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.
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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. |
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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 |
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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.
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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.
Stand / Last Revision: 19.10.02