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Old 11-24-2021, 11:06 PM
RobVG RobVG is offline
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Default 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 11:53 PM.
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  #2  
Old 11-26-2021, 02:32 AM
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Tau Zero Tau Zero is offline
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Quote:
Originally Posted by RobVG
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.
RobVG,

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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  #3  
Old 11-28-2021, 08:53 PM
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HenryMac HenryMac is offline
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Quote:
Originally Posted by RobVG
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


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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Old 11-29-2021, 09:03 AM
aeppel_cpm aeppel_cpm is offline
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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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  #5  
Old Yesterday, 10:20 PM
RobVG RobVG is offline
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Thanks for your thoughts!
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