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 129-242 F,
-it's just not up to the challenge of staying together above Mach 3!
-
-So, for this build, the plan was to use essentially the same design, but
-switch to one of the Cotronics high-temperature epoxies. 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 for all of the
-fin can laminations might be the easiest way to go. After studying the
+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](http://cotronics.com) 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](https://www.cotronics.com/vo/cotr/pdf/4461.pdf)
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.
+expensive. I just need to make another fin can or two with it before the
+shelf life expires!
## Design Details
-This is basically a "2 fins and a nose cone" design, using a single 5 foot
-length of filament would fiberglass airframe, a filament would nose cone
-with aluminum tip, and plywood fins covered with tip to tip carbon fiber.
+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](http://lokiresearch.com) 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
## 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. 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.
+
+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!
+
projects / web / gag.com / blobdiff