¡Muy Fuerte!

Introduction

Next in line is another minimum diameter kit-bashed Performance Rocketry kit, this time a 4″ Intimidator 4. I plan to fly this guy on anything from a K-N motor and eventually use it as a booster for a Balls project. The rocket is named after the Loose Cannons cd Muy Fuerte, which also seems fitting since it should be a tough rocket (Would a shred would be embarrassing? Sí).  I learned a few things from my last build, so this should turn out even better. At first glance the fins seem to be a little too beefy for my liking at 3/16″ thick and weighing 0.9lb each! Lets see how it goes.

Fin Mounting

Before getting the fins put in to place I needed to do a few things. First was to trim off the extra tab length on the root edge of the fin using a table saw. This was pretty simple and came out cleaner than when I dremeled off the Comp3 fins. I also made a fin jig to increase the bevel on the fins since they came a little too dull for my liking. Lastly was to rough up the fin surface to get a better bond on the tip-to-tip lamination. I noticed after the destruction of the Comp3 that carbon fiber did not bond well to the scuffed up surface of the G10. So this go around I went a bit extreme to make sure the laminate would bond well. I set the table saw blade just above the surface of the table, maybe a 1/32″ or so and raked the fins back and forth to remove the polished finish and reveal a rough surface. This worked great, and after everything was all said and done each fin went down in weight by 25% and still seem a bit overkill with additional reinforcement to come.

Once the surfaces were sanded and cleaned, I taped the fin slots on the inside of the airframe to tack the fins in place. After the tape was smoothed out I inserted the motor case for the fins to butt up against while the JB Weld cured. Once all the fins were in place, I laid in the fin fillets with some more JB Weld. I vigorously slid a steel washer back in forth in the joint to “score” a line to tape off for the fillets. After the tape is in place I pour the epoxy and then run the same washer over the joint to provide a smooth and evenly distributed fillet.

Tip-to-Tip Carbon Fiber Fin Reinforcement

Vacuuming bagging is the way to go, however, I’ve had pretty good luck with hand layups using peel ply and some cheap tools. I start by carefully creating a template with a sheet of paper so that I can cut out the carbon fiber to fit the tip-to-tip joint. It is helpful to put masking tape on the carbon fiber where you intend to cut so that you don’t fray the ends of the pieces. If you really want to make this look good, you’re going to have to spend of a lot of preparation time here.  You will have to make several small slits in the carbon fiber (and peel ply, purchased from aircraft spruce item number 01-1482) in the areas above and below the fins to make the transitions to the airframe as smooth as possible.

After prepping the pieces I covered the entire area with epoxy so that the carbon fiber is completely saturated. The peel ply (that is also slit in the appropriate areas) is then placed over the carbon fiber, and with a small foam roller I evenly distribute the epoxy. The peel ply I use is a Nylon fabric that sometimes has wrinkles in it. If you place the wrinkled peel ply over the layup you will get wrinkles in the finished product (assuming you’re not using a vacuum bag). All you need to do is use an iron on your ironing board to fix this before its used. I didn’t do this on my first layup and you can see the crease in the picture below, I’ll have to sand this carefully.

The foam roller can be purchased at home depot for a few bucks and trust me this cheap tool makes a big difference. I’ve done this process without the roller and the end product tends to come out wavey and non-uniform.

As the epoxy starts to setup, you want to trim and removes the excess carbon. You can do this after it has dried but it is much more difficult, especially the areas where the carbon meets from joint to join. I then lightly sand the surface with some 100 grit and then 320 grit sand paper. You don’t want to sand too much of the layup but rather smooth the surface. In some areas where the carbon overlaps you need to grind it down a bit, but just the bare minimum. Next I thinned some epoxy with some acetone and proceeded to rub a very very fine layer over the fin can using a soft lint free cloth. Do this in a well lit area and look at the reflection to spot out the areas you need to wipe down. I repeated this process of lightly sanding with 320 grit and then polishing until I got a finish that made me happy.

More coming soon.

  1. #1 by Phil Babcock on December 1, 2010 - 11:09 am

    That looks like a Performance Rocketry Intimidator 3. I am planning on doing a similar minimum diameter rocket for my L3 attempt. It will be Performance Rocketry G3 with some modified fins and a Von Karman nosecone. I’ll be very interested to see your build!

  2. #2 by James on December 1, 2010 - 5:11 pm

    Good eye Phil, it’s an Intimidator 4 that I’m converting. You can see my Competitor 3 build as well under the vehicle tab.

  3. #3 by Phil Babcock on December 1, 2010 - 8:18 pm

    What did you do for motor retention on the Competitor 3?

  4. #4 by James on December 2, 2010 - 3:22 am

    Since the recovery harness was attached directly to the bulkhead of the motor I wasn’t too worried about loosing the case. I just did the old masking tape friction fit.

  5. #5 by Phil Babcock on December 2, 2010 - 12:09 pm

    James :
    Since the recovery harness was attached directly to the bulkhead of the motor I wasn’t too worried about loosing the case. I just did the old masking tape friction fit.

    Do you think that you pass for an L3 certification or would they want more positive retention?

  6. #6 by James on December 2, 2010 - 10:37 pm

    Well you need to ask your TAP on that, but in this particular configuration I bet you could have it approved. Otherwise you could also rig a bulkhead up the airframe with a threaded feature to screw the motor into with the threaded forward closure.

  7. #7 by Phil Babcock on December 5, 2010 - 8:07 pm

    I will make sure to double check with my TAP before I go to far on the build. What is your experience with having the fins one body diameter up from the bottom to increase performance? I have heard that it helps to smooth out the airflow over the airframe.

  8. #8 by James on December 6, 2010 - 6:24 pm

    That’s a good question, I normally do not consider the aerodynamic advantage of the fins further up the rocket as I would imagine it would be a minimal payoff. I would normally do this to reduce the chance of the fins being damaged on landing, but it’s a trade off with the stability margin since you’re reducing it as you move the fins up typically (and then potentially adding more mass to the nose). I may be wrong but I doubt there is a huge benefit, but may be worth it for an altitude attempt. Right now I’m actually running a CFD analysis to check on this, if I have some decent results i’ll post them.

  9. #9 by Phil Babcock on December 7, 2010 - 7:32 am

    James :
    That’s a good question, I normally do not consider the aerodynamic advantage of the fins further up the rocket as I would imagine it would be a minimal payoff. I would normally do this to reduce the chance of the fins being damaged on landing, but it’s a trade off with the stability margin since you’re reducing it as you move the fins up typically (and then potentially adding more mass to the nose). I may be wrong but I doubt there is a huge benefit, but may be worth it for an altitude attempt. Right now I’m actually running a CFD analysis to check on this, if I have some decent results i’ll post them.

    Thanks, I will be interested to see how it turns out.

  10. #10 by James on December 13, 2010 - 11:11 pm

    I just added a page with the CFD analysis under the miscellaneous tab, check it out.

Comments are closed.