Monday, June 27, 2016

Baby Chesapeake Sea Monster, the good, the bad and the ugly.

Despite the fact that the Chesapeake Sea Monster has remained unproven for around 9 months, I got the crazy idea to build a small one. With no other builds on the bench, this ate on my brain like a bug from outer space. I started liking A10's again after flying the finless, ducted Cosmic Top Secret beta test rocket. So, I started planning a proper build, with nice BT-55 parts from FlisKits: Balsa nose and tail cones, mini Tres mount and pre-cut 13mm mounts.  It was getting too pricey so I decided to see what I had on hand.

I found a piece of Apogee thin wall 29mm tubing, a nose cone that fit, and a section of BT-5...I ran with it. This was a fast real-time design/build. I found enough scrap balsa for all the fins. The rear spar is 1/4x1/4 basswood, also a leftover. Some of the dimensions are scaled from its big brother, some aren't. My canted mount isn't canted as much a Tres mount since three 13mm tubes really need a BT-55 or there abouts. The mount is not perfect. I shaped the basswood spar to fit in the center of the canted motor mount. It was aligned by eyeball. Looked perfect when first installed, less so once the fins were getting installed. It slopes down just a bit so I adjusted the rear stabilizer to keep the rear wing parallel to the airflow. The side fins were attached during the Game of Thrones season finale and also aligned by eyeball.

Inside, I used a motor block from a PS-II kit to further support the 13mm mounts. A Kevlar leader is attached via the epoxy clay that I used to plug the gaps around the motor mounts.

My first stab at nose weight, with the three canted mounts loaded, brings the CG to just in front of the main wings, just like the big one. I wanted it stable without any of the forward mounts being used. It will get a swing test before it flies.

I haven't ever needed to look at the thrust curve of an A10 and was surprised at what I found. As expected, an A10 has a big initial kick of over 12 Ns but then settles quickly to ~1.5 Ns for about 3/4 of the burn. They are actually an A2.35.

This build weighs in at ~6 oz with three A10's. Will the A10's be enough? If you use the actual average, the answer would be 'NO'. If you use the initial thrust, the answer is 'YES!'  Will a 36 Ns spike rip it apart? How about 60 Ns? Or 108 Ns? What about those A3's? I'll have to sleep on it.

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