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Simming a short, stubby -booster-

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Charles_McG

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So I’ve built this lovely 1/5 scale Tater.
DAC73B69-3283-405B-B96D-33902F203EC9.jpeg

I’m having trouble with stability in the sim. The booster is built on shipping tubing and turned out relatively heavy. Even with the electronics and batteries in the Recruit, it’s not long enough to get the weight forward enough. Without extra weight in the Terrier interstage, it’s simming at Cp=Cg.
So what I’m asking is when is it appropriate to use the base drag correction hack. The booster is 24” long and 5.75” diameter. So about 4:1. But the whole stack is much longer.
So is it reasonable to use the sim hack, or not?
 

BABAR

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So I’ve built this lovely 1/5 scale Tater.
View attachment 428077

I’m having trouble with stability in the sim. The booster is built on shipping tubing and turned out relatively heavy. Even with the electronics and batteries in the Recruit, it’s not long enough to get the weight forward enough. Without extra weight in the Terrier interstage, it’s simming at Cp=Cg.
So what I’m asking is when is it appropriate to use the base drag correction hack. The booster is 24” long and 5.75” diameter. So about 4:1. But the whole stack is much longer.
So is it reasonable to use the sim hack, or not?
I only hack taters if I want fries.
 

UhClem

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Not exactly sure what you mean by base drag correction hack but I suspect the answer is yes. But only if you fly the booster by itself.

I also suspect that it will not alter the calculated CP of the full stack enough to matter. When you simulate the full stack what does the CP location do as it comes off the rod? (This of course will vary with wind and motor selection so use something worst case.)
 

Charles_McG

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Not exactly sure what you mean by base drag correction hack but I suspect the answer is yes. But only if you fly the booster by itself.

I also suspect that it will not alter the calculated CP of the full stack enough to matter. When you simulate the full stack what does the CP location do as it comes off the rod? (This of course will vary with wind and motor selection so use something worst case.)
By ‘the hack’ I mean the trick of adding a 3:1 conical massless nosecone to the aft of the rocket to force a better estimate of how base drag moves Cp.

It’s suggested for rockets with less than 10:1 L to D. Most used for 5:1-ish.
If I apply it in this sim, it goes from a static stability of 0 to 0.6.
I haven’t looked at stability v velocity. I’ll have to check that out. I would be boosting at ~15G and >4X the cross wind at the top of the rod, even at 12mph.
 

UhClem

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If I apply it in this sim, it goes from a static stability of 0 to 0.6.
With the high L/D of the full stack the CP shift is going to be greater than you expect so a static margin of 0.6 (based on booster diameter?) is unlikely to be sufficient.

Something that bugs me about the base drag hack is that base drag is lower while the motor is producing thrust.
 

Charles_McG

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That was tested in wind tunnels apparently- and the jet from the motor is so narrow that it’s not a huge effect.
 

Charles_McG

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I’d be tickled with a stability of 0.6. If it was a Black Brant XI (next year’s project) then a stability of 2-3 would be easy. But that skinny, short Recruit is just a bugger. I’m thinking of rebuilding it in fiberglass to move weight forward.
I’ve been challenged by the idea of stability this year. My Nike Tomahawk-12 has a static stability of over 3 and boosts straight as an arrow in winds up to 15mph. And I’ve got 5 flights, most in 10-12mph winds, so I know it’s not a fluke. But it does buck the common wisdom.
 

UhClem

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and the jet from the motor is so narrow that it’s not a huge effect.
Bigger than you think. As I recall NACA did some research on the subject. But Topics in Advanced Model Rocketry kind of punts because apparently research was lacking at the time. I did locate one document that offered an equation but the dang thing is anonymous so it's reliability is questionable.

I can believe that Nike Tomahawk flies well with a static margin of 3 since it has a fairly high L/D.

I believe that the way to go is to see how much CP movement you get in the sim (worst case is as it exits the rod/rail) and then add enough margin so that you feel comfortable.
 
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