Aerodynamics question

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DrewW

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When a rocket stops being radially symmetric, and you have wings or the like are there any rules of thumb for maintaining stability? Right now I'm just assuming that the minimum fin projected area in any view needs to have at least 1 cal of stability as a starting point...but that's more of a assumption.
 

mjennings

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Yup that's a good starting point. Cardboard cut out method is generally conservative so if you have a cut out for each view find the CP and assume the least stable answer is true CP. You can do more that a top and side view as well. Take the 45 degree angles too and get the CP from those projections.
 

Jeff Lassahn

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Yup, doing the CP calculation at different angles and making sure the worst one is still OK is definitely the right thing to do. Note that OpenRocket generally does the right thing here automatically.

Two other things to consider:
1. Make sure the thrust vector of the engine points through the center of mass of the rocket. If the center of mass isn't on the symmetry line you might need to offset or tilt the engine slightly to keep things lined up right.
2. Make sure the lift and drag forces from any asymmetric fins and stuff are close to even around the thrust axis. If not the forces will tend to pull the rocket into flying a loop instead of a straight flight.

#1 and #2 are separate problems, you have to solve both of them, you can't e.g. have a off center thrust vector an compensate by adding off center drag in the opposite direction.
You can cope with a little bit of off-center force of either kind by making the rocket roll, so that it corkscrews rather than loops. But not too much...
 

DrewW

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2. Make sure the lift and drag forces from any asymmetric fins and stuff are close to even around the thrust axis. If not the forces will tend to pull the rocket into flying a loop instead of a straight flight.
@Jeff Lassahn thank you! I didn’t even have a notion this could happen, though it explains the action I saw from someone else’s rocket glider this weekend. Any thoughts on how close is enough between the perpendicular views? 1 cal? 2?
 

Jeff Lassahn

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It's OK for the different perpendicular directions to have very different CP positions.
The thing to watch out for is if one of the views is lopsided, so that it produces a torque trying to pull to one side.
 

DrewW

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It's OK for the different perpendicular directions to have very different CP positions.
The thing to watch out for is if one of the views is lopsided, so that it produces a torque trying to pull to one side.
@Jeff Lassahn I'm not usually this dense, but let me parrot back in my words to check if I'm understanding correctly. As long as adequate stability exists center of pressure is not the issue I need to be concerned with, however, differences in drag forces will induce a torque moment? Presumably, high drag on one side of the rocket slows that side down allowing the other side to accelerate, causing a desire for pointy end down behavior?
 

Jeff Lassahn

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It sounds like you're saying what I'm meaning.
Somebody has a quote that goes something like "the biggest problem with communication is the illusion that it has occurred", and this might be one of those cases. Hopefully we're both making sense, and all is well.
 

jqavins

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Yup, doing the CP calculation at different angles and making sure the worst one is still OK is definitely the right thing to do. Note that OpenRocket generally does the right thing here automatically.
And so does RockSim.

Unequal drag is, of course, an issue, but it's not one you need to obsess over until it's pretty large. Look at the Quest Force 5, as an example. The ventral fins, which you can't see in the picture, are just two parallel small strakes. Not much drag on that side, as compared to the dorsal side with the wing dihedral, the winglets on top of that, and the vertical stabilizer (dorsal fin). That should make it want to pitch up. But the big wings give it a lot of pitch restoring force, and it flys fine.

Yes, those side pods are offset to the ventral side, which will make the drag a little closer to equal. But I'm sure it's still quite unequal, since the wings are offset to the dorsal side and the wing to pod junction is surely very draggy.
 

DrewW

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Yup, doing the CP calculation at different angles and making sure the worst one is still OK is definitely the right thing to do. Note that OpenRocket generally does the right thing here automatically.
@Jeff Lassahn @jqavins So I just ran an engineering experiment in OR and sure enough, OR presents CP as the position in the least stable axis...I didn't realize that was the case, makes me feel much better about first order stability in some non-standard designs I'm fooling around with.
 

BABAR

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Some real world rockets with asymmetric fin designs that were squirrelly but successful and enjoyable flyers. Doubt they would sim out as stable.


post 4







the last one is basically a Picasso version of your basic 3FNC

all had net straight trajectories, and decent altitudes.
 

DrewW

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Some real world rockets with asymmetric fin designs that were squirrelly but successful and enjoyable flyers. Doubt they would sim out as stable.
@BABAR please teach me this forbidden knowledge!
 

BABAR

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I think I started down the dark side with this, designed by Bruce Levinson.

The plans link doesn't work anymore, maybe @hcmbanjo can chime in for a source, JonRocket.com doesn't sell the kit anymore. But it gives you an idea what is possible.

Keys to my success are in alignment, even when offset planar and tube fins always aligned with long axis of rocket.

I'll leave you with this two planar fin rocket

 

hcmbanjo

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I think I started down the dark side with this, designed by Bruce Levinson.
The plans link doesn't work anymore, maybe @hcmbanjo can chime in for a source, JonRocket.com doesn't sell the kit anymore. But it gives you an idea what is possible.
Keys to my success are in alignment, even when offset planar and tube fins always aligned with long axis of rocket.
I'll leave you with this two planar fin rocket
The Corkscrew was an Odd'l Rockets kit for quite a while.
I wrote Bruce Levison and got his permission to produce the kit.
I liked the design being simple and flight performance lived up to it's name.
I have plans to reissue it using standard 18mm motors. The first kit was 13mm.
I just haven't had the time lately for any new kit production.
 

DrewW

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@hcmbanjo any thoughts on resource material for exploring less traditional rocket builds for someone with a fairly technical background? You, @jflis, @BABAR, and @Daddyisabar all seem to have some pretty deep intuition about these things.
 

Daddyisabar

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So simple. Search your feelings. What do you love and can you fly it? I like old warbirds, trains, monsters and science fiction. Flown them all. Follow the basic rules but keep pushing the envelope. Gain experience building kits, do some kit bashing and then scratch build.
 

DrewW

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I think I started down the dark side with this, designed by Bruce Levinson.

The plans link doesn't work anymore, maybe @hcmbanjo can chime in for a source, JonRocket.com doesn't sell the kit anymore. But it gives you an idea what is possible.

Keys to my success are in alignment, even when offset planar and tube fins always aligned with long axis of rocket.

I'll leave you with this two planar fin rocket

@BABAR @hcmbanjo from the historical threads it looks like a BT20 ~20-24" long, with a ~1.5" BT80 ring fin, and a PNC-18A? Streamer recovery I'm guessing?
 

BABAR

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@hcmbanjo any thoughts on resource material for exploring less traditional rocket builds for someone with a fairly technical background? You, @jflis, @BABAR, and @Daddyisabar all seem to have some pretty deep intuition about these things.
Not so sure about intuition as experience.

There is the old joke with various versions around.

The one I heard was the reporter talking to a famous General with a long record of winning battles

The reporter asked, "how did you manage to win so many battles?"

The general replied, "good decisions."

The reporter asked," General, how did you make so many good decisions?"

The General replied, "experience."

The reporter then asked how the general got so much experience.

The general replied, "Bad decisions."
 

BABAR

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@BABAR @hcmbanjo from the historical threads it looks like a BT20 ~20-24" long, with a ~1.5" BT80 ring fin, and a PNC-18A? Streamer recovery I'm guessing?
Any body tube size will work. I'd say scale the picture for a BT-20. Any nose cone will work. Use the scale for length and size of the ring. Not sure if Bruce split the ring, I just put the inside ring on the outside diameter of the BT-20. I used two launch lugs, one on each side of the BT-20 / Ring joint, for strength. You could go with on lig and replace the other with a 1/8" balsa strip (aka a "cheater") Quaker cardboard round oatmeal canisters (not the cheap generic cannisters), pringles cans, and mixed nuts cans (which are usually cardboard with a metal lid and base) can be substituted for the ring.

Streamer recovery is great, the ring has so much drag, it comes down slow anyway once you blow the nose.

@hcmbanjo @Daddyisabar , here's an interesting question: I know real research sounding rockets occasionally used canted fins or even a helical launch tube to impart spin to rockets to minimize weather cocking. Would corkscrew rockets also be relatively immune?
 

BABAR

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This may help
Build thread on BT-5 KIT
 

jflis

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@hcmbanjo any thoughts on resource material for exploring less traditional rocket builds for someone with a fairly technical background? You, @jflis, @BABAR, and @Daddyisabar all seem to have some pretty deep intuition about these things.
Yeah, as some have mentioned, it’s more an “experience” thing... 58 years of rocketry and you tend to gain and learn from a lot of experiences... you learn more from the mistakes though, and I’ve sure made my share of mistakes... :)
 

hcmbanjo

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Any body tube size will work.
Well, not really -
The longer the body tube, the less of the corkscrew effect.
Too short a body tube and it would be unstable.
I'm referring to the main, longer tube length, not the offset ring at the bottom.
If the main tube is too long it could go straight up with no noticeable corkscrew wiggle.
 

BABAR

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Well, not really -
The longer the body tube, the less of the corkscrew effect.
Too short a body tube and it would be unstable.
I'm referring to the main, longer tube length, not the offset ring at the bottom.
If the main tube is too long it could go straight up with no noticeable corkscrew wiggle.
My bad, I was thinking diameter. You are already planning BT-20 upscale, I am sure the concept would work with larger tubes. Proportionately to the ring, lengthwise there will be a sweet spot which will be stable but corkscrewy. Although I have my doubts about toooooo long, my suspicion is that just about any realistic length will still corkscrew, but not sure the optimum length for (nor definition OF) optimum “corkscrewyness”.

a helical trail from an 18 mm Q-Jet would be cool, although not sure the ring could handle the load.
 

BABAR

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Yeah, as some have mentioned, it’s more an “experience” thing... 58 years of rocketry and you tend to gain and learn from a lot of experiences... you learn more from the mistakes though, and I’ve sure made my share of mistakes... :)
so long as no one is hurt and no property other than possibly the rocket itself is damaged, I don’t mind mistakes as along as they teach me something. Then again, I am L-0, so I don’t have a lot invested in casings and electronics.

It’s when the rocket goes catywampus and there is no good explanation (and thus nothing to do differently to keep it from doing it AGAIN) that is frustrating.

and it’s always nice if you have a field without a lot of spectators so you are less embarrassed when things don’t go as planned!
 

Daddyisabar

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so long as no one is hurt and no property other than possibly the rocket itself is damaged, I don’t mind mistakes as along as they teach me something. Then again, I am L-0, so I don’t have a lot invested in casings and electronics.

It’s when the rocket goes catywampus and there is no good explanation (and thus nothing to do differently to keep it from doing it AGAIN) that is frustrating.

and it’s always nice if you have a field without a lot of spectators so you are less embarrassed when things don’t go as planned!
The hard part is when you live in a metro area and the club's RSO is the toughest I have ever seen. Failure is not an option, we use NASA standards. So young man where is your wind tunnel data, your rocsim, your swing test video, your cardboard cut outs? Well Sir the rocket is so asymmetrical that I couldn't do a rocsim and the cut out method didn'twork either. Do to the design I couldn't find a way to attach a swing line. Don't have access to wind tunnel. But I do have plenty of power if the entire cluster lights, plenty of nose weight to bring the CG to appropriate mindsimed position and a really long and thick stainless steel launch rod! Everything is well built and it looks real Purdy!...
 

DrewW

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The gentleman who regularly RCOs for my club is on the NFPA Committee and fairly risk averse, he gives my kids’ E2X rockets a thorough eval before flights.
 

shockie

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Any body tube size will work. I'd say scale the picture for a BT-20. Any nose cone will work. Use the scale for length and size of the ring. Not sure if Bruce split the ring, I just put the inside ring on the outside diameter of the BT-20. I used two launch lugs, one on each side of the BT-20 / Ring joint, for strength. You could go with on lig and replace the other with a 1/8" balsa strip (aka a "cheater") Quaker cardboard round oatmeal canisters (not the cheap generic cannisters), pringles cans, and mixed nuts cans (which are usually cardboard with a metal lid and base) can be substituted for the ring.

Streamer recovery is great, the ring has so much drag, it comes down slow anyway once you blow the nose.

@hcmbanjo @Daddyisabar , here's an interesting question: I know real research sounding rockets occasionally used canted fins or even a helical launch tube to impart spin to rockets to minimize weather cocking. Would corkscrew rockets also be relatively immune?
What's the difference between spin,rotation or cork screwing? Different types of the same phenomenon? It seems to me a moment/force relationship is involved?
 

BABAR

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What's the difference between spin,rotation or cork screwing? Different types of the same phenomenon? It seems to me a moment/force relationship is involved?
Somebody probably will come up with a better answer than me.
Here is my take.


pure spin is rotation strictly around the long axis of the rocket. It is usually caused by either a canted fin or fins or a non-neutral airfoil. (For high performance in subsonic range, rounded front and tapered tail airfoil = good, non-neutral [think airplane or glider wing] not so good.). If you look at flight vids, MOST low power rockets have a moderate amount of spin that you, watching the rocket from the ground, really don’t notice at all. The smoke trail is straight or nearly straight. The rotation costs minimal performance loss, unless it is really pronounced not significant for a sport model,

corkscrewing to me is where the nose of the rocket is tilted Slightly off axis, which would normall cause a St Louis Arch flight with a prang. The cause can still be a canted fin, but for intentionally corkscrewing rockets is usually asymmetric drag, exaggerated on one side of the rocket, (Bruce Levinson’s single Ring Fin attached at one edge directly to the body tube, is a classic and extreme example.) But even though the nose is off axis, the rocket is rotating, so even though the nose is off axis, the rotation is continually correcting the NET trajectory. Just like the nose is off axis, the tail and the nozzle plume is off axis, so the smoke trail is helical. The appearance of the helix also often changes between boost and coast phases. Expect a significant hit in maximum altitude, but these birds are not altitude seekers anyway.

there is also something called “coning”, which I read about when I wander into the high power forum. I don’t pretend to understand that, except that it is apparently bad.
 

DrewW

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The Corkscrew was an Odd'l Rockets kit for quite a while.
I wrote Bruce Levison and got his permission to produce the kit.
I liked the design being simple and flight performance lived up to it's name.
I have plans to reissue it using standard 18mm motors. The first kit was 13mm.
I just haven't had the time lately for any new kit production.
How much nose weight would you recommend for the 18mm Corkscrew variety?
 
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