Seeking Insights on Water Rocket Fins Optimization

[Anonymous]

Hello Rocketry Enthusiasts!

I hope this post finds you all soaring high in your rocketry adventures! I'm a high school student currently navigating through the fascinating world of water rocketry for my research paper. My focus is on the optimization of water rocket fins in terms of their size, placement, and shape to achieve maximum stability and efficiency during flight.

My journey for shape optimization so far has led me to delve into the induced drag equation of a fin and explore the lifting line theory in an attempt to find the optimal fin shape. Where the theory suggests that elliptical fin shape produces the least induced drag and is therefore the best.

However, I've encountered a contradiction too: while the lifting line theory provides a foundation, I've also stumbled upon discussions suggesting that, particularly in the realm of smaller rocketry (in relation to nuances related to the Reynolds number), a simple rectangular fin might actually provide superior or at least comparable performance to more complex, theoretically optimized shapes.

In my research, there is a theoretical part, where I try to find the best placement, size and shape through various means (such as those stated above) and then test it in a simulation software and an experimental part, where I then try and build and launch that rocket. I was looking at various simulators and was leaning towards RockSim since they have a specific function related to water rockets, but I was also considering OpenRocket. Could any of you vouch for one or the other in relation to specifically water rocket simulation?

I would appreciate if any of you could give me pointers/more sources to be able to expand my research and knowledge. Thanks in advance!

 

[lakeroadster]

Hello Rocketry Enthusiasts!

I hope this post finds you all soaring high in your rocketry adventures! I'm a high school student currently navigating through the fascinating world of water rocketry for my research paper. My focus is on the optimization of water rocket fins in terms of their size, placement, and shape to achieve maximum stability and efficiency during flight.

My journey for shape optimization so far has led me to delve into the induced drag equation of a fin and explore the lifting line theory in an attempt to find the optimal fin shape. Where the theory suggests that elliptical fin shape produces the least induced drag and is therefore the best.

However, I've encountered a contradiction too: while the lifting line theory provides a foundation, I've also stumbled upon discussions suggesting that, particularly in the realm of smaller rocketry (in relation to nuances related to the Reynolds number), a simple rectangular fin might actually provide superior or at least comparable performance to more complex, theoretically optimized shapes.

In my research, there is a theoretical part, where I try to find the best placement, size and shape through various means (such as those stated above) and then test it in a simulation software and an experimental part, where I then try and build and launch that rocket. I was looking at various simulators and was leaning towards RockSim since they have a specific function related to water rockets, but I was also considering OpenRocket. Could any of you vouch for one or the other in relation to specifically water rocket simulation?

I would appreciate if any of you could give me pointers/more sources to be able to expand my research and knowledge. Thanks in advance!

I would download Open Rocket and give it a try. It's pretty awesome. Start with the sample rocket simulations, work your way up from there.

The fact that your dealing with "water rockets" should have very little bearing on the simulation.

 

[cls]

My journey for shape optimization so far has led me to delve into the induced drag equation of a fin and explore the lifting line theory in an attempt to find the optimal fin shape. Where the theory suggests that elliptical fin shape produces the least induced drag and is therefore the best.

Elliptical tips are great for a Spitfire in a dog fight, but terrible for rockets.

Go read Harry Stein, Handbook of Model Rocketry, chapter on fins shape. Bottom line, trapezoid or clipped delta. Round the leading edge, or foil the whole span if you can do it with precision and utmost consistency.

The thrust equations of water rockets are different than chemical propellant. But you should be able to graph the thrust and weight, and make your own .ENG file for use with Open Rocket.

Gave fun!

 

[Voyager1]

You can run water rocket simulations in apps such as these
https://polyplex.org/rockets/simulation/
http://www.aircommandrockets.com/sim/simulator.htmand extract the thrust curves to create .eng files to use in OpenRocket to simulate your rocket designs.

 

[GuyNoir]

Go read Harry Stein

That’s “Stine”, not “Stein”.

 

[c0c0m0ke]

To: Anonymous (Rocketry enthusiast)


One of the early "Rocketry Show" podcasts, CG and Geem interviewed a fellow who launched a full scale port-a-potty. He thought long fins would stabilize the PP, but his OpenRocket simulations were disagreeing with him. After more time and research, he concluded that fins, to be effective, had to stick out from the rocket. The OR simulations agreed! There is an optimum distance, where beyond a certain distance out, drag and weight become a liability. Listen to the podcast, don't take my word for it

Carlos,

 

[neil_w]

I would download Open Rocket and give it a try. It's pretty awesome. Start with the sample rocket simulations, work your way up from there.

The fact that your dealing with "water rockets" should have very little bearing on the simulation.

It could be fun to include an example water rocket in the program, if we had the details and an appropriate thrustcurve file....

 

[Rschub]

Since you won't be approaching mach, an elliptical nosecone and elliptical planform fins with a decent span and taper, and a tailcone like an Astron Sprint is theoretically the most efficient shape.
https://2.bp.blogspot.com/-GrxuU_sM...73YovCBtgv-iLZFvrx3xMspvMg/s1600/DSCN2192.JPG
I await the inevitable disagreement.

Hint: for the right reynolds number range, this is actually more effecient:

https://sustainableskies.org/wp-content/uploads/2010/07/goldschmeid-body-528x279.jpg

 

[NTP2]

How far down the rabbit hole do you want to go? From your post it looks like very far so I say give openfoam a shot it a free CFD package.
Ps you could write a script to optimize a fin…
Ps a lot of the super optimized fin shapes are really really hard to actually make in reality.

 

[Anonymous]

How far down the rabbit hole do you want to go? From your post it looks like very far so I say give openfoam a shot it a free CFD package.
Ps you could write a script to optimize a fin…
Ps a lot of the super optimized fin shapes are really really hard to actually make in reality.

When talking about a script, are you referring to one made directly in the CFD? I don't have much experience in coding either, so how hard would it be? Thanks

 

[OzHybrid]

When looking for the answer to a magic equation, you need to summon the correct magician. @air.command @air.command @air.command

 

[NTP2]

When talking about a script, are you referring to one made directly in the CFD? I don't have much experience in coding either, so how hard would it be? Thanks

Openfoam is basically a C library so you should be able to find a tutorial for a algorithm online in C just modify it for openfoam and your off to the races.
Edit a google search for C optimization algorithm should do the trick and openfoam is very well documented and there are lots of tutorials online to.

 

[OzHybrid]

The shape of fins is not just about min drag. It's about minimum achievable drag taking into account survivability. This might be about chosing the strongest point of the body to mount the fins and the shape. Or strengthening where you intend to mount.
Moving the fins further back gets you more stability bang from your fin. Regardless of the shape.

 

[Anonymous]

At this point in my research I am doubting whether to do an Openrocket simulation with different fins shapes or an Openfoam one with different fin shapes. The thing is, how do I take into account in OpenFoam that the fin is attatched to the water rocket. Do i have to make a 3D model of the water rocket or just a 3D model of the fin? Also, how extensive (time and power wise) would running a CFD like OpenFoam be?

The experimental part I would posteriorly do would just be to compare the experimental data to a real world application of it, making one or two rockets and launching them a couple of times.

 

[NTP2]

At this point in my research I am doubting whether to do an Openrocket simulation with different fins shapes or an Openfoam one with different fin shapes. The thing is, how do I take into account in OpenFoam that the fin is attatched to the water rocket. Do i have to make a 3D model of the water rocket or just a 3D model of the fin? Also, how extensive (time and power wise) would running a CFD like OpenFoam be?

The experimental part I would posteriorly do would just be to compare the experimental data to a real world application of it, making one or two rockets and launching them a couple of times.

With openfoam the results are very very good you probably don’t need to do the experiment but you can if you want, and you can probably find a file for a water rocket on a 3d printing site you then modify the fins. Edit you could probably run it on a laptop but you would get best results with a good machine. Does your school have a CAD class? if so ask if you can use those machines.

 

[Rschub]

achieve maximum stability and efficiency during flight.

Maximum stability and maximum efficiency are competing goals, there will need to be a trade-off between them.

If you are just concentrating on the fins, there are a few things to consider. First, the fins are a relatively small percentage of the total rocket drag. People looking for fin drag improvements are looking for a tiny performance improvement after or while optimizing everything else.

The fins size will greatly depend on the CG shift as the "fuel" is used. If the rocket can be designed with a very small CG shift, the fins can be smaller if the CG is well forward. Generally, the design of the fin will greatly depend on the other rocket characteristics.

Elliptical fins are perfectly fine, the reason you don't see them on airplanes is due to reasons other than aerodynamic efficiency. For supersonic conditions swept airfoils are used to reduce transonic drag, which you will not likely see, although there some pretty impressive projects out there.

The interference drag at the body/fin intersection is likely to be higher than the drag of the entire fin, so attention is needed there as well.

The airfoil to use is a whole subject of its own.

Some rules of thumb.

Use a symmetrical airfoil with a maximum thickness at the 30-40% chord line. Taper the thickness from 9-11% at the root to as thin as structurally possible at the tip. The planform should be elliptical or a tapered rectangle with a tip chord of 50%ish of the root chord. The aspect ratio around 3-4.

You could use the sim program to vary some parameters to see how they affect drag and put them in a chart for your project.

Good luck!

Edit: A good but ponderous reference is TOWS [Theory Of Wing Sections] here should be a pdf floating around online.

 

[Anonymous]

With openfoam the results are very very good you probably don’t need to do the experiment but you can if you want, and you can probably find a file for a water rocket on a 3d printing site you then modify the fins. Edit you could probably run it on a laptop but you would get best results with a good machine. Does your school have a CAD class? if so ask if you can use those machines.

I currently have a desktop computer at home with 32 GBs of RAM, a AMD Ryzen 7 5800x and a RTX 3070, I am guessing that will be powerful enough to be able to run a few simulations

 

[NTP2]

I currently have a desktop computer at home with 32 GBs of RAM, a AMD Ryzen 7 5800x and a RTX 3070, I am guessing that will be powerful enough to be able to run a few simulations

yup it’s plenty.

 

[Anonymous]

I was considering biting the bullet and buying RockSim after finding out that they allow for much easier setup of water rocket simulations and running through a series of simulations while varying parameters, since for my work I am not looking at doing a simple literature review rather a hands-on experience with my own part of research. An optimization with CFD software, especially taking into account the motion of a water rocket seems overkill and harder to program. If I am trying to do an optimization, would you suggest that I choose, just as an example 5 different shapes such as clipped delta, elliptical, rectangle, triangle and trapezoid and vary parameters such as root chord length on all of them and compare the effects it has? The only ambiguity I see is that since shapes are a data type that do not admit continuous change (e.g: a shape is either a triangle or a circle) I am unable to create a continuous graph that models the effect the change of shape has on the height. Would you believe this to be the best way to go about finding the optimal shape? Or is there another method or way that would be better and more insightful? Do you have any suggestions on parameters to vary and optimize?

 

[NTP2]

I was considering biting the bullet and buying RockSim after finding out that they allow for much easier setup of water rocket simulations and running through a series of simulations while varying parameters, since for my work I am not looking at doing a simple literature review rather a hands-on experience with my own part of research. An optimization with CFD software, especially taking into account the motion of a water rocket seems overkill and harder to program. If I am trying to do an optimization, would you suggest that I choose, as an example 5 different shapes (clipped delta, elliptical, rectangle, triangle and trapezoid) and vary parameters such as root chord length on all of them and compare the effects it has, the problem is that since shapes are a data type that do not admit continuous change (a shape is either a triangle or a circle) I am unable to create a continuous graph that models the effect the change of shape has on the height. Would you believe this to be the best way to go about finding the optimal shape? Or is there another method or way that would be better and more insightful?

That sounds like a pretty good plan to me.

 

[Anonymous]

Do you have any suggestions on parameters to optimize or look at? I was considering doing the basics such as root chord length, number of fins, etc. Just to reiterate and summarize my previous reply, there is no way to "parametrize" (either in Rocksim or OR) the shape itself, such as making a variable that slowly rounds the corners to be able to elaborate a graph that starts with a rectangle fin and ends in an elliptical for example? Am I restricted to just comparing the common fin shapes and within the fin shapes doing an analysis of lengths and areas? Once again, I thank you all for your answers and contributions, it has helped me tremendously in being able to give my research a clearer focus.

 

[Alan15578]

First you nave not clearly defined your optimization. Second, your approach can at best find an optimum of the software model, not real world performance. My question would be how might an optimal fin shape vary between a water rocket and solid propellant rocket?

I will also say that Anonymous seems quite advanced for a high school student.

 

[Anonymous]

First you nave not clearly defined your optimization. Second, your approach can at best find an optimum of the software model, not real world performance. My question would be how might an optimal fin shape vary between a water rocket and solid propellant rocket?

I will also say that Anonymous seems quite advanced for a high school student.

Sorry, by optimization I meant the fin shape (defined by the simulation results), size (defined by the simulation results) and placement (I will define it through Barrowman's equations) that accomplishes the highest altitude for a water rocket with all the other parameters (such as bottle size and shape, quantity of water and pressurized air, etc.) constant. Although rocket fins mostly affect the stability, I assumed that an increase in stability transfers into an increase in height, since there will be less induced drag acting from the rocket tilting constantly and therefore the rocket will assume a higher altitude.

The second part of my research is an investigation aimed at the second point you made, finding out whether the simulation is accurate to a certain degree, this is not to criticize the simulator used, rather to make sure that the simulator's results transfer to real world applications and being able to contrast the two.

Now that you are commenting "out loud" on the solid propellant rocket distinction I did realize that there is probably little change, the only possible change being that since the mass is constantly changing in a water rocket, we must ensure that even with little to no water left, the CG is in front of the CP.

I will take your last statement as a compliment I guess, but the reason for my methodology is that this is intended for a "research paper" for my high school diploma and I want it to be more than a superficial review of the current literature (which wouldn't be graded well either).

 

[Alan15578]

Sorry, by optimization I meant the fin shape (defined by the simulation results), size (defined by the simulation results) and placement (I will define it through Barrowman's equations) that accomplishes the highest altitude for a water rocket with all the other parameters (such as bottle size and shape, quantity of water and pressurized air, etc.) constant. Although rocket fins mostly affect the stability, I assumed that an increase in stability transfers into an increase in height, since there will be less induced drag acting from the rocket tilting constantly and therefore the rocket will assume a higher altitude.

The second part of my research is an investigation aimed at the second point you made, finding out whether the simulation is accurate to a certain degree, this is not to criticize the simulator used, rather to make sure that the simulator's results transfer to real world applications and being able to contrast the two.

Now that you are commenting "out loud" on the solid propellant rocket distinction I did realize that there is probably little change, the only possible change being that since the mass is constantly changing in a water rocket, we must ensure that even with little to no water left, the CG is in front of the CP.

I will take your last statement as a compliment I guess, but the reason for my methodology is that this is intended for a "research paper" for my high school diploma and I want it to be more than a superficial review of the current literature (which wouldn't be graded well either).

As a general rule, I try not to do students work for them, except when I grade/correct their homework. But I've already stepped in it. I graduated high school in 1974, so there many things I do not know. How does a student properly cite reference sources, such as Anonymous, in on-line forums in their formal research papers? I have no experience with the sort of water rockets that high school students may be familiar with. However, your project is similar to what I did for my Senior Project in Aerospace Engineering in 1978, for a BP Dual Egglofter, which I also entered in NARAM-22 R&D competition. Do not look for my report, but I might find it interesting to compare with your uninfluenced results and conclusions.

For the solid propellant rockets, the aft body is just a small diameter cylinder, possibly with a small boat-tail. Fuel does burn off during the flight. but the fuel cannot slosh as a water rocket will. The raw eggs do provide considerable roll damping, but to keep modeling reasonable and make the results more general, I used the hard boiled simplification. I'm guessing that your water rocket is something like a two liter soda bottle with lots of aft curvature, and structural challenges, that could well affect your optimal fin size and shape.

With a water rocket you could afford to make hundreds of flights and record altitude with a small barro-altimeter, until you get a statistically valid result. I had to write my own high fidelity 6DOF simulation program for a general configuration rocket. Simulation is not a bad approach, as long as long as it models your type of water rocket very well.

I did lifting line theory college, not high school. You should not be concerned about induced drag, but you should be concerned about lift-curve slope. This means you will want high aspect ratio fins, but not so high that they flex in flight. You also may find sweep important. You could potentially define an arbitrary fins shape with a dozen or so coordinates and use a genetic algorithm to
optimize the shape. However, we know a lot about wings and fins and you can parameterize with a small number of parameters such as area, aspect ratio, sweep, etc. which will speed your optimization problem. Indeed you might find it helpful to find an optimal static margin and damping ratio first, and then fine tune with fin shape integrated into the flow field of your water rocket.

 

[NTP2]

As a general rule, I try not to do students work for them, except when I grade/correct their tests. But I've already stepped in it. I graduated high school in 1974, so there many things I do not know. How does a student properly cite reference sources, such as Anonymous, in on-line forums in their formal research papers? I have no experience with the sort of water rockets that high school students may be familiar with. However, your project is similar to what I did for my Senior Project in Aerospace Engineering in 1978, for a BP Dual Egglofter, which I also entered in NARAM-22 R&D competition. Do not look for my report, but I might find it interesting to compare with your uninfluenced results and conclusions.

For the solid propellant rockets, the aft body is just a small diameter cylinder, possibly with a small boat-tail. Fuel does burn off during the flight. but the fuel cannot slosh as a water rocket will. The raw eggs do provide considerable roll damping, but to keep modeling reasonable and make the results more general, I used the hard boiled simplification. I'm guessing that your water rocket is something like a two liter soda bottle with lots of aft curvature, and structural challenges, that could well affect your optimal fin size and shape.

With a water rocket you could afford to make hundreds of flights and record altitude with a small barro-altimeter, until you get a statistically valid result. I had to write my own high fidelity 6DOF simulation program for a general configuration rocket. Simulation is not a bad approach, as long as long as it models your type of water rocket very well.

I did lifting line theory college, not high school. You should not be concerned about induced drag, but you should be concerned about lift-curve slope. This means you will want high aspect ratio fins, but not so high that they flex in flight. You also may find sweep important. You could potentially define an arbitrary fins shape with a dozen or so coordinates and use a genetic algorithm to
optimize the shape. However, we know a lot about wings and fins and you can parameterize with a small number of parameters such as area, aspect ratio, sweep, etc. which will speed your optimization problem. Indeed you might find it helpful to find an optimal static margin and damping ratio first, and then fine tune with fin shape integrated into the flow field of your water rocket.

Good advice listen.

 

[Anonymous]

I am finally in my real-world construction phase of the investigation. And I am looking for some insight into attachment modes of the fins to the bottle itself. After doing some research online, I found out that most people either: a) tape the fins directly to the body b) use some sort of glue or silicone c) make tabs with glue that go into the bottle itself. My investigation is looking to compare different fin shapes in the real world too, so my first question is: Should I just construct several water rockets with different fin shapes attached? Or will that make the different flights vary (even if I am using the same measurements for all the cuts? Or is there another way to make them removable, so I can quickly switch them out (I don't want a ring of fins that come out from the actual body)?

Also, I am not planning on using a parachute, seeing as I am simply interested in seeing the max altitude, and designing a parachute mechanism that works is tedious. Therefore, will my wings detach if I'm only using glue/tape/silicone? Will they break? (I am planning on 3D printing them with PLA plastic).

One final question regarding design is that I am also making a 3D printed nose cone with a seemingly elliptical shape, it is not hollow. How would you suggest I attach it to the bottom part of the bottle, which has been cut out? I had initially thought to straight up use tape or make a ledge with some type of water-resistant material on the inside of the bottle, so I can glue the nose cone to the body. Also, I wanted to include an Estes altimeter somewhere in the nose cone, to achieve it would you suggest making a small hollow compartment to fit it in (which can just be taped or covered to save it from the water)?

I once again cannot thank you all enough for all the help you have provided with my research

 

[NTP2]

I am finally in my real-world construction phase of the investigation. And I am looking for some insight into attachment modes of the fins to the bottle itself. After doing some research online, I found out that most people either: a) tape the fins directly to the body b) use some sort of glue or silicone c) make tabs with glue that go into the bottle itself. My investigation is looking to compare different fin shapes in the real world too, so my first question is: Should I just construct several water rockets with different fin shapes attached? Or will that make the different flights vary (even if I am using the same measurements for all the cuts? Or is there another way to make them removable, so I can quickly switch them out (I don't want a ring of fins that come out from the actual body)?

Also, I am not planning on using a parachute, seeing as I am simply interested in seeing the max altitude, and designing a parachute mechanism that works is tedious. Therefore, will my wings detach if I'm only using glue/tape/silicone? Will they break? (I am planning on 3D printing them with PLA plastic).

One final question regarding design is that I am also making a 3D printed nose cone with a seemingly elliptical shape, it is not hollow. How would you suggest I attach it to the bottom part of the bottle, which has been cut out? I had initially thought to straight up use tape or make a ledge with some type of water-resistant material on the inside of the bottle, so I can glue the nose cone to the body. Also, I wanted to include an Estes altimeter somewhere in the nose cone, to achieve it would you suggest making a small hollow compartment to fit it in (which can just be taped or covered to save it from the water)?

I once again cannot thank you all enough for all the help you have provided with my research

I would make the nose hollow for mass savings, if you have a printer you can probably do some sort of quick replace with the fins, if you want I can even do the CAD.

 

[Anonymous]

If I do end up making the nose hollow, how would you suggest I place the altimeter? Since the water rocket is essentially just nosecone and bottle body (where the water is, so a no-go for the altimeter). What do you mean by quick replace? My plan was just making a whole set of different fins and a set of nosecones and making different bottle rockets (with same bottle model and measurements) to launch. I already have modified a sort of elliptical nosecone, the file of which is adjoined below. It was originally solid, but upon your suggestion, I have hollowed it out.

The only part I have yet to design are the fins, but they should be fairly simple to do considering I am planning on doing them without airfoil, just because in my optimizations I have separately varied the root chord length, on the one hand, in increments of 1 cm, from 5 cm to 20 cm (having to adjust the tip chord length as a ratio to preserve the shape) and the span of same way, since it turned out the feature of parametrization of RockSim had been removed, unfortunately, and the only parameters which are consistent throughout different common fin shapes (clipped delta, trapezoidal, rectangular, triangular, elliptical...), since elliptical fins, as an example, don't have a sweep angle nor a concrete tip chord length. My reasoning behind the airfoil argument was that seeing as there is no way to vary cross-section in increments in the simulation (a part from selecting round, square or airfoil) I think I will just let it be, even though it may be slightly more optimal. The problem really stems from the fact that I don't know exactly what airfoil RockSim uses when it says airfoil, so even if I wanted to then create a CAD file (a part from it being a task and environment I am not that well versed in) I wouldn't know what measurements to do, as to imitate the ones made in Rocksim. If you have any ideas on the topic, I would appreciate it, but that is my current plan.

P.S I was going to print both the nose cone and the fins in PLA plastic

 

[NTP2]

If I do end up making the nose hollow, how would you suggest I place the altimeter? Since the water rocket is essentially just nosecone and bottle body (where the water is, so a no-go for the altimeter). What do you mean by quick replace? My plan was just making a whole set of different fins and a set of nosecones and making different bottle rockets (with same bottle model and measurements) to launch. I already have modified a sort of elliptical nosecone, the file of which is adjoined below. It was originally solid, but upon your suggestion, I have hollowed it out.

The only part I have yet to design are the fins, but they should be fairly simple to do considering I am planning on doing them without airfoil, just because in my optimizations I have separately varied the root chord length, on the one hand, in increments of 1 cm, from 5 cm to 20 cm (having to adjust the tip chord length as a ratio to preserve the shape) and the span of same way, since it turned out the feature of parametrization of RockSim had been removed, unfortunately, and the only parameters which are consistent throughout different common fin shapes (clipped delta, trapezoidal, rectangular, triangular, elliptical...), since elliptical fins, as an example, don't have a sweep angle nor a concrete tip chord length. My reasoning behind the airfoil argument was that seeing as there is no way to vary cross-section in increments in the simulation (a part from selecting round, square or airfoil) I think I will just let it be, even though it may be slightly more optimal. The problem really stems from the fact that I don't know exactly what airfoil RockSim uses when it says airfoil, so even if I wanted to then create a CAD file (a part from it being a task and environment I am not that well versed in) I wouldn't know what measurements to do, as to imitate the ones made in Rocksim. If you have any ideas on the topic, I would appreciate it, but that is my current plan.

P.S I was going to print both the nose cone and the fins in PLA plastic

You could
A glue the altimeter in
B make something that is glued to the inside of the nose and has a spot for the altimeter
As for a quick replace, I was imagining a ring glued to the water bit and pins, when you need to change a fin you remove the pins and put a new one in.

Ps why different noses it’s not very good idea as you have just removed a constant.

 

[Anonymous]

If I do end up making the nose hollow, how would you suggest I place the altimeter? Since the water rocket is essentially just nosecone and bottle body (where the water is, so a no-go for the altimeter). What do you mean by quick replace? My plan was just making a whole set of different fins and a set of nosecones and making different bottle rockets (with same bottle model and measurements) to launch. I already have modified a sort of elliptical nosecone, the file of which is adjoined below. It was originally solid, but upon your suggestion, I have hollowed it out.

The only part I have yet to design are the fins, but they should be fairly simple to do considering I am planning on doing them without airfoil, just because in my optimizations I have separately varied the root chord length, on the one hand, in increments of 1 cm, from 5 cm to 20 cm (having to adjust the tip chord length as a ratio to preserve the shape) and the span of same way, since it turned out the feature of parametrization of RockSim had been removed, unfortunately, and the only parameters which are consistent throughout different common fin shapes (clipped delta, trapezoidal, rectangular, triangular, elliptical...), since elliptical fins, as an example, don't have a sweep angle nor a concrete tip chord length. My reasoning behind the airfoil argument was that seeing as there is no way to vary cross-section in increments in the simulation (a part from selecting round, square or airfoil) I think I will just let it be, even though it may be slightly more optimal. The problem really stems from the fact that I don't know exactly what airfoil RockSim uses when it says airfoil, so even if I wanted to then create a CAD file (a part from it being a task and environment I am not that well versed in) I wouldn't know what measurements to do, as to imitate the ones made in Rocksim. If you have any ideas on the topic, I would appreciate it, but that is my current plan.

P.S I was going to print both the nose cone and the fins in PLA plastic

I meant constantly the same nose cone, but considering I was going to make many rockets, I would need more than 1.