# Problems with the "How to Design, Build and Test Small Liquid-Fuel Rocket Engines" text

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.

• 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? Mar 17, 2018 at 22:09
• 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. Mar 17, 2018 at 22:29
• 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 Mar 17, 2018 at 22:32
• 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. Mar 17, 2018 at 22:56
• @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, Mar 17, 2018 at 23:40