Corporate Colors


This is the airframe Bdale built for his second attempt at hitting Mach 3. This is a step along a learning path building confidence before trying to assemble a minimum-diameter airframe to fly a CTI O3400 someday.

The first attempt was made with a very similar airframe that was assembled entirely using West System epoxy, including the fin can tip to tip laminations that were all done in a single vacuum bagging operation. That airframe flew at Airfest in 2016 on a CTI M2245, and was at about Mach 2.91 when the fin can came apart. Post-flight analysis suggested the leading edge of the tip to tip laminations got hot enough for the epoxy to soften allowing the carbon fiber to be ripped off... West System is and will continue to be my go-to epoxy for normal airframe builds, and worked great on a previous project that got to Mach 2.21... but with a glass transition temperature of 129-142 F, it's just not up to the challenge of staying together above Mach 3!

So, for this build, the plan was to use the same design and build techniques, but switch to one of the Cotronics high-temperature epoxies. Because high temperature epoxy is seen as expensive, others have talked about using lesser epoxy for the bulk of the fin build-up laminations and then just using Cotronics as a top coat, or the thicker version to build up leading edges. But it seemed to me that using the lower viscosity type and staying with the same build approach would both be the easiest way to go, and from a learning perspective the idea of "change only one variable at a time" really appealed to me. After studying the options, I chose Duralco 4461 which is supposed to be good to 500 F with a suitable post-cure. A pint kit with shipping cost me nearly $130, but I used much less than half the kit building this airframe. So, in the grand scheme of things, it's not that expensive. I just need to make another fin can or two with it before the shelf life expires!

Design Details

This is basically a "3 fins and a nose cone" design, using a single 5 foot length of filament wound fiberglass airframe, a filament wound nose cone with aluminum tip, and plywood fin cores covered with tip to tip carbon fiber.

Due to the CTI M2245 reload that was used for the first attempt not being available for a while, the M3464 Loki Blue from Scott Kormeier at Loki Research was chosen to power this attempt.

The fins were made using high quality 1/8" Baltic birch plywood cores glued into slots milled in the airframe tube, then 3 layers of 5.8 oz 2x2 twill carbon fiber were laminated "tip to tip" across the airframe through each valley. My normal peel-ply and breather were used, and the entire fin can was laminated and vacuum bagged in one operation to yield a full chemical bond across all fin edges.

Actually, the inner carbon fiber layer was cut smaller to not go all the way to the fin edges, the outer two were big enough to go past the edges slightly to allow for sanding back to the final fin shape after initial cure. The middle layer was rotated 45 degrees from the inner and outer layers, giving us fibers in 4 directions.

The OpenRocket design file is CorporateCollors.ork, and that design file plus all content on this page are released under the Creative Commons Attribution-ShareAlike 4.0 International license.




I've put all the build photos I took together in one place.

Result and Lessons Learned

The airframe flew on 8 July 2017 in Argonia, Kansas, at a Fun Fly hosted by the Kloudbusters at their rocket pasture. The motor was a Loki M3464 Blue, and everything performed perfectly until apogee. The maximum velocity was 1047 m/s, or right at Mach 3.1, on the way to an apogee of 32,635 feet above ground.

Unfortunately, while telemetry shows the electronics correctly fired the apogee ejection charge, clearly the nose cone did not successfully separate. The resulting ballistic return impacted about 1.1 miles down range to the south-south-west, at 37 9.1739 N, 97 44.8809 West.

With the final telemetry frame received from about 200m altitude on the way down, we know impact was at about 2/3 Mach. Not surprisingly, then, what we found in the middle of the wheat stubble was a 3" diameter hole with 3 slots radiating outward, and quite a bit of visible purple paint on the sides of the hole. Probing with a shovel handle, we learned the aft end of the airframe was on the order of 18 inches below ground level, and the aft end of the motor nozzle was at least 4 feet down! Curious to know the fate of the fin can that was the focus of this project, we took turns shoveling until the fin can was sufficiently exposed to reveal two perfectly intact fins and the third sheared off by impact with a fist-sized rock several inches below ground level. Given the heat, and lacking either a backhoe or an army with shovels, after taking a bunch of photos and logging GPS coordinates, the decision was made to abandon recovery and just fill in the hole.

So, two big lessons learned.

Yes, Bdale can build a fin can that can survive Mach 3!

Getting so focused on one part of the project that you forget things
you know you should do elsewhere to ensure success leads to loss...

What I mean by the second is that while this is the first time I've personally put an airframe above 30,000 feet... I've hung around other people who do it successfully, and I've listened to details of what they did. In hindsight, I "coulda, shoulda, woulda" put more attention on the apogee ejection event. More black powder in the charge. More confinement to allow more of the powder to burn before being dispersed in the lower-pressure environment at altitude. Using one of the spare TeleMega channels to fire an up-sized backup charge. Flying a TeleMetrum for full ejection event redundancy instead of just a TeleGPS for redundant tracking. But I didn't do any of those things, and lost the airframe and everything in it as a result. Yep, lesson learned!