Carl,Since your tanks were also part of the airframe, how did you handle the tank end caps? What was the shape and were they welded? Also, did you route cables and feedlines through the tanks or around the outside?
-Bob On 10/25/2013 02:04 PM, Carl Tedesco wrote:
Sent from my iPhoneOn Oct 24, 2013, at 8:47 PM, Robert Watzlavick <rocket@xxxxxxxxxxxxxx <mailto:rocket@xxxxxxxxxxxxxx>> wrote:Carl,Thanks for the info and link to Richard's page. I haven't decided on the exact coupler configuration yet so there's still room to improve the design. I assume one "caliber" means one diameter?Yes, one caliber = one diameter.I saw that on many other HPR websites where the construction materials were composites or fiberglass. Some of them even suggested two or three diameters. I can't believe that you would need a 5-10 inch long coupler to hold two 5 inch diameter tube sections together.In hindsight, they may be talking about the sections that separate for parachute deployment, since these sections usually have 2 to 4 nylon shear screws. The one caliber rule insures the section stays put during high speed flight.An airplane fuselage is many feet in diameter and they usually only have a single frame section with a few inches on either side. Of course there stringer section running the length of the airplane.Ours was about 2-3 inch long coupler for our 8-inch diameter airframe. If I do it this way again I would probably use two rows of fasteners offset; still the same number of fasteners.I can envision alignment issues though, especially if the ends aren't completely round and the fastener holes aren't drilled perfectly square to the ends. I was hoping to mitigate some of that by making the coupler a bit thicker than needed so it wouldn't deform as everything is bolted together.Was your fuselage skin also the pressure tank?Yes, the pressurized tanks were also the airframe. ---CarlI had to go with 16 #10-32 screws on the 5 inch diameter to keep from exceeding the bearing allowable stress on the skin. With a conservative 2.0 factor of safety, the tank end caps have to withstand around 18000 lbf each (which works out to about 1100 lbf per #10 fastener in shear).-Bob On 10/23/2013 02:17 PM, Carl Tedesco wrote:Bob, Just getting to this post...Here are some comments on the couplers on our SDSU rocket <http://eon.sdsu.edu/%7Erocket/>:1. Our SDSU rocket was 8" diameter, 0.072" wall. 2. The coupler was made from the same tube; we slit the tube longitudinally to remove enough so that it could be fit into the original 8" tubing. 3. We riveted the coupling to the airframe/tank tube for sections that we did not intend to seperate. 4. For sections we wanted to separate we used twenty (20) 8-32 screws. We had a few screws strip in the thin 0.072" wall, so your method with inserts is appealing. 5. Our big problem was alignment of airframe sections, which many on this list gave some good tips. 6. Picture of our inter tank adapter here <http://4.bp.blogspot.com/-y9c5oqkLSw4/URW_PgeNrnI/AAAAAAAAAFc/Wpq8c8LMobA/s320/Rocket+project+1.jpg> 7. An HPR guy once told me to make the coupler insertion length at least one caliber. We did not follow that advice and hence could have contributed to our alignment issues given the tolerances of our "rolled" couplers.Why could you not use your "inner sleeve" as the coupling tube? Seems like one extra part.Richard Nakka has a nice little webpage on calculating loads on your airframe here <http://www.nakka-rocketry.net/fusestru.html>. It doesn't address loads at if your rocket has any angle-of-attack, but it's still a nice guide.--- Carl
