2
$\begingroup$

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.

$\endgroup$
  • 1
    $\begingroup$ 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? $\endgroup$ – Russell Borogove Mar 17 '18 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. $\endgroup$ – Luke Jarboe Mar 17 '18 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 $\endgroup$ – Luke Jarboe Mar 17 '18 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. $\endgroup$ – Organic Marble Mar 17 '18 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, $\endgroup$ – Luke Jarboe Mar 17 '18 at 23:40
1
$\begingroup$

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.

$\endgroup$

protected by Community Jan 17 at 11:45

Thank you for your interest in this question. Because it has attracted low-quality or spam answers that had to be removed, posting an answer now requires 10 reputation on this site (the association bonus does not count).

Would you like to answer one of these unanswered questions instead?

Not the answer you're looking for? Browse other questions tagged or ask your own question.