Cobbled together from German V2 missiles captured at the end of World War II and an early US sounding rocket, the Bumper WAC project was the first two-stage liquid propulsion rocket. First flown at the White Sands Missile Range in 1948, Bumper WAC became the first rocket to fly from Florida's Cape Canaveral on July 24, 1950.

This page will document the construction of a 1/26.5 scale model of the Bumper WAC rocket in preparation for the FAI 15th World Spacemodeling Championships, to be held at the Polish Air Force Academy in Deblin, Poland, in September, 2004. The model will be flown in the S5C Scale Altitude event, in which Tony Reynolds and I will both be flying Bumper WACs as we attempt to bring home a Gold Medal for the USA.

The current S5 rules greatly favor the Bumper WAC prototype, at least in theory, as it has a booster-to-sustainer diameter ratio of almost 6-1. This allows the modeler to meet the minimum diameter requirement of 50mm easily, while still maintaining a reasonably small sustainer diameter. The rules also specify a minimum overall length of 650mm, and the 1/26.5 scale model conveniently comes in at 651mm. (Download Excel Bumper WAC Dimensions.) Our rockets will lift off using a standard Estes B6 motor in the V2 booster, and will stage to a Delta B2 in the WAC sustainer. Or so we hope...

A major challenge will be weight management, as the rules specify a maximum weight of 150 grams, or about 5.3 ounces.

The fin mold was created by Tom Campbell, a friend and very gifted modeler. Tom engineered the masters and molds for the Bumper WAC models flown by the U.S. S5C team (Tom, Dr. Bob Kreutz, and myself) at the 2002 World Championships. The Bumper WAC has also been flown in FAI competition by Slovakian spacemodeler Jan Kohuta.

This pour was made with an experimental mixture of casting resin and micro-balloons, in an effort to keep weight down. Before trimming the excess resin, each fin weighs in at ~16 grams. That's too stinkin' heavy, unfortunately, so they'll need to be recast as a hollow piece.

A beautiful mandrel for the V2 air was machined for the project by Michael Herndon, a fellow member of the Austin Area Rocketry Group. Had Michael not stepped up to machine the mandrel for me, I would have been forced to go to a commercial supplier. I'm told that this would have been a VERY expensive component to have machined out in the "real world." Rocket people are great, huh?

Students of the V2 will note that no effort has been made to model the distinctive hammered appearance of the missile. Why? Because FAI Spacemodeling judges (a fairly joyless bunch) don't seem to like the look, and apparently want V2 models to appear "clean." Accuracy be dammed, then...

The mold box is a cut-down $5.99 plastic mailbox, courtesy of Home Depot, and the mold material is Synair 333, sourced from Bare l Foil. Given the volume of mold material needed, this turned out to be one of the more expensive parts of the project. So far, about three gallons of Synair 333 have been consumed, at the rate of $70 for each two-gallon set. This unpleasant expense is offset by the fact that beer in Poland is cheap, about .50￠ US.

Here's the finished mold for the forward V2/Bumper section. The pennies help align the mold halves during the casting process. My lovely wife, Sarah, would like to point out that I'm playing with this noxious mold goo atop her $1200 kitchen island. Ain't love grand?

Here's another view of the mold-making process, with a shot of the aft V2/Bumper mold still in the mold form.

Fresh out of the mold box, the aft V2/Bumper mold halves are shown with the mandrel.

Plater of Paris was poured into the silicone molds to create positive vacuum form mold halves.

The plaster mold halves had a few air bubbles, which were quickly fixed with a bit of filler putty. The vacuum form table is hooked up to a shop vac. A sheet of styrene plastic (.015" thick in this case) is secured in a styrene , then heated in a 375o oven. When the plastic gets droopy, the is placed on the vacuum table and the shop vac is turned on.

The warm plastic is sucked around the part by the vacuum pressure. Here's a view of a finished vacuformed part on the vacuum table. For a short video of the vacuform process, click here (1.8MB).

...and another view of a finished part. After the parts are trimmed, the two halves will be joined to create a finished part. Unlike fiberglass, the styrene parts are almost immediately ready to be painted.

Here are the finished forward V2 halves, ready to be trimmed and joined. Two complete sets of parts are being produced.