| . | IMEX V2 PMC | . |
| . | Construction | . |
The basic design parameters are set by the kit. The chief challenges are appropriate structural reinforcements and balancing to make the kit flight ready.
The kit is designed with a see-through half to allow display of the motor and fuel tanks. Consequently, the two body halves are different types of plastic. The clear side is noticably heaver and stiffer than the white side, and each side has slightly different response to glue. The basic approach chosen for structure was to use a LOC 29mm motor tube extending the full length of the rocket to carry the thrust load and add rigidity to the airframe. Plywood centering rings are used at four points to help maintain a circular profile. A coupler tube is used to create the separation point (LOC motor adapter MMA1). Because of this design, no chute wadding is required because no exhaust gasses enter the chute compartment.
The V2 was designed to carry a 2000 pound bomb in the nose, and has fins small enough to allow it to be transported on railroad cars through German railroad tunnels in WWII. Because of this, building an accurate reproduction always requires considerable nose weight.
A significant limiting design parameter is weight. NAR competition is limited to a maximum weight of 1500 grams, but the effective limit is the maximum liftoff weight for a G80: 1470 grams. For a rocket of this size, weight would not ordinarily be a problem, but plastic is considerably heavier than cardboard, and the V2 has its own special weight problems.
To maximize performance on a variety of engines while meeting the NAR
weight limit, a system of removable weights was designed. Oval lead fishing
weights are twisted onto a threaded rod which threads into a metal cap in the
tip of the nose. The threaded rod can be removed to change weights for flight
with G and H motors. Approximately 8 ounces of nose weight is required to
balance a G80.
The program VCP was used to experiment with a variety of weight configurations. Because of the approximations in the program, the computed CP is more accurate if the boat tail is not modeled. I created simulations both with and without the boat tail to see the difference. Since the root area of the fins is blocked from the relative wind by the rocket body, the fins do not have as much effect as the same size fins on a rocket with straight sides.
Accurate weight and placement information for all components is also critical for a reliable computation. A digital scale accurate to 1 gram was used to weigh the components throughout the assembly process to keep a running tally of stability and total weight as epoxy and other items are added to the airframe. These quantities are small and difficult to estimate, but have a significant effect on total weight.
Ordinarily, this is not much of a problem, but with a V2, every bit
of extra weight in the tail cone must be balanced off with additional
weight in the nose - four ounces of epoxy turns into a half-pound of
liftoff weight. A constant running watch on weight was necessary to
ensure the total weight wouldn't run over the limit.
You can see most of the materials and equipment used in the photo at right. Since this kit is manufactured using a vaccum molding process, all parts must be cut out from a large sheet of plastic. A Dremel tool with a cutting wheel was used to cut out the body halves. This could also be done with an X-acto saw. After cutting is complete, a good mating of the halves is achieved by careful sanding on a very large block. This is particularly critical near the nose where tolerances are close.
Fins were cut out using a very sharp x-acto knife and sanded to shape on
a large block. During this step, careful attention to the edge thickness is
critical to preservation of the fin's airfoil shape and proper fitting on the
airframe in later assembly.
Several different adhesive types were used during assembly. An epoxy designed
for joining wood was used to attach the plastic skins to the basswood fin cores.
90 minute epoxy was used to join the motor tube, fin cores, and bulkheads. A
special glue I have found only at Ace Hardware was used to assemble the airframe
halves and join the fins to the body. This glue is called "Plastic Bonder". It
performed very well on both the clear and white plastic, and sands like a dream.
I used some epoxy odds and ends to finish out some gluing on the airframe seam,
and one of these reacted with Rustoleum paint causing a slight orange peel effect
along the seam. This problem seemed to disappear after about a week of curing.
It may not have been a compatibility problem, but merely a curing problem resulting
from the immelsely deep layers of red oxide primer I used to hide the seam.
Scraps left over from cutting out the airframe were used to reinforce the
airframe seam. Of course, gaps must be left were the bulkheads will be placed.
Binder clips were used to clamp the strips on one half. The plywood bulkheads
were then glued into place on the motor tube, basswood roots for the fins were
glued into place, and the separation point was cut. Blind T-nuts were added to
the rear bulkhead to allow for motor retention. The inside of the airframe
halves must be sanded with coarse grit paper where each bulkhead will attach to
an outer wall. After drying, the two airframe halves were glued around this core
to form the basic airframe.
After the airframe assembly is dry, the separation
point can be cut in the body. A razor saw was used to make this cut just below
the upper main bulkhead. When the nose is separated from the body, another scrap
of plastic can be used to make a lip inside the seam to help keep the nose and
body aligned.
The kit includes a metal nose, shown at left, but this proved to be insufficiently
strong for landing. It was broken off on the first trial flight. The metal nose was
replaced by casting an entire nose of epoxy to protect the plastic at landing.
The epoxy nose was cast using Play-Dough as a mold. The basic nose shape was
created by poking the rocket into the Play-Dough, then appropriate enlargements
were made with a pencil to create the desired shape. After curing, the nose was
shaped with a dremel grinder to get the desired shape. This technique worked fairly
well, and I've used it on other rockets since then. The Play-Dough doesn't stick
to the epoxy, and its very easy to bring into shape with the Dremel and sandpaper.
The basswood fin cores will now serve as the fin attachment points.
The kit is designed with plastic ridges on the side of the rocket body
to which the fin skins attach. These attachement points were used as an
epoxy point to help strengthen the overall structure. I really can't imagine
how the fins skins alone would be strong enough to hold up even a display model if
the rocket were built for static display.
To ensure adequate rigidity, the entire fin must be filled with wood and epoxy. The core must be tapered to fit the airfoil shape of the fin skin. The plastic skin is very weak by itself, but forms a very tough and rigid structure once it is epoxied onto the wood core. Sanding the entire inside of the fin skin is essential for good adhesion. Again, binder clips were used to hold the skin tightly against the basswood cores while the epoxy cures.
After fin assembly is complete, plastic bonder was used as a joining material
and filler at the fin root. A section of aluminum pipe with a 3/8" ID was used as
the launch lug with a balsa stand-off to ensure clearance against the curved tube.
The model is finished in the scheme of V2 number 2, shown at right. Paint data was taken from Peter Alway's Rockets of the World.
Scale detailing of the kit fins do not match any of the White Sands rounds. The fin actutator covers are teardrop shaped, rather than rectilinear as shown in Always' drawings. There are also elliptical trailing edge pods that do not match any White Sands missions. It appears to be modeled after one of the German experimental rounds which carried antennas in pods that were used to cut-off the motor, but I'm still looking for detailed information to confirm this.
In any case, I left the inaccurate fin details alone and corrected the nose
to approximate the shape of round two. I like the accent of the pods, but I
do not model any Nazi V2s, so even if the issue of accuracy is settled, I won't
be painting subsequent rockets to model a Nazi round.
After completing basic shaping with plastic bonder, minor filling was done with Squadron putty, followed by a heavy coat of Rustoleum Red Oxide primer. I used Rustoleum paint for the entire project. The red oxide primer is particularly good for filling minor blemishes and can be sprayed on absurdly thick without worrying about runs.
After much sanding, the entire rocket was painted black, then masked to paint
the yellow areas. Low-tack painter's tape was used to mask off the areas to be
painted. (thanks to James Duffy for this tip). Brushwork was used to patch up
the inevitable oversprays.
After all was said and done, I'm satisfied with the project. There are finishing and scale problems that could be corrected, but the structure of the rocket is sound and I think it will fly for a long time. I have socked away another IMEX kit and I'll be tackling this again sometime with the lessons I learned the first time through. One thing I like about this approach is that it presents the gently tapered tailcone of the V2 in a way that just isn't accurately modeled by using nosecones and a section of body tube to recreate the outline.
The picture at right shows the V2 before I added the final touch of silver at
the nose and tailcone joints.