Pendulum Canards

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jqavins

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Has anyone made a rocket with canards that are free swinging and weighted at their trailing edges?

Let me modify that. I know someone has, because I read it here recently, but I 1) don't remember who, and 2) don't know why. It seems to me, by a first order thought experiment, that that could help ensure a vertical flight, but with lots of details to be worked, details that may kill the whole thing.

The first step in any further investigation is to research the state of the art, and that starts with asking around. So I'm asking.
 

jqavins

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Do they serve a purpose, other than getting the appearance without the CP penalty?

(Nice kit.)
 

Funkworks

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Here's Apogee's take on Cosmodrome's Black Brant V:

"The rocket is unique in that the forward fins rotate. The reason for this is to neutralize any forces they create. Normally, fins near the front of the rocket are destabilizing and could make the path erratic. But since they each pivot, any forces they create just cause them to rotate the fin, and not the rocket itself.

To be honest, the rocket has such huge fins in the rear, that this feature is overkill. Even if you just mounted these forward fins permanently to the tube, the rocket would still be stable and give you straight flights."


So in model rockets this size, I think they really only adds drag.
 

kuririn

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Maybe would make sense on large forward canards?
 

Funkworks

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Maybe would make sense on large forward canards?
Maybe but I wouldn't know how large. At the professional level, I think forward fins are only for active guidance. So the only actual utility of free-floating canards might be to stabilize very large scale models (but Cosmodrome's BBVC is the only model I know of that has some).
 

burkefj

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I don't believe you need the weight on them either, just put the pivot as far forward as you can, only the portion ahead of the pivot will count against you as fin area from what I remember doing the analysis. This thread covers a talos with lots of discussion, it was finished but not flown iirc

Frank
 
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David_Stack

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a few more past threads on the subject:


and



r/
Dave
 

KenECoyote

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I've been working on it since early this year and it was mid fins. Plan to do more testing this winter and last version had shifting internal pendulum weight that locks the fins off boost so they don't flutter on coast (13mm Tomahawk SLCM) and it worked wonderfully; however ejection debris has fouled the mechanism, so next versions will be enclosed with an ejection bypass system.
I had mostly posted this to Facebook and I think it was the Estes forum and maybe even a Viking bulk pack build thread.
 

KenECoyote

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Oh, my first version was a mini Der Red Max with weighted pendulum canards. Flew very well and nose cone can be used on other rockets.
 

jqavins

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Great, you're the one.

Most of the responses (I haven't followed the links) seem to be about killing the negative stability effect the canards have. Your previous mention of the pendulum canards was about something more that than, was it not? A passive vertical trajectory system?

Would you be so kind as to post more about it? I'm not going to sign up on FB just for this.
 

KenECoyote

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Great, you're the one.

Most of the responses (I haven't followed the links) seem to be about killing the negative stability effect the canards have. Your previous mention of the pendulum canards was about something more that than, was it not? A passive vertical trajectory system?

Would you be so kind as to post more about it? I'm not going to sign up on FB just for this.
Sure, however I may not be able to for a bit since I have a few current projects I have to finish by the next club launch early next month and I'm currently away. I do want to do more testing and build bigger versions (along with control versions) to test this, but that's a winter project at this point. I'll see what I can do, but feel free to message me if you have any specific questions. I thought this up because one of my long term goals for years has been a large scale HP Tomahawk Cruise Missile that uses stabilization so it doesn't become a cruise missile! Lol
 

georgegassaway

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Has anyone made a rocket with canards that are free swinging and weighted at their trailing edges?

Let me modify that. I know someone has, because I read it here recently, but I 1) don't remember who, and 2) don't know why. It seems to me, by a first order thought experiment, that that could help ensure a vertical flight, but with lots of details to be worked, details that may kill the whole thing.

The first step in any further investigation is to research the state of the art, and that starts with asking around. So I'm asking.
No, it's a fallacy. It won't work. I'm at work and do not have time to write more right now. Pendulum Guidance has come up on TRF before.
 

georgegassaway

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OK, I found this thread: https://www.rocketryforum.com/threads/rocket-guidance.30537

It was about guidance overall, but drifted to pendulum guidnance in message #19.

And here is a reply I made, after others made good posts pointing out the problems with gravity based guidance. This looks at what would really be happening onboard, totally ignoring gravity, but totally focusing on any system that would respond to any horizontal force. (that is, perpendicular to the rocket's body, not the Earth, no matter which way the rocket was pointed).

(from: https://www.rocketryforum.com/threads/rocket-guidance.30537/#post-328689)
--------------------------------------------------------------------------------------------
"A lot of good explanations of why a pendulum system won't work for rockets.

Here's another bug, that does not involve why gravity won't make it work.

It's lateral acceleration, or "sideways" G-forces relative to a rocket that for the sake of this beginning properly, at the launch is (was) pointed vertical.

Assuming a perfectly smooth vertical launch, the flight acceleration G force is perfectly straight down the length of the rocket, a line from the nose tip thru the center of the engine nozzle.

But then, a little bit of wind causes the rocket to have a very tiny angle of attack to the airflow, and therefore a tiny little lateral (sideways) G-force.

It's now doomed to rapidly force itself off-vertical and either hit the ground or do loops in the sky.

Note that the little lateral G-force in this case is not gravity-related, it's due to the airflow at a very slight angle of attack and therefore a slight lateral g-force. The rocket could be perfectly vertical but a wind gust could cause this lateral G-force.

The Pendulum guidance responds to this by producing a small corrective deflection to make it "turn" the other way to counter it the presumed off-vertical tilt, but that very corrective force causes its own G-load..... in the same direction as the original disturbance.

The Pendulum system then responds to its own self-generated lateral G-load by moving the control surfaces even more, causing even more of a lateral G- force, and in response to that more corrective "control" which feeds on itself rapidly until the Pendulum system reaches maximum control surface deflection and locks up completely, making the rocket do loops around and around until burnout (and then a few more loops till it slows down enough).

Actually what I described in the above paragraph would happen in a fraction of a second, from first tiny control response attempt to maximum lock-up.

So, if anyone does not quite believe that gravity won't make a free-flying pendulum work, then ignore the gravity part and note the above as to why it won't work for vertical guidance for rockets, it's own control response forces would lock itself up into a loop (if it did not hit the ground first)."
---------------

I hope linking to that 2011 thread does NOT bring about yet another round of "guidance is not legal" crap. It is, long as it's unable to target anything (using guidance to fly "up" is fine).

- George Gassaway
 

KenECoyote

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I think what I'm doing is somewhat different and the dynamics are different at lower impulse/thrust. Will test more this winter and update.
 

jqavins

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OK, I found this thread: https://www.rocketryforum.com/threads/rocket-guidance.30537

It was about guidance overall, but drifted to pendulum guidnance in message #19.

And here is a reply I made, after others made good posts pointing out the problems with gravity based guidance. This looks at what would really be happening onboard, totally ignoring gravity, but totally focusing on any system that would respond to any horizontal force. (that is, perpendicular to the rocket's body, not the Earth, no matter which way the rocket was pointed).

(from: https://www.rocketryforum.com/threads/rocket-guidance.30537/#post-328689)
Yup (he said sadly) that sounds right. I should have realized that myself. Consider the project cancelled. (Back to the active version pipe dream. Yes, others have done it, but for me it's a pipe dream until my budgeted funds, time, and ambition are all increased.)
 

georgegassaway

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Has anyone made a rocket with canards that are free swinging and weighted at their trailing edges?

Let me modify that. I know someone has, because I read it here recently, but I 1) don't remember who, and 2) don't know why. It seems to me, by a first order thought experiment, that that could help ensure a vertical flight, but with lots of details to be worked, details that may kill the whole thing.
Oh, you may have misinterpreted what someone was doing. A few people have claimed that they have built rockets with canard fins, which avoided the instability part by having the canard fins freely pivoting so the airstream keeps them straight into the flow, rather than reaching an angle of attack to the airflow that would cause instability. It sounds like it works, but I've never tried it myself. Perhaps you read that, and misinterpreted it to mean flight path guidance?
 

jqavins

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No, I conceived it as flight path guidance for myself, after reading of someone else's (Ken E. Cayote's) canard pendula. "Why would someone do that. Hmm. Well, if the canard always points down, then..."

Consider a rocket going "straight", in that its velocity and thrust vectors are aligned, but not going vertical, i.e. the velocity vector is not aligned with the gravitational field vector. Now, add a canard that is aligned with gravity. The moment that the canard applies to the rocket is in the desired direction, rotating the rocket toward gravitational alignment.

That much is correct. So all one needs is to put the canard's CG well aft of its pivot and there you go, right? A plumb bob is probably a better analogy than a pendulum.

But there are many complications. The air stream will want to push the canard into line with the airframe, but that's easily solved. Put the pivot right at the fin's CP. Since a plane fin's CP and CG are probably not far apart, you'd need to add mass to the aft end to change that. Again, think plumb bob.

But it will swing like a pendulum, creating reaction forces as the rocket's CG oscillates laterally. How bad would that be? I probably should have some damping in the bearing. But how much? Too much damping won't let the canard react quickly enough to do any good.

OK, step one on this has to be to ask if anyone has done it (or just tried it) before and get the benefit of his/her experience.

And that's why this thread exists. And, as so well explained in the thread that you linked to, the premise is fundamentally and fatally flawed from the start, because a plumb bob doesn't work when the bob and the pivot are accelerating together relative to the gravitational frame. The pivoting canard with the aft mass will not align itself to gravity, and the entire notion goes up in smoke. Or rather, it was only made of smoke to begin with because, refer to the final sentence of the first paragraph, the canard will not always point down.
 

Kelly

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Now, add a canard that is aligned with gravity.
That's the whole problem. There's no way to do this, unless the rocket is sitting still on the pad. As soon as it takes off, the rocket is accelerating, and the pendulum will be aligned with the axis of acceleration, whether that is up, down, or sideways. As soon as the engine burn is done, the rocket is decelerating, and the pendulum will once again be aligned with the axis of acceleration, regardless in what direction this points. Eventually the rocket will slow, and both the rocket and pendulum will be in free fall, and the pendulum will point in a random direction.

But, at no time during flight, will the pendulum have any way whatsoever to align itself with gravity.
 

KenECoyote

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No, I conceived it as flight path guidance for myself, after reading of someone else's (Ken E. Cayote's) canard pendula. "Why would someone do that. Hmm. Well, if the canard always points down, then..."

Consider a rocket going "straight", in that its velocity and thrust vectors are aligned, but not going vertical, i.e. the velocity vector is not aligned with the gravitational field vector. Now, add a canard that is aligned with gravity. The moment that the canard applies to the rocket is in the desired direction, rotating the rocket toward gravitational alignment.

That much is correct. So all one needs is to put the canard's CG well aft of its pivot and there you go, right? A plumb bob is probably a better analogy than a pendulum.

But there are many complications. The air stream will want to push the canard into line with the airframe, but that's easily solved. Put the pivot right at the fin's CP. Since a plane fin's CP and CG are probably not far apart, you'd need to add mass to the aft end to change that. Again, think plumb bob.

But it will swing like a pendulum, creating reaction forces as the rocket's CG oscillates laterally. How bad would that be? I probably should have some damping in the bearing. But how much? Too much damping won't let the canard react quickly enough to do any good.

OK, step one on this has to be to ask if anyone has done it (or just tried it) before and get the benefit of his/her experience.

And that's why this thread exists. And, as so well explained in the thread that you linked to, the premise is fundamentally and fatally flawed from the start, because a plumb bob doesn't work when the bob and the pivot are accelerating together relative to the gravitational frame. The pivoting canard with the aft mass will not align itself to gravity, and the entire notion goes up in smoke. Or rather, it was only made of smoke to begin with because, refer to the final sentence of the first paragraph, the canard will not always point down.
You're on to what I was thinking! :cheers:

Also some other points that may not be taken into consideration in earlier tests by others:
  • Use of canards instead of the rear fins...forward fin movement can have much more affect on the flight path due to axis of motion (?)
  • Gravity has more of an effect at lower acceleration. High thrust can negate/mitigate/lessen the effect which I think was what earlier testers found.
My next tests will involve slower rockets like Der Red Max and involve single pair of fins in the nose cone where I can test it with the weighted pendulum and launch it at 29 or less degrees to vertical. For control, I'll then remove the pendulum weight (add equal weight back to same area) and test with just free moving front canards without the weight. I suspect it will be clear that there is a weighted pendulum effect on the flight path based on some of my observations of what I perceived to be flight corrections on my small mini motor rocket tests. More fun to come later, but much other stuff is on my plate right now (job search is heating up!).
 

jqavins

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That's the whole problem. There's no way to do this, unless the rocket is sitting still on the pad...
Yes, see post #18 and the final paragraph of post #20, where I wrote just that.

You're on to what I was thinking! :cheers:

Also some other points that may not be taken into consideration in earlier tests by others:
  • Use of canards instead of the rear fins...forward fin movement can have much more affect on the flight path due to axis of motion (?)
  • Gravity has more of an effect at lower acceleration. High thrust can negate/mitigate/lessen the effect which I think was what earlier testers found.
My next tests will involve slower rockets like Der Red Max and involve single pair of fins in the nose cone where I can test it with the weighted pendulum and launch it at 29 or less degrees to vertical. For control, I'll then remove the pendulum weight (add equal weight back to same area) and test with just free moving front canards without the weight. I suspect it will be clear that there is a weighted pendulum effect on the flight path based on some of my observations of what I perceived to be flight corrections on my small mini motor rocket tests. More fun to come later, but much other stuff is on my plate right now (job search is heating up!).
Do keep us updated on your results. With a sufficient quantity of sufficient quality data, I've been very effectively persuaded that you will find there's no help. I would be delighted (and puzzled) to be wrong.
 
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KenECoyote

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As soon as the engine burn is done, the rocket is decelerating, and the pendulum will once again be aligned with the axis of acceleration, regardless in what direction this points. Eventually the rocket will slow, and both the rocket and pendulum will be in free fall, and the pendulum will point in a random direction.

But, at no time during flight, will the pendulum have any way whatsoever to align itself with gravity.

btw - I think some are a bit dismissive of gravity not affecting the pendulum. I think we have to acknowledge that gravity does have an effect on every rocket (aim it at 29 degrees on a low/mid-thrust rocket and it will arc over to more than that...well known effect of trajectory I believe), so I don't see how a free swinging weighted item within the rocket is suddenly not affected by gravity in any way. I'm not saying it's a big effect, but there are ways to make it bigger. :)
 

Kelly

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btw - I think some are a bit dismissive of gravity not affecting the pendulum.
Of course, the pendulum is still affected by gravity. The trouble is, the rocket frame is affected by gravity in exactly the same way. There is no relative force, or acceleration, between the rocket and the pendulum, so the pendulum cannot control the rocket in any deterministic way.

Imagine you are inside a big, windowless hollow ball, holding a pendulum. I drop the ball - with you and the pendulum - off of a very tall building. Although gravity is pulling on you, the ball, and the pendulum, you don't really feel it. You will become, effectively, weightless, as will the pendulum. You cannot look at the pendulum, and figure out which way is 'up'.
 

KenECoyote

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Of course, the pendulum is still affected by gravity. The trouble is, the rocket frame is affected by gravity in exactly the same way. There is no relative force, or acceleration, between the rocket and the pendulum, so the pendulum cannot control the rocket in any deterministic way.

Imagine you are inside a big, windowless hollow ball, holding a pendulum. I drop the ball - with you and the pendulum - off of a very tall building. Although gravity is pulling on you, the ball, and the pendulum, you don't really feel it. You will become, effectively, weightless, as will the pendulum. You cannot look at the pendulum, and figure out which way is 'up'.
There is another force you're not accounting for in the example...canards on the same ball would have another influence and if they are angled, they would affect the fall...attach a pendulum and the air resistance would then fight against the same gravity pulling down the weight, but wouldn't affect the same gravity that is pulling down on the ball the same way
 

Kelly

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There is another force you're not accounting for in the example...canards on the same ball would have another influence and if they are angled, they would affect the fall...attach a pendulum and the air resistance would then fight against the same gravity pulling down the weight, but wouldn't affect the same gravity that is pulling down on the ball the same way
I'm not sure I entirely understand the setup you're envisioning here, but let me take a stab... you're saying the ball (from my story) has external canards, that are controlled by the pendulum?
So, first, as I've said, if the ball is in free fall ('free' meaning unaffected by aerodynamics) then the pendulum just floats, and can't control anything. But if, as you say, the canards have an aerodynamic effect (drag or lift) then you're correct the ball is not in free fall, and the pendulum will... uh... dangle. But in which direction does it dangle? The direction will always be in the opposite direction of the aerodynamic force. Let's say the ball is falling vertically: drag will exert an upward force on the ball/canard system, but not on the pendulum, so the pendulum will hang down. But say, instead, I shot the ball off the tower horizontally: now the drag is coming from the side, and the pendulum will stretch out sideways. If I throw the ball up, then the drag is pushing from the top, and the pendulum will actually be inverted - it will dangle up!

So, now the pendulum is just reacting to the direction of travel - it still has no idea which way gravity is pointing, it just knows which direction the ball is moving.
 

KenECoyote

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I'm not sure I entirely understand the setup you're envisioning here, but let me take a stab... you're saying the ball (from my story) has external canards, that are controlled by the pendulum?
So, first, as I've said, if the ball is in free fall ('free' meaning unaffected by aerodynamics) then the pendulum just floats, and can't control anything. But if, as you say, the canards have an aerodynamic effect (drag or lift) then you're correct the ball is not in free fall, and the pendulum will... uh... dangle. But in which direction does it dangle? The direction will always be in the opposite direction of the aerodynamic force. Let's say the ball is falling vertically: drag will exert an upward force on the ball/canard system, but not on the pendulum, so the pendulum will hang down. But say, instead, I shot the ball off the tower horizontally: now the drag is coming from the side, and the pendulum will stretch out sideways. If I throw the ball up, then the drag is pushing from the top, and the pendulum will actually be inverted - it will dangle up!

So, now the pendulum is just reacting to the direction of travel - it still has no idea which way gravity is pointing, it just knows which direction the ball is moving.
Yeah, it's a bit of a brain buster, but I'm pretty sure about my theory...just a bit hard to explain. I'll try to draw it out when I get a chance and may also do some other experiments not requiring launching and I'll report back; however patience would be appreciated since I have a lot of commitments ahead first. This one I enjoy doing as a side experiment to the other things going on and sadly/happily (?) I have a LOT of other hobbies right now fighting for my time. :p 😄 😋 (Plus currently doing extensive research for a job position I'm applying for...has to do with the commercial tire industry.)
 

Kelly

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DEFINITELY not trying to dissuade you from playing around with different ideas, nor trying to rush you. Have fun with it! Just trying to present some of the physics behind it.
 
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