Fin size, body length?

The Rocketry Forum

Help Support The Rocketry Forum:

Senior Space Cadet

Well-Known Member
TRF Supporter
Joined
May 23, 2020
Messages
259
Reaction score
91
Is there a percentage of body length you like the CP to be at?
Is there a body length to tube diameter ratio that you prefer?
One of you guys said they like short rockets with heavy nose cones. Exactly what I thought was wrong with my first design.
What are the advantages and disadvantages of a short rocket with heavy nose cone vs. long rocket with light nose cone?
Gotta go get ready for my hike. Trailhead parking fills up early.
 

rklapp

NAR# 109557
TRF Supporter
Joined
May 8, 2020
Messages
362
Reaction score
199
Location
Oahu, Hawaii
All I know is that the CG needs to be about an inch ahead of CP with engine. The CP is a factor of the fin shape and stuff. Consequently, the OR cal should be between 1.0 and 2.0 with engine. Sorry if I’m rehashing.

Have fun on the hike.
 

dhbarr

Amateur Professional
Joined
Jan 30, 2016
Messages
6,373
Reaction score
1,020
The most rockety-shaped rockets are about 10-12 times as long as they are wide, with the nosecone taking up about 3 of those.

With three or four fins of appropriate size this gives a caliber or two of stability, usually without much added nose weight.

If you look at sounding rockets and unguided missiles from the cold war, they're going to all fairly well fall in that range.

But! The range of possible safe flight extends from
to
 

neil_w

Hunkered down and slowly going crazy
TRF Supporter
Joined
Jul 14, 2015
Messages
9,071
Reaction score
2,400
Location
Northern NJ
Is there a percentage of body length you like the CP to be at?
I don't believe there is any significance to this, just as it doesn't matter if the CG is far behind the mid-point. Just make sure the CG is sufficiently ahead of CP.
Is there a body length to tube diameter ratio that you prefer?
Everyone has their own preferences. Rockets can range from extremely short and stubby to absurdly long and skinny. All can fly fine if designed correctly. Build what you like.
 

Antares JS

Well-Known Member
Joined
Mar 5, 2020
Messages
278
Reaction score
156
Location
Eastern Shore, VA
What are the advantages and disadvantages of a short rocket with heavy nose cone vs. long rocket with light nose cone?
Short, fat rockets with heavy noses are usually pretty draggy and fly low and slow, and some people like that because they can watch the entire flight without the rocket going too fast or too high. Other people just like the aesthetic. It's not all about reaching the maximum altitude, speed, or some other performance metric for a lot of us, it's just about doing what you think is fun.

That said, I tend to prefer really long, skinny rockets that might be 20-30 or more times as long as they are wide. That length typically allows for smaller fins than would typically be seen on a rocket of its weight class. I also try to keep the span of my fins low and make them longer if they need to be bigger. I don't do this for any performance reasons, it's just the aesthetic I prefer. I generally go with 2-3 stability calibers to give myself extra margin if something causes the rocket to oscillate in flight.

It only matters where your CG and CP are in relation to each other. It does not matter where they are in relation to the rocket's length or any other geometry.
 
Last edited:

rklapp

NAR# 109557
TRF Supporter
Joined
May 8, 2020
Messages
362
Reaction score
199
Location
Oahu, Hawaii
No, it should be at least 1 caliber: in BT diameter. it will vary due to tube diameter (and a few other factors)
I agree. I used "about" as a relative term, but your explanation is more precise. Adding the different type of engines will kick the CG to the aft so 2-3 calibers with an unladen rocket is beneficial.
 

neil_w

Hunkered down and slowly going crazy
TRF Supporter
Joined
Jul 14, 2015
Messages
9,071
Reaction score
2,400
Location
Northern NJ
I agree. I used "about" as a relative term, but your explanation is more precise. Adding the different type of engines will kick the CG to the aft so 2-3 calibers with an unladen rocket is beneficial.
Stability of an unladen rocket doesn't matter at all. Stability with motor is the *only* thing that counts.

Certainly, stability will usually* be greater without a motor, but exactly how much greater is highly dependent on the details of the rocket and the motor, so there is little benefit in trying to guess at it.



*not with tractor motors!
 

lakeroadster

Lonewolf.... No Club
TRF Supporter
Joined
Mar 3, 2018
Messages
2,007
Reaction score
830
Location
Central Colorado
Stability of an unladen rocket doesn't matter at all. Stability with motor is the *only* thing that counts.

Certainly, stability will usually* be greater without a motor, but exactly how much greater is highly dependent on the details of the rocket and the motor, so there is little benefit in trying to guess at it.
*not with tractor motors!
Tractor Motor Footnote :goodjob:
 

Attachments

Senior Space Cadet

Well-Known Member
TRF Supporter
Joined
May 23, 2020
Messages
259
Reaction score
91
The most rockety-shaped rockets are about 10-12 times as long as they are wide, with the nosecone taking up about 3 of those.

With three or four fins of appropriate size this gives a caliber or two of stability, usually without much added nose weight.

If you look at sounding rockets and unguided missiles from the cold war, they're going to all fairly well fall in that range.

But! The range of possible safe flight extends from
to
By a caliber, do you mean one body tube diameter?
 

Senior Space Cadet

Well-Known Member
TRF Supporter
Joined
May 23, 2020
Messages
259
Reaction score
91
I'm just looking for general guidelines. Rules of thumb experienced builders go by when designing a rocket. I realize there is a lot of variation in there, otherwise it wouldn't be much fun. If everyone built rockets that were exactly the same, all there would be is a little flame and smoke.
I still like the idea of a tractor motor, but don't want to just build a fantasy rocket. Haven't come up with anything remarkable yet.
I'm also toying with the idea of alternate power sources, such as electric jet and propeller. Probably heresy, and maybe no longer, technically, a rocket, but not subject to fire bans.
 

Senior Space Cadet

Well-Known Member
TRF Supporter
Joined
May 23, 2020
Messages
259
Reaction score
91
Short, fat rockets with heavy noses are usually pretty draggy and fly low and slow, and some people like that because they can watch the entire flight without the rocket going too fast or too high. Other people just like the aesthetic. It's not all about reaching the maximum altitude, speed, or some other performance metric for a lot of us, it's just about doing what you think is fun.

That said, I tend to prefer really long, skinny rockets that might be 20-30 or more times as long as they are wide. That length typically allows for smaller fins than would typically be seen on a rocket of its weight class. I also try to keep the span of my fins low and make them longer if they need to be bigger. I don't do this for any performance reasons, it's just the aesthetic I prefer. I generally go with 2-3 stability calibers to give myself extra margin if something causes the rocket to oscillate in flight.

It only matters where your CG and CP are in relation to each other. It does not matter where they are in relation to the rocket's length or any other geometry.
Long skinny rocket with small fins is where I was going with my arrow vane fin designs. I've built two with 6 vanes and pretty confident they will fly well, once I get the CG right.
 

Antares JS

Well-Known Member
Joined
Mar 5, 2020
Messages
278
Reaction score
156
Location
Eastern Shore, VA
Long skinny rocket with small fins is where I was going with my arrow vane fin designs. I've built two with 6 vanes and pretty confident they will fly well, once I get the CG right.
In that case, if you are looking for general guidelines, for a long skinny rocket like I described that is 20-30 times as long as it is wide, you should give yourself more margin than 1-2 calibers (which does mean body diameters) distance between the CG and CP, because CP tends to shift a bit more at varying angles of attack with this kind of geometry. You should go with a distance between your CG and CP of 10% of the rocket's length for safety.

To repeat, it does not matter where your CG and CP are along the rocket's length. It only matters where they are in relation to each other. Don't waste your time thinking you need to get your CP 2/3 of the way down the length or whatever.

And if you find yourself thinking you need to use a lot of nose weight, stop and rethink what you're doing. I have never had to add nose weight to a rocket with this kind of geometry. The only reason you would need to is if your fins are really ridiculously small. An extreme example of a long, skinny rocket is the Estes Mean Machine kit. You can see how small the fins are in relation to its size and keep in mind that it needs no extra nose weight to fly perfectly straight. I gave one of mine a pair of strap-on boosters, adding significant weight to the tail end, and it STILL requires no nose weight to fly straight.

Edit: It also might be worth pointing out to you that rockets with this kind of long and skinny geometry do not perform well in swing tests. If they are swung around in circles, the angle of attack varies too much along their length to get a realistic result unless the circle they are being swung in is really big (i.e, you use a really long string) at which point trying to get it going in the first place is impractical.
 
Last edited:

Senior Space Cadet

Well-Known Member
TRF Supporter
Joined
May 23, 2020
Messages
259
Reaction score
91
In that case, if you are looking for general guidelines, for a long skinny rocket like I described that is 20-30 times as long as it is wide, you should give yourself more margin than 1-2 calibers (which does mean body diameters) distance between the CG and CP, because CP tends to shift a bit more at varying angles of attack with this kind of geometry. You should go with a distance between your CG and CP of 10% of the rocket's length for safety.

To repeat, it does not matter where your CG and CP are along the rocket's length. It only matters where they are in relation to each other. Don't waste your time thinking you need to get your CP 2/3 of the way down the length or whatever.

And if you find yourself thinking you need to use a lot of nose weight, stop and rethink what you're doing. I have never had to add nose weight to a rocket with this kind of geometry. The only reason you would need to is if your fins are really ridiculously small. An extreme example of a long, skinny rocket is the Estes Mean Machine kit. You can see how small the fins are in relation to its size and keep in mind that it needs no extra nose weight to fly perfectly straight. I gave one of mine a pair of strap-on boosters, adding significant weight to the tail end, and it STILL requires no nose weight to fly straight.

Edit: It also might be worth pointing out to you that rockets with this kind of long and skinny geometry do not perform well in swing tests. If they are swung around in circles, the angle of attack varies too much along their length to get a realistic result unless the circle they are being swung in is really big (i.e, you use a really long string) at which point trying to get it going in the first place is impractical.
OK, this brings up a question I've been pondering. You say you don't need added weight in your rockets, if I'm understanding you correctly. Sounds like a lot of other people do, with all the talk about epoxy and BBs and clay. Some even say they prefer heavy nose cones. So my question would be, how many of you experienced builders need to put weight in your nose cones and how much?
I've already stated, several times, that the arrow vane rockets, I've made, perform especially well in the swing test. Better than my rocket with the huge fins.
I don't claim to know why this is, but it's true. Currently, I'm going with 6 vanes on a 14", 24mm body tube with a 3" ogive nose cone.
 

Antares JS

Well-Known Member
Joined
Mar 5, 2020
Messages
278
Reaction score
156
Location
Eastern Shore, VA
I think this is a case of mistaken perception. Extra nose weight is typically only needed on weird rockets or scale models with tiny fins (Saturn V's being an obvious example). Most of us have experience with adding nose weight because we have had to add it to SOME of our rockets for these reasons. Most of our rockets don't need it though, and there is no reason a standard-configuration 3-or-4FNC rocket would need it. If you need to spread your CG and CP out more, it's usually more advantageous to make your rocket longer to shift the CG forward or make your fins bigger to pull your CP back than to add otherwise-useless lead weight to your rocket.

Your rocket probably performs well in the swing test because it's long for its width, but not that long overall. Actual length has more influence on what I said about swing testing before than the diameter-to-length ratio.
 

lakeroadster

Lonewolf.... No Club
TRF Supporter
Joined
Mar 3, 2018
Messages
2,007
Reaction score
830
Location
Central Colorado
OK, this brings up a question I've been pondering. You say you don't need added weight in your rockets, if I'm understanding you correctly. Sounds like a lot of other people do, with all the talk about epoxy and BBs and clay. Some even say they prefer heavy nose cones. So my question would be, how many of you experienced builders need to put weight in your nose cones and how much?
I've already stated, several times, that the arrow vane rockets, I've made, perform especially well in the swing test. Better than my rocket with the huge fins.
I don't claim to know why this is, but it's true. Currently, I'm going with 6 vanes on a 14", 24mm body tube with a 3" ogive nose cone.
One of the optimization tools that Open Rocket provides allows you to have the program change the length of the rockets body tube. If you don't want any nose cone ballast... try that.

My Hammer head rocket has 7.2 ounces (F42T-8 power),
F-79 has 4.5 ounces (dual D12-3 powered),
X-Wing has .88 ounces (single D-12 power).

As Antares eluded to, short rockets that look less like an arrow and more like a plane, a missile / armament typically, or an odd-roc are the ones that need nose weight*

*except tractor motors (thanks Neil)

002.jpg001.JPG
000.JPG001.JPG
001.JPG010.JPG
 
Last edited:

neil_w

Hunkered down and slowly going crazy
TRF Supporter
Joined
Jul 14, 2015
Messages
9,071
Reaction score
2,400
Location
Northern NJ
As I've said before, typical low-powered rockets of tradition design do not need much if any nose weight. But be careful of trying to generalize; rockets come in an infinite variety of designs, in various sizes, with different flight objectives. As a perfect example above, @lakeroadster 's creations are anything but typical; those nose weight numbers will tell you exactly nothing about how much you should expect to need in a 3FNC or even a 6FNC.

You have a free tool at your disposal that will help you answer your questions in a far more useful and informative way, I can't figure out why you're so reluctant to use it.

Swing tests are fine for small rockets of strange design for which you can't trust the simulation. But once the rocket size starts increasing it becomes impractical. When @OverTheTop and crew built and flew a full-scale V2 they didn't swing-test it off the top of a mountain. They built a sim model. (BTW V2 is a good example of a design that requires nose weight).

Have a rocket that seems to behave in an unexpected way? Post the sim and we'll comment. We still have not seen a sim of your small rocket with large fins that supposedly need a lot of nose weight.
 

dr wogz

Fly caster
Joined
Feb 5, 2009
Messages
5,353
Reaction score
711
Location
Land of Poutine!
Most (if not all) of my LPR fleet have no nose weight, or just what came with the kit. (I tend to use OR and design accordingly for Scratch models.) Most LPR don't require nose weight due to their size & such. And, most A-B-C motors differ in weight by only a few grams, if that..

Now, when you get into MPR & HPR, you are starting to deal with a greater weight range in motors. For example: My LOC Goblin can fly on G- to K motors.. We're talking a 2 or 4 (or more!) fold increase in motor weight, so the CG can move back quite a ways.. I intend to fly it mainly on H to J motors, so I've added weight for the biggest J I plan. I have my CP marked (generally required for HPR), and the day I do want to put a K1050 in it, I will know that I will likely need more nose weight based on fully loading / prepping it and balancing it a caliber at least forward of the marked CP.. (I also have OR, so I can also do a quick check with said motor, and confirm: "yeah, another 18 oz in the nose is needed..")

Coming back to your arrow vane rocket: We aren't contradicting you, we are merely saying the overall span of the fins seems small / short. And, to rectify a fixed fin size (small in your case) is typically countered with nose weight. Your design may very well be stable & be a great flyer. Or, if may sky-write all over the place, and plant itself in the top of the vendors' trailer.. (and this may be due to other factors, such as the fins being floppy or being lost due to ..)
 

JohnCoker

Well-Known Member
Joined
Apr 13, 2013
Messages
1,690
Reaction score
330
I'm just looking for general guidelines. Rules of thumb experienced builders go by when designing a rocket.
I generally build models of actual sounding rockets/missiles, so the proportions are usually defined. As far as "designing a rocket," that so much depends on where you are on the oddroc to competition spectrum that it's going to be hard to give rules of thumb. You can make an unusual shape fly if necessary, but as others have said most normal shapes work as-is. (The simulators are you friends here for determining the CP and you can determine the dry CG directly.)
 

Senior Space Cadet

Well-Known Member
TRF Supporter
Joined
May 23, 2020
Messages
259
Reaction score
91
I think this is a case of mistaken perception. Extra nose weight is typically only needed on weird rockets or scale models with tiny fins (Saturn V's being an obvious example). Most of us have experience with adding nose weight because we have had to add it to SOME of our rockets for these reasons. Most of our rockets don't need it though, and there is no reason a standard-configuration 3-or-4FNC rocket would need it. If you need to spread your CG and CP out more, it's usually more advantageous to make your rocket longer to shift the CG forward or make your fins bigger to pull your CP back than to add otherwise-useless lead weight to your rocket.

Your rocket probably performs well in the swing test because it's long for its width, but not that long overall. Actual length has more influence on what I said about swing testing before than the diameter-to-length ratio.
Wow! That is an earth shattering revelation. Now, I have heard that a rocket that is too light won't coast far enough, after engine burnout, so won't reach it's potentially highest apogee. But this information will greatly simplify my larger rocket builds as I was concerned about how I was going to weight the nose cones. Makes my work on Open Rocket a little harder.
 

Senior Space Cadet

Well-Known Member
TRF Supporter
Joined
May 23, 2020
Messages
259
Reaction score
91
I generally build models of actual sounding rockets/missiles, so the proportions are usually defined. As far as "designing a rocket," that so much depends on where you are on the oddroc to competition spectrum that it's going to be hard to give rules of thumb. You can make an unusual shape fly if necessary, but as others have said most normal shapes work as-is. (The simulators are you friends here for determining the CP and you can determine the dry CG directly.)
I once talked to a guy at the Smithsonian Institute about boat plans. The Smithsonian has the plans for historic watercraft. I told him I was interested in building a much, much smaller version of a J-class yacht, the huge yachts they used to race in the America's Cup. He just about called me an idiot. In fact he told me that was the dumbest thing he'd heard. He might need to work on his people skills, but he's right. You can't just make a smaller version of a boat. Basically, it would sail like crap. Making it smaller changes the dynamics of everything. You can design a boat that looks similar to a J-yacht, but you can't just shrink the dimensions. Obviously, you've had success building smaller versions of sounding rockets, and Estes even has kits, but wouldn't just shrinking them in size change the flight dynamics of the design, or are you making subtle changes?
 

Senior Space Cadet

Well-Known Member
TRF Supporter
Joined
May 23, 2020
Messages
259
Reaction score
91
Luckily, I haven't wasted a ridiculous amount of parts and you've go me on the right track before starting my larger rocket builds.
I have one last question, which could potentially simplify my large racket builds.
Other than breaking more easily, is there any problem with making long, skinny fins? That is fins with a short base relative to their height.
I'm not going to say why I'm asking this because you will think I'm an idiot, but there is madness to my method.
 

Senior Space Cadet

Well-Known Member
TRF Supporter
Joined
May 23, 2020
Messages
259
Reaction score
91
Critique this for me, please?
Obviously a very preliminary design, but is there anything that just glares at you? Too long, too short, fins too small, too large? Or does this look like the start for a sound design?
Screenshot 2020-07-03 04.30.13.png
 

dhbarr

Amateur Professional
Joined
Jan 30, 2016
Messages
6,373
Reaction score
1,020
That's a good looking bird, go for it!
 

Steve Shannon

Well-Known Member
TRF Supporter
Joined
Jul 23, 2011
Messages
6,303
Reaction score
2,858
Location
Butte, Montana
Critique this for me, please?
Obviously a very preliminary design, but is there anything that just glares at you? Too long, too short, fins too small, too large? Or does this look like the start for a sound design?View attachment 423044
That looks good. Add a parachute and a motor and see where the Cg and Cp are. Then adjust it to be your ideal. For instance: scale the fins down and see how far the Cp shifts forward. Then scale them up and see how far it shifts aft.
 

Steve Shannon

Well-Known Member
TRF Supporter
Joined
Jul 23, 2011
Messages
6,303
Reaction score
2,858
Location
Butte, Montana
Luckily, I haven't wasted a ridiculous amount of parts and you've go me on the right track before starting my larger rocket builds.
I have one last question, which could potentially simplify my large racket builds.
Other than breaking more easily, is there any problem with making long, skinny fins? That is fins with a short base relative to their height.
I'm not going to say why I'm asking this because you will think I'm an idiot, but there is madness to my method.
Nope, that’s perfectly fine. Fins with a short root but long span are popular in some designs. They will have different dynamics, but that’s not a bad thing. The Binder Design Dragonfly really epitomizes that type of fin.
BE5D4296-3C2F-42FC-98C1-AEAABB8055DB.jpeg
 

Antares JS

Well-Known Member
Joined
Mar 5, 2020
Messages
278
Reaction score
156
Location
Eastern Shore, VA
Other than breaking more easily, is there any problem with making long, skinny fins? That is fins with a short base relative to their height.
Everyone else is right as well, I just want to add that if you ever do high power, fins like that are a bad idea for a rocket expected to reach transonic speeds due to the likelihood of flutter. No problem at all for low power though.

As for your sim, your overall shape is fine, just make sure you complete the simulation by adding your recovery system, motor mount (if not a minimum diameter), and motor, and make any needed adjustments before you build it.
 

neil_w

Hunkered down and slowly going crazy
TRF Supporter
Joined
Jul 14, 2015
Messages
9,071
Reaction score
2,400
Location
Northern NJ

dr wogz

Fly caster
Joined
Feb 5, 2009
Messages
5,353
Reaction score
711
Location
Land of Poutine!

for some other examples.

Just remember, the longer the fins are, the more likely they are to break on landing. (The longer the fin, the longer the lever. So, less force required to break a fin at its root.)

As for your boat analogy, look at R/C airplanes. There are many kits for the coveted p-51, Spitfire, BF-109, Fw-190, etc.. but if you look, really look, their proportions are off; they aren't to scale. (And you'll see things like "Sport Scale" to better describe them.) A true linear down-scale of a plane won't fly. It's control surfaces are way too small at even a 1/4 scale (more so the smaller you go) Modelers do try to get as scale as possible, but compromises need to be made in order for it to fly.. well.


And as for J boats:


and these look like a hoot!
 

JohnCoker

Well-Known Member
Joined
Apr 13, 2013
Messages
1,690
Reaction score
330
... Wouldn't just shrinking them in size change the flight dynamics of the design, or are you making subtle changes?
No doubt it does, but the size gets swamped by other differences. The envelope of what's flyable is so large that a straight-forward scale down falls within it nearly all the time.

Commercial/military rockets are designed for a mission, usually something like "deliver a payload of this shape and mass to that location" and are mostly motor. That mission (as well as inevitable cost constraints) drive an optimization process that provides a clear guide for "designing" the rocket.

As hobbyists, we generally don't have the same goals, so "designing a rocket" isn't constrained in the same way. Before we can talk about designing, we need to talk about what you're designing "for" (the mission).

Competition fliers usually have constraints on the vehicle and motor and a given payload and are optimizing for something specific such as altitude or reliably gentle landing.

Scale modelers are trying to duplicate the appearance and perhaps some of the behavior of a rocket or missile.

Oddroc builders are trying to come up with something fun that no one has done before or make something fly that looks like it shouldn't.

I think you need to decide what the "mission" is for a rocket before you can design it.
 
Top