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Preparing for Certification

posted Feb 7, 2016, 10:07 AM by Randall Shanyfelt   [ updated Feb 7, 2016, 12:14 PM by James Cramton ]
Preparing for Certification

As a Level 3 certification holder in both NAR and Prefect for Tripoli rocketry associations, I have witnessed quite a few Level 1 certification attempts.  The reason I say attempts is because there have been quite a lot of failures that would not have happened if the fliers had done the proper amount of homework, learning as much as they could about proper build techniques, stability issues, and safety.

Planning your build is the first big step.  Whether you chose to build a kit or scratch build your rocket, the same basic principles apply.  I always suggest that for a certification flight you keep it as simple as possible.  You can always try more complicated steps after you get your certification.  Motor eject, electronic eject or both at Apogee is a stronger bet than trying dual deployment or other more complicated methods.  I have had several people try for their certification using dual deploy, minimum diameter, GPS tracking, and as large an I motor as they can find.  Guess what, most of them failed.  A simple build with single deploy, minimal electronics or none and a small H motor in a medium size rocket with 3 or 4 fins and a nose cone will be successful most of the time.  Know the safety codes and stability factors, such as center of gravity compared with center of pressure and what factors affect this relationship.  You need to know the weight of your rocket compared to the amount of initial thrust of the motor you choose.  What is that limit?  I am not going to give you all the answers, that is part of doing your homework and being able to answer correctly when the person doing your certification asks.   If you don’t know the answers to these basic questions, you shouldn’t be allowed to fly.  Having a safe stable rocket is our main goal.

If you are building a cardboard tube rocket with wood centering rings and bulk heads, you need to have good tolerances where your couplers and nose cone are inserted into the airframe.  The reason for this is, if they are too loose, you might get drag separation when your motor burns out.  The momentum of the rocket will tend to want to have the heavier pieces of the rocket keep going up, such as the nose cone.  The pieces with more drag, such as the booster section with the fins, will tend to slow down faster.  This is drag separation, and it will cause your parachute to deploy early causing the shock cord to zipper the airframe.  If you get a big zipper, you will not pass your certification.  If you have the proper tolerances, you should be able to pick up your rocket by the nose cone and not have it separate.   You should be able to shake it a little and then have it separate.   You can adjust the fit by using a little masking tape, if it is too loose or sand the coupler a little, if it is too tight.  A little talcum powder at these separation points will help also.  If you have a fiberglass rocket or if your nose cone is heavy, you will want to use sheer pins at the separation points.  These are small nylon screws, usually #56, that are inserted thru holes in your airframe at the separation points.  Your black power charge(s) should have enough pressure to sheer these pins and cause separation.   It is important to know how much black powder to use, and the way you do that is to ground test your rocket.  You want to have enough pressure to separate your rocket, but not enough to blow it up.  There are websites that you can use to calculate the amount of powder so you have a safe amount to start with.

More and more kits that are being sold are made of fiberglass.  This presents some build issues that you don’t run into with cardboard and wood.   Epoxy will not stick to fiberglass unless it is roughed up first.  By this I mean get your 40 or 60 grit sandpaper and scratch the surface of the fiberglass anywhere there is a joint with fiberglass and epoxy.   If it is a thru the wall fin design, follow the manufacturer’s directions for gluing on the fins.  Most will suggest that you add filets where the fin root attaches to the motor tube.  This can be done either by drilling holes in your airframe and using a syringe to inject epoxy or leaving the aft centering ring off until you filet the fin roots to the motor tube.  Again, make sure you have a good bond on the bulkheads for the nose cone and booster sections.  These are where your eye bolts are attached for your shock cords.  There is a lot of tension on these during deployment.   A good way to add strength for these bulkheads is to recess them a little ways into the nose cone or booster section and glue in ¼ inch wide band of material above the bulk head.  Again, scratch, scratch, scratch the fiberglass.  Adding a little ground fiberglass powder to your epoxy with help to strengthen the bond.  You can get this at several rocket suppliers or composite suppliers.

One way that we can all learn, especially those that are going to try for their level 1, is to talk to other members about their rockets and what has worked for them and what has not.  I have personally learned a lot from talking to others and by observing how they do things. I have never run into anyone that wasn’t excited to show off what they have done and to share new ideas.  I know that it is a little intimidating to go up to someone you don’t know and ask for their help.  As part of the certification process, it is my responsibility to make sure the rocket is safe to fly and to make sure the flyer is aware of why he or she did certain things.  I can’t do the rocket setup myself, but I can observe, and if I know that, for instance, the way the parachute is packed, it probably won’t open, I can stop them from continuing.   In a case like that, I would demonstrate the proper way and then let them do it themselves.  It would have been better if they had asked a fellow flyer to show them so they could do it correctly the first time themselves.  If you are not sure, ask someone who knows, if you think the answer you get is not correct, ask someone else and have them explain it.

Part of launching a rocket with pretty good knowledge of what it is going to do is to run simulations.  There are several programs available for this.  Apogee rocketry sells one called RockSim.   Another program that is free is called Open Rocket.  There is a little bit of a learning curve with each of these, but the results are worth it.  How high will your rocket go, will it bust the waiver?  What is a waiver?  All of these things are the responsibility of the flier.  Know what factors affect how your rocket flies; wind, drag, shape of fins and nose cone, weight, etc., and what the affects will be.

This seems like a lot to learn for you Level 1 certification, but if you do it will be a safe and fun hobby.

 

Art Just

NAR level 3 Number 87145

Tripoli Prefect Level 3 Number 12693

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