not like a hobby rocket but a sounding rocket.

Ditch all nonessential components and recover only the essential. Though, even in the desert launching things without the expectation of at least attempting recovering them is frowned upon.....

Break the rocket into two pieces. Each section deploys a chute in a deployment bag with a steamer on the bag ring. Retain the chute in the bag with a zip tie as a choker and have the zip tie pass through dual line cutters. One cutter is the primary, one cutter is the backup. Either one cuts the tie, the streamer pulls the bag off the chute. On the chute, install a rising-ring "slider" on the shroud/suspension lines as a disreefing mechanism to delay the inflation of the chute to avoid the shock of a sudden chute inflation.

Treat it like what it is.

Only hobby rocketeers try and recover things all tied together. Your project and its launch area does not warrant all the components to be tied to a single system. There is no reason for it. Treat each piece as it's own independently recoverable system. Way more appropriate approach.

 

[StreuB1]

Your main bridle ("shock cord") will be z folded and taped. Your chute will be all the way at the end. Your line cutter charge firing lines will run the length of the bridle back to your electronics. You will use breakaway loops in the firing lines as strain reliefs and between these you will use cotton string to bind the lines to the bridle using cross hitches. Use very fine strand copper conductors with high strand count. Don't use silicone insulated wires for your line as it likes to kink. Teflon jacket is preferred but PVC is just fine.

 

[Dan Griffing]

If the booster section is going ballistic, then your drogue isn't big enough. It means the drogue is coming down faster than the booster section when it's sideways, which means the booster is coming down uncontrolled. You don't have a controlled decent of the whole rocket. You need to redesign the recovery system before the point the main is deployed.

I disagree about the drogue being too small when the rocket comes in ballistic. You want the airframe parts coming down in a flat spin, and too big a drogue prevents this.

I use a drogue that’s just big enough to organize the descent so that the airframe halves are spinning in an “inverted Y” beneath the drogue and descending at 60-80 fps.

You want to air friction on the horizontal airframe sections to be the determinant of the descent speed instead of the size of the drogue.

 

[Handeman]

I disagree about the drogue being too small when the rocket comes in ballistic. You want the airframe parts coming down in a flat spin, and too big a drogue prevents this.

I use a drogue that’s just big enough to organize the descent so that the airframe halves are spinning in an “inverted Y” beneath the drogue and descending at 60-80 fps.

You want to air friction on the horizontal airframe sections to be the determinant of the descent speed instead of the size of the drogue.

I tend to agree with that reasoning, but if your booster is turning ballistic, it's not falling in an inverted Y. The booster has to be falling slower than your drogue to be able to tilt all the way down in a ballistic manner. Since it isn't in an inverted Y, I still think you need a slightly larger drogue.

 

[AllDigital]

Or you can place another small drogue at the harness a little upstream from the main shrouds to destabilize the lower airframe to keep it from getting to the ballistic orientation.

In this HEDD configuration, when the main deploys it pulls out a small avBay which then pulls out the main from the apex. I placed a second small pilot chute on the avBay, along with additional Z fold and it substantially minimized the shock. The suggestion of another chute was very helpful. The vehicle still momentarily reoriented, but not a full 180 degrees, only about 120 degrees and then softly came back up to vertical under main.

Below is a photo with all the laundry out. I still might consider a version where the drogue/nose/avbay float separate, but in the test this weekend everything worked flawlessly. The drogue/main/avbay/pilot chute stayed above the main and out of the way. Everything deployed very smooth.