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Tim's First Rocket Motor Design

July 31, 2008

I'm Tim Patterson, the webmaster of this site for the past 19 or so years. I became interested in rocketry when my dad took a job in Washington, D.C. and we spent a week touring the Smithsonian and Air & Space Museum when I was 10 years old. Seeing the old Mercury/Gemini/Apollo space capsules in person (under plexiglass) left an indelible mark on me.

September 1994 Phoenix New Times | Amateur Rocketry

Then I learned of the existence of "high power rocketry (HPR)" and "amateur rocketry" back in September 1994 when I read an article in the local Phoenix New Times paper. I was excited to find out organizations like Tripoli and the RRS existed.

Phoenix New Times, September 1994 [PDF, 6.7MB]

I had played with Estes models like everyone else, but didn't realize that people were actually building such large rockets. And I had no idea that average people actually designed, built and launched liquid bi-propellant powered rockets. Incredible!

Soon after I joined both the Tripoli Rocketry Association and Reaction Research Society (RRS). I built a couple of large rockets (3.5" diameter, with 29mm and 38mm engine mounts) with the intention of certifying with Tripoli, but got side-tracked. I ended up getting more interested in liquid-fueled rocket motors and design.. I never did get Tripoli certified.

So, after reading and learning a bunch more, I tried to design my own small liquid fueled rocket motor in late 1995. I finished my first 20lb thrust motor, to be powered by methyl alcohol (methanol, easy to get) and gaseous oxygen. I decided on gaseous oxygen (at least for the test model) since it would be easier to handle than liquid oxygen, which would have required a special dewar and added other complications with venting, etc..

Rocket Motor Design Booklet Cover

First Rocket Motor Design (Calculations + CAD Drawings) [PDF, 4.9MB]

Once my initial calculations and designs were complete I really wanted some feedback from more experienced people. So I made a bunch of copies, then bound my calculations, designs and CAD drawings in report folders, and mailed them off to a few members of the RRS who I knew had experience with liquid fueled motors.

Scott Claflin - Motor Review [PDF, 3.4MB]

Tom Mueller - Motor Review [PDF, 1.8MB]

David Crisalli - Motor Review [PDF, 1.7MB]

I was surprised (I was new and didn't realize they were such cool and helpful people) to receive feedback from 3 of the top guys in the Reaction Research Society with some really useful constructive critisism and tips to improve on my design. I incorporated lots of their tips into my second revised design. One glaring omission from my design, Scott Claflin was kind enough to point out, was the lack of any o-rings or sealant. Tom and Dave also gave me some great tips. I've scanned their replies to me, available for download above.

Revised Rocket Motor Design (CAD Drawings) [PDF, 3.4MB]

Rocket Motor Injector and FittingsShortly after incorporating the feedback into my new design I took my CAD drawings down to a local shop, Atomic Rockets (no longer in business), where they had a machinist begin fabrication on the injector out of aluminum for my small rocket motor. I got it back with everything but the injection holes drilled. I was still unsure about how I wanted to do the injection, and had not yet finished the redesign for swapping the fuel and oxidizer so that the fuel sprayed along the chamber wall and the oxygen in the middle to avoid unnecessary errosion of the graphite combustion chamber.

» Click here for photos of the injector and fittings

Due to job changes, a move to Austin and other life stuff, I ended up putting the project on the shelf. One of these days I do plan to get back to the project and hopefully finish and test fire it.. Maybe I'll do something like they do in the software industry and "open source" it, and make it a group project with other interested folks. Might be fun. :)