Stability, nose weight, and fin size at mach 1+ speeds

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TRFfan

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I've been designing an ultra high preformance 24 mm MD rocket for flights over 11,000 ft. To get over 11,000 ft in the sims, i had to add 54 grams of nose weight and reduce the fin area to a small size. The rocket's top speed is mach 1.5 (and stability at top speed is 1.6 cal according to OR and RASAero) and so i was wondering if it still could go unstable due to the small size of the fins (or end up getting less altitude). I've attached the ORK file here.

stability-time graph
View attachment 314753
screenshot
View attachment 314754

ORK file

View attachment 314755
 

Nytrunner

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Moving away from "calibers" of stability, a design point is to try and keep your CG/CP separation at 8-15% of the rocket length. Your 1.6 caliber is barely 9%, and I believe high performance guys would recommend at least 2 calibers for mach+ flight.

Then again, you may have to push some boundaries if you want to get high performance.
 

TRFfan

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Moving away from "calibers" of stability, a design point is to try and keep your CG/CP separation at 8-15% of the rocket length. Your 1.6 caliber is barely 9%, and I believe high performance guys would recommend at least 2 calibers for mach+ flight.

Then again, you may have to push some boundaries if you want to get high performance.
I was told that 2 cal is what most people use, but above 1.5 cal is fine too (if you're pushing the boundries). The sim lists the rocket as being over 1.5 cal during the whole flight but since the fins are so small i was thinking that it might go unstable anyway due to the mach transition and the small fin span.
 

Nytrunner

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Also didn't you just ask this question in a different thread you started?
 

Buckeye

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Add some wind to your simulation and look at stability when the rocket leaves the rail. That is likely your weak point. Going Mach sounds sexy and all, but the mundane liftoff is more important.
 

TRFfan

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Add some wind to your simulation and look at stability when the rocket leaves the rail. That is likely your weak point. Going Mach sounds sexy and all, but the mundane liftoff is more important.
I increased the wind speed to 10 mph and stability was 1.65 cal.
 

tfish

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Here's is a link to a post I made awhile back

https://www.rocketryforum.com/showthread.php?132566-Upper-level-wind-limit(s)&p=1557771#post1557771

a couple of things to add. At some point you have to get off of sims and just go fly it and see what works and what doesn't. I've seen a lot of altitude attempts out at BR. It's not as easy as it my seem. I've seen as many as 8 flights to set a record. Which included a few misplaced or shreeded rockets and some flights classified as..well that sure didn't work.

Tony
 

timbucktoo

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At some point you have to get off of sims and just go fly it and see what works and what doesn't. I've seen a lot of altitude attempts out at BR. It's not as easy as it my seem. I've seen as many as 8 flights to set a record. Which included a few misplaced or shreeded rockets and some flights classified as..well that sure didn't work.

Tony
Exactly what Tony said. A friend attempted a Tripoli F altitude record and finally did after losing 2 rockets and finally achieved it on his 4th attempt. These altitude records are trial & error!
 

TRFfan

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Exactly what Tony said. A friend attempted a Tripoli F altitude record and finally did after losing 2 rockets and finally achieved it on his 4th attempt. These altitude records are trial & error!
Yeah that's true. My actual altitude record rocket has 2.7 cal and 2 cal at top speed so it should be fine. The fins are larger too.

THis is about my other altitude rocket (im trying to push the boundries with this one) and that involes cutting the stability to the bare minimum.
 

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