Article: 1651
By: Dave Randall

Beyond Masking Tape – Understanding  Why You Should Use Shear Pins

Are you a rocketeer that’s relatively new to mid or high power rockets?  Did you get your Level 1 Certification recently?  Chances are you want to fly faster, bigger, heavier, taller, fatter rockets than you’ve done in the past.  I know when I started flying rockets with G,  H and I motors for a little while, I began to notice folks were using ‘shear pins’ to hold their rockets together.  My first thought?  ACK!  I want it to come apart, not stay together! I have seen a ballistic recovery; it wasn’t pretty.   But, considering I had also just witnessed normal recoveries with shear pins, I was intrigued and asked for more information.  Here’s what I learned:

What’s a shear pin?

A shear pin is a small plastic rod that is inserted in hole drilled at the coupling between two tubes. It is designed to hold the tubes together under normal flight stresses, but when the ejection charge fires, the force of the coupler tube sliding past the body tube slices or shears the pin. Shear pins are most useful because they hold a rocket together when some event occurring during the flight might otherwise cause it to come apart at the wrong time.   There are a lot of moments during the flight that are the wrong time – I’ll cover those next.  The goal of the shear pins is to help guarantee that your rocket recovery system is deployed at the right time.  The right time is at apogee or at a specific low altitude in a dual-deploy configuration.   *A side note- if you aren’t familiar with dual-deployment, this is a technique to recover your rocket by using two parachutes – one is small (the drogue chute) and is deployed at apogee; the second parachute (the big main chute) is deployed at low altitude, usually below  1,000’.  This allows your rocket to come down quickly and avoid drifting far away, yet land gently.

The “wrong time” for deployment

Generally speaking, the wrong time to deploy your recovery system (usually a parachute or streamer) is anytime other than at apogee.  There’s a lot of time between when your motor first ignites, and when the rocket finally lands on the ground.  The flight has several phases; thrust, coast, apogee, descent, and landing. Typically, the transition from one phase to another is when shear pins are most valuable.

  1. Drag Separation – At the moment your motor burns out, the thrust is no longer increasing the velocity of your rocket.  At this point in time, other forces acting on your rocket start to have a bigger influence on the flight.  Air has been passing over the fins to help keep the rocket stable and pointed in one direction.  At burnout, that air will now start to slow the rocket down.  Because the fins are at the aft end of the rocket, you can almost imagine the rocket being pulled from behind.  Inertia wants to keep parts moving up; friction from the air wants to slow other parts down. This problem can be more pronounced when your rocket is nose-heavy – either due to added nose-weight or the simple mass of your recovery system, but can also happen to a perfectly balanced rocket.  The common failure is called “Drag separation” because the drag forces on the fins and the inertia of the rocket above the fins cause the rocket to separate.  High thrust, short duration motors (V-Max or Warp 9 being the most extreme examples) can be more susceptible to this problem, as well as loosely coupled airframes.  Usually, this failure causes the chute deployment to occur when the rocket is travelling at its fastest velocity.  When the chute deploys and fills with air, the shroud lines and shock cord typically tear through the body tube, causing a “zipper”.
  2. Shifting Mass – At the moment your motor ignites and begins moving up, any loose components (say… a tightly wound chute) in the rocket will want to stay where they were while the rocket airframe tries to go up.   Because of the law of inertia (yes, junior high physics do actually come in handy!), the chute is slammed against the motor or the forward end of your altimeter bay (for a main chute in a dual-deploy configuration).  At motor burnout, the rocket airframe begins slowing down. The faster your rocket decelerates, the more pronounced this problem can be.  Because of inertia, your loose components don’t slow down at the same rate as the rocket airframe.  Now, your components move forward in the rocket and slam against the altimeter bay or the nosecone.  Any loosely coupled sections will separate.  When the sections separate, your rocket is still going awfully darn fast. The chute deploys and fills with air, the shroud lines and shock cord typically tear through the body tube, again, causing a “zipper” and possibly shredding the parachute.
  3. Apogee – Wait, apogee? Isn’t that when the chute is supposed to come out?  Yep!  But when you have a dual deployment configuration, it’s common to have two points of separation on the rocket.  One separation point is for the drogue parachute; one is for the main parachute.  You only want to have one separation at apogee to deploy your drogue parachute.  So, if your rocket is not coupled tightly enough where your main chute will come out, you could have both sections separate and then both your main and drogue chutes will come out at apogee.  When one chute comes out and the rocket is travelling too fast, the chute inflation happens more violently, and it creates what is often called a ‘crack the whip’ deployment.  If your rocket components are loosely coupled, the separation doesn’t need to be violent, only strong enough to pull the tubes apart.
  4. Oversized Ejection Charge – Your ejection charges can be integral with the motor or separately controlled by electronics.   You should ground test your charges to be sure of the charge size required to properly separate the rocket.   Vern Knowles has an excellent article to help you calculate the size of charge required based on the math & science of it all.  But, let’s assume your ejection charge is too large and causes a very forceful separation.  That separation causes your shock cords to extend their full length, and then the inertia of the rocket components causes separation of your main parachute section.
  5. Incorrect Ejection Timing – In the dual deployment rocket, an ejection charge integral with the motor is fired after a delay (motor eject).  If the rocket uses an altimeter, the ejection charge is fired at apogee.  Most altimeters do not have a problem detecting apogee and firing the charge.  Often times, motor ejection is used as a backup.  In both cases, however, if the deployment charge fires too early or too late, there will be enough velocity in the rocket that when your drogue chute deploys, it can cause all sections of the rocket to separate.  Again, you may get a crack-the-whip type deployment with zippers and/or a shredded parachute.

So, those are the five “wrong times” to separate your rocket.  Now you know why you would consider using shear pins in your rocket.  For the last section of this article, I’ll give some guidance in helping you answer the next question: Should you use shear pins?

Let’s get past square one here first with a few quick tests.  The first test is the River City test.  Take your fully assembled rocket as if you were taking it out to the launch pad, grab it by the section just above your motor section and hold it out in front of you.  Is part of your rocket on the carpet?  Nope?  Then grab the next section up on the rocket and again, hold it out in front of you.  Continue doing this, working your way up through each sections of the rocket, including the nosecone.  If at any point, some part of your rocket was overcome by gravity and made its way to the floor while you were still holding on to another part of the rocket, there’s trouble in River City my friend.  We’ll call this condition “Trouble in River City”.  But, if you made it past the River City test, it is time for the next test.  This one is the Gutentite test.  Just like you did for the River City test, grab each section of your rocket and hold it in front of you.  Now move your rocket up and down to exert some force on your couplings.   Work your way up through each section, and if you get to the top section with the entire rocket in your hand and no parts on the floor, you pass the Gutentite test!  If you have some parts on the floor, well, your rocket has the “Not Gutentite” condition.  If you want to take the testing to the next level, give the “Osotite” test a try.  Just like you did for previous tests, grab each section of your rocket and hold it in front of you.  Now forcefully move your rocket up and down to exert a significant amount of force on your couplings.   You can also invert the rocket and do the same test to add a bit more gravity to the stress test. If your rocket passes this test, ground test your charges so you can be confident it will separate properly.

Both the “Trouble in River City” and “Not Gutentite” conditions can usually be solved temporarily at a launch by putting masking tape around the coupler surface.  Increasing the thickness of the coupler increases the friction between the two components.  With the right amount of friction, your rocket can pass both tests.   Are you done?  Maybe for now, but not really …   While effective for any one given flight, masking tape is subject to heating up in the sun, “leaking adhesive” around the edges, compressing, and its fit will likely change from one launch to the next.  Repeatability is the goal in everything rocketry.  A masking tape fit is not a “repeatable” solution.  If you have a large gap between your tubes, use a more permanent solution to fix this condition.  I will typically put one or more layers of fiberglass and epoxy over the coupler tube to add thickness.  It can be sanded down to get a proper fit. After you have a more permanent solution, you can also use shear pins to improve the repeatability and predictability of your deployment.

Looking at the five conditions from above, however, you need to decide whether you should use shear pins.   Let’s take them one at a time:

  1. Drag separation – If your rocket has relatively heavy mass forward of any couplings, you should consider shear pins.   If your rocket has large fins or flat leading edge surfaces on the fins causing a lot of drag, you should consider shear pins. Put them in, and of course, ground test.
  2. Shifting mass – If your rocket has chutes, shroud lines, and shock cords that easily move around inside the body tube, you should consider shear pins, especially if they are heavy chutes.    You can test this easily by holding your fully prepared for flight rocket horizontally and shaking it left and right.  If you get a lot of movement, and your parts start separating, you have two options here.  You can add shear pins to prevent the separation, and you can also add the “Newman Chute Shelf”.   Make an internal shelf for your recovery components by inserting a coupler tube with a centering ring inside your body tube.  You can adjust the shelf to allow enough room for the recovery components and yet minimize movement.   This method can also be coupled with using shear pins. And then, of course you will ground test.
  3. Apogee – In my experience, rockets that pass the Gutentite test are typically not subject to failure at apogee.  If your rocket can’t pass the Gutentite test, or even the River City test, fix that first, and consider shear pins if you want added security. And then ground test.
  4. Oversized Ejection Charge – Proper ejection charge sizing starts with a calculation based on the volume you need to pressurize and the force needed to separate the parts.  I won’t recommend using shear pins based on your ejection charge size.  You need to decide first if you need shear pins, then size your ejection charges appropriately to shear the pins.  And then ground test.
  5. Improper Ejection Timing – My opinion here is similar to the apogee condition.  If your rocket can’t pass the Gutentite test, or even the River City test, fix that first.  Generally speaking, relying on motor ejection delay has its place, but isn’t nearly as precise, repeatable and predictable as a good altimeter.  As with each of the other cases, consider shear pins if you want added security. And then ground test.
  6. For Added Measure – You may find that even though your rocket doesn’t meet those five criteria, you want the added security of shear pins anyways.  That’s perfectly fine!  Ground test your charges and your rocket in its ready to fly configuration to ensure you have everything working properly.

So, you should now be armed with enough information to decide whether you want to use shear pins in your projects.  Building your rockets with designs that promote repeatability and predictability are keys being a successful flyer.   Using shear pins is one way that you can help built both of those to elements into your flights. And if it’s not obvious by now, ground testing your rocket with or without shear pins helps you understand how your rockets’ deployment system operates.

A few final notes about shear pins.  First, if you have a very tight friction fit and you’re going to use shear pins, consider reducing the overall friction of your fit.  Remember, repeatability is the goal here, so if your friction fit is subject to variability through temperature, humidity and changes in your tube coupling size (it is), you could find yourself with a very tight fit plus shear pins.  The risk is that your coupling is so tight that your ejection charge is too small.  Second, you may choose not to use shear pins for various reasons.  For aesthetic purposes, you may choose to keep a friction fit.  You may also find that your rocket gives very repeatable results with friction fit in your ground testing and simply choose to stick with friction fit.  Lastly, I’ve mentioned ground testing a bunch.  Take a look at this Perfect Flight article on how to create inexpensive ejection charges, both for testing and for in-flight use.

There are two more questions to answer in this whole shear pin discussion.  How do I install shear pins? And, What shear pin should I use?  For answers to those questions, I refer you to Kent Newman’s excellent article on Shear Pins.