What I (think) I've learned from designing and building my first rocket. Thoughts on fin design.

[Senior Space Cadet]

I haven't actually launched it, so I'm still learning from this project.
I decided to start with 18mm motors. I think that was a good decision.
Since low powered rockets aren't supersonic, it seemed obvious that I wanted a rounded nose cone, despite not looking cool. I would have thought that rounded nose cones, for 24mm body tubes, would be plentiful, but that isn't the case. I found some balsa ones, that I decided not to use and one superlight plastic one, which I also decided not to use. So I ended up using the shortest pointy nose cone I could find. Estes makes a great rounded nose cone for a larger tube size. Wish they made it for the BT-50.
After toying with the idea of making a minimum diameter (18mm) rocket, I decided to go with 24mm, to make swapping engines quick, clean and easy. I also ended up using the engine mount for attaching my shock chord. So I don't regret that decision.
I went with the recommended minimum body length of ten times the tube diameter. I'm finding that, despite having oversized fins, I'm having to add a lot of weight to the nose cone to get stable flight. On my next rocket, I'm going to go with a longer body tube.
As I just said, I have (three) huge fins and am still struggling for stability. I don't think it's the fault of the fin design. Plenty of surface area and center of pressure is well back. Fin design is the one place I really have creative control, in a traditional rocket design. I have some ideas I'm going to play with. I don't think that surface area or how far back the center of pressure is, is the whole story. On an airplane, the tail, which controls yaw is ninety degrees to that motion and the stabilizer which controls pitch is ninety degrees to that motion. A fin that is in line with the motion you want to correct is doing no good no matter the size or placement. The closer to ninety degrees, the better. I think going to four fins is one step in the right direction, but I have some other ideas.
I still like the idea of "tractor" motor placement, despite all the problems.

 

[OverTheTop]

For the same amount of surface area, the further back the fins are they more effect they have on stability (further from the CG). Note that rockets with fins hanging out the back are prone to damaging fins on landing. Just a couple of things to consider .

 

[neil_w]

Since low powered rockets aren't supersonic, it seemed obvious that I wanted a rounded nose cone, despite not looking cool.

Why is that obvious? What is your objective?

I went with the recommended minimum body length of ten times the tube diameter. I'm finding that, despite having oversized fins, I'm having to add a lot of weight to the nose cone to get stable flight. On my next rocket, I'm going to go with a longer body tube.

What does the rocket look like? What is your nose cone made of? What are your fins made of? Etc. etc.

Have you tried creating a model in OpenRocket? You can get all your questions answered about 1000 times more quickly that way than by trial and error.

 

[Senior Space Cadet]

Here are photos.

 

[Antares JS]

That rocket looks pretty standard and I don't see why you would be struggling with stability, unless your fins are made of an unusually heavy material. How do you know this rocket struggles with stability? Like neil_w said, using OpenRocket or RockSim would probably help you answer your questions a lot quicker than waiting on forum responses.

 

[Senior Space Cadet]

My goal was form follows function. I don't mind if my rocket looks good. In fact I want my rocket to look good. But my priority is that it fly well. By fly well, I want it to fly straight and reach a respectable height for the engine I'm using. I don't expect to set an records, but I don't want some five year old, showing up at the next launch, with a rocket that flies higher. Right now, I think it's a possibility.
My concern, not having other rockets to compare it to, is that it will be heavy. I think it would perform "better" with a rounded nose cone, since this would result in the least drag on a rocket that is subsonic. The fins are large and located at the very tail, as well as being swept back, so the need for a lot of weight in the nose cone is a bit surprising to me but, again, I have nothing to compare it to.

 

[Senior Space Cadet]

I tested for stability by swinging it around my head, out in the street, at 3:00 am.
I would call myself technologically challenged, which is why I haven't used any of the simulation programs. I don't even own a cell phone. I have used boat CAD programs, but that's pushing my limits. Just being honest.
There may be nothing wrong with the stability. It's just that I added what seemed like a ton of weight to the nose cone and it still didn't want to point forward in the spin test. Maybe I just need two tons and that isn't as weird as it seems to me.

 

[boatgeek]

I tested for stability by swinging it around my head, out in the street, at 3:00 am.
I would call myself technologically challenged, which is why I haven't used any of the simulation programs. I don't even own a cell phone. I have used boat CAD programs, but that's pushing my limits. Just being honest.
There may be nothing wrong with the stability. It's just that I added what seemed like a ton of weight to the nose cone and it still didn't want to point forward in the spin test. Maybe I just need two tons and that isn't as weird as it seems to me.

If you can use boat CAD programs, you can definitely use OpenRocket. It's really not too bad at all. Maybe also try a longer string for the spin test?

 

[neil_w]

There's no way that rocket should require a lot of nose weight unless the fins are really heavy. How much does the rocket weigh without the nose weight? Where's the center of gravity? How much nose weight did you put in?

I tested for stability by swinging it around my head, out in the street, at 3:00 am.
I would call myself technologically challenged, which is why I haven't used any of the simulation programs. I don't even own a cell phone. I have used boat CAD programs, but that's pushing my limits. Just being honest.

If you care about optimizing your rocket performance, then something like OpenRocket is a necessity. It's not hard at all, and there's plenty of support to be had around here. One thing I would expect it to tell you, for example, is that you'll have more to gain by optimizing those fins than by switching nose cones.

 

[BABAR]

Here are photos.View attachment 419188View attachment 419189

Looks great!

Most of my rockets that use tubes are 18” long? Why? Because that’s the most common tube size I buy. Longer rockets tend to be more stable, which makes sense.*. The further your nose cone is from the current Center of Gravity (CG) the less nose weight (if any) you need in the nose to shift CG forward, you have a longer “lever arm.” In fact, on most of my 18” rockets just the weight of the balsa or plastic nose cone and the weight of the body tube itself are enough the bring the CG forward. The downsides of longer rockets for low power (up to a point) are minimal, a longer body tube is going to be proportionally heavier (especially with primer and paint), and subject to more structural stress, although up to 18” both are pretty minimal. It also costs a bit more, after all, I could build two 9 inch, three 6 inch, or 6 three inch (can you say Mosquito?) rockets with one 18” body tube. I’d need more nose cones and fin material, though. My understanding is that for most low power kits, the most expensive part for the manufacturer is the nose cone.



*as always, there are always exceptions. Reeeeeealllllly short stubby rockets also tend to be more stable due to something called “base drag”, but ignore that for the moment.

 

[Speaknoevil]

I tested for stability by swinging it around my head, out in the street, at 3:00 am.
I would call myself technologically challenged, which is why I haven't used any of the simulation programs. I don't even own a cell phone. I have used boat CAD programs, but that's pushing my limits. Just being honest.
There may be nothing wrong with the stability. It's just that I added what seemed like a ton of weight to the nose cone and it still didn't want to point forward in the spin test. Maybe I just need two tons and that isn't as weird as it seems to me.

Within the first year of building rockets, I ceased doing swing tests. I could not get enough velocity for any rocket to stabilize, even tried and true fliers like the Estes Eliminator and the Aerotech Initiator. Started using Rocksim and never looked back.

 

[Antares JS]

What Speaknoevil said. The swing test is not reliable and should not be taken as gospel. I also had rockets I knew to be stable refuse to fly nose first in a swing test. I haven't bothered with them since I was 12.

If you're that reluctant to use a computer, you can calculate CP location by hand. On a rocket like that, it should be easy. There's just a lot of arithmetic. When checking its location relative to CG, make sure you put the motor and recovery wadding in.

https://mae-nas.eng.usu.edu/MAE_5900_Web/5900/USLI_2010/Flight_Mechanics/Barrowman.html

 

[Senior Space Cadet]

I do have a second goal. I want my rockets to be original.
I can't make nose cones.
I can't make body tubes.
But I can make fins and I can use tubes in an original way.
I've been looking through a lot of photos of model rockets on the internet and haven't seen any fins like what I plan on making.
Most likely, someone some where has done something similar, but not many and none exactly the same. But I don't want to be different just for the sake of being different. I'd like it to move the ball forward. I think if I have one asset, it is that I can think outside the box.

 

[neil_w]

Fundamentals of rocket stability still apply to original and/or out-of-the-box designs. Certainly, weird enough designs can be hard to analyze. But for a simple design like you're showing here, you should be able to know exactly what's happening and why.

Learn the fundamentals, learn how to analyze, learn how to use OR or Rocksim, and then have at it.

 

[boatgeek]

I do have a second goal. I want my rockets to be original.
I can't make nose cones.
I can't make body tubes.
But I can make fins and I can use tubes in an original way.
I've been looking through a lot of photos of model rockets on the internet and haven't seen any fins like what I plan on making.
Most likely, someone some where has done something similar, but not many and none exactly the same. But I don't want to be different just for the sake of being different. I'd like it to move the ball forward. I think if I have one asset, it is that I can think outside the box.

I get all that and do a lot of it myself. One challenge you run up against is that if you truly want to be optimized, your fins are going to look a lot like everyone else's. It's the same reason that all hatchbacks look the same--they've been optimized and the laws of physics plus some basic market demands end up forcing them into a particular shape. When you get to altitude optimization, you'll see that everybody has the same fins for the most part for the same reason.

That's not to say you can't do stuff that's different. Lots of us do stuff that's unique and cool (see, for example, just about any neil_w design). It probably will be notable for looking good rather than winning altitude records. Also, when it comes to wringing the last bit of performance out, workmanship matters an awful lot. That's why I don't play there as much.

 

[KilroySmith]

I admire your clear goals; but I would caution that making a rocket that works is a lot less frustrating than making a bunch of rockets that don't, unless your goal is just to build rockets. OpenRocket can be used at a very basic level (just using external components) to calculate Cp; you can build and balance to determine Cg. Or, you can go deep and include everything including the weight of the lines to your chute to have it give an estimate of Cg (which still has to be verified against reality) and be able to model flights.

Fins provide a remarkable amount of opportunity for creativity. For example, Binder Designs does a lot of creative fin work - I'm a fan of the Terrordactyl:
https://www.binderdesign.com/store/page2.htmlIt won't go the highest, or the fastest, but it sure looks cool. I think there's a lot of area here for being artistic in your particular way - whether that's pure function or pure fantasy or something in between.

 

[Ronz Rocketz]

If you can use boat CAD programs, you can definitely use OpenRocket. It's really not too bad at all. Maybe also try a longer string for the spin test?

I've had good luck with using discarded rubber shock cords for the swing test. Seems to keep the string on the CG at higher swing velocities.

 

[rocketgeek101]

I'll echo what others have already said that you should really give a simulation program like OpenRocket a try. I suspect you'll find it easy enough to learn at a basic level, and that's all you really need. If you find yourself struggling, this forum is the official place to seek support with installing and using the program (many of its developers are members here) so you are already in the right place!

If you choose to try OpenRocket, see this thread for the easiest (and best) way to install it on your computer: https://www.rocketryforum.com/threa...d-mac-to-solve-all-your-java-problems.143540/

I took the liberty of creating your design in OpenRocket and can confirm that it should be stable (though I had to estimate the dimensions from the photo you posted, so I doubt this is totally accurate - nonetheless, it seems like you should have a pretty healthy stability margin).



I've attached the file incase you do decide to try OpenRocket, as you may find it a useful starting point. Feel free to do whatever you want with it.

 

[BABAR]

I do have a second goal. I want my rockets to be original.
I can't make nose cones.
I can't make body tubes.
But I can make fins and I can use tubes in an original way.
I've been looking through a lot of photos of model rockets on the internet and haven't seen any fins like what I plan on making.

If you can roll a piece of cardstock into a cone, YOU can make a nose cone. Google (or search on this forum). WAC Corporal. It was a real sounding rocket, used a straight cone. Now, you want Ogive, that’s a bit tougher.

Google Estes Pop Fly. Or take a look at this

https://www.rocketryforum.com/threads/hail-mary.62920/
Or this

https://www.rocketryforum.com/threads/tank-killer-update-11-15.139310/#post-1741174
Got any left over Plastic Easter Eggs? Instant Big Bertha cones

BTW, nose cones don’t have to be round


https://www.rocketryforum.com/threa...ir-brake-recovery-rocket.145077/#post-1782279
Yup, I am biased, but I think this as cool as any nose cone out there, probably cost me a Quarter in materials, and made it in 5 minutes.

And yes, YOU can make tubes. Roll your own from cardstock, or recycle Pringle’s cans or mailing tubes.

Again, BODY TUBES DON’T HAVE TO BE ROUND EITHER.

check out New Way rockets

https://www.erockets.biz/newway-rocket-kits/
Maybe you are goin be the Rocketry Forum “Fin-ster”, and come up with new materials or techniques for making fins, your Arrow fletching vane thingies were pretty cool, not sure if original or not, but definitely different.

Take a look at this

https://www.rocketryforum.com/threads/fliskit-tiddlywink-helicopter-duration-model.128965/
And this

https://www.rocketryforum.com/threa...non-competition-helicopter.39114/#post-364481
Who says fins have to come off perpendicular to the long axis of the rocket? And at least two guys figured out independently dual use fins, they act like fins on boost and contribute significant surface area to Rotors on descent.

Whatever you build and fly, it needs to be SAFE, and part of that is being STABLE. But there are a zillion outside the box ways to do that (@Daddyisabar probably has done at least half of them!)

People on this forum love pictures and drawings. If Swing Tests aren’t your thing, and you can’t do OpenRocket (in my case, I REALLY can’t, it doesn’t do square rockets well, and lately most of mine are helis or airbrakes, so you aren’t alone) then post either pics or ideas on the Scratch Forum under the “half-baked” thread and people will be glad to “mind-sim” it for you.

https://www.rocketryforum.com/threads/neil_ws-half-baked-design-thread.140478/page-35#post-1970088


If you are going for speed, altitude, or duration records, you’ve got a steep road, because people have been working over half a century in model Rocketry to optimize models for those. Records are made to be broken, but your not likely to do it as a beginner.

 

[125cc]

As a beginner I am still learning the fundamentals, although as a pilot I do have an understanding of aerodynamics, CP, CofG etc. Designing from scratch however is a different story!

I personally see it as something of an art form in design, if all rockets were 3/4FNC on a stick it would be pretty boring! I am attracted to unusual designs that appeal to my sci-fi imagination. Some of the Estes models in this regard are very cool. I also like tall rockets. My ambition is to design and build my own sci-fi rockets of around a meter high with larger delta wing surfaces. Probably high drag but makes for an easier recovery! I am building a few kits first to understand the fundamentals of construction whilst flying my current RTF model for launch experience. I'm going to have a go at OpenRocket as well to knock up some designs. I have a selection of Estes sci-fi nose cones just begging to top some interesting designs I have in my head...

 

[Daddyisabar]

Start out and build more advanced kits like Flis & Pembetton. Get some flight experience with a club. Then scratch build from rocsim and maybe then to mindsim for crazy scifi designs.

 

[Ronz Rocketz]

Unfortunately, OR doesn’t do spaceships very well, especially angled tubes and rings. Rocsim can apparently if you can afford it. AC Supply has a nice collection of spaceships for sale.

 

[OverTheTop]

Try Sirius Rocketry. They have some nice kits
https://www.siriusrocketry.com/
I have personally built their Interrogator and the SS Cestris. Good kits to build, and look great when completed.