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Rocksim Question: CP for pods and fins

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Howie

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I have a LOC / Precision 5.5" R2 (stretched V2). I'm considering adding the fairings for the fin tip stabilizers to enhance realism, and perhaps the CG / CP relationship as well. Before I start, I wanted to see what effects this addition would have in Rocksim. ;The picture below shows what I'm seeking to accomplish. Note: I do not know how to change the color or lighting of the 3D view in Rocksim, so if any of you have advice on this, feel welcome to message me.

V2 3D view.JPG
To start, I used a Rocksim file for the 5.5" V2 from Rocket Reviews, and added 5" at the front of the body tube to render the stretched V2 that I am modeling. No other changes. I verified the basic fin dimensions, but trust the original author modeled the fin and boat tail accurately. Here is a 2D view, including the dimensions for the fin.

V2 2D-1.JPG

To give an easy approximation of the fairings, I used Rocksim to add them at the outer edge of the fins, like a winglet on an aircraft. These fin fairing dimensions are as follows: ;root length 3.0", sweep length 2.3", top length of 0.5", and a taper for the remainder of the length; with a max. fin span of 0.4". The method I used in Rocksim was to add a pod to each fin, shift those back to where the leading edge of the fairings start, added the 2 fins which are automatically spaced 180 degrees apart, and made sure the radial position of these were perpendicular to each fin. Here's the 2D view, with the fairings in red and the remainder of the rocket in blue. The CP with this change is 0.31" further aft. Here's how it looks:

V2 2D-2.JPG

But I wondered if this was accurate and decided to try placing the fairings about 1.25" in from the outer edge, about where they should be on my model according to the data I have in hand. For this Rocksim file, I split the fins into inner and outer sections, as the location of the fairings would be along this imaginary joint. Starting with the first file, I trimmed the fins down to render the inner section. ;Next I added the pod, then added the outer fins. Here I ran into a snag: the tip edge on the original is swept back nearly 90 degrees, but Rocksim only permits a trapezoidal or elliptical fin to be added. To keep the fin area constant compared to my original baseline, I added the span of the forward and aft ends of the original tip edge, and took the average for the span. This way I could keep these trapezoidal fins the same area as the original actual fins for the V2. To my surprise, the result of this change was the CP was shifted forward from the baseline by 3.6", quite a change considering the fin area is constant or nearly so. Here's the 2D side view of this iteration, showing the line where the inner / outer fin boundary and the pod connecting them is located:

V2 2D-3.JPG

Lastly I added the fairings to this file to see what the net change would be for the CP. Adding these fairings in the same manner as the baseline to the last iteration shifted the CP rearward 0.54". Here's the 2D view, again with red fairings and the remainder of the rocket blue.

V2 2D-4.JPG

So after that lengthy introduction here's my question: what IS the CP for this rocket, with these added fairings??!! Does my methodology for these make sense, or is there a more accurate way to use Rocksim to determine what I'm searching for? And why such a big disparity in the CP results between the original baseline and the inner / outer fin approximation I used for the third design file? I can only conclude that Rocksim uses a different method to model fins added to pods versus standard fins.

So Rocksim users (and developers if you're tuned in), what are your thoughts? Open Rocket users, feel free to contribute. And if any researchers have knowledge on CP data on an actual V2 that can be used to determine how this simulation compares, I'd be very curious to know. Lastly, have any of you scale enthusiasts built a V2 with these fin fairings added, have you noticed any change in stability compared to an otherwise comparable V2?
 

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markkoelsch

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What stability calculation method are you using? I would recommend the Rocksim method as Barrowman does not do fins on a boat tail.

I do think the fins you are talking about will get toasted.

Remember, almost all V2 take a fair amount of added nose weight.
 

dhbarr

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I can only conclude that Rocksim uses a different method to model fins added to pods versus standard fins.
Yup. All our sims are approximations, and when two approximations bump into each other sometimes they act crazy. Tube fin minimum length on OR has absurd results in some scenarios, for instance.

Your winglets, provided they're straight and not absurdly heavy, will add a tiny bit of stability. Given how far aft they are, it's not much.

I'm not aware of any sim software that handles any aspect of winglets, and definitely not interference drag that close to the laminar flow boundary. Watch the speed limit, pushing one of those babies through a machwave would put a pretty tremendous force on each fin.
 

Howie

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Thanks for the replies. About the simulation method, I use the Rocksim method as it seems to model the boat tail of this rocket more accurately. When I got the unexpected results for the third and forth simulations, the first thing I checked was to make sure I was still using the Rocksim method and not Barrowman.

This rocket has flown several times already - I got it assembled and primered just in time for the Midwest Power launch last November, and also flown since then. I was intending to finish this over the winter but I did not find good reference material for what I wanted to model until recently. This will be a replica of Rocket #31 flown at White Sands, NM. It was one of the Blossom flights - these had the length extended by one body diameter to accommodate a capsule large enough to house a monkey, plus equipment to monitor it during the flight.

When I built this I took the supplied amount of lead shot inside the nose tip and added another ounce and a half for insurance. I originally intended to fly this on nothing bigger then an 38/720 AT motor. Once I plugged my finished weight in I saw I had enough stability margin for bigger motors and since then I've used motors as large as a CTI 38mm 6XL. This is the biggest motor this physically fits, and even the Imax load will not push this faster the 570MPH, so I don't think I need to worry about transonic effects for this rocket. Good point though, as those fins will induce flow changes to the air passing around them.

Just so I'm clear, these will not be fin tip winglets, but will be placed about 1.2" inboard from the tip. They will have a chord of 3.0 inches and stand 0.4" tall, so they are not remotely slender. As long as they are applied straight and built reasonably strong, I can't see how flight loads would damage them. As for landing damage they should be far enough up and in that they would make no contact on landing unless the fin really digs into the ground.

In short I'm not looking to these fairings as a band-aid to correct an unstable rocket. I'm just wanting to be sure this realism enhancing addition will push the CP far enough back to compensate for their weight, and perhaps more.
 
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