For those that have read or consulted the text: How to Design, Build and Test Small Liquid-Fuel Rocket Engines (Rocketlab/China Lake). Otherwise, see steps 10-13 here: https://risacher.org/rocket/example.html

I am currently designing an amateur rocket engine of my own. I am working through the book's calculations before I calculate dimensions for my own design. When calculating the gap for the water (coolant) there seems to be a mistake. The book displays: D2=(.0151)^(1/2)=0.123 ft = 1.475 inch, but I have done the calculation over and over again and continue to get D2=(.0139)^(1/2)=0.118 ft = 1.416 inch. I have used the books values and convert from inches to ft when appropriate. Do you know what's going on here? Does D2=0.123 ft = 1.475 inch (which is what the book says), or is this a mistake?

For those of you that have read this text, there seem to be various other discrepancies, rounding errors, and overlooked items that I have questions about as well.

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    $\begingroup$ Where do the values 0.0151 and 0.0139 come from? You've "done the calculations over and over again"; did you expect the square root of 0.0139 to change if you keep computing it? $\endgroup$ Mar 17, 2018 at 22:09
  • $\begingroup$ By over and over again, I mean that I have checked and rechecked my calculations. In the book, D2 = SQRT((4*Ww)/(Vw * pi * rho) + (D1)^2)). Where Ww = 0.775lb/s, Vw=30ft/s, D1=1.3875 inches, and rho=62.4 lb/ft^3. D1 must therefore be converted to ft. $\endgroup$ Mar 17, 2018 at 22:29
  • $\begingroup$ The book displays D2=(0.0151)^(1/2) but when the values above are substituted into the equation, you get D2=(.0139)^(1/2). See steps 10-13 here: risacher.org/rocket/example.html $\endgroup$ Mar 17, 2018 at 22:32
  • $\begingroup$ I am not saying this is your only problem but if Dc = 1.2 in, tw = 0.0225 in, and D1 = Dc + 2tw you should check your D1 value. $\endgroup$ Mar 17, 2018 at 22:56
  • $\begingroup$ @OrganicMarble tw= 0.09375. It says this in the line just after the one you are referencing. It is set to this higher value to increase safety and allow for welding, $\endgroup$ Mar 17, 2018 at 23:40

1 Answer 1


One of the online versions of this book, the one found here, includes an "Additions and Corrections" page.

The fifth bullet point on this page states:

In the example calculation, I think there may be arithmetic errors in the calculation of the coolant flow gap. Readers are strongly encouraged to redo all calculations for themselves.

I have now seen three different people come up with ~1.416 for the answer, so it seems somewhat confirmed.


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