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While I said just a few hours ago that "There's almost no such thing as a dumb question! this one just might sound like one.

I'm making some slides about first principles thinking applied to engineering problems, and Elon Musk has several good quotes, one for example at a TED talk where near the end he says something like “...boil things down to their fundamental truths and reason up from there.” I'm using both Tesla model S and Falcon 9/Falcon Heavy as examples.

For the Tesla Model S, you can get the approximate required battery size easily from a range and speed (400km, 110kph), cross-sectional area (25 square feet), and drag coefficient of 0.24, plus the rolling resistance of good tires, 90% overall efficiency and another kilowatt or two for air conditioning, computer&display, and maybe some music.

I wanted to add the drag coefficient of a Falcon 9 rocket as a tangential comment only. I don't need an exact number, but something in the ballpark, and I have no idea where to begin.

Assume sub-sonic, flying straight, with a reasonably large payload fairing. It doesn't have to be a Falcon 9 - any rocket somewhat representative is fine.

If there is a plot of drag coeficient vs speed that extends to supersonic, that would be really very helpful also. This is back-of-the-envelope I'm doing here.

Is it better or worse than the "Long Cylinder" below?

Measured Drag Coefficients

Image from https://en.wikipedia.org/wiki/File:14ilf1l.svg

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    $\begingroup$ Braeunig's Saturn V simulation page has a graph of Cd versus Mach number; it's a rough approximation based on the figures for the Atlas rocket, but better than nothing. $\endgroup$ Jun 29, 2016 at 17:02
  • $\begingroup$ Indeed, there is useful discussion and plots at http://www.braeunig.us/apollo/saturnV.htm. $\endgroup$
    – uhoh
    Jun 29, 2016 at 22:07
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    $\begingroup$ That page is, like, the source of 1/3 of my points on here. $\endgroup$ Jun 30, 2016 at 2:20
  • $\begingroup$ Oops! Didn't mean to reveal trade secrets. $\endgroup$
    – uhoh
    Jun 30, 2016 at 4:55
  • $\begingroup$ Haha, not at all -- I normally link it, but I was on my phone when I made my first comment. $\endgroup$ Jun 30, 2016 at 14:47

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It's much better than the long cylinder from your example, because its drag is dominated by the flat face on the front. Count on 0.25 for a long cylinder terminated by a nose cone (maybe a bit more due to the reduction in diameter below the fairing).

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  • $\begingroup$ I see. The same way the long cylinder is lower than the short cylinder, the nose-coned long cylinder is better than the cone alone. Thanks for being clear and quick! $\endgroup$
    – uhoh
    Jun 29, 2016 at 10:06
  • $\begingroup$ Actually, looking at the second plot with the subtitle "Drag coefficient of blunt nose and rounded nose cylinders versus fineness ratio l/d", would an F9 with a big fairing be around l/d of about 8 to 10, with a drag coefficient of about 0.2 to 0.25 or am I reading it wrong? $\endgroup$
    – uhoh
    Jun 29, 2016 at 15:59
  • $\begingroup$ You're right, 0.4 is too high. IDK how the step in diameter from the fairing to the stages affects this though. $\endgroup$
    – Hobbes
    Jun 29, 2016 at 16:43
  • $\begingroup$ Ahh, I see what you mean. $\endgroup$
    – uhoh
    Jun 29, 2016 at 22:02

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