## Formulas from the Decker paper

Moderator: Mike Everman

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### Formulas from the Decker paper

I was re-reading the Decker paper on subsonic ramjet design and was thinking of taking his design and scaling it down for my first attempt at a ramjet. (6.67 inch diameter ramjet is a beast to have as my very first attempt)

In the process of scaling it down, I wrote down all the formulas that he used and just changed the input area. This is where I ran into problems. Its been a long time since I was in school, but one of the real simple formulas that has stuck in my head was the area of a circle. area = pi*radius^2 If you know the area and want to find the diameter, it would be: diameter = (sqrt (area/pi))*2 His formula on the other hand is: diameter = (area * 4)/pi

It just doesn't add up (pun not intended) Right at the top of page 3, he lays out the formula for the diffuser entrance diameter as 10(4)/3.14 = 3.56 which when I plug into any calculator I have comes up with 12.74

It seems like every formula he uses for a diameter is wrong. Am I doing something wrong or are all the formulas in that paper wrong? I could take the numbers for each dimension he has and scale them down to the size I want to use, but I'd rather use the formulas if possible.

-Aaron

marksteamnz
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You are correct and he's wrong. You are sure he's not looking at Circumference = pi*d?
Cheers
Mark Stacey
www.cncprototyping.co.nz

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### Decker's formulas

He is solving for diameter (diffuser entrance/exit, nozzle exit, etc)

Attached is an excel spreadsheet that has all the formulas in it. Put the desired entrance area in cell B1 and the rest will calculate out for you. (correctly even) Using this spreadsheet and a program called VCP (used in model rocketry) I have created all the templates for my first ramjet. I'll start cutting next week. I don't have a welder (yet) so I'm going to use silver solder and 26 gauge mild steel until I get one. (I know the engines wont last, but they're easy to build)

I'll post build pics once I have something to show.

-Aaron
Attachments
Decker_formulas.xls