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CFTC2 - Clear FTC #2

Using the experiences of the past backgliding rockets, the next one should be built more by calculated design than by trial and error. A neighbour's friend brought me some 31 mm diameter clear FTCs and a 21.3 mm white PVC pipe as an ideal launch tube from USA. Thanks, Markus Schäufele, for all the trouble you had!

 I built 2 Water rockets from it: CFTC1 with a payload compartment for an altimeter in the future and CFTC2, a plain rocket, described here.

The rocket top has a simple internal bulkhead. As usual, bottle necks are used as a bulkhead. The flange, being greater than the internal diameter of the FTC, had to be grinded down a little to make a tight fit with the FTC. It's easy with a lathe, but can be done manually as well.

Glue: I am still testing. Here I used "Bison Polyurethane Holzleim für aussen" (=wood glue for outdoors), a honeylike substance. I put a drop of water into the glue, stirred thoroughly, and applied it. It foams nicely, generating a yellowish foam. Fast cure time due to the added water.

CFTC2 nosecone assembledCFTC2 nosecone with tape 

Then, a plain bottle cap or one with a selfmade nosecone ( as on the pics) is screwed on. The small remaining gap can be closed with tape

CFTC2 fin assembly

The rocket nozzle is an EXTERNAL bulkhead - I need the flange for the Clark type launcher. I used a CA type of glue with special polyolefin primer. It can be used only on tight fits like here.

The laminated cardboard fins are not glued to the FTC directly, but to a movable sleeve, made of a scrap piece of FTC. The sleeve is taped in place.


Backglide stability

Getting the BCP far back of the CG is no problem with this skinny rockets.

Using my CLA calculator, I tried at first to calculate a position of the finned sleeve, where the rocket would backglide without weight in the back, by moving the sleeve forward. I could not find a position with theoretical backglide stability this way: the CG did move forward as well as the CLA, the distance CG-CLA was never sufficient. Since FTCs are so rare here in Germany, I do believe in my calculations and did no test launch with this attractive weightless method. However, this could eventually work with 3 fins only and less and/or lighter fin glue.

Next, I adjusted fin size and back weight. The distance CG-CLA was attuned to 45 mm on CFTC1 and 50 mm on CFTC2 = ca. 1,5 calibers.

Then came the great moment:
Would the test launches in the field prove these calculations?

The answer:
both these rockets backglided well, directly from the work bench.
This was a good feeling: calculate it, build it, and it works! (this is not always so ...).

See a launch report with a video of CFTC2 here.




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