Saturday, August 26, 2006

Snowbird Zero

I wrote in a previous post about the joys of paper rockets. Snowbird Zero was my second foray into this cheap but challenging field.

I didn't intend to go big into paper rockets, but just as Currell Graphics got me started, Jimmy Yawn inspired me to go farther. Jimmy's series of pages on his homegrown Snapdragon are a well written, fully illustrated how-to guide to rolling your own rockets. He has detailed instructions on how to roll nosecones and body tubes out of paper or cardstock. I was planning on making tubes for 18mm (A through C) or 24mm (D and E) motored rockets. Then I went to Wal-Mart and found four-packs of A10-3Ts for $4.97. At that price, I couldn't not buy a pack, but I didn't have any mini-motored birds in which to use them. I couldn't find any that I liked in the local stores, and I didn't have time before the launch to order any online. Time to roll my own!

The key to body tubes is having a nonstick former to roll them around, something that white or wood glue won't stick to. I found a 9/16" brass cylinder for a couple of bucks and started rolling body tubes. The key to rolling body tubes is to remember that they cost nothing in time or money, so you might as well roll a bunch and chose the handful of perfect ones to use in your rockets. It took me a couple of sessions to get very good at it, and even then every third tube came out crooked, wrinkled, or otherwise ugly.

I also tried rolling nose cones, but I had less success. I decided to bite the bullet and buy a pack of PNC-5 nose cones for 13mm body tubes. A pack of five nose cones was five bucks. So if could make everything else myself, the cost of each of my mini-motored birds would be one dollar--a few cents less than the motors that would push them. I had body tubes down and nose cones in hand, so now all I needed were fins.

I'm lazy and I hate sanding, sealing, and aligning fins. You don't have to sand or seal paper fins, but alignment is still a potential pitfall. Then I found Art Applewhite's 13mm Six, a six-sided paper rocket. The fin can for the Six is a one-piece unit that is cut and folded into shape. If you do it right, you get three fins spaced evenly around the rocket and perfectly aligned. The method works for any number of fins, but if you use the one-piece fin can the number of fins and their leading edge sweep are inextricably linked. Each fin is folded double along its leading edge, so the leading edge angle (from the body tube) multiplied by the number of fins must equal 180. A three-fin can will have fins swept back at 30 degrees. Four fins must be swept at 45 degrees, six fins would be steeply raked at 60 degrees, and so on. If this doesn't make any sense, compare the plans shown below with the photos of the finished rocket.

Pretty much everything about this rocket was dictated by the fact that it was intended to fly on A10-3T motors. A short, light bird will simply disappear on those motors. I at least wanted to get the nose cone back. The pack of nose cones included a couple of short blunt ones and three long ogive types. The shorties would be great for making a downscale clone of the Big Bertha, and the long ones would make a killer 13mm Der Red Max, but I didn't trust rockets that short not to go stratospheric on the A10s. I decided to make a sort of "sport scale" clone of the Estes Bluebird Zero, a nice long-bodied model from the days before Estes fell in love with boring, plastic-finned 4FNCs. I don't have a color printer and I was too lazy to go to Kinkos to print out the parts in color, so I decided that my rocket would be black and white. Hence the plans with "Blackbird Zero" printed on them. But the all-black body wrap didn't turn out so well, so I left it off, and in the end I named the rocket Snowbird Zero.

The very long body tube is made from two hand-rolled tubes and a more tightly-rolled coupler. I used the forward fin can to cover up the join. If the front of the body tube looks bent, it's because I used a through-the-wall shock cord mount and covered it with a long body wrap (the body wrap was another idea borrowed from the indefatigueable Jimmy Yawn). The motor block and launch lug are also made of rolled paper.

A note on the body tube diameter. Estes mini-motors are 13mm in diameter. The brass tubes at the hobby store came in sixteenth-inch increments. Half an inch is 12.5mm, just too small to admit a motor, so I went for the next size up. But 9/16" is just a smidge too big. The motors and the nose cone were sliding all over the place. I made single-turn internal wraps out of sketch paper to decrease the internal diameter of the body tube at either end.

The flight was fast, straight, and not so high that the rocket disappeared. It probably helped that I packed the nose cone with handy-tack beforehand. The streamer deployed just as planned at apogee, and the rocket came down just fine. But on recovery I saw that the ejection charge had blown out something inside the rocket, because the front of the body tube was occluded by a wad of scorched paper. It's either the thrust ring or the internal wrap for the nose cone. I haven't investigated enough to figure out which. I know that's lame. I really need to get it fixed. For one thing, I need to know what went wrong so I can make the next rocket better. And I still have three A10s left to burn and nothing else to fly them in.

Anyway, I try to learn something from every rocket, and this one was a good introduction to a lot of techniques that I hadn't tried before. I will definitely build more like this.


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