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Anyone use the Rocksim / Openrocket base drag trick?
In the Apogee newsletter #154 Bruce Levison shows how to use a zero mass cone at the back of the rocket to simulate base drag.
Openrocket understates the stability of short stubby rockets and this trick is supposed to compensate for it. My problem is, he didn't clearly define just what qualifies as a short stubby rocket. He said the trick is for rockets with "much less than the normal 10 to 1" length to diameter ratio. He's pretty exact with how to draw the cone but not with what rockets you should apply it to. Thanks Last edited by RobVG : 11-24-2021 at 10:53 PM. |
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Quote:
I used this technique on a slightly upscaled Centuri Vector-V ("Good heavens!") 10 years ago: https://forums.rocketshoppe.com/sho...127&postcount=1
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Jay Goemmer "Centuri Guy"/"Tau Zero" YORF Member 28 Semroc SAM #0029 NAR 86131 "I think about organizing things all the time. Never seems to happen. I find something that piques my interest and I'm off on a quest. Or a Centuri. " --Bill Eichelberger, 02/22/2022 “Centuri fret buzz in an updated form.” Bill “Wallyum” Eichelberger re: Estes Flutter-By 03 Sept 2014 |
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Quote:
Agreed... WTH is "much less than the normal 10 to 1"? I guess since the Estes Fat Boy is 4.6:1 that less than 5:1 is the definition? |
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That article is about an earlier version of Rocksim - but Openrocket is supposed to have the same issue: it calculates the amount of base drag ok, but doesn't position it correctly.
I've also used it to convince myself that certain squat rockets would be okay to fly. From a discussion over on the Forum That Shall Not Be Named (I don't have a link handy) the general feel was that the hack could always be applied, no matter the length of the rocket - but that as the L goes up, it's a smaller and smaller fraction of the overall forces. By 10:1, it's negligible.
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Charles McGonegal Ciderwright AEppelTreow Winery & Distillery Ad Astra Tabernamque! |
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Thanks for your thoughts!
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I have an Estes Big Daddy Rocksim file that I got from somewhere and one that I long ago modified with the base drag virtual cone. Just ran some sims:
Loaded with D12-5 - 0.38 stability factor With virtual cone & loaded with D12-5 - 1.02 stability factor Considering how close the latter is to the rule of thumb 1.0, perhaps even Estes uses the virtual cone method to create a safe design without specifying a lot of unnecessary nose weight. |
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