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Loaded 45 lbs CG 80 inch CP 117 inch stability 10.5 caliber |
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Empty 27 lbs CG 76 inch CP 117 inch stability 11.6 caliber |
Motor N-1000 10 second burn time 15 lbs N2O (approx) 2.5 lbs PVC fuel grain (approx) *numbers are approximate |
The Nitrous tank, combustion chamber, and payload section are comprised of 3.25" OD by 3.00" ID 6061 Aluminum tube. There was nothing unusual about cutting the aluminum tubes, with the possible exception of squaring the ends. The ends of the tubes need to be close to square, but not perfect since aluminum couplers and screws will be used to connect the parts.
Squaring the ends of the tubes |
The tubes were squared by hand using a file, a flat plate, and 4 angles. Although this seems tedious the work progressed quickly and required less than an hour to square 6 tube ends. |
Polishing the tubes |
Polishing the tubes, or as I like to call it.. one more example why women live longer than men. This is a picture of a 66" tube on a lathe half as large, with additional support provided by a steady rest clamped to a drill press, and also supported by a JawStand. |
Cleaning the Nitrous Tank |
Cleaning the Nitrous tank with Acetone. |
While making the nosecone, my wood lathe seized. When I removed the motor and cracked open the motor case, ALL the dark sawdust, seen in the foreground of this picture, fell out of the poor guy. The sawdust must represent years of accumulation. I cannot imagine how the lathe worked at all. In a typical Tim Allen style move, I bought a replacement motor with MORE POWER. It didn't fit of course, but after an all-nighter bending parts and removing some of the metal case, I was back in business.
The rough shape of each fin is cut from sheet aluminum on a bandsaw. In the future I must remember to cut the fin from interior aluminum only. After the fin can was completed I noticed the factory cut edge of the aluminum sheet was a little rough. The edge wasn't bad enough to warrant remaking any of the parts.
Just like the old recommendation in the instructions in Estes kits, the fins are clamped together and ground until they are identical. Grinding the fins using a belt sander was simple, quick, and easy.
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This is a photo after grinding one side of the fin. The results are perfect! |
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Next a section of 3/4 x 3/4 x 1/8 aluminum angle is cut. One leg of the angle is shorted by 1/2". Holes are drilled, the leading edge is angled, and the fin and angle are screwed together. |
The final step before mounting the fins to the fin can is polishing the parts. A polishing wheel and red iron oxide is used. After polishing, the fins have a surface almost as perfect as a bathroom mirror. Below are before and after pictures.
Next the fins are mounted to the fin can.
The radial location of the fin is determined using a piece of wrap-around paper, a ruler, and a diamond glass cutter to mark the location on the aluminum. Using magnifier glasses helps a bunch to get good measurements and good markings. After the locations of the fins are marked on the fin can, a protractor provides a quick sanity check.
A fin mounting jig was cut from high quality furniture grade wood. This fin mounting jig is so simple to make that I normally just throw-away the jig when Im finished. The results are excellent. (I need to write a how-to page for the fin mounting jig.. coming soon..)
The fins are mounted one at a time. I used JBWeld metal based epoxy to attach the fins to the fin can. In tests I was able to put opposing fins on blocks of wood and stand on the middle of the fin can without the part bending or breaking. The aluminum angles provide a lot of surface area for the metal based epoxy to grip. If this rocket were to exceed mach 2, I think a better attachment protocol would be needed, but this design should work fine for Solus-N.
The radial position has been marked, the fin jig creates perpendicular alignment, and then I use my Photoshop Alignment Method to make sure everything is perfect before the JBWeld sets. The Photoshop Alignment Method is described here.
The injector is a 5 port injector in a Urbanski-Colburn (UC) configuration. Four of the five ports are plugged, and the remining port doubles as a fill tube for the motor. At ignition, the plugs and Nylon fill tube burn-though, allowing theNitrous to flow into the combustion chamber. See the photos in the Ignitor and Pre-heater section for more details.
The injector face and back plate were machined from stock 6061 aluminum. The holes in the injector face were drilled on a lathe using a 4 jaw independant chuck instead of a drill press so that perpendicular threads could be started. Usually perpendicular threads can be cut using a tap in the drill press chuck, but large NPT tapers require too much force for me to turn by hand. The lathe chuck worked fine.
Drilling injector using 4 jaw independent chuck |
Hand threading the injector face |
Running a sensor tap through the injector to measure combustion chamber pressure proved more difficult than I imagined. Several methods were tried and failed before I settled on the hole in the screw pictured above. The photo below shows one failed design where the injector explosively disassembled when pressurized. The bolts failed in tension.
The fuel grain is comprised primarily of gray Schedule 80 PVC pipe, available from a plumbing supply house. The forward end of the PVC contains 4 notches which align with the 4 stainless bolts on the face of the ignitor. Red RTV is used to ensure an air tight seal against the injector. The aft end of the grain is ground flat and red RTV is also used to ensure a tight seal against the nozzle. Additional O-rings and machining are not necessary when RTV is used.
Next, the PVC grain is wrapped in a sheet of EPDM rubber. The width of the EPDM rubber is 1.25x the circumference of the PVC tube. In other words the EPDM overlaps itself by 25%. The purpose of the EPDM is a) insulate the case hardware from heat b) the web thickness of the PVC is very close to a burn through. The EPDM provides just enough thickness to alleviate the risk. c) make the fit between the fuel grain and the case hardware tight.
Finally, the PVC and EPDM are wrapped in brown craft paper. EPDM rubber doesn't want to slide into aluminum. It sticks. The paper allows easy motor prep.
The ignitor and pre-heater are simplified solid rocket fuel grains designed to provide significant heat and flame in order to a) open the UC valves and b) light the hybrid rocket motor. The Nylon tubes and Nylon plugs burn away releasing the pressurized Nitrous Oxide. The pressurized Nitrous flows through the injectors, into the burning pre-heaters, and combustion of the PVC solid grain commences. Even after the pre-heater extinguishes, the Nitrous and PVC continue to burn.
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Pre-heater formula Ammonium Perchlorate 200u Aluminum -325 mesh HTBP Dioctal Adipate Isonate 143L |
75% 1.75% 14% 7% 2.25% |
Engineering the pre-heater was one of the most challenging parts of this project. Various attempts, videos, and discussions appear elsewhere on this web site. Below is a picture of the design which ultimately worked.
Discussion of this design and video here.
