Need help designing a model rocket

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dee

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Hi! I'm new here and just wanted to ask a few questions. I'm a total beginner so I apologise if these are too basic or obvious.

I need to design a model rocket that is less than 180 cm in length and is capable of carrying a 1 kg payload, deploying it at 1km and then safely landing after.

So my questions are:

1. Is there a particular equation to use when designing the nosecone? I found a video using FreeCAD to design nosecones and have currently made an elliptical nosecone using our required dimensions (length and diameters). Is there a way to check if it'll perform well?
2. We started out with an OpenRocket simulation and are using the dimensions from the final optimised simulation to do the CAD modelling. How are the dimensions of the tail cone determined? What are the parameters that influence the design of the tail cone?
3. Similarly, how does one go about designing the fins? We have a trapezoidal set from our OpenRocket simulation that provides decent stability. Do we just convert it to CAD directly? How can we test whether it's a good design?

For context, the OpenRocket simulation gave favourable results, we got the desired velocity, weight and apogee which is why we decided to replicate that design in our CAD models. Right now, I'm planning to use the nosecone from FreeCAD and then build the rest of the body tube on SolidWorks including the couplers, bulkheads and the engine mount - all as separate parts so I can insert the payload and avionics bays later. Is that a good approach or is there a better alternative?

I've come across several equations but I can't seem to know where I should start from or what I should be looking for or how I can use them in the designing process. I've also read about the 5:1 ratio for the tail cones and would love to know how these results are arrived at and their significance.

I'd appreciate any help and advice regarding the designing process in general.

Thank you.
 
Nose cone shape doesn't have a huge impact on aerodynamics at sub sonic speeds. It has some effect, but nose cone shape is way down the list of things you need to do correctly.

Can you post your openrocket design?

What motors are you considering? Do you have access to commercial motors like Aerotech? Will this be a high power rocket? What country are you in?
 
One way to answer questions such as "is this the best design for fins/nose cone/other parts?" is to change the design in OR and see what happens to altitude, speed, or whatever parameter is important. It's not a perfect solution but it will usually provide insight on what is important. Try changing the rocket diameter and see what happens to altitude. Then try changing nose cone shape and/or length, or the root length or tip length etc. of the fins. Playing around with OR will provide a lot of education. And it's a lot cheaper than making a new set of fins for each test flight.
 
1kg is a lot of mass, not in the realm of a beginner's rocket. 1km altitude is achievable, with motors available to beginners, but with zero or perhaps 30g payload.

Whoever set up this task for you did it wrong, they may not understand how things are done in the hobby.

But, please go ahead, make your design, and then look realistically at what it takes to achieve it. I think you'll need "Level 1" certification, at least.
 
Hi! I'm new here and just wanted to ask a few questions. I'm a total beginner

I need to design a model rocket that is less than 180 cm in length and is capable of carrying a 1 kg payload, deploying it at 1km and then safely landing after.
Ignore this post if @dee is proposing a simple intellectual or "on paper" academic exercise. OTOH, if the plan is to actually build and fly such a rocket, the following applies

Okay, there have to be others that have problems with the above opening statements.

I wish the @dee the best with this, but if a real project, this isn't do-able without a level 1 cert from SOMEBODY involved locally in the process. I started this before @cls post above, but he beat me to the button.

Find a local club and they will likely be a great additional resource for your project.

Good Luck!
 
If you're trying to hit a target altitude with a particular motor, you may not actually want things to be as slick or as light as possible.

If you are trying to minimize the power required, that's another story. It also matters whether you are using a motor with a quick burst, or a motor with a long burn. With a quick burst, there will be an optimum weight for any given rocket design, and it might even be significantly heavier than 1 kg. If so, you don't have to make everything as light as possible. If not, or if the motor is a long burning, low thrust one, saving weight helps.

A subsonic, optimized rocket, for flight in the lower atmosphere, probably includes a very smooth pod, just large enough for a payload, with a tail boom just long enough for the fins to maintain stability, and with a boom just large enough for the motor you use. In the case of a long burning motor, or one which gives an optimum weight close to or below 1kg, approximate unobtainium as well as you can in order to save weight. ;-)

The following is a picture of a test body for research on low drag shapes. If I'm not mistaken, it was dropped in a deep lake. However, the same shape would make a great payload carrying, single stage rocket if you had impossibly thin rocket motors. Think of it as a caricature:
https://commons.wikimedia.org/wiki/..._tel_que_rapporté_par_Bruce_H._Carmichael.png
Bernard de Go Mars, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons


With all the CAD, it sounds like you might be thinking of 3D printing parts of the rocket. However, keep in mind that most 3D printing materials are flexible and weak for their weight, compared to things like balsa, plywood, or composites with carbon fiber, kevlar, or even fiberglass. I suspect even the cardboard they make body tubes with is superior. 3D printing might allow you to make some wild shapes, but they'll be heavy, fragile, or both. You can probably do a pretty good job without any exotic materials.

When designing, keep in mind the materials, tools and methods you are familiar with, have on hand, and/or prefer. I'm sure a wide variety of good designs are possible, and far more bad ones.

If you don't need to get the last bit of performance, or if you don't have time, go with something entirely conventional.

1.
As Zeus-cat points out, the nose cone is not terribly critical, though I suppose you could make a terrible one if you tried to. An ellipse ought to be fine until you are trying to tweak the last one percent of performance. It would be quite easy to do an elliptical nose cone in Solidworks, BTW.

2.
You can just fiddle around with Openrocket and get close enough, unless you want to get sophisticated. Remember that a boat tail slightly reduces stability.

3.
If Openrocket says it's stable with those fins, it probably is. Make the fins thick enough so that they are unlikely to break or flutter with the materials you prefer to use. This is one place where balsa and/or plywood are superior materials, unless you do something clever with 3D infill. Come to think of it, if you're good with CAD, you could make a 3D printed core for a fin out of honeycomb, and then sheet it with veneer. But we're getting fancy here. Fins have a fair amount of the drag of the rocket. Round edged fins will have much less drag than square edged ones. Fins that are round edged in front and tapered in the back will have less than that. Then, an elliptical cross section that is longer than it is thick for the leading edge will have somewhat less drag than a circular one. On a subsonic rocket, an airfoil on the fin will have less drag than that, and allow more thickness without a drag penalty. Here's a way to get accurate shapes in wood: https://charlesriverrc.org/articles...ithout-templates/markdrela_airfoilshaping.pdf

The fin planform is not critical. Go with something easy to make. A trapezoid with the tip maybe half as wide as the root is probably a good way to go.
--------
When designing in Solidworks, it can sometimes be helpful to design one important part and then make it part of an assembly. When making the parts that fit with it, you can edit a part within the assembly, and you can also check the fit. However, it may take a while to get comfortable doing this. Unless you are comfortable with the CAD program, designing a simple rocket may be just as fast on paper.

As far as equations go, if you mention which ones we might be able to tell you which ones may be useful. I'm not familiar with the 5:1 ratio for tail cones, and I suspect that's just a rough rule of thumb. I'm sure the optimal value, whatever it is, depends on the Reynolds number, boundary layer thickness, etc.

You may learn a lot by examining known, successful designs that aren't too different from what you're doing. No need to reinvent the wheel. OTOH, if you are looking at bad designs, you may be importing square wheels.
 
Nose cone shape doesn't have a huge impact on aerodynamics at sub sonic speeds. It has some effect, but nose cone shape is way down the list of things you need to do correctly.

Can you post your openrocket design?

What motors are you considering? Do you have access to commercial motors like Aerotech? Will this be a high power rocket? What country are you in?
Thank you! I guess we'll go with the elliptical one for now.

I'm sorry, but this is for an upcoming model rocketry competition and we've been instructed to not share anything online until the competition is over. I'll be happy to email it to you perhaps?

The motors will be provided by the organisers, they've just given us the length, diameter and motor capacity limit. We're allowed to use aluminium, cardboard or PVC and we'll be provided with a motor according to which material we use. We do have access but we're not allowed to use anything other than what the organisers will provide. Yes, this is a high-power rocket. We're considering using cardboard right now and the weight (from OpenRocket) is about 10 kg, including the motor and the payload. I'm from India.
 
One way to answer questions such as "is this the best design for fins/nose cone/other parts?" is to change the design in OR and see what happens to altitude, speed, or whatever parameter is important. It's not a perfect solution but it will usually provide insight on what is important. Try changing the rocket diameter and see what happens to altitude. Then try changing nose cone shape and/or length, or the root length or tip length etc. of the fins. Playing around with OR will provide a lot of education. And it's a lot cheaper than making a new set of fins for each test flight.
Thank you so much! We'll try that out with the fins. This is for a competition and we have been given certain constraints regarding the dimensions, total weight and the material we can use - the entire rocket should be less than 180 cm, and it should be able to carry a payload that is 40 cm long and 15 cm in diameter.
 
1kg is a lot of mass, not in the realm of a beginner's rocket. 1km altitude is achievable, with motors available to beginners, but with zero or perhaps 30g payload.

Whoever set up this task for you did it wrong, they may not understand how things are done in the hobby.

But, please go ahead, make your design, and then look realistically at what it takes to achieve it. I think you'll need "Level 1" certification, at least.
Yes, that makes sense. This is actually for a model rocketry competition and a couple of us signed up as a team, hoping to learn along the way. We don't expect to win but we'd like to do it the right way for as long as we can. Thank you for your response!
 
Ignore this post if @dee is proposing a simple intellectual or "on paper" academic exercise. OTOH, if the plan is to actually build and fly such a rocket, the following applies

Okay, there have to be others that have problems with the above opening statements.

I wish the @dee the best with this, but if a real project, this isn't do-able without a level 1 cert from SOMEBODY involved locally in the process. I started this before @cls post above, but he beat me to the button.

Find a local club and they will likely be a great additional resource for your project.

Good Luck!
I'm sorry, I didn't mean to come off as entirely ignorant. We're engineering students who signed up for a model rocketry competition. We don't expect to win, we'd just like to get to know the process and work with it in any way we can. We're meeting with a professional soon to gain a bit more clarity on how to proceed, hoping it'll go well! Thank you for your response!
 
Yes, that makes sense. This is actually for a model rocketry competition and a couple of us signed up as a team, hoping to learn along the way. We don't expect to win but we'd like to do it the right way for as long as we can. Thank you for your response!
Where are you located?
 
If you're trying to hit a target altitude with a particular motor, you may not actually want things to be as slick or as light as possible.

If you are trying to minimize the power required, that's another story. It also matters whether you are using a motor with a quick burst, or a motor with a long burn. With a quick burst, there will be an optimum weight for any given rocket design, and it might even be significantly heavier than 1 kg. If so, you don't have to make everything as light as possible. If not, or if the motor is a long burning, low thrust one, saving weight helps.

A subsonic, optimized rocket, for flight in the lower atmosphere, probably includes a very smooth pod, just large enough for a payload, with a tail boom just long enough for the fins to maintain stability, and with a boom just large enough for the motor you use. In the case of a long burning motor, or one which gives an optimum weight close to or below 1kg, approximate unobtainium as well as you can in order to save weight. ;-)

The following is a picture of a test body for research on low drag shapes. If I'm not mistaken, it was dropped in a deep lake. However, the same shape would make a great payload carrying, single stage rocket if you had impossibly thin rocket motors. Think of it as a caricature:
https://commons.wikimedia.org/wiki/File:Dolfin,_corps_laminaire_de_North_American_Aviation,_tel_que_rapporté_par_Bruce_H._Carmichael.png
Bernard de Go Mars, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons


With all the CAD, it sounds like you might be thinking of 3D printing parts of the rocket. However, keep in mind that most 3D printing materials are flexible and weak for their weight, compared to things like balsa, plywood, or composites with carbon fiber, kevlar, or even fiberglass. I suspect even the cardboard they make body tubes with is superior. 3D printing might allow you to make some wild shapes, but they'll be heavy, fragile, or both. You can probably do a pretty good job without any exotic materials.

When designing, keep in mind the materials, tools and methods you are familiar with, have on hand, and/or prefer. I'm sure a wide variety of good designs are possible, and far more bad ones.

If you don't need to get the last bit of performance, or if you don't have time, go with something entirely conventional.

1.
As Zeus-cat points out, the nose cone is not terribly critical, though I suppose you could make a terrible one if you tried to. An ellipse ought to be fine until you are trying to tweak the last one percent of performance. It would be quite easy to do an elliptical nose cone in Solidworks, BTW.

2.
You can just fiddle around with Openrocket and get close enough, unless you want to get sophisticated. Remember that a boat tail slightly reduces stability.

3.
If Openrocket says it's stable with those fins, it probably is. Make the fins thick enough so that they are unlikely to break or flutter with the materials you prefer to use. This is one place where balsa and/or plywood are superior materials, unless you do something clever with 3D infill. Come to think of it, if you're good with CAD, you could make a 3D printed core for a fin out of honeycomb, and then sheet it with veneer. But we're getting fancy here. Fins have a fair amount of the drag of the rocket. Round edged fins will have much less drag than square edged ones. Fins that are round edged in front and tapered in the back will have less than that. Then, an elliptical cross section that is longer than it is thick for the leading edge will have somewhat less drag than a circular one. On a subsonic rocket, an airfoil on the fin will have less drag than that, and allow more thickness without a drag penalty. Here's a way to get accurate shapes in wood: https://charlesriverrc.org/articles...ithout-templates/markdrela_airfoilshaping.pdf

The fin planform is not critical. Go with something easy to make. A trapezoid with the tip maybe half as wide as the root is probably a good way to go.
--------
When designing in Solidworks, it can sometimes be helpful to design one important part and then make it part of an assembly. When making the parts that fit with it, you can edit a part within the assembly, and you can also check the fit. However, it may take a while to get comfortable doing this. Unless you are comfortable with the CAD program, designing a simple rocket may be just as fast on paper.

As far as equations go, if you mention which ones we might be able to tell you which ones may be useful. I'm not familiar with the 5:1 ratio for tail cones, and I suspect that's just a rough rule of thumb. I'm sure the optimal value, whatever it is, depends on the Reynolds number, boundary layer thickness, etc.

You may learn a lot by examining known, successful designs that aren't too different from what you're doing. No need to reinvent the wheel. OTOH, if you are looking at bad designs, you may be importing square wheels.

This is for a competition and the motor will be provided by the organisers depending on whether we use aluminium, PVC or cardboard to build the rocket. It'll likely be a quick burst motor because the weight limits we have to adhere to are significantly greater than 1 kg. We're currently considering cardboard and the weight limit is 10 kg including the motor and the payload.

Thank you! I'll keep that in mind. The motor they'll provide for cardboard rockets is almost 10 cm in diameter.

We're not sure right now but we are leaning towards cardboard for all the main parts and 3D printing just the payload and avionics bays. I'll do that, thanks!

I guess we'll go with the elliptical one for now. Thank you for the advice! We're currently using trapezoidal fins but I'd like to explore a few more designs, so I'll keep those in mind.

Yes, I'm currently designing the parts separately so we can adjust them later on. There are also a couple of us on the team, so it's easier to work when we can share our individual parts and assemble them later.

I was thinking about these (https://www.nakka-rocketry.net/RD_body.html) when I posted the question. I'm not sure if they figure directly in the designing process, I was just curious how to utilise them if they did.

That sounds like a good idea! I'll try that out. Thank you for your response!
 
Maybe next year we will remember when someone starts a thread like this. It is always some team in India with this same challenge.
Not always India, but generally from outside the US. And there will always be indignant replies that the assignment is bad, beginners shouldn't be building large(ish) high power rockets like this, blah blah blah. It's as predictable as glue threads.

I admit that assigning inexperienced rocketeers to build something of this size has always rubbed me the wrong way (in whatever country), but it seems to be pretty common and who am I to judge. ¯\_(ツ)_/¯
 
This is for a competition and the motor will be provided by the organisers depending on whether we use aluminium, PVC or cardboard to build the rocket. It'll likely be a quick burst motor because the weight limits we have to adhere to are significantly greater than 1 kg. We're currently considering cardboard and the weight limit is 10 kg including the motor and the payload.

Thank you! I'll keep that in mind. The motor they'll provide for cardboard rockets is almost 10 cm in diameter.

We're not sure right now but we are leaning towards cardboard for all the main parts and 3D printing just the payload and avionics bays. I'll do that, thanks!

So, you are 3D printing cardboard? Cardboard works, but you might learn more by selecting aluminum and learning to use university machining equipment. Sometimes I am amazed that you may be earning college credits for playing around with commercial hobby rocketry.
I guess we'll go with the elliptical one for now. Thank you for the advice! We're currently using trapezoidal fins but I'd like to explore a few more designs, so I'll keep those in mind.

Yes, I'm currently designing the parts separately so we can adjust them later on. There are also a couple of us on the team, so it's easier to work when we can share our individual parts and assemble them later.

I was thinking about these (https://www.nakka-rocketry.net/RD_body.html) when I posted the question. I'm not sure if they figure directly in the designing process, I was just curious how to utilise them if they did.
That is a very good reference and it might help you address some issues in a written report. Most hobby HPR seems to be done on a "just do it" basis, with learning from trial and error.

You mentioned optimization, but you did not tell us what you are trying to optimize. Is there a penalty for exceeding the target altitude, or are you trying to maximize altitude? You mention that you had simulated a design to the target altitude, so it is not clear what you are trying to improve.
 
So, you are 3D printing cardboard? Cardboard works, but you might learn more by selecting aluminum and learning to use university machining equipment. Sometimes I am amazed that you may be earning college credits for playing around with commercial hobby rocketry.

That is a very good reference and it might help you address some issues in a written report. Most hobby HPR seems to be done on a "just do it" basis, with learning from trial and error.

You mentioned optimization, but you did not tell us what you are trying to optimize. Is there a penalty for exceeding the target altitude, or are you trying to maximize altitude? You mention that you had simulated a design to the target altitude, so it is not clear what you are trying to improve.
We've not decided on what we will be 3D printing, we've just done most of the simulations using cardboard so far. We are looking into which material is both affordable and accessible for us. Thank you, that is something we're exploring since we also have lab instructors who could help us.

We do have a design report to submit before we can begin building the rocket, so I suppose it'll be helpful along the way.

Yes, there is a penalty for exceeding the 1km altitude. We mainly optimised our design to attain an apogee greater than 1km as our initial design's apogee was around 800 m. We also optimised the stability to obtain a value between 1 and 2. We were mainly trying to improve our design to attain the required apogee while also maintaining stability, given the mass and dimensional constraints. Thank you for your response!
 
Not always India, but generally from outside the US. And there will always be indignant replies that the assignment is bad, beginners shouldn't be building large(ish) high power rockets like this, blah blah blah. It's as predictable as glue threads.

I admit that assigning inexperienced rocketeers to build something of this size has always rubbed me the wrong way (in whatever country), but it seems to be pretty common and who am I to judge. ¯\_(ツ)_/¯
I'm sorry if my post came across as disrespectful.

Competitions like this are often common in India and mainly serve as a learning experience - we're not expected to do it perfectly or professionally, although there are teams that do. As engineering majors, it's simply an exciting opportunity for us to gain some practical experience as opposed to the heavily theoretical education that is generally prevalent in the Indian curriculum.
 
Not always India, but generally from outside the US. And there will always be indignant replies that the assignment is bad, beginners shouldn't be building large(ish) high power rockets like this, blah blah blah. It's as predictable as glue threads.

I admit that assigning inexperienced rocketeers to build something of this size has always rubbed me the wrong way (in whatever country), but it seems to be pretty common and who am I to judge. ¯\_(ツ)_/¯

You know who in the USA is exempt from NFPA rocket regulations like certs?
Universities, Colleges , and Businesses engaged in rocketry.

The reason our College teams want Certs, is they want to be able to fly at NAR or TRA launch fields rather then their own property. I have seen some Universities have their own acquired launch areas as well.
 
Do they still need waivers?

Of course, that is with the FAA. That has nothing to due with certs. Universities can get waivers as can just about anyone.

Back in the old days, only NFPA 1122 existed, no 1127. TRA was an organization and soon corporation engaged in rocketry so it's members could be exempt from NFPA 1122 and get those Aerotech and Vulcan motors over the normal limit at the time.

They got FAA waivers for their launches also. Back then you needed a waiver for over 1lb into airspace generally above 1200 feet.

Later NFPA 1127 was created to make HPR more mainstream and the NAR started supporting it also.
 
Do you have a thrust curve for the available motors, or at least average and total impulse?
No. We've been told that the motor capacity cannot exceed 2800 Newton-seconds of total impulse.

For now, we're expected to submit a report detailing our proposed design. We've only been given the motor dimensions so far.
 
Can you use electronics? I know there are altimeters that deploy on the way DOWN, is there a way to deploy a streamer or something to “promptly” slow the rocket on the way UP at say 900 meters? (Obviously you don’t want a sudden stop that will disassemble the rocket mid flight).

How many practice shots do you get?
 
Can you use electronics? I know there are altimeters that deploy on the way DOWN, is there a way to deploy a streamer or something to “promptly” slow the rocket on the way UP at say 900 meters? (Obviously you don’t want a sudden stop that will disassemble the rocket mid flight).

How many practice shots do you get?

Currently College teams in competitions are using "Air Brakes" in advanced designs to hit the target.
 
Cardboard is probably a good choice. It's relatively easy to work with. PVC is quite heavy for its stiffness and strength.

Is the 10kg a maximum weight, or a minimum weight? I should think there'd be no need for a maximum weight as the rockets just wouldn't perform if overweight. As a minimum weight, 10kg seems absurdly high.

I had just assumed that the apogee was supposed to be 1km. Tim Smith has a valid point, at least if that's how the actual rules are worded.

Where I live, wood is more affordable than aluminum or PVC. It's also available in different densities, though balsa gets kind of expensive, at least per board foot. Paulownia is quite light, if you have it in your area. Spruce, if you have something like that around, is traditional for aircraft, and is strong for its weight. Birch and maple are heavy and strong. I imagine that the species of wood available over there may be different than those found here in the northeastern part of the USA.

A good design that leaves enough time for building and testing is better than a perfect design that doesn't. If testing is allowed, it will probably be helpful. An extra rocket, just in case the first has a problem, might be good, if there's time.
 
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