Is there a way to sim a rocket with spherical fins?

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A sim may show them as relatively dragless. But they definitely will be quite draggy.

For me given the length I'd bet if the nose is about as heavy as the fins+motor, it'll be over stable.
 
All this mindsiming is scarring me and fear is the mind killer. Off to the Bunker! It's not totally finless, at least it has something down there.

I have been burning reload AT F12-3-BJs this winter. Might be a good choice for this one at 24mm. Nice power delivery that won't rip your balls off at the pad.
 
neil_w said:
They sim with plenty of drag but not base drag.
Click to expand...

Base drag is a term in OR that only applies to the blunt aft end of the airframe or a transition with a diameter. Since the aft of the sphere (bastardized transition) has no diameter, OR thinks the base drag is zero. As far as I can tell, no other component is allowed to have base drag.

Base drag is just pressure drag, so I don't know why this redundant term was introduced decades ago.

In reality, there is net pressure drag on the spheres due to high pressure on front, low pressure on the rear ("base", if you like). Does OR's pressure drag capture this? Unlikely. CFD needed.

1706275889829.png

terryg said:
I have no doubts it will be stable. I am just wanting to get altitude estimates and the delay times needed.
Click to expand...

However OR got there, its CD prediction of 1.8 is probably in the ballpark. Go for it in your altitude and optimal delay analysis.
 
Buckeye said:
Base drag is a term in OR that only applies to the blunt aft end of the airframe or a transition with a diameter. Since the aft of the sphere (bastardized transition) has no diameter, OR thinks the base drag is zero. As far as I can tell, no other component is allowed to have base drag.
Click to expand...
Fins also produce base drag, unless they are set to "airfoil".
 
Well the swing test results are discouraging. It difficult to swing test a long rocket due to the angle of attack issues. This requires a larger swing radius. The results were that it tended to stay in the starting orientation. If started nose first it stayed nose first. If started with an up/down orientation it it tended to stay that way. If it started "balls first" it tended to stay that way. Unfortunately during testing I damaged the "balls" end of the tube. So what I am going to do is shorten it a bit and add 3 fins between the balls. I have ordered more table tennis balls and will try the original design again keeping the weight in rear as low as possible.
 
terryg said:
Well the swing test results are discouraging. It difficult to swing test a long rocket due to the angle of attack issues. This requires a larger swing radius. The results were that it tended to stay in the starting orientation. If started nose first it stayed nose first. If started with an up/down orientation it it tended to stay that way. If it started "balls first" it tended to stay that way. Unfortunately during testing I damaged the "balls" end of the tube. So what I am going to do is shorten it a bit and add 3 fins between the balls. I have ordered more table tennis balls and will try the original design again keeping the weight in rear as low as possible.
Click to expand...

Nooooooo! Can you give it a try sans fins on a smallish motor in a safe area? Your TRF colleagues are dying to know.

Swing tests are notoriously difficult and inconclusive.
 
In my best Joe Pesci voice; "Doze wise guy swing tests are bustin' your balls!"

Damaged balls? How are you attaching the balls? There are many ways to attach the balls for higher power. No need for ugly fins. No need for a stubby, short rocket if long, lean and lovely is so desired. Give into the Dark Side and fly the rocket you love. Power, nose weight and sloppy, spherical base drag are your dark arts allies. Not the kind of rocket you would see a Jedi flying, but hanging out with oddroc scum can be fun. Feel the dark side forces and vortices flowing around those balls as they accelerate off the pad. Feel the power of the F12 3 Black Jack propellant! :)
 
Buckeye said:
Nooooooo! Can you give it a try sans fins on a smallish motor in a safe area? Your TRF colleagues are dying to know.

Swing tests are notoriously difficult and inconclusive.
Click to expand...
Ok, will do! It will be 5" shorter after I remove the damaged tubing and reattach my "balls".
I am leaning toward using a D24T although a E35W would be more spectacular sky writing.
 
terryg said:
Ok, will do! It will be 5" shorter after I remove the damaged tubing and reattach my "balls".
I am leaning toward using a D24T although a E35W would be more spectacular sky writing.
Click to expand...
With white lightning or blue thunder you will need firmly attached balls.

No on purpose skywriting! All Jedi hate that.

Like Goldilocks you have to get the thrust curve just right! Baby bear rules!
 
terryg said:
Well the swing test results are discouraging. It difficult to swing test a long rocket due to the angle of attack issues. This requires a larger swing radius. The results were that it tended to stay in the starting orientation. If started nose first it stayed nose first. If started with an up/down orientation it it tended to stay that way. If it started "balls first" it tended to stay that way. Unfortunately during testing I damaged the "balls" end of the tube. So what I am going to do is shorten it a bit and add 3 fins between the balls. I have ordered more table tennis balls and will try the original design again keeping the weight in rear as low as possible.
Click to expand...
You tested the rocket with a motor in place.. in the "all up" flight ready configuration, right?​
​
Rocket Stability by Vern Estes.jpgRocket Stability Diagram by Vern Estes.jpg​
 
lakeroadster said:
You're doing the test wrong then.​
Click to expand...
Listen to The King of Swing! A well built and balanced Balls To The Wall rocket like this one is easily swingable. No excuses and failure is not an option.

A wise old Jedi once told me if you can't computer sim it, don't fly it! Trust the Machines!

Cardboard cut outs for symmetrical designs! Stay on the path to the Light!

Do the Math! Build the wind tunnel. Rocket Science Rules!

Do not be seduced by dangerous mindsiming.
 
Daddyisabar said:
A wise old Jedi once told me if you can't computer sim it, don't fly it! Trust the Machines!
Cardboard cut outs for symmetrical designs! Stay on the path to the Light!
Do the Math! Build the wind tunnel. Rocket Science Rules!
Do not be seduced by dangerous mindsiming.
Click to expand...

Wow, I'm of almost exactly the opposite mindset. I scratch build all the time and have NEVER "simmed" a rocket (mostly because I don't know how to use OpenRocket), but seriously, who want to spend time in front of a PC when you can build instead? Most rockets are not that hard. I can mind-sim for the most part and a decent swing test will confirm my thoughts.

Dude, fly it as-is -- just be sure you're wearing something bulletproof, safety glasses, and a hard-hat. Once you have flown successfully, you won't be scared of flying it the next time with a bigger and badder motor. Build/destroy/build-again. It's a much more fun process than all this "simming" -- simming is for people who are concerned because there might be small children getting injured when the rocket goes sideways, but never fear, if you're by yourself, you're the only one who can get injured!
 
techrat said:
Wow, I'm of almost exactly the opposite mindset.
Click to expand...
And I've just realized that I'm a "darksider" -- which BTW was a term I first heard from the motorcycle subculture -- people who put small automobile tires on their motorcycles. But I also think that's cool because then you get 50,000 miles from a tire instead of the usual 5,000. But I'm not starting that debate here....
 
techrat said:
Wow, I'm of almost exactly the opposite mindset. I scratch build all the time and have NEVER "simmed" a rocket (mostly because I don't know how to use OpenRocket), but seriously, who want to spend time in front of a PC when you can build instead? Most rockets are not that hard. I can mind-sim for the most part and a decent swing test will confirm my thoughts.

Dude, fly it as-is -- just be sure you're wearing something bulletproof, safety glasses, and a hard-hat. Once you have flown successfully, you won't be scared of flying it the next time with a bigger and badder motor. Build/destroy/build-again. It's a much more fun process than all this "simming" -- simming is for people who are concerned because there might be small children getting injured when the rocket goes sideways, but never fear, if you're by yourself, you're the only one who can get injured!
Click to expand...
Those who can, do. Those who can't, take unnecessary risks.​
​
I run a sim for every rocket, and I swing test every rocket.... before launch.​
​
You do you. Careful giving "questionable" advice.​
 
techrat said:
Wow, I'm of almost exactly the opposite mindset. I scratch build all the time and have NEVER "simmed" a rocket (mostly because I don't know how to use OpenRocket), but seriously, who want to spend time in front of a PC when you can build instead? Most rockets are not that hard. I can mind-sim for the most part and a decent swing test will confirm my thoughts.

Dude, fly it as-is -- just be sure you're wearing something bulletproof, safety glasses, and a hard-hat. Once you have flown successfully, you won't be scared of flying it the next time with a bigger and badder motor. Build/destroy/build-again. It's a much more fun process than all this "simming" -- simming is for people who are concerned because there might be small children getting injured when the rocket goes sideways, but never fear, if you're by yourself, you're the only one who can get injured!
Click to expand...
I have never computer simmed, swung test, cut out or wind tunneled. They are just useful tools that are far from infallible. Flew with a very strict RSO who had zero tolerance for unstable oddrocs. The fear was mind numbingling palpable, but that made it fun. Flew a lot of weird stuff and never unstable except for once when I pushed the envelope too far. Fortunately that RSO was not at that launch and it landed close to the away pad. Experience goes the furthest but it takes time, build a lot of complex kits first.
 
Spherical fins aren't fins and don't behave aerodynamically like fins; they only contribute drag. Yes they will have base drag, but OR won't model it (we in general don't get that for tail cones; we need to at some point...). Note that the drag off the balls is always straight off the back, so the "base drag hack" won't work to better model their behavior.

If I were RSO i'd want to see this thing make its first flight on a motor/launch rod combination that had motor burnout very close to being still on the rod (I've got an odd rocket I call the Great Pumpkin that consists of a foam rubber jack o lantern with a motor mount glued into it. It flies on an A10-0, goes about 20 feet high, uses nerf recovery. Great fun).
 
If I was bringing something like this to a club launch, I'd also bring a video of the test flight. Although I do acknowledge not everybody has the space for that.
 
terryg said:
Well the swing test results are discouraging.
Click to expand...
You can't swing it fast enough, to get pressure drag from the balls. Use a VERY punchy motor with short burn time. (Get going as fast as possible before leaving rod, stays low.)

Lauch with only a few other people around, from a long ways away. (Hope for the best; plan for the worst.)
 
I'm so confused, so much conflicting advice. Tiny motor on first flight for safety right? They say you should always test fly a 3-4FNC on the lowest recommended motor for safety and proof of concept. That is being a good boy right? But the lack of speed on such an oddroc abomination "flight" tells you nothing, just like a slow swing test. It doesn't even get off the rod and everyone has a good laugh at the Youngling's oddroc. Or it flops off the rod, same result, back to square one and the computer sim is still being wonky.

Be a very, very naughty boy! Rip it off the pad! The draggy balls rekitted! Cool! I still know nothing but it was awesomely cool. right? Oopsie, in big trouble with the nice boy's as I try to find all the non biodegradable ping pong balls and other bits in the field. I would have bulletproofed but it would have been too heavy on the hind end, requiring an even bigger motor. Big $$$.

I'm still confused but I think making a tiny Micromax model might be the trick to prove stability of my upscale!..what's that you say professor... you can't just scale up and make the aerodynamics and all the other factors work? Darn, these silly oddrocs are hard.

Maybe such a Balls to the Wall oddroc idea is just too silly? No, it is going to be fun. I have puzzled until my puzzler is sore and I have figured it out! No more confusion, it's all so clear to me now! Like the Grinch I have filtered out all the noise and will build it to the best of my ability. Build light, build strong, fly safe. Out to the far far away pad and push the button. Just as Chancellor Palpatine says: "DO IT! "
 
neil_w said:
I'm fairly sure they're *supposed* to. ;)

Will check on this.
Click to expand...
I took a look at it yesterday (and described what's going on in the vNARCON talk today). I turns out OR only reports base drag for "symmetric components", which means nose cone, body tube, and transition. It actually computes both pressure and base drag for other components, but then lumps them together and reports the sum as pressure drag.
https://github.com/openrocket/openrocket/issues/2448
 
What motors are you going to fly it with? E, maybe F, a big G? Build it. Talk to the RSO and LCO. Probably fly it off the HPR pads 100, 200, or 300 ft out where distances make it safe. If it works, you can move it in closer. Just talk to the RSO and LCO first.

Should be fun.
 
I found that I had another full length of BT60 coupler tube. So I am rebuilding it with that, rather then using the shortened tube. I have rebuilt the motor mount and eliminated 1.5 oz off the balls end of the rocket and I am going to use a thinner kevlar shock cord instead of the mule tape I was using.
I will probably use a D24T for its first flight and an Q-Jet E35W later. It is a shame that CTI 24 mm reloads require hazmat as they might be a good choice. It took a surprising amount of force to rip the balls off, so I do not think that will be an issue with a reasonable motor choice.
 
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