The fins

MatthewL
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The fins

Post by MatthewL »

Would it be better if the fins where small or big?
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Zioly
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Re: The fins

Post by Zioly »

Definitely big... definitely...
Bottle Rockets: 5th
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Scrambler: 29th (with a failed run too  ;))
Mousetrap Vehicle
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Experimental Design (or other inquiry :P)

...Yes, my profile picture is G2 apEX at the PGL Major Qual.  :lol:
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Re: The fins

Post by MatthewL »

Ok thank you
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Re: The fins

Post by kendreaditya »

MatthewL wrote:Ok thank you
I got 14 sec at states with small fins
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Re: The fins

Post by Zioly »

kendreaditya wrote:
MatthewL wrote:Ok thank you
I got 14 sec at states with small fins
I've gotten 15. Yes, it'll get you those times, but those are its max, I believe. If you want great times, such as 20-25 seconds, large fins are best.
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Re: The fins

Post by andrewwski »

It's not that simple.

Larger fins will cause more drag, which slows down the ascent, limits the altitude, and thereby shortens the flight time. But if you don't have large enough fins, the rocket may not be stable. You want the rocket's center of gravity (CG) to be ahead of the center of pressure (CP) - the further ahead the CG is, the more stable the rocket. Although as the water is discharged, the CG will naturally move forward, so the rocket should get more stable throughout its flight.

But - that's only the "up" part. Unfortunately, the attributes (stability) that make the rocket go up nicely also make it come down nicely - too nicely. The more stable the rocket is, the more of a nosedive it will take. If you can change the shape of the rocket at apogee, then you can add drag to slow the descent. But I don't have the rules for this year, frequently that is not allowed.

However, the center of pressure a static quantity - and hence not located at the same place during all points of the flight. Rather, it is also dependent on the angle of the air stream with respect to the rocket (the "angle of attack"). There exist designs - most commonly the "backslider" design - which exploit this to allow for different behavior depending on the angle of attack. In the "backslider", when the rocket is traveling forward, the CG is ahead of the CP for low angles of attack, but behind the CP for high angles of attack.

What essentially happens during a backslider launch is that the angle of attack is initially low - the velocity vector is along the rocket's axis - and the CG is ahead of the CP, so the rocket is stable. Once the rocket reaches apogee, the direction of its velocity switches (so that it's coming down). This means that essentially "forward" and "backward" have switched, so that the bottom end of the rocket is now the "forward" end. But if this is the case, then the CG is now *behind* the CP, so the rocket isn't stable!

When this happens, the rocket will start to rotate over as it is momentarily unstable. In a non-backslider configuration, if the CG and CP are relatively independent of the angle of attack, the rocket will tumble over such that the nose is pointed down - and now the CG is ahead of the CP again, along the velocity vector. So the rocket is stable, but it's nosediving!

In the backslider design, however, as the rocket begins to tip over at apogee (due to its instability), the angle of attack now increases. At high angles of attack, the backslider has the CP closer to the nose than the CG - the reverse of the low angle of attack case! So now, the rocket will want to come down backward! But remember that this stability is at a high angle of attack, so it won't be coming straight down - rather, it will appear to "glide". This gives you higher drag - which you want on the way down!

There exist some convenient approximations to determine where the CP is in both cases - the Barrowman equations give the CP for the low angle of attack case, while the "Center of Lateral Area" gives the CP for the high angle of attack case. You can find explanations of these pretty easily with a quick Google search.

OK, so what's the point of all this and how does it relate to fins? Fins serve to move the CP aft. The location of the CP with respect to the CG determines the stability. The stability is important to understand at every stage of the flight.

So there's not a binary answer as to whether you want "large fins" or "small fins". If the fins are too small, the CP will likely be too far forward and you won't get good results (unstable on the ascent). If they're too large, you'll get added stability but more drag (you'll notice that pretty much every large rocket used for space launches does not have fins for this reason - rather they use active control by gimbaling the engines - which cuts down on drag). If you are able to use parachutes or similar to slow the descent, then you probably want to make the fins just large enough so that the ascent is stable. But if you cannot do that (again, read your rules - I don't have them), then the sizing is driven far more by the design for stability.
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Re: The fins

Post by kendreaditya »

Zioly wrote:
kendreaditya wrote:
MatthewL wrote:Ok thank you
I got 14 sec at states with small fins
I've gotten 15. Yes, it'll get you those times, but those are its max, I believe. If you want great times, such as 20-25 seconds, large fins are best.
I tried big fins they didn't work so well, I got 11 seconds.

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Re: The fins

Post by Zioly »

kendreaditya wrote:
Zioly wrote:
kendreaditya wrote:
I got 14 sec at states with small fins
I've gotten 15. Yes, it'll get you those times, but those are its max, I believe. If you want great times, such as 20-25 seconds, large fins are best.
I tried big fins they didn't work so well, I got 11 seconds.

Image
Hmmm... don't be so quick to blame the size of the fins for the time. As we've learned from the Wright brothers, you never know which parts work and which don't simply by testing the final product! If you don't know what I mean, read up on the legacy of the Wright brothers... in short, their testing apparatus and particularly their method out-shined others through a perfection of individual pieces and not a full machine.

Anyways, back to the present, I'm 99% sure that it's not the size of the fins at fault; rather, your fin material and body design are what jump out at me as most flaw-probable. :D
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Re: The fins

Post by Zioly »

andrewwski wrote:It's not that simple.

Larger fins will cause more drag, which slows down the ascent, limits the altitude, and thereby shortens the flight time. But if you don't have large enough fins, the rocket may not be stable. You want the rocket's center of gravity (CG) to be ahead of the center of pressure (CP) - the further ahead the CG is, the more stable the rocket. Although as the water is discharged, the CG will naturally move forward, so the rocket should get more stable throughout its flight.

But - that's only the "up" part. Unfortunately, the attributes (stability) that make the rocket go up nicely also make it come down nicely - too nicely. The more stable the rocket is, the more of a nosedive it will take. If you can change the shape of the rocket at apogee, then you can add drag to slow the descent. But I don't have the rules for this year, frequently that is not allowed.

However, the center of pressure a static quantity - and hence not located at the same place during all points of the flight. Rather, it is also dependent on the angle of the air stream with respect to the rocket (the "angle of attack"). There exist designs - most commonly the "backslider" design - which exploit this to allow for different behavior depending on the angle of attack. In the "backslider", when the rocket is traveling forward, the CG is ahead of the CP for low angles of attack, but behind the CP for high angles of attack.

What essentially happens during a backslider launch is that the angle of attack is initially low - the velocity vector is along the rocket's axis - and the CG is ahead of the CP, so the rocket is stable. Once the rocket reaches apogee, the direction of its velocity switches (so that it's coming down). This means that essentially "forward" and "backward" have switched, so that the bottom end of the rocket is now the "forward" end. But if this is the case, then the CG is now *behind* the CP, so the rocket isn't stable!

When this happens, the rocket will start to rotate over as it is momentarily unstable. In a non-backslider configuration, if the CG and CP are relatively independent of the angle of attack, the rocket will tumble over such that the nose is pointed down - and now the CG is ahead of the CP again, along the velocity vector. So the rocket is stable, but it's nosediving!

In the backslider design, however, as the rocket begins to tip over at apogee (due to its instability), the angle of attack now increases. At high angles of attack, the backslider has the CP closer to the nose than the CG - the reverse of the low angle of attack case! So now, the rocket will want to come down backward! But remember that this stability is at a high angle of attack, so it won't be coming straight down - rather, it will appear to "glide". This gives you higher drag - which you want on the way down!

There exist some convenient approximations to determine where the CP is in both cases - the Barrowman equations give the CP for the low angle of attack case, while the "Center of Lateral Area" gives the CP for the high angle of attack case. You can find explanations of these pretty easily with a quick Google search.

OK, so what's the point of all this and how does it relate to fins? Fins serve to move the CP aft. The location of the CP with respect to the CG determines the stability. The stability is important to understand at every stage of the flight.

So there's not a binary answer as to whether you want "large fins" or "small fins". If the fins are too small, the CP will likely be too far forward and you won't get good results (unstable on the ascent). If they're too large, you'll get added stability but more drag (you'll notice that pretty much every large rocket used for space launches does not have fins for this reason - rather they use active control by gimbaling the engines - which cuts down on drag). If you are able to use parachutes or similar to slow the descent, then you probably want to make the fins just large enough so that the ascent is stable. But if you cannot do that (again, read your rules - I don't have them), then the sizing is driven far more by the design for stability.
Agreed. The science behind it is precise and at times overwhelming. However, I believe there's a portion of the fins' impact that you have acknowledged, but overlooked, in my opinion. That impact is the drag the fins create on the descent. Large fins will slow the rocket's descent exponentially compared to the slight increase in distance with smaller fins.

While it's true that the fins control the CG, CP, and other things such as CLA, the applications of these are strictly limited to, in my opinion, whether the rocket will nose-dive or not and whether it'll fly straight or not. Once a rocket have achieved a simple ascent, flip at apogee, and horizontal descent, the next step it to try to maximize the time upon that simple structure.

So, if the "simple structure" or that formula was created with CG, CP, and CLA calculations, the next bit is moreso strategy and "weighing the benefits." For example, small fins will make the rocket fly higher, but they also won't create as much drag as larger fins would. Larger fins would cause a rocket to lose some altitude, but ultimately, well-built large fins will entail a very slow descent.

In my opinion, it's the descent that matters--not how high a rocket goes. A slow descent is exponentially more impactful on time than a slightly higher altitude. That being said, your original point is still valid: balance. For example, as you said, it's important to strike a balance between large fins and small fins to ensure that the rocket does in fact stay stable, but that's merely the surface of this great event.

In my haste, I probably overlooked the fact that others might not have had experience properly balancing a rocket without calculations, so that it's practically second nature, but when I said that bigger fins were better, I meant that the strategy they promoted ultimately led to higher times. Additionally, my tip meant to focus on larger fins and to perfect them.

I hope I didn't ramble on too badly and that my point was made clear. If not, please ask, and I'd be more than happy to answer any questions and clear up any confusion. Also, I was in no way discrediting your post, andrewwski; I was just giving my own insight on the strategy of the fins.

EDIT: Sorry for the double post... :?
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Re: The fins

Post by kendreaditya »

Zioly wrote:
andrewwski wrote:It's not that simple.

Larger fins will cause more drag, which slows down the ascent, limits the altitude, and thereby shortens the flight time. But if you don't have large enough fins, the rocket may not be stable. You want the rocket's center of gravity (CG) to be ahead of the center of pressure (CP) - the further ahead the CG is, the more stable the rocket. Although as the water is discharged, the CG will naturally move forward, so the rocket should get more stable throughout its flight.

But - that's only the "up" part. Unfortunately, the attributes (stability) that make the rocket go up nicely also make it come down nicely - too nicely. The more stable the rocket is, the more of a nosedive it will take. If you can change the shape of the rocket at apogee, then you can add drag to slow the descent. But I don't have the rules for this year, frequently that is not allowed.

However, the center of pressure a static quantity - and hence not located at the same place during all points of the flight. Rather, it is also dependent on the angle of the air stream with respect to the rocket (the "angle of attack"). There exist designs - most commonly the "backslider" design - which exploit this to allow for different behavior depending on the angle of attack. In the "backslider", when the rocket is traveling forward, the CG is ahead of the CP for low angles of attack, but behind the CP for high angles of attack.

What essentially happens during a backslider launch is that the angle of attack is initially low - the velocity vector is along the rocket's axis - and the CG is ahead of the CP, so the rocket is stable. Once the rocket reaches apogee, the direction of its velocity switches (so that it's coming down). This means that essentially "forward" and "backward" have switched, so that the bottom end of the rocket is now the "forward" end. But if this is the case, then the CG is now *behind* the CP, so the rocket isn't stable!

When this happens, the rocket will start to rotate over as it is momentarily unstable. In a non-backslider configuration, if the CG and CP are relatively independent of the angle of attack, the rocket will tumble over such that the nose is pointed down - and now the CG is ahead of the CP again, along the velocity vector. So the rocket is stable, but it's nosediving!

In the backslider design, however, as the rocket begins to tip over at apogee (due to its instability), the angle of attack now increases. At high angles of attack, the backslider has the CP closer to the nose than the CG - the reverse of the low angle of attack case! So now, the rocket will want to come down backward! But remember that this stability is at a high angle of attack, so it won't be coming straight down - rather, it will appear to "glide". This gives you higher drag - which you want on the way down!

There exist some convenient approximations to determine where the CP is in both cases - the Barrowman equations give the CP for the low angle of attack case, while the "Center of Lateral Area" gives the CP for the high angle of attack case. You can find explanations of these pretty easily with a quick Google search.

OK, so what's the point of all this and how does it relate to fins? Fins serve to move the CP aft. The location of the CP with respect to the CG determines the stability. The stability is important to understand at every stage of the flight.

So there's not a binary answer as to whether you want "large fins" or "small fins". If the fins are too small, the CP will likely be too far forward and you won't get good results (unstable on the ascent). If they're too large, you'll get added stability but more drag (you'll notice that pretty much every large rocket used for space launches does not have fins for this reason - rather they use active control by gimbaling the engines - which cuts down on drag). If you are able to use parachutes or similar to slow the descent, then you probably want to make the fins just large enough so that the ascent is stable. But if you cannot do that (again, read your rules - I don't have them), then the sizing is driven far more by the design for stability.
Agreed. The science behind it is precise and at times overwhelming. However, I believe there's a portion of the fins' impact that you have acknowledged, but overlooked, in my opinion. That impact is the drag the fins create on the descent. Large fins will slow the rocket's descent exponentially compared to the slight increase in distance with smaller fins.

While it's true that the fins control the CG, CP, and other things such as CLA, the applications of these are strictly limited to, in my opinion, whether the rocket will nose-dive or not and whether it'll fly straight or not. Once a rocket have achieved a simple ascent, flip at apogee, and horizontal descent, the next step it to try to maximize the time upon that simple structure.

So, if the "simple structure" or that formula was created with CG, CP, and CLA calculations, the next bit is moreso strategy and "weighing the benefits." For example, small fins will make the rocket fly higher, but they also won't create as much drag as larger fins would. Larger fins would cause a rocket to lose some altitude, but ultimately, well-built large fins will entail a very slow descent.

In my opinion, it's the descent that matters--not how high a rocket goes. A slow descent is exponentially more impactful on time than a slightly higher altitude. That being said, your original point is still valid: balance. For example, as you said, it's important to strike a balance between large fins and small fins to ensure that the rocket does in fact stay stable, but that's merely the surface of this great event.

In my haste, I probably overlooked the fact that others might not have had experience properly balancing a rocket without calculations, so that it's practically second nature, but when I said that bigger fins were better, I meant that the strategy they promoted ultimately led to higher times. Additionally, my tip meant to focus on larger fins and to perfect them.

I hope I didn't ramble on too badly and that my point was made clear. If not, please ask, and I'd be more than happy to answer any questions and clear up any confusion. Also, I was in no way discrediting your post, andrewwski; I was just giving my own insight on the strategy of the fins.

EDIT: Sorry for the double post... :?
I read most of it but i was kinda confusing. Also, I am a rocket with a 4 fins close to perfect cause it took 2 1/2 to make them. But when I tested them they were fine the nosecone was the issue, the nosecone created to much drag so it didn't get any height at all. The good part is i think i found a good fin size. In addition, I used mylar with a 2.5 ft nosecone and a 4 ft nosecone they still got 11 seconds. I think I might switch back to the T12 or T8 tubes. What type of nosecone did you use and what length did you have?

P.S. is anyone going to rustian invitational in PA
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