Need help estimating CP on an odd build.

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Ravenex

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So the story behind this rocket... My dad flies rockets, I fly rockets, both my kids fly rockets, my wife... doesn't fly rockets. But she does come to all the launches. So for a while I've been trying to convince her to help the kids with theirs, or build her own, or even let me build her one to fly so she doesn't have to sit all day and do nothing. So after years of repeated "you need a rocket"s my wife comes home one day and anounces she has a surprise for me. She has finally decided to get a rocket... tatoo. So being not so slow myself, and really actually liking her tatoo, I say "If I build one of those, will you fly it?"

My estimations convinced me it should fly fine with some reasonable nose weight. I mean hell I got a troll shaped piece of tagboard with long pink hair and grass and flowers to fly. So I have now modelled up her rocket, and I attempted to put it in Rocksim and Open Rocket and they both agree the center of pressure is 84 inches... in front of the nose. The cardboard cutout method in Rocksim does give an estimate pretty close to where I guest it would be.

So I would like to here from the experienced Mindsim users, approximately where do you think the CP is? For a idea of scale, from tip to the end of the fins is approx. 34in, the max diameter is approx. 12.5in and it has a 54mm motor mount.

Jens Tatoo.jpg Jens Rocket.JPG Jens Rocket 2.JPG
 
I built and fly a rocket almost identical to that. Although a bit smaller it needed 10oz. of nose weight.
I'll measure and post size for reference tomorrow for ya.
https://www.rocketryforum.com/threa...cket-formerly-known-as-black-its-back.140511/
Charlie sold over 100 custom kits made. Flies straight as an arrow.
Could not get it right in Rocksim...took a lot of guestimation.

20170508_123736.jpg

If you follow links to Vasstas web site. you will find original size version almost identical to size you want to build. Around 5-7lbs of weight in nose.
 
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If that retainer were less rounded down it would produce a lot more base drag, which you'll probably want. Is that planned for off-the-shelf or custom?
 
Whatever Jim's CP is, I would pull yours aft a hair, due to the fourth fin, no holes in the fins, and the bell nozzle.
 
If that retainer were less rounded down it would produce a lot more base drag, which you'll probably want. Is that planned for off-the-shelf or custom?

I'm not sure what you mean by rounded, the retainer is a standard aeropack retainer, it is just recessed a little into the bell to hide it. Here's a cross section. The blue in the body is foam, the whole thing will be made much like my dad (Python Rocketry) makes his nose cones.

Jens Rocket Cross Section.JPG
 
If you notice the actual fins on mine, are pretty close to yours.
We added the G-10 web to enlarge them, and bring CP back a bit.
In build thread you can see how we started with G-10 core and added wood to both sides getting the profile.
It made a big difference for CP.
 
I'm not sure what you mean by rounded, the retainer is a standard aeropack retainer, it is just recessed a little into the bell to hide it. Here's a cross section. The blue in the body is foam, the whole thing will be made much like my dad (Python Rocketry) makes his nose cones.

I misunderstood. If that aft bell were more /\ and less () you'd increase base drag which could mean less nose weight.
 
My guess (and I mean guess as in GUESS) is that Dave F. hit it pretty close.
 
Open rocket agreed with Rocksims result of -84". Dave's estimate is close to mine. If that's right then I don't need much nose weight at all.
 
Even with 1lb of permanent nose weight the rocket will only weight 7-8 lbs empty. The solid models calculate the empty CG as shown below:
Jens Rocket CG Empty.JPG


With a K740 and 1.5lb of removable weight in the nose the CG is here: Jens Rocket CG K740.JPG
 
With a rocket with that much base drag, you can get away with a much smaller stability ratio than 1 cal.
 
The tapering nature of the design will severely reduce base drag effects in my flow behavior estimation.

Once a CP is figured on, I'd try and keep the CG 3.5-5" ahead of it for a stubby rocket like that.
 
A simple way of working with stability is to figure the mass of the rocket body and its weight will tend to correlate -- for example wide rear has both CP and CG shifted back. Then you put a motor in it and need to counterbalance that with nose weight so the CG doesn't move. Finally some small fins gives some actual stability margin. Or for finless rockets with the CP aft of center, if the nose weight and engine weight are the same then then their CG is forward of CP, providing the margin.

I'm not sure this is adequate with the fins that far forward, as they are more forward than the TRF logo rocket or https://www.rocketryforum.com/threads/cats-in-space-trf-logo-rocket-flies-at-last.57713/

As to "base drag", I have had several pieces of evidence there is no such thing, besides this shape wouldn't make much of it anyway. The subject of calibers of stability is unrelated, it has to do with the inherent geometry flaw of of using the diameter of the rocket -- which has almost nothing to do with the margin required as a fraction of length anyway -- as a measure of stability. It's like measuring using the width of a ruler instead of along its length.
 
As to "base drag", I have had several pieces of evidence there is no such thing, besides this shape wouldn't make much of it anyway.

I have to disagree with the general idea that Base Drag is "nonexistent" . . .

If that were true, boat-tails would have no effect on performance.

Dave F.
 
Boat tails have a negative effect on stability. Expanding transitions toward the rear improve CP, but at the transition itself if that's not the back. A less draggy shaped one has less effect, even with the same base size. That's one piece of evidence, along with practical experience with flying finless rockets. Open Rocket is already an optimist.

The fake nozzle should certainly be included in the sim, there behind the boat tail section it's substantial, but then let the S/W do the job. Although it does seem something is wrong in this case. Much more so than any of this!
 
Have to agree with Dave on this one. Base drag is a known and (with difficulty) quantifiable fluid dynamic effect on vehicles (on ground and in flight).

That's the very reason boattails and tailcones reduce drag: allowing the flow to settle back together, decreasing flow separation, and minimizing the low pressure region created by a blunt rear face of the vehicle. It's not as substantial an effect as aerodynamic surfaces like fins, but it exists. (and openrocket is overconservative in how it accounts for tailcones)
 
^^-- I was talking about base drag as a stabilizing force. Of course it is a drag, quite a large one. In most or all cases where it has been considered that base drag is needed to explain a rocket's stability, I believe it is better explained otherwise.

I also built and flew a rocket somewhat like the one being discussed, but with conventional shaped fins and more elongated (2/3rds tailcone). Open Rocket showed an almost unbelievable low drag and flight performance, a 5.5" rocket flying like a conventional 3" one or better, and was overoptimistic on that. Stability calcs seemed perfect though.

Barrowman's equations for rocket stability consider these items for stability: Nose cones, fins, and any kind of expanding or tapering transitions. Boat tails reduce stability. Meanwhile, the regular cut off straight tube back end has no effect positive or negative.
 
I imagine someone performing the hammer throw, minus the throwing part, lol.

PERFECT !

Dave F.

tumblr_ngwno9AIZ81svpfhbo1_500.gif
 
I've used wind testing rather than swing testing. It tricky to get usable results because it really shows how narrow an angle of attack is required for stability of most rockets -- any larger shift of wind direction and you're sideways.
 
Well I just tried making this rocket on Open Rocket and yeah it hates it. It basically uses Barrowman's equations and that's what they do, nose cone and tailcones are destabilizing factors, and when that's the whole rocket it apparently overwhelms that scheme. There should be a theory that explains it, but theories that explain other deviations from Barrowman may not help, even if they're correct.

For the rocket I built, it was more stretched with large fins at the rear and I made it work in Open Rocket. The rocket flew great and never weathercocked, so wasn't particularly overstable.

For other rockets such as Jim's above and Cats in Space, the fins were at least more towards the rear of the rocket than the OP's drawings, and the rockets are a little more elongated.

The problem with "base drag" being a stabilizing factor here is two, the main one being: this rocket has lower base drag than even conventional rockets. The only sharply cut off part of the rocket or "base" is the false nozzle, which is small. OR will actually give you credit for the nozzle, but based on its leading side not its cutoff (base) side. The problem is it's saying everything else about the airframe/body is destabilizing, to a degree that huge fins are required. The taper of the second half of the rocket doesn't need to be turned into a benefit, just less of a hindrance.

On the plus side, an ordinary football could be used as an aerodynamic test dummy.


I've read all those before. I definitely recommend angle of attack analysis in Open Rocket (under Component Analysis). Of course, it's going to tell you the same thing as that first link, because it's based on it, but for your specific rocket. However, the article still makes the mistake of not questioning "calibers" as a unit of stability margin when discussing long and short rockets. If you use a better measure of rocket size, say length+width of the airframe, rather than just diameter, and consider the needed stability margin a fraction of that, you end up finding that unusually long and short rockets need a similar stability margin. It's less a factor of aerodynamics than geometry. Again, why would diameter be the primary determiner of stability margin's scale unless all rockets considered have a similar aspect ratio? That explains that deviation for all rocket-shaped rockets, no base drag effects needed to explain it, leaving only saucer types to explain. In those cases I believe the leading side of the shape has more to do with it than the trailing shape, is all.
 
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