0FNC: Some thoughts

I made attempts at simulations of the Arcturus, Candlestick,* and American Rocketeer Finless Rocket. Here they are along with the Rxiiiab.finlessnessThere was a fair bit of guesswork involved and the CG must be somewhat uncertain. The CP from OpenRocket coincides for the Arcturus with that given (presumably calculated by hand from the Barrowman equations) by James, gratifyingly enough.

In all four cases the Barrowman CP is near or a little forward of the tail cone. The Arcturus’s section of airframe behind the cone, which contains the motor mount, pulls the CG back significantly. Nevertheless in my simulation I put in only 0.5 oz of nose weight and the CG is more than a caliber ahead of the CP — if by caliber you mean the diameter of the main part of the rocket. (Of course OR uses the maximum diameter. But the “1 caliber stability” rule of thumb was devised for cylindrical, finned rockets, and I have no idea whether or how it can be correctly applied to rockets like these. I just content myself with observing the CG is solidly forward of the CP.) There’s also 0.5 oz in the payload section of the Candlestick, despite Chandler’s insistence that you should use at least 1 ounce. For the Finless Rocket I used the specified 0.4 oz, and for the Rxiiiab it’s 0.16 oz — which is what it is as of last night when I scraped almost half the clay out. Again, though, the CG should be regarded as a rough guess, though it’s pretty close on the Rxiiiab.

As for the CP, are the Barrowman equations applicable? I’m not sure. But I’d think Barrowman might put the CP too far forward, given that there’s presumably some base effect not being accounted for.

So I’m arguing James and Chandler and whoever did the Finless Rocket were being quite conservative with their nose weight recommendations. I’m not sure I’ve convinced myself though.


* Candlestick. By Jeff Chandler. Okay, I see what you did there.


5 thoughts on “0FNC: Some thoughts

  1. Rich, the CP may actually be further back than OR calculates. Take a look at the Peak of Flight newsletter issue #154 (https://www.apogeerockets.com/education/downloads/Newsletter154.pdf), which suggests that rockets with large base diameters have “base vortex” with will add to the stability. This is the principle that allows the seeming unstable Spool Rocket to fly (http://modelrocketbuilding.blogspot.com/2011/08/spool-rocket-build-finished.html). I used the method in the newsletter to simulate the Spool Rocket in OR, which would otherwise predict the Spool is pathological.

  2. Thanks, I’d missed that build! I take it you tried a C engine with less than happy results.

    I think the shape is similar enough to the Rxiiiab that I can use your results (stable with B, not with C) to estimate pretty well where the CG needs to be. I’ll try fine tuning my Finless sim to see where the CG with B and C motors is, then use that to decide how much nose weight the Rxiiiab needs.

  3. Pingback: 0FNC: Some further thoughts | Rich's Rockets

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