Little Joe booster undergoing testing at North American Aviation, Los Angeles, California
Boilerplate Development
A simple boilerplate version of the Little Joe model is being constructed to develop prep procedures and to test flight characteristics. As noted previously, the scale of this particular model is dictated by the selected airframe material, a 4.476" outside diameter tube sourced from Totally Tubular. Once a styrene fascia layer is added to the tube, the O.D. will be in the neighborhood of 4.59". Laser cut centering rings for the final model have been sourced from Balsa Machining Service, while the rings on the bolerplate were hacked out of foamcore board. At this point, plans call for the model to fly on a cluster of 4x 24mm D12 and 4x 13mm A10 motors. That may change before construction of the final airframe begins...
A simple simulated Marman clamp is added to the transition shroud of the boilerplate booster using Tenax, followed by a quick test fit of the capsule. (Be sure to click on the Marman clamp link above; I was fascinated to learn who the inventor was!)
After drawing the fin pattern in Vectorworks, the pattern is used to cut boilerplate fin sides out of 3/32" balsa. While the boilerplate fins lack the detail that will be found on the final model, every effort is being made to duplicate the mass and aerodynamic profile of the final part.
With the fins bolted into place we can put the boilerplate on it's feet for the first time! Just for grins I stacked the capsule, escape tower, and escape motor parts to see how things look. Note that the tower components are not glued but simply stacked at this point, so they might look a bit wonky to the critical eye. I also checked the overall length of the entire stack, and the dimension is accurate to within 2mm.
The components were also weighed, as the FAI S7 event has a maximum weight limit of 1 kilogram (2.2 lbs). We're currently at 283 grams, which leaves us a generous margin for motors, paint, parachutes, and whatever else I've forgotten.
Cluster whip
The Little Joe boilerplate is designed to fly on a cluster of four Estes D12-3 motors, so a cluster whip will be required for ignition. After stripping the wires, connecting the clips, and painting flux onto the area to be soldered, I asked myself "is this the soldering iron I blew out in the Czech Republic when I connected it to a 240V outlet?" Sure enough, it was. (Hey, I was an Asian Studies major, not an engineer, okay?) Time to head to Fry's for a replacement soldering iron, it seems.
Well, the trip to Fry's ended in success, and the cluster whip is complete. There are matched, color-coded pairs of leads for each motor in the Little Joe cluster. The other end of the leads terminate in bare wire tinned with solder; I'll put some sort of fancy connector on there someday.
Astute viewers will note that there are five pairs of igniter leads, not four. Hmmm...
Boilerplate details
With the cluster whip project temporarily sidelined, we'll move on to some of the minor details required to fly the boilerplate. First, a pair of 1/4" launch lugs are glued into place and aligned using a launch rod as an alignment guide. Next, we turn to the capsule and tower assembly. I have a bad habit of scorching parachutes, especially when they're packed into tiny spaces. For that reason I like to create "cold compartments" for recovery systems, eliminating the need for recovery wadding. This has the added benefit of speeding prep time, something that can be critical under the time constraints presented by FAI competition. The component to the right of the capsule is what we'll call the "recovery spindle." The recovery spindle will slide into the capsule core tube with the parachutes held in the cold compartment between the two centering rings. At ejection, the recovery spindle will slide out of the capsule core tube like a piston, with the parachutes safely protected from ejection gasses. (I must credit Tom Campbell with giving me this idea, as he used a similar approach on his 2000 and 2002 Bumper WAC models.) The tower assembly will be glued to the top of the recovery spindle.
These two horribly composed, ill-exposed, and out-of-focus photographs show the completed tower before and after the addition of the recovery spindle. The parachutes for the tower and capsule will wrap around the spindle, then the entire assembly will be inserted into top of the capsule. All of the parts used for the boilerplate are "reject" parts created during the fabrication process.
Stacking the parts
With the tower and capsule complete, we're able to stack the complete stack again, and actually have everything aligned and connected. A quick check of the overall height of the model shows that our error is less than 2mm, or less than 1%! The FAI rules allow maximum points for each major dimension that falls below the 1% error threshold.
It finally flies!
After putting the inevitable off for several months, the LJ boilerplate finally flew on August 20. The flight can be characterized as a partial success (which is, ironically, exactly in line with the results of the actual LJ-1A flight in November '59). The model boosted flawlessly, moving out well on the cluster of 4 Estes D12 motors. Unfortunately, I used an insufficient quantity of "dog barf" recovery wadding, and the nylon chute melted into a lump. Amazingly, the booster only snapped a couple of fins (which are still intact), and should be useful for future development work. The capsule managed to catch a small thermal, and now resides in the top of an oak tree. In spite of these annoyances, the test flight established that the basic design of the model is sound. (Rocketry enthusiasts will note the liberal interpretation of the term "blast deflector" in the above photo.)
A short video of the flight is available here (2.2MB, QuickTime 7 is required). This was taped using my whiz-bang new high-definition video camera, and the footage is fascinating to watch at full resolution; the igniter plugs can actually be seen bouncing around after ignition.
Back to part 1: Capsule assembly...
Back to part 2: Escape Tower assembly...
Back to part 3: Transition assembly...
On to part 6: Airframe assembly...
On to part 7: Paint and Markings...
On to part 8: Motor Mount assembly...