Does someone have the fin dimensions for the Goblin? I looked up the plan on jimZ's but they won't print out. I can't use a rocsim file because I don't have it. I need numbers!

I am using photometrics from the K55 document (assumption: BT-55 OD is 1.325 inch).

The fin ratios should be on the money, but the actual dims are going to +/- 5%.

Root edge ......: 2.0000 inch
Leading edge ...: 2.5231 inch
Trailing edge ..: 1.8010 inch
Fin tip ........: 1.4535 inch

Greg

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Thank you for a fast reply and the dimensions. I haven't herd of photometrics, but I like what I see. Is it a program you added on yourself, or is it hidden somewhere in the jimz plans site?

































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I had a .pdf of the Goblin plans, probably from the JimZ site. From there I took a screen capture of the fin area and did a paste into AutoCAD. I had the proportion of the fins, but no scale to go by. I ran a line on a known dimension of the rocket (where the nose cone meets the airframe) and scaled that line to equal the rocket airframe OD. I ran a line along the fin root, since it was the least likely dimension spoiled by perspective and used that as a basis for deriving the other fin dimensions. It was close to 2 inches, but not quite. In human factors, dimensions are typically rounded to give the user, i.e., a person, an easy time of doing their work. So I rounded the root dimension to 2 inches even, then scaled the fin accordingly.

Upshot: I used two programs, Acrobat and AutoCAD to derive the information. It was part science and part art, like most things in rocketry.

BTW, when I was a kid I made a Goblin, but have absolutely no idea what happened to it. I painted it florescent yellow and black, a very cool but LOUD color scheme.

Greg

I have my original yet unfinished (it's in the sanding sealer stage) Goblin at home....I will try to remember to measure it and confirm later this evening. Greg's numbers look quite plausible, but I don't have the rocket in front of me here.

You could cheat and buy one from me.

.

Or just buy the decals from me and the parts from SEMROC.

It's not that hard to do.

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"I'm a sandman. I've never killed anyone. I terminate runners when their time is up." Logan from "Logan's Run"

http://sandmandecals.com/

If you are having difficulty printing the fin template in the right size, check to make sure that you have the image file's dpi set correctly. All of the plans at JimZ's site are scanned at 300 dpi. When you click on the fin pattern graphic from the site, it will often display at your browser's default resolution, which for Windows browsers, is usually 96 dpi. Then if you try to print the pattern from the browser, it will be the wrong size (usually way too large). The solution is to actually download the graphic and save it to your disk, and then open it in a graphics editing program like MS Paint. From there you can correct the graphic resolution, which will usually cause the template to print at the correct size. Be sure that your graphics program is not set to "automatically resize the image to fit the page" (I don't open templates in either OpenOffice or PowerPoint for that very reason) and that the only thing you change is the image resolution. And when you print it, make sure that you do not have the printer set to resize the image to fit the page.

All of the Goblin's plan pages at JimZ's site are in TIFF format. I have converted the page in question to an Adobe Acrobat (.pdf) file and attached it to this post. When you print the page, set the Page Scaling box in the Print Dialogue to "None" and it will print at the correct size.

I just flew my Goblin clone (built from the plans at JimZ's site) yesterday at my club's Spooky Sport Launch. I built my clone with parts from Semroc, but I cut the fins myself using the template in the plans. I obtained the decals from Excelsior. I flew it on a C6-5 that I had friction-fit inside a spent 24mm motor casing (after grinding out the nozzle from the spent case). Despite rather breezy conditions, it had a nice straight and high flight, and the 5 second delay seemed to be just right. At NARAM in August, my clone had its maiden flight on a B6-4 (also friction-fit into a spent 24mm casing) and it had a beautiful flight on that motor as well. I used a 9" nylon chute instead of a streamer in both flights instead of the usual streamer because I wanted to soften the landings somewhat, and because the flights would not really be high enough to require streamer recovery. It did drift a bit in the breeze in yesterday's flight, but it stayed within the field and was easily retrieved.

MarkII

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k-55e-letter-size.pdf (19.8 KB, 136 views)

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SAM # 0011

If anyone wants an Acrobat version of the entire set of Goblin plans that are posted at JimZ's site, send me a PM with your email address. I will need to send the file via email because it is way too big (1.88 Mb) to post as an attachment here.

I have tried to contact Jim Zalewski several times to send him my Acrobat conversions of the plans at his site, but I have never gotten a response.

Many of the plans at JimZ's have PDF versions already, but the majority do not. I have created PDF versions of nearly all of the remaining TIFF-only plans, so if anyone wants one of those files, let me know. Although some of these files are sizable (a few are almost 2 Mb or more), they are still much smaller than a set of the pages in TIFF format, and in my PDF versions, all of the pages for each kit are in one file. The PDF versions are also much easier to print than the TIFF versions.

MarkII

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SAM # 0011

OK - just measured my original Goblin fin template. The dimensions from the CAD measurements of the scan are plenty close enough. Here's what I have:

Root: 2 inches
Tip: 1 7/16 inches
Leading edge: 2 9/16 inches
Trailing edge: 1 13/16 inches.

Mark, I did the Constellation all into a .pdf and emailed JimZ asking if he wanted the file....and never heard from him either.