The L3 Armageddon 

 Doc's Level 3 Certification Rocket Project

This page last updated on 01/15/2006

Note: Picture is shown with motor loaded but CG is shown without motor. CP is actual.

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Materials used:

All surfaces were joined using West Systems epoxy, Jet Glue (Cyanoacrylate)  or Titebond (Aliphatic Resin) wood glue where indicated. Fiberglass used was 6 oz.

Figure 1      

     Figure 2

Figure 2

Figure 3

Securing the MMT and centering rings into the booster section:

     The MMT assembly was constructed per the instruction. The hardware was tightened and secured with epoxy.

     The motor mount tube was epoxyied into the airframe by pouring 25 mL of epoxy onto each of the centering rings through the fin slots with a syringe (see fig 2). The assembly was first epoxied with the aft section up, to apply epoxy to the aft sides of the centering rings. Once the epoxy was cured, it was examined with a fiber optic bore scope. There were two locations that appeared not to be glued. Epoxy was again inserted into the airframe through the fin slots and the assembly was tilted in the direction of the questionable areas to allow the epoxy to flow into the area. When it appeared to be complete, the assembly was turned over and the aforementioned process was repeated to apply epoxy on the forward side of the centering rings.

     No fiberglass was used for the airframe of this rocket. The 5.5” tube used in the construction of this kit was tested in my laboratory and exhibited a strength of 1312.7 lbs-force before yielding. However, if any side forces are applied during acceleration we must examine column buckling loads. Column buckling formula are fun and exciting. But to test the column buckling in real life, a piece of 5.5” airframe was tested by compression at a 8.5 degree angle. The force at yield was 886.4 lbs force.  

     Compressive and column buckling forces do not apply to the MMT/sustainer section due to the reinforcement by the fins and the MMT assembly. We therefore only need to look at the section forward of the fins. The forward section, (with weights) including altimeter weighs ~5 pounds.  The Hypertek M740 will exert a g-force of 23.16. Thus: 5 x 23.16 = 115 lbs. this is the maximum force applied to the airframe in an un-reinforced area.  Inversely, we can surmise that we have a tube capable of withstanding 38.27 g's (886.4 /23.16 lbs = 38.27)

              Figure 4      

Figure 5

Figure 6

Figure 7

     Compressive forces can be seen in the fin area during accent as well as landing, however as heretofore mentioned, most of this force is transferred to the CR's and mmt tube via the epoxy and thus its strength depends on the shear force of the epoxy. Therefore, there are virtually no axial forces on the tube at in this area. Virtually all of the forces on the tube itself are circumferential via any torque forces applied during flight or bending moment applied on the fins during landing. 

     To combat any bending moment applied to the fins, I looked at data seen in the RMR HPR Strength of Materials Test. One of the discoveries made during the testing was on the fin-cans. The fin can that appears to be the strongest was one done by Mark Simpson, see figure 5. He used a strip of wood material along the fin body joint. I used this method on the ID. Marks method increased the strength of the joint over the next highest strength by 30%, and by 200% over the average. It also prevented the tube from yielding at all during the fin-can test, and thus transferred the force to the fin making the joint stronger than the fin. I have no doubt that if the fin material was stronger, the value would have been higher. This appears to be sufficient to replace the fiberglass on the tube in this area. 

Fin assembly 

    I duplicated Mark Simpson's wood fillets in the L3 Armageddon as seen in figure 6. Pieces of corner molding were used for this. The fins were epoxied in place, then the molding was inserted with copious amounts of epoxy. The aft CR was then epoxied into place and the nozzle fins were added.

     The large fins of this rocket are only ¼” thick. To lessen the effect of possible fin flutter, a laminate of fiberglass was applied to the surface of the fins. This was accomplished using standard fiberglass lay-up practices as seen in figure 7.

Note: the nose cone, altimeter bay and the parachutes are for illustration purposes only. They are not the ones used for the rocket. J

     Recovery will be the typical two stage method: drogue at apogee, and main at 500'. One drawback with this kit is that the recovery for the main is aft of the payload section and the drogue is forward, or so it would appear. The section forward of the altimeter is only 11" and has one attach point. The section aft of the altimeter is 29" and has the two attach points at the MMT. There is just not enough room forward of the altimeter bay to comfortably fit the main. 

     All shock cords are 9/19 tubular nylon. The drogue cord is 20' in length. The main consists of three cords. There is a 3-point attachment cord at the mmt attached  to a 20' length of cord at which the parachute is attached. A 30' cord is then attached from the parachute to the altimeter bay.

     I didn't do much in the way of aerodynamic shaping of the fins. "Air foiling" the fins is done to reduce drag for greater altitudes. (but I like them low and slow) A more blunt leading edge increases drag by breaking up the air over the fins.

Figure 18

     Motor retention! Kind of late to be thinking about this isn't it? I did put  blind nuts in the aft centering ring during assembly. The 6" long all-thread looked, well, kind of clumsy.  I happened to pick up a 75mm Aeropack retaining system so I thought I would use it. It was hell trying to put it on after the nozzle fins were already glued on! I used a Dremel saw to cut a gap between the fins and the MMT long enough to insert the retainer. (Don't try this at home!) It made more work for me than expected! The retainer was glued in with JB Weld and the gap between the fins and the retainer was filled with wood putty. OY!

Rail Buttons

Figure 19

Motor Data