{"id":1651,"date":"2009-11-22T20:22:31","date_gmt":"2009-11-23T04:22:31","guid":{"rendered":"http:\/\/rocketsnw.com\/?page_id=1651"},"modified":"2009-11-23T06:06:07","modified_gmt":"2009-11-23T14:06:07","slug":"shear-pins-a-beginners-guide","status":"publish","type":"page","link":"https:\/\/northwestrocketry.com\/?page_id=1651","title":{"rendered":"Shear Pins- A Beginners Guide"},"content":{"rendered":"

Article: 1651
\nBy: Dave Randall<\/p>\n

Beyond Masking Tape \u2013 Understanding \u00a0Why You Should Use Shear Pins<\/strong><\/p>\n

Are you a rocketeer that\u2019s relatively new to mid or high power rockets?\u00a0 Did you get your Level 1 Certification recently?\u00a0 Chances are you want to fly faster, bigger, heavier, taller, fatter rockets than you\u2019ve done in the past. \u00a0I know when I started flying rockets with G, \u00a0H and I motors for a little while, I began to notice folks were using \u2018shear pins\u2019 to hold their rockets together.\u00a0 My first thought?\u00a0 ACK!\u00a0 I want it to come apart, not stay together!<\/em> I have seen a ballistic recovery; it wasn\u2019t pretty.\u00a0\u00a0 But, considering I had also just witnessed normal recoveries with shear pins, I was intrigued and asked for more information.\u00a0 Here\u2019s what I learned:<\/p>\n

What\u2019s a shear pin?<\/strong><\/p>\n

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. \u00a0\u00a0There are a lot of moments during the flight that are the wrong<\/em> time \u2013 I\u2019ll cover those next.\u00a0 The goal of the shear pins is to help guarantee that your rocket recovery system is deployed at the right<\/em> time. \u00a0The right time is at apogee or at a specific low altitude in a dual-deploy configuration.\u00a0 \u00a0*A side note- if you aren\u2019t familiar with dual-deployment, this is a technique to recover your rocket by using two parachutes \u2013 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 \u00a01,000\u2019.\u00a0 This allows your rocket to come down quickly and avoid drifting far away, yet land gently.<\/p>\n

The \u201cwrong time\u201d for deployment<\/strong><\/p>\n

Generally speaking, the wrong time to deploy your recovery system (usually a parachute or streamer) is anytime other than<\/em> at apogee.\u00a0 There\u2019s a lot of time between when your motor first ignites, and when the rocket finally lands on the ground.\u00a0 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.<\/p>\n

    \n
  1. Drag Separation<\/strong> \u2013 At the moment your motor burns out, the thrust is no longer increasing the velocity of your rocket.\u00a0 At this point in time, other forces acting on your rocket start to have a bigger influence on the flight.\u00a0 Air has been passing over the fins to help keep the rocket stable and pointed in one direction.\u00a0 At burnout, that air will now start to slow the rocket down.\u00a0 Because the fins are at the aft end of the rocket, you can almost imagine the rocket being pulled from behind. \u00a0Inertia 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 \u2013 either due to added nose-weight or the simple mass of your recovery system, but can also happen to a perfectly balanced rocket. \u00a0The common failure is called \u201cDrag separation\u201d because the drag forces on the fins and the inertia of the rocket above the fins cause the rocket to separate.\u00a0 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.\u00a0 Usually, this failure causes the chute deployment to occur when the rocket is travelling at its fastest velocity.\u00a0 When the chute deploys and fills with air, the shroud lines and shock cord typically tear through the body tube, causing a \u201czipper\u201d.<\/li>\n
  2. Shifting Mass<\/strong> \u2013 At the moment your motor ignites and begins moving up, any loose components (say\u2026 a tightly wound chute) in the rocket will want to stay where they were while the rocket airframe tries to go up.\u00a0 \u00a0Because 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). \u00a0At motor burnout, the rocket airframe begins slowing down. The faster your rocket decelerates, the more pronounced this problem can be.\u00a0 Because of inertia, your loose components don\u2019t slow down at the same rate as the rocket airframe.\u00a0 Now, your components move forward in the rocket and slam against the altimeter bay or the nosecone.\u00a0 Any loosely coupled sections will separate.\u00a0 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 \u201czipper\u201d and possibly shredding the parachute.<\/li>\n
  3. Apogee<\/strong> \u2013 Wait, apogee? Isn\u2019t that when the chute is supposed<\/em> to come out?\u00a0 Yep!\u00a0 But when you have a dual deployment configuration, it\u2019s common to have two points of separation on the rocket.\u00a0 One separation point is for the drogue parachute; one is for the main parachute.\u00a0 You only want to have one separation at apogee to deploy your drogue parachute.\u00a0 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.\u00a0 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 \u2018crack the whip\u2019 deployment.\u00a0 If your rocket components are loosely coupled, the separation doesn\u2019t need to be violent, only strong enough to pull the tubes apart.<\/li>\n
  4. Oversized Ejection Charge<\/strong> \u2013 Your ejection charges can be integral with the motor or separately controlled by electronics.\u00a0\u00a0 You should ground test your charges to be sure of the charge size required to properly separate the rocket.\u00a0\u00a0 Vern Knowles has an excellent article<\/a> to help you calculate the size of charge required based on the math & science of it all.\u00a0 But, let\u2019s assume your ejection charge is too large and causes a very forceful separation.\u00a0 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.<\/li>\n
  5. Incorrect Ejection Timing <\/strong>\u2013 In the dual deployment rocket, an ejection charge integral with the motor is fired after a delay (motor eject).\u00a0 If the rocket uses an altimeter, the ejection charge is fired at apogee.\u00a0 Most altimeters do not have a problem detecting apogee and firing the charge.\u00a0 Often times, motor ejection is used as a backup. \u00a0In 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.\u00a0 Again, you may get a crack-the-whip type deployment with zippers and\/or a shredded parachute.<\/li>\n<\/ol>\n

    So, those are the five \u201cwrong times\u201d to separate your rocket.\u00a0 Now you know why<\/em> you would consider using shear pins in your rocket.\u00a0 For the last section of this article, I\u2019ll give some guidance in helping you answer the next question: Should you use shear pins?<\/p>\n

    Let\u2019s get past square one here first with a few quick tests.\u00a0 The first test is the River City test.\u00a0 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.\u00a0 Is part of your rocket on the carpet?\u00a0 Nope? \u00a0Then grab the next section up on the rocket and again, hold it out in front of you.\u00a0 Continue doing this, working your way up through each sections of the rocket, including the nosecone.\u00a0 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\u2019s trouble in River City my friend.\u00a0 We\u2019ll call this condition \u201cTrouble in River City\u201d.\u00a0 But, if you made it past the River City test, it is time for the next test.\u00a0 This one is the Gutentite test.\u00a0 Just like you did for the River City test, grab each section of your rocket and hold it in front of you.\u00a0 Now move your rocket up and down to exert some force on your couplings.\u00a0\u00a0 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!\u00a0 If you have some parts on the floor, well, your rocket has the \u201cNot Gutentite\u201d condition.\u00a0 If you want to take the testing to the next level, give the \u201cOsotite\u201d test a try.\u00a0 Just like you did for previous tests, grab each section of your rocket and hold it in front of you.\u00a0 Now forcefully move your rocket up and down to exert a significant amount of force on your couplings.\u00a0\u00a0 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<\/span><\/em> your charges so you can be confident it will separate properly.<\/p>\n

    Both the \u201cTrouble in River City\u201d and \u201cNot Gutentite\u201d conditions can usually be solved temporarily at a launch by putting masking tape around the coupler surface.\u00a0 Increasing the thickness of the coupler increases the friction between the two components.\u00a0 With the right amount of friction, your rocket can pass both tests.\u00a0 \u00a0Are you done?\u00a0 Maybe for now, but not really \u2026\u00a0\u00a0 While effective for any one given flight, masking tape is subject to heating up in the sun, \u201cleaking adhesive\u201d around the edges, compressing, and its fit will likely change from one launch to the next.\u00a0 Repeatability is the goal in everything rocketry.\u00a0 A masking tape fit is not a \u201crepeatable\u201d solution.\u00a0 If you have a large gap between your tubes, use a more permanent solution to fix this condition. \u00a0I will typically put one or more layers of fiberglass and epoxy over the coupler tube to add thickness.\u00a0 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.<\/p>\n

    Looking at the five conditions from above, however, you need to decide whether you should use shear pins.\u00a0\u00a0 Let\u2019s take them one at a time:<\/p>\n

      \n
    1. Drag separation<\/strong> \u2013 If your rocket has relatively heavy mass forward of any couplings, you should consider shear pins.\u00a0 \u00a0If 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.<\/li>\n
    2. Shifting mass<\/strong> \u2013 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.\u00a0\u00a0 \u00a0You can test this easily by holding your fully prepared for flight rocket horizontally and shaking it left and right.\u00a0 If you get a lot of movement, and your parts start separating, you have two options here.\u00a0 You can add shear pins to prevent the separation, and you can also add the \u201cNewman Chute Shelf\u201d.\u00a0\u00a0 Make an internal shelf for your recovery components by inserting a coupler tube with a centering ring inside your body tube.\u00a0 You can adjust the shelf to allow enough room for the recovery components and yet minimize movement.\u00a0 \u00a0This method can also be coupled with using shear pins. And then, of course you will ground test.<\/li>\n
    3. Apogee<\/strong> \u2013 In my experience, rockets that pass the Gutentite test are typically not subject to failure at apogee.\u00a0 If your rocket can\u2019t 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.<\/li>\n
    4. Oversized Ejection Charge<\/strong> \u2013 Proper ejection charge sizing starts with a calculation based on the volume you need to pressurize and the force needed to separate the parts.\u00a0 I won\u2019t recommend using shear pins based on<\/em> your ejection charge size.\u00a0 You need to decide first<\/em> if you need shear pins, then size your ejection charges appropriately to shear the pins.\u00a0 And then ground test.<\/li>\n
    5. Improper Ejection Timing<\/strong> \u2013 My opinion here is similar to the apogee condition.\u00a0 If your rocket can\u2019t pass the Gutentite test, or even the River City test, fix that first.\u00a0 Generally speaking, relying on motor ejection delay has its place, but isn\u2019t nearly as precise, repeatable and predictable as a good altimeter.\u00a0 As with each of the other cases, consider shear pins if you want added security. And then ground test.<\/li>\n
    6. For Added Measure <\/strong>\u2013 You may find that even though your rocket doesn\u2019t meet those five criteria, you want the added security of shear pins anyways.\u00a0 That\u2019s perfectly fine!\u00a0 Ground test your charges and your rocket in its ready to fly configuration to ensure you have everything working properly.<\/li>\n<\/ol>\n

      So, you should now be armed with enough information to decide whether you want to use shear pins in your projects. \u00a0Building your rockets with designs that promote repeatability and predictability are keys being a successful flyer.\u00a0\u00a0 Using shear pins is one way that you can help built both of those to elements into your flights. And if it\u2019s not obvious by now, ground testing your rocket with or without shear pins helps you understand how your rockets\u2019 deployment system operates.<\/p>\n

      A few final notes about shear pins.\u00a0 First, if you have a very tight friction fit and you\u2019re going to use shear pins, consider reducing<\/em> the overall friction of your fit.\u00a0 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. \u00a0The risk is that your coupling is so tight that your ejection charge is too small.\u00a0 Second, you may choose not to use shear pins for various reasons.\u00a0 For aesthetic purposes, you may choose to keep a friction fit.\u00a0 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.\u00a0 Lastly, I\u2019ve mentioned ground testing a bunch.\u00a0 Take a look at this Perfect Flight article<\/a> on how to create inexpensive ejection charges, both for testing and for in-flight use.<\/p>\n

      There are two more questions to answer in this whole shear pin discussion.\u00a0 How do I install shear pins? And, What shear pin should I use?\u00a0 For answers to those questions, I refer you to Kent Newman\u2019s excellent article on Shear Pins<\/a>.<\/p>\n","protected":false},"excerpt":{"rendered":"

      Article: 1651 By: Dave Randall Beyond Masking Tape \u2013 Understanding \u00a0Why You Should Use Shear Pins Are you a rocketeer that\u2019s relatively new to mid or high power rockets?\u00a0 Did you get your Level 1 Certification recently?\u00a0 Chances are you want to fly faster, bigger, heavier, taller, fatter rockets than you\u2019ve done in the past. […]<\/p>\n","protected":false},"author":1,"featured_media":0,"parent":257,"menu_order":0,"comment_status":"open","ping_status":"open","template":"","meta":{"ngg_post_thumbnail":0},"_links":{"self":[{"href":"https:\/\/northwestrocketry.com\/index.php?rest_route=\/wp\/v2\/pages\/1651"}],"collection":[{"href":"https:\/\/northwestrocketry.com\/index.php?rest_route=\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/northwestrocketry.com\/index.php?rest_route=\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/northwestrocketry.com\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/northwestrocketry.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=1651"}],"version-history":[{"count":5,"href":"https:\/\/northwestrocketry.com\/index.php?rest_route=\/wp\/v2\/pages\/1651\/revisions"}],"predecessor-version":[{"id":1656,"href":"https:\/\/northwestrocketry.com\/index.php?rest_route=\/wp\/v2\/pages\/1651\/revisions\/1656"}],"up":[{"embeddable":true,"href":"https:\/\/northwestrocketry.com\/index.php?rest_route=\/wp\/v2\/pages\/257"}],"wp:attachment":[{"href":"https:\/\/northwestrocketry.com\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=1651"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}