Level 3 Prototype

Notice the strakes, the long, thin "fins" that run the rocket length. There are aft strakes (higher) and forward strakes. The problem is that the strakes, in order to stay true to scale, have to run over and forward of the altimeter bay. That's because, there must be enough room above the bay to accommodate the parachute. The Level 3 will be more than 9' tall and 8" in diameter. The strakes, I thought, cannot simply be epoxied to the airframe surface. They would shred at maximum velocity. I thought that a long motor mount tube with 4 or 5 centering rings, would not only handle that aft fins, but also the aft third or so of the strakes mounted  through-the wall. The length of the motor tube would be limited by the need for enough room for the drogue parachute. I was also concerned that if I did a traditional construction, the structural integrity of the airframe may be compromised. That is, normally the fins would be mounted between centering rings down to the motor mount tube. Slots would be cut up the airframe to accommodate the fins as the entire fin and motor mount piece is inserted into the aft end of the airframe and epoxied into place. The very aft end of the airframe would be held together with let's say a large clamp (looks like a radiator hose clamp) until the epoxy was completely cured. In the case of the AGM-78, 3/4 of the airframe would have to be cut to deal with such a long fin set. So a prototype was need to develop a design that could be scaled up for the L3. In addition, working with RockSim, I would need to design and build a flyable, stable rocket that was not very heavy. If the usual fin material was used, usually birch ply, the fins alone would add 10 or more pounds to the L3. And I was only going to fly the L3 with a small M, the M1297W. So weight would be critical. I decided on something a bit unusual. I would use Blue Tube for the 4" body, but cut slots for the fins and strakes in sections. The aft fins would have there own slot for through-the-wall mounting. The aft third or so of the strake would be mounted the same way with 2 tabs for the prototype and three for the L3 to anchor the strake. The slot for the rest of the strake would have a base created by airframe "liners" (doublers) that were actually Blue Tube couplers placed inside the airframe starting at the end of the motor mount tube centering ring. The strake would "rest" in the slot all the way up the airframe ending with a small offset epoxied to the airframe. The strakes would be epoxied into those slots. With fillets of epoxy and filler running the length of the strakes, I thought the entire fin set would be strong enough to handle the launch stresses. To keep weight to a minimum, I planned to use a relatively new fin material made of a honeycomb of Nomex and fiberglass that weighed about 1/3 that of birch ply and maybe a little stronger. The material comes in several thicknesses. And to remain true to scale based on photos, I would use 1/4" for the strakes and 3/8" for the fins. Though I could probably use 1/4" for both strakes and fins. The photos I've seen show fins that are very thin and seem to have the same thickness throughout. But I can't really tell, and I can"t find anyone at Raytheon in Tucson who will. Depending on how the prototype flies, I might consider 3/8" thickness throughout for the L3. So, this portion of the site will show my progress with the prototype project.

I chose a .29 scale of the AGM-78 to allow for a 4" airframe.

Here's the RockSim file shown first without a motor:
 


Airframe


  Blue Tube

            


Blue Tube cut to length 43.5" (.29 scale).A blue tube coupler sanded to prep for insertion in the airframe. The 54" motor mount with three centering rings.

   The Motor Tube and Harness Attachment

   I'm just test fitting the centering rings on the phenolic 54mm moter tube. I'm also test fitting the Aero Pack motor retainer. The important thing here is that that aft centering ring will be touching the aft fins' tabs. There 
   has be enough room between the last ring and the motor retainer.


                                                                                                           


   Here are a couple of angles on the motor tube and centering rings all epoxy fillets completed. I marked the position of the motor retainer on the motor tube. Because of the length of the airframe, I have to permanently attach the
   the recovery harness. I will not be able to reach in and remove it. This rocket will use a K motor for it's test flight. With motor deploy at apogee, I'm expecting a jolt and the possibility of a zipper. So I'm using two   
   attachment points, and I'm using 1/2" Kevlar harnesses (over-kill) that will handle the deployment charge flame. They are tied in a loop and a 4" Fireball is attached. I'll begin with the attachment points. I'm using a 1/4" forged eye
   bolt and a 1" zinc plated U-bolt.

       

   Here is the U-bolt with a length of 1/2" Kevlar strap. Note that I've done the usual three wraps and a cinch knot. But I have added epoxy to the knot and the tag end of the Kevlar to keep it from fraying.

   I've done the same with the eye bolt. In another view from behind, I have added epoxy to lock the nuts.

        


          

Here's the Fireball set up to just sit half-way out of the airframe. That should be enough I hope to prevent a zipper. In the picture, I'm showing the Fireball, but I have not adjusted the harness to pull it into the airframe. Right, angled and aft view with centering rings epoxied with slow cure 30 minute 2-part hobby epoxy. The motor is flush and the aft ring that was temporary used to hold the motor tube in place has been removed in preparation for strake and fin placement.

      

                                                                                                                                                                                   
    


Fins and Strakes
July 2, 2010

After letting every thing sit for about a month,  I finally starting to think about the fins and strakes. Working off the RockSim file, I cut out a mock up using 1/4" foam poster board. When I was satisfied, I cut the strakes from a laminated honeycomb from Giant Leap Rocketry. It's made of a layer of Nomex sandwiched between two thin layers of fiberglass. The strake is 1/4" thick and the aft fins 3/8". If I made this from birch ply, the final rocket weight of the prototype would be too high. I had to think of a material that would also work for the L3, or its weight would be too high for a planned Aerotech M1297.

 
              

Below, I have placed a set of strakes and fins for fit. I'm doing a set for vertical alignment. (I did the same with the opposite after these were epoxied with slow cure 2-part hobby epoxy). I'm just testing out an idea for using #64 rubber bands to hold the strakes in place until the epoxy cures. I started
with the strakes and then vertically lined up the fins to them. I didn't use a fin guide for this. I just set up the slots horizontally using a level and then set up the first fins at 90 degrees. I can adjust the tension of the rubber bands on one side or another to hold a fin in place. Then, I can site in the vertical  position with a level and hold it right there for a few hours until the fin is pretty solid. I used regular 30 minute hobby epoxy for this job. Below center is a better view of the rubber band setup.

            

A finished set




Below is the AGM with fins epoxied in place. I mixed a batch of PML's 2 part (1:1) hard foam. It has completely filled the aft cavity. In just a few hours, this is rock hard. Now I have to begin what I think is fun and others find brutal. I get out the Dremel and start cutting out all the excess foam right down to the fin tabs. I have to be able to get that last centering ring in and still have room for a perfect motor retainer fit.


          



Here is what I end up after cutting wheels and little drum sanders. It's very clean. Next is the aft centering ring. First, I'll test fit the motor retainer and confirm I have the room I planned for the ring. Perfect.

       

Here is the aft ring in place with the retainer ready for epoxy. I used West Systems for this. J-B Weld was used for the retainer.
   
  

The nose cone

The PML 4" cone shape is perfect for the AGM-78 prototype. The cone needs to accommodate enough lead shot for proper CG/CP. This cone will have a final weight of 20-22oz. But the bulkhead is removable so additional weight can be added.  Here is the sequence of construction of the nose cone.

The nose cone's shoulder is cut off to accommodate the bulkheads. I made bulkheads from the leftover pieces of the 54mm centering rings I made for this prototype. The band sander is used to clean these up for the best fit into the cone shoulder. The holes in the bulkhead are enlarged to accommodate the 1/4" all thread rod that will eventually be epoxied into the nose cone. Here the bulkhead is held in place with 1/4" x 1 1/4" fender washers and 1/4" nuts.

              

                                                                                                                                                                                    
Next I would normally scuff the inside of the cone (80 grit), add 3-4 oz of 2-part hobby epoxy and push in the entire inside bulkhead and all thread. The next day I would move on to these next steps. But, I'm just test fitting for now.

In the center,  I've placed a 1/4" x 1 1/4" fender washer over the nut holding the bulkhead in place. You could add a number of nuts to create space for whatever amount of lead shot needed for balance. The second bulkhead is placed over the washer. This bulkhead's hole is drilled for a slightly loser fit over the all thread and the bulkhead itself is very slightly lose fitting inside the cone shoulder. This is for easy removal later if more or less shot is necessary. Another washer and nut hold it all in place.


       

Finally I attach the 1/4" all thread coupler and a 1/4" forged eye bolt. The holes for the nose cone shear pins have to be drilled to deal with the position of the bulkheads.



Good to go. Now I can go back and epoxy the all thread into the cone.

Continuing with Strakes and Fins
July 24 and 25, 2010

I've been impressed by just how time consuming the project is. The strakes are clearly most difficult as each cap has its own mitering. After working on the L3 for weeks now, I'm coming back to the prototype for more ideas. The L3 has been glassed, and fins and strakes can't be far behind. So, based on my experience with the prototype,  L3  fins will be capped with 3/8" x 3/8" birch. These will be sanded down to create sharp leading and trailing edges. Only the leading edges of the L3 strakes would be 3/8" x 3/8". They can be shaped for aerodynamics. However, I'm going to just cap the tops and aft edges of the L3 strakes with 3/8" x 1/8" birch.

Here is more work on the prototype in the past few days. I've found it easier to use a combination of soft clamps and rubber bands to hold the strake caps in place while the epoxy cures. I've also found that I can only epoxy only a couple of mitered pieces at a time. Note the leading edges of the strakes below. The close-up shows how the lead allows air to flow over the cap and over the trailing edge. This design should minimize forces that could pull the pieces apart during maximum acceleration. However, the real challenge is to tapper the leading edge of the lower strake into the cap of that strake, then taper the leading edge of the upper strake into the cap of that strake.


    


   

So far so good

I completed all four strakes and moved on to the easier fins. The fins receive birch caps that are 1/8" x 3/8" on the top and trailing edges and 3/8" square for the leading edges.

August 8, 2010

I sanded all edges with 80 grit paper using a palm sander, then 120 grit by hand. I also used the 80 on the strakes and fins simply to remove dried epoxy that dripped and hardened during the cap gluing.I'm very careful not to overdo it. It's easy to sand right through the thin fiberglass. The close ups are simply to show the edges and the flow of leading edges up and over the others to avoid cap shredding under aerodynamic stress. I consider this the "first pass" with the fins. I'll go ahead and fillet the strakes and fins using West Systems 105/206/404 high density filler. I'll go back and fill any gaps between the fins/strakes and their caps with white Squadron putty and sand with 150-220 grit. The prototype will fly with only one coat of automotive gray primer/filler.


              


          

Fin fillets and Fin Finishing
August 9 and 10, 2010

With the first pass sanding of the fins and strakes, it's time to do the fillets.  I've stretched 1/2" blue masking tape the length required to leave a few inches aft of the airframe and forward of the strake. From a different angle, I have leveled the rocket so that epoxy will generally stay in place once applied. I mask the shape of the most forward end of the fillet for a shape that will best blend into the strake. I also mast the trailing edge of the fin. There is a gap between the fins and strakes. That doesn't matter. I'll create a dam on the untapped side to catch extra epoxy. When it gets a little tacky, I can pull off the dam and remove the excess creating a defined gap.  I try to keep my setup pretty clean. This can get messy or not. You can see in the picture below that the frame is rolled for a symmetrically shaped fillet. For each of the fillets,  I'll use one pump each of 105/206 West Systems with 1 1/2 teaspoons of 404 high density filler. I use a tongue depressor sized stick to mix this all to the consistency of thick syrup. I spread just enough of the syrup to fill  the valley defined by the tape. I then drag the tip of one of the plastic spoons the length of the tap plowing excess epoxy onto the tape. In seconds,  I'll pull off the masks at each end of the fillet then pull each of the long tapes up and away from the frame in one steady step.


                             


               


 After doing a couple of fillets. They will be cured sufficiently in a few hours to roll the airframe and do the rest, all in about 2 days. Then, I can go on to filling some of the small defects where the caps meet the fins. Here,  I'm using green Squadron putty. After it all dries,  I'll palm sand with 220 grit.


                  




Ready to prime



Primed white and loaded with the Cesaroni K445 delay 11 seconds (RockSim) pad rail ready




Pad ready weight is 9.6 pounds theoretical, actual 10 pounds
There are 39 oz of lead shout in the nose
CG theoretical is 32.4 inches from nose, actual 31.5.
Spherachute 84" should produce a descent rate of ~15.5'/sec






   


  

  



Flight #1
August 14, 2010 Rainbow Valley, AZ

The ignition of the Cesaroni K445 was instant. This was my first K motor launch. After some minor wobble off the 8' rail, the rocket quickly stabilized with a straight  flight without any apparent roll. At 11 seconds into the flight, the motor deploy worked perfectly, and the Sperachute 84" main opened without incident. The wobble was predicted by RockSim at launch winds of 8-15 mph as were the conditions at launch. The nose cone was not apparent however. Upon recover, there was no nose cone. Only the 1/4' eye bolt was attached to its Quick-Link with threads undamaged. The nose cone weighed 2 pounds, 6 ounces. The cone had a 1/4' all thread embedded in epoxy. The all thead was coupled to the eye bolt. It's probable that the coupling was not sufficiently tight to prevent the unscrewing of the eye bolt. There was minor cosmetic damage to two fin tips when the rocket was dragged along the desert by a full chute and ~15 mph ground winds. Thanks for a great photo Moe!! Are they mach diamonds in that exhaust flame?

Flight #2
September 11, 2010
Rainbow Valley, AZ
Cesaroni K445

         

Two successful motor deploys. The design is stable. Time to finish the Level 3. Still on target for October 23, 2011

Flight #3 Full paint scheme
May 14, 2010
Rainbow Valley, AZ
Cesaroni K500 RL





Successful flight.