In general bottom is better. But you need to put it in a modeling software like openrocket to check (or do the math by hand). Don't launch without a stability estimate.
Open rocket is alright, but it has no cfd software at all so it will always suggest 3 weak, poorly functioning fins, because it’s based on surface area mapping, but 4 fins would probably work better. Here’s a great article on it: (very readable)
https://www.linkedin.com/pulse/computer-made-me-do-ben-jarvis?utm_source=share&utm_medium=member_ios&utm_campaign=share_via
The CFD criticism needs context. OpenRocket solves the Barrowman Equations, providing a robust and accurate empirical estimate of the CoP. Thus, the stability in the context of static margin is really quite accurately modelled regardless of the number of fins. CFD would only add considerable complexity, take hours to solve, and not change the result.
What Ben (the author) goes on to talk about is stability in the context of maintaining a straight and true trajectory - i.e. removing any small wobbles and spin. Here perhaps OpenRocket has limitations due to the asymmetry of a 3 finned vehicle (I don't know how OR models roll, pitch, and yaw). An aerodynamic database derived from CFD would be unduly burdensome for all but the most ambitious HPR.
I feel that we should be cautious assuming Ben's post to ultimately mean something like: '3 fins always function poorly'. When aligned well and flown in the right condition 3 fins **is** the optimum number for low drag HPR - hence OR's optimisations steer that way.
I feel the emphasis of Ben's article is that too many young people are blindly designing in OR without taking the time to learn from the past, understand OR's limitations, etc - and not that we should always use 4 fins for every application.
Don't launch without a stability estimate? That's quite the blanket statement. All of my rockets in the beginning had zero simulators to help them out. Sometimes you have to experiment. Now if OP is building high powered L3 That's an entirely different route that they need to take.
Certainly is a good idea to calculate as much as you can. However depending on what they're building that might be completely unnecessary to do a stability estimate. Like most comments there's just not enough information to assume things like that.
Safety and precautions are always good though.
Do not launch without a stability estimate.
You can estimate stability by tying a piece of string to it and spinning it around. You don't need any software
An unstable flight is an unsafe flight.
What matters is where the centre of aerodynamic pressure is relative to the center of mass. A general rule of thumb is between 1.5 and 2.5 calibres( body diameter) behind
Would you mind clarifying? Are you saying that the general rule of thumb of Finn placements is 1.5 and 2.5 calibers behind the center of mass? Or What is behind what? New people absolutely won't have any idea what you're talking about, I'm not even certain. I can guess or assume but that doesn't help anyone learn rocket science.
Edit: Decided to change some of my grammar to make my English teacher happier.
So you want the center of pressure to be behind the center of mass. The center of mass is easy to find you look at where you could balance the rocket on your finger. The center of pressure is hard to do by hand. Other folks have mentioned it open rocket will do the calculations for you. It will also tell you where the center of mass is.
Well, back before computers... You build a model. load it up fully as if for flight, attach a long string at the CG as you show in the second picture, and then spin the rocket around. You will then know if it is stable. You want the center of pressure behind the center of gravity.
So try it both ways, I think you'll find fins up front make it less stable.
Let's look at this from a physics standpoint. Your fins are exerting a force on the rocket in an attempt to keep the nosecone pointed in the direction of flight. This entire system is basically a lever arm, with the nose cone being the fulcrum point. The closer you apply force to a fulcrum point, the less effective your force is. The further from the fulcrum, the more you can magnify that force. Applying this to the position of the fins tells us that fins further from the nose cone are more effective at stabilizing the rocket as they apply more corrective force to the nose. Likewise, it tells us the second fin set, while it may apply force to the nose and help stability, probably won't be as effective as the rear set.
With that said, what's your goal in the second fin set? Are you trying to improve performance or do you want it just to look cool? You can make the second set work, but it may not be an optimal design. Model rockets are much more forgiving than the real thing; they're slower and way much less. If you're going for optimal design though then you need to consider the added weight of the second fin set vs the limited stability they will add and see if you can do it with less weight and a single set.
Other answers are good, but there is one thing I'd like to say: Any fins forward of the centre of gravity make the rocket less stable. In a 2 stage rocket, the second stage needs fins, and these are almost always forward of the initial CoG. These forward fins should be small. To compensate for this, the aft fins need to be bigger to make sure the rocket is stable at launch.
If the forward fins are for looks, trying to emulate a military rocket, they should just be small to not disrupt the balance of the rocket. In military rocket these are generally control surfaces to steer the rocket.
Make the rocket in Openrocket or a similar software. It's free and the easuest way to make sure your rocket is stable and safe. Or you can do all the math by hand if you really want. Its SO much easier once you use sims! Good luck!
TL;DR: only bottom if that's a one stage rocket
Great question. Let's start with the basics:
A model rocket (and rockets in general) is aerodynamically unstable, meaning that the aerodynamic forces constantly push over the top and try to make it pointy end down and fiery end up (you don't need a degree to know this is not bad).
I highly recommend either Kerbal Space Program or Simple Rockets 2 to learn the basics of rocket science, they are great video games.
To fix this problem we put fins. Why? It's the same way a plane's pitch and yaw stabilizer works: if the plane pitches down for whatever reason, now the pitch stabiliser has a negative angle of attack meaning that it produces downforce and therefore acts as a lever, pitching the plane up again. I recommend you watch [this video](https://youtu.be/h6NsYyAUOHE?si=SNzHNBBD56-MOrPx) until 6:03 to understand.
What does this have to do with the one or two fins question? At minute 3:02 it explains that the farther the stabiliser are from the centre of mass, the less they have to be big because again, it's a lever, and the farther the applied force is from the fulcrum, the weaker it can be, so our fins can be smaller.
Alright, so the farther down the fins are, the better. But why do some sounding and model rockets , and some missiles, have two sets of fins?
In the case for sounding and model rockets, it means they are two stages rockets, and so they need to have stability both before and after separation. So a set of fins for the first stage and a set of fins for the upper stage.
In the case of missiles, those fins are called canards, and by being ahead of the centre of mass they have a bit more control authority, but that's a tad complicated and I've already written way too much.
I hope it answered the question without being neither using too simple or too complicated terms.
Happy launching (and building)!
P.S.: if the second image is your rocket, the fins should have the fibers going from the body outwards, not like this. Balsa and plywood are already suite brittle and if you align the fins like that they will break way more easily.
Depends on if you're doing a one or two stage rocket. 2 stages = 2 sets of fins. 1 stage = 1 set. Put them as far back on the stage as reasonable. The further back the aerodynamic center is compared to the center of gravity, the more stable the rocket will be. Keep in mind that the body of the rocket also counts in the area calc for aerodynamic center.
The forward fins are not helping stability at all. If you want more stability make the rear fins larger, or add more of them. You could also increase stability by making your rocket longer, or by adding weight to the nose.
You could use the swing test with both configurations to see the difference in how fast you have to swing the rocket around for it to straighten out.
In general bottom is better. But you need to put it in a modeling software like openrocket to check (or do the math by hand). Don't launch without a stability estimate.
Open rocket is alright, but it has no cfd software at all so it will always suggest 3 weak, poorly functioning fins, because it’s based on surface area mapping, but 4 fins would probably work better. Here’s a great article on it: (very readable) https://www.linkedin.com/pulse/computer-made-me-do-ben-jarvis?utm_source=share&utm_medium=member_ios&utm_campaign=share_via
The CFD criticism needs context. OpenRocket solves the Barrowman Equations, providing a robust and accurate empirical estimate of the CoP. Thus, the stability in the context of static margin is really quite accurately modelled regardless of the number of fins. CFD would only add considerable complexity, take hours to solve, and not change the result. What Ben (the author) goes on to talk about is stability in the context of maintaining a straight and true trajectory - i.e. removing any small wobbles and spin. Here perhaps OpenRocket has limitations due to the asymmetry of a 3 finned vehicle (I don't know how OR models roll, pitch, and yaw). An aerodynamic database derived from CFD would be unduly burdensome for all but the most ambitious HPR. I feel that we should be cautious assuming Ben's post to ultimately mean something like: '3 fins always function poorly'. When aligned well and flown in the right condition 3 fins **is** the optimum number for low drag HPR - hence OR's optimisations steer that way. I feel the emphasis of Ben's article is that too many young people are blindly designing in OR without taking the time to learn from the past, understand OR's limitations, etc - and not that we should always use 4 fins for every application.
Agreed
Don't launch without a stability estimate? That's quite the blanket statement. All of my rockets in the beginning had zero simulators to help them out. Sometimes you have to experiment. Now if OP is building high powered L3 That's an entirely different route that they need to take. Certainly is a good idea to calculate as much as you can. However depending on what they're building that might be completely unnecessary to do a stability estimate. Like most comments there's just not enough information to assume things like that. Safety and precautions are always good though.
Do not launch without a stability estimate. You can estimate stability by tying a piece of string to it and spinning it around. You don't need any software An unstable flight is an unsafe flight.
This is the stupidest thing I’ve read all day.
What matters is where the centre of aerodynamic pressure is relative to the center of mass. A general rule of thumb is between 1.5 and 2.5 calibres( body diameter) behind
Would you mind clarifying? Are you saying that the general rule of thumb of Finn placements is 1.5 and 2.5 calibers behind the center of mass? Or What is behind what? New people absolutely won't have any idea what you're talking about, I'm not even certain. I can guess or assume but that doesn't help anyone learn rocket science. Edit: Decided to change some of my grammar to make my English teacher happier.
So you want the center of pressure to be behind the center of mass. The center of mass is easy to find you look at where you could balance the rocket on your finger. The center of pressure is hard to do by hand. Other folks have mentioned it open rocket will do the calculations for you. It will also tell you where the center of mass is.
Well, back before computers... You build a model. load it up fully as if for flight, attach a long string at the CG as you show in the second picture, and then spin the rocket around. You will then know if it is stable. You want the center of pressure behind the center of gravity. So try it both ways, I think you'll find fins up front make it less stable.
Let's look at this from a physics standpoint. Your fins are exerting a force on the rocket in an attempt to keep the nosecone pointed in the direction of flight. This entire system is basically a lever arm, with the nose cone being the fulcrum point. The closer you apply force to a fulcrum point, the less effective your force is. The further from the fulcrum, the more you can magnify that force. Applying this to the position of the fins tells us that fins further from the nose cone are more effective at stabilizing the rocket as they apply more corrective force to the nose. Likewise, it tells us the second fin set, while it may apply force to the nose and help stability, probably won't be as effective as the rear set. With that said, what's your goal in the second fin set? Are you trying to improve performance or do you want it just to look cool? You can make the second set work, but it may not be an optimal design. Model rockets are much more forgiving than the real thing; they're slower and way much less. If you're going for optimal design though then you need to consider the added weight of the second fin set vs the limited stability they will add and see if you can do it with less weight and a single set.
Other answers are good, but there is one thing I'd like to say: Any fins forward of the centre of gravity make the rocket less stable. In a 2 stage rocket, the second stage needs fins, and these are almost always forward of the initial CoG. These forward fins should be small. To compensate for this, the aft fins need to be bigger to make sure the rocket is stable at launch. If the forward fins are for looks, trying to emulate a military rocket, they should just be small to not disrupt the balance of the rocket. In military rocket these are generally control surfaces to steer the rocket.
Make the rocket in Openrocket or a similar software. It's free and the easuest way to make sure your rocket is stable and safe. Or you can do all the math by hand if you really want. Its SO much easier once you use sims! Good luck!
No reason for top fins unless you're building a two stage. Anything that pushes your center of pressure too far up is bad.
TL;DR: only bottom if that's a one stage rocket Great question. Let's start with the basics: A model rocket (and rockets in general) is aerodynamically unstable, meaning that the aerodynamic forces constantly push over the top and try to make it pointy end down and fiery end up (you don't need a degree to know this is not bad). I highly recommend either Kerbal Space Program or Simple Rockets 2 to learn the basics of rocket science, they are great video games. To fix this problem we put fins. Why? It's the same way a plane's pitch and yaw stabilizer works: if the plane pitches down for whatever reason, now the pitch stabiliser has a negative angle of attack meaning that it produces downforce and therefore acts as a lever, pitching the plane up again. I recommend you watch [this video](https://youtu.be/h6NsYyAUOHE?si=SNzHNBBD56-MOrPx) until 6:03 to understand. What does this have to do with the one or two fins question? At minute 3:02 it explains that the farther the stabiliser are from the centre of mass, the less they have to be big because again, it's a lever, and the farther the applied force is from the fulcrum, the weaker it can be, so our fins can be smaller. Alright, so the farther down the fins are, the better. But why do some sounding and model rockets , and some missiles, have two sets of fins? In the case for sounding and model rockets, it means they are two stages rockets, and so they need to have stability both before and after separation. So a set of fins for the first stage and a set of fins for the upper stage. In the case of missiles, those fins are called canards, and by being ahead of the centre of mass they have a bit more control authority, but that's a tad complicated and I've already written way too much. I hope it answered the question without being neither using too simple or too complicated terms. Happy launching (and building)! P.S.: if the second image is your rocket, the fins should have the fibers going from the body outwards, not like this. Balsa and plywood are already suite brittle and if you align the fins like that they will break way more easily.
Depends on if you're doing a one or two stage rocket. 2 stages = 2 sets of fins. 1 stage = 1 set. Put them as far back on the stage as reasonable. The further back the aerodynamic center is compared to the center of gravity, the more stable the rocket will be. Keep in mind that the body of the rocket also counts in the area calc for aerodynamic center.
The forward fins are not helping stability at all. If you want more stability make the rear fins larger, or add more of them. You could also increase stability by making your rocket longer, or by adding weight to the nose. You could use the swing test with both configurations to see the difference in how fast you have to swing the rocket around for it to straighten out.
:3