Tube vs. fin stabilized

[Symbiosis]

I am new to the forum. Did my share of Estes models as a kid (30 years ago), but have been trying to get my 13-year-old into rocketry. We joined the MDRA last summer and are currently working on a Level 1 project.

My question is: what are the aerodynamic pros and cons to tube stabilization? Presumably it is not the most efficient or else it would be seen as more than just a novelty.

 

[rocket9005]

Tube fins are easier to glue to the main body tube, and are easier to align properly along the flight axis of the rocket. You don't have to seal them before you paint. But they are harder to paint the inside of the tube, and they weigh more that their balsa counterparts, and have a lot more drag.

Having said that I think that they look cook. I have built a whole lone of scratch builds using tube fins, most using 24mm motors and BT-60 tubes, and one HPR using LOC 54mm tubing with a 38mm motor mount.

 

[Symbiosis]

thanks 9005. That's pretty much as I expected. I agree, the tube stabilized rockets have an interesting look. I made a model recently that was fins attached to tubes. I raked back the leading edge of the tubes to the same angle as the leading edge of the fins in hopes that would reduce drag somewhat.

It flew fine.

 

[troj]

I like tube fins; I like ring fins even better.

That said, my understanding is that they create a lot more drag that conventional fins. So, if you're after performance, they're not a good design.

But they're still really cool.

-Kevin

 

[Symbiosis]

I've never tried ring fins. Would it make a difference if they were installed very close inboard to the body tube? I wonder what would be the minimum distance to offer sufficient stabilization?

Here's another question: what, if any, value is there to canards (forward fins, if my terminology is correct)?

 

[Pem Tech]

I've never tried ring fins. Would it make a difference if they were installed very close inboard to the body tube? I wonder what would be the minimum distance to offer sufficient stabilization?

Here's another question: what, if any, value is there to canards (forward fins, if my terminology is correct)?

How about ring fins AND regular fins?

Our Screemin' Green Meenie' has both...


Or what about a long sloping (we call it sculpted) tube fin?

Our King Kraken has proven to be extreamly low drag for a tube fin. She flys more like a standard flat fin rocket.

Tube Fins!
Gotta lovem'

 

[troj]

I've never tried ring fins. Would it make a difference if they were installed very close inboard to the body tube? I wonder what would be the minimum distance to offer sufficient stabilization?

It's a function of the overall area of the ring fin. Take the surface area of the fin and project it out as a set of fins. If those fins are sufficient to provide stability, then your ring is sufficient.

Here's another question: what, if any, value is there to canards (forward fins, if my terminology is correct)?

On guided rockets, they often help with maneuverability. On model rockets using nothing but aerodynamic stabilization, they can crease issues by causing instability -- they move the center of pressure forward.

How about ring fins AND regular fins?

Layne, you're just a deviant...

-Kevin

 

[Rocketjunkie]

One other thing, tube fin rockets don't weathercock.

 

[Symbiosis]

Layne,

King Kraken looks similar to the design I described above.

 

[luke strawwalker]

I've never tried ring fins. Would it make a difference if they were installed very close inboard to the body tube? I wonder what would be the minimum distance to offer sufficient stabilization?

Here's another question: what, if any, value is there to canards (forward fins, if my terminology is correct)?

Canards are a big NO-NO unless you're using some type of active guidance (like George Gassaway's Sunseeker rocket) and are actually very DESTABILIZING, requiring extra-large rear fins to compensate. Dogfighting missiles have them, but they're actively controlled by the missile's guidance system to steer them close to the target for detonation. There has been some interesting stuff done in using SWIVELING canard fins on rockets, strictly for looks-- by mounting them a certain way on a pivoting 'axle' so they can move freely, realistic looking dogfighting missiles can be built without the destabilizing influence of fixed forward fins... (do a search and I'm sure you'll come up with the thread on this... )

A ring fin has to be large enough to get out into the airstream to actually stabilize the rocket-- if it's say, a BT-60 ring on a BT-55 tubed rocket, it's going to be too small to actually stabilize the rocket, much like fins that are too small. There is a boundary layer of turbulent air close to the surface of the rocket in flight, and this air is not very effective at acting on fins (ring or flat) to correct the flight path. That's why when making regular fins bigger, increasing the span out from the tube is more effective than increasing the length of the fin along the tube.

I don't know of the rule of thumb for sizing a ring-fin, but I'd guesstimate that at least twice the body tube diameter is a good starting point, and at least two body tube diameters in length. Of course moving the ring fin back behind the rocket on swept fins greatly increases it's effectiveness by increasing the moment of intertia and moving the CP rearward.

Good luck! OL JR

 

[Pem Tech]

Layne, you're just a deviant...

-Kevin

Thanks!

 

[shrox]

Tubes good. Canards, not so scary, just make them straight and not too big. Here is one of my old kits I used to offer. It has sensibly sized canards.

 

[MarkII]

One other thing, tube fin rockets don't weathercock.

Or flutter.

MarkII

 

[jflis]

I am new to the forum. Did my share of Estes models as a kid (30 years ago), but have been trying to get my 13-year-old into rocketry. We joined the MDRA last summer and are currently working on a Level 1 project.

My question is: what are the aerodynamic pros and cons to tube stabilization? Presumably it is not the most efficient or else it would be seen as more than just a novelty.

First, welcome to the forum and welcome back to rocketry!

Tube finned rockets are more for looks and challenge than anything. You can really do some fantastic things if you think about tubes as fins in addition to body tubes

We have the following fun examples:

Borealis

Corona
Corona-2 booster
D-Nelson Tomahawk
HERC-5
Long OverDue

Good stuff and lots of fun, tube fins

jim

 

[RocketT.Coyote]

Built a commercial tube-fin rocket from a kit. Went unstable during coast. Came down tail first and landed before deployment of recovery device.
Coyote luck, I guess.

 

[Tom W]

One other thing, tube fin rockets don't weathercock.

Absolute Truth.

When the wind picks up I reach for the tube finned rockets. It still amazes me that they always go where they're pointed.

 

[accooper]

How do you guys cut those wonderful angle cuts on those tubes?

Andrew From Texas

 

[MarkII]

How do you guys cut those wonderful angle cuts on those tubes?

Andrew From Texas

Elfin magic.

MarkII

 

[MarkII]

Actually, I use a template generator to create a bevel template. I happen to like this one.

To get FSI-style parallel bevels at the two ends of the tube, first I use the Fin Wrapper tool in VCP to create a two-fin wrapping guide for the size of tube that I am using. I use it to mark two lines on the tube that are 180° apart.

Then I create the bevel template, sizing it so that the two ends just meet when it is wrapped around the tube. I wrap it around the tube and use a strip of tape to hold the two edges together, Then I rotate it on the tube until the joint between the two edges is right over one of the lines, and the tip of the template is even with the top of the tube. I mark the tube for the bevel, and then slip the template off.

I turn the template around and slip it back onto the tube, so that the tip is on the other end. I rotate the template around so that the joint between the edges is even with the other line that I marked, 180° away from the first line. Again, the tip of the bevel is even with the top of the tube. I mark the outline for the other bevel cut.

I put a new blade in my hobby knife and lightly trace the cut line for one of the bevels with it. If I have anything that will fit into the tube and support it, I will slip it in while I am cutting. Otherwise I just very carefully cut the first bevel using multiple light passes with the knife. I repeat the process for the other end. Finally, I sand the bevels lightly by stroking them on a sheet of 320 grit sandpaper placed on my bench. I use care to only sand just enough to clean up the cut edges. If you do too much sanding, you can unintentionally change the angle of the cut.

After filling and sanding the spirals smooth on the outside and inside of the tube, I coat the inside with thin CA and then sand it smooth. Occasionally I need to do this a second time in order to get the inside of the tube completely smooth.

MarkII

 

[cjl]

A note about canards: You don't need to completely avoid them. They can be used (usually for looks or for scale models), you just have to be quite careful about your stability, and you will likely need noseweight. Here's one of my rockets that has quite sizable canards, and as a result, needed 10lbs of lead in the nose. It flies great.

 

[Pippen]

I am new to the forum. Did my share of Estes models as a kid (30 years ago), but have been trying to get my 13-year-old into rocketry. We joined the MDRA last summer and are currently working on a Level 1 project.

.

Gald to have you on board. Welcome back to model rocketry!

 

[Symbiosis]

Ok, I am out of my depth on this one, but just curious:

Are all two-stage rockets staged at the bottom? Are any staged like a conventional (non-model) rocket? I would assume you'd need an independently stable upper stage and that would seem to require canards there. Electronic ignition of upper stage?

 

[jflis]

How do you guys cut those wonderful angle cuts on those tubes?

Andrew From Texas

If you take a tube cut at an angle (a straight cut, nothing fancy) and slice open the tube and flatten it out you will see that the shape of the flattened tube is a sine wave (or cosine, depends on where you slice it... )

For fancier designs (like the cuts on our Night Whisper model) I cut and shaped the tube by hand till I got the right look. Then I sliced open the tube, photographed the flattened tube and then drew the curve in a CAD program.

hope this helps!
jim

 

[luke strawwalker]

A note about canards: You don't need to completely avoid them. They can be used (usually for looks or for scale models), you just have to be quite careful about your stability, and you will likely need noseweight. Here's one of my rockets that has quite sizable canards, and as a result, needed 10lbs of lead in the nose. It flies great.

Yeah, I guess I was a bit unclear and overvociferous about canards-- MANY rockets use them. Just realize that they do NOT contribute to stability; quite the opposite as they are DESTABILIZING (by moving the CP forward) and you have to add either extra fin area at the rear to counteract their effects (move the CP rearward again), or noseweight to move the CG forward.

That said, they REALLY make a rocket look cool-- but ya do have to be careful designing with them... OL JR

 

[luke strawwalker]

Ok, I am out of my depth on this one, but just curious:

Are all two-stage rockets staged at the bottom? Are any staged like a conventional (non-model) rocket? I would assume you'd need an independently stable upper stage and that would seem to require canards there. Electronic ignition of upper stage?

Staged 'at the bottom'??? Not sure what you mean by this...

If you refer to the common practice of taping a booster motor to an upperstage motor using a short 'booster stage' airframe as is common in most Estes-style two stagers, then yes that is probably the most commonly used method, but it's not the ONLY one...

There is also GAP STAGING, which has the lower stage consisting of an airframe of up to 12 inches in length with the booster motor installed at the bottom, and the upperstage engine mounted in the rear of the upperstage significantly forward of the lower stage engine. Usually, you either want a minimum diameter (same diameter as the engine) lower stage, or use a stuffer tube to contain the hot particles and gases that ignite the upperstage engine, and you have to use ports to vent the air from the stage when the lower engine 'blows through' to ignite the upperstage, usually by placing a pair of holes at the top of the first stage or through the stuffer tube just below the second stage motor and through the outer body tube or through the centering rings at the back of the first stage to allow the air to vent from the first stage. When the first stage engine 'burns out' hot chunks of burning black powder (BP) and hot gases travel up the tube and into the nozzle of the upperstage engine, igniting it, which then blows the first stage off. This can allow you to make longer more 'real' looking first stages.

Of course, this only works on BP motors... composite propellant (APCP) motors cannot be ignited this way-- for one, they're ignited from the head end, not the tail end as BP motors are. APCP also requires a fairly significant 'pressure spike' to really get the motor lit, whereas BP only requires a hot enough ignition source (burning BP particles, hot gases, infrared, etc.) To stage APCP motors, you MUST use some form of electronic ignition. There are various systems out there, from relatively simple electronic staging timers that sense when the first stage lifts off and then ignites the upperstage engine a preset number of seconds after the rocket lifts off (generally a little greater than the burn duration of the first stage engine) up to more sophisticated systems that use logging altimeters coupled with G-force sensors to detect the burnout of the first stage motor and fire the second stage ignitor. You have to have a GOOD power source and a highly reliable type of ignitor to use with these systems to do everything possible to get upperstage ignition-- definitely more involved than igniting BP upperstages using blow-through....

Hope this helps! OL JR

 

[luke strawwalker]

Ok, I am out of my depth on this one, but just curious:

Are all two-stage rockets staged at the bottom? Are any staged like a conventional (non-model) rocket? I would assume you'd need an independently stable upper stage and that would seem to require canards there. Electronic ignition of upper stage?

Yes, the upperstage must be 'independently stable' and also must be stable with the lower stage installed-- usually requiring larger fins on the lower stage to increase stability because of two influences-- 1) the weight of TWO engines in the back moves the CG rearward, which is destabilizing by moving it closer to the CP and 2) the upperstage fins (especially on gap-staged rockets) are further forward on the combined first and second stage rocket, which moves the CP forward because being further forward makes them 'act' like canards until the first stage seperates (presumably right behind the upperstage fins).

Adding canards up near the nosecone would be VERY destabilizing in this situation for two reasons-- 1) the canards would move the CP even MORE forward than it already is on a gap-staged rocket, and 2) the weight of the first and second stage engines STILL moves the CG rearward, which is destabilizing...

That said, it CAN be done, but it is VERY TRICKY... TLP sold a kit of the Nike-Ajax, which I've seen flown as a two-stager, and it has canards up on the nosecone just like the real Nike-Ajax did, and of course the upperstage fins at the back of the upperstage, with a larger body tube first stage with LARGE fins on the back... it requires VERY careful work though to ensure that enough noseweight is used to bring the CG forward, and to make sure the CP is still at least 1 caliber behind that CG since the canards and 'mid body' upperstage fins (when the first stage is attached) move the CP significantly far forward at liftoff and prior to staging.

As I said, it CAN be done, but if you need the explanation you're not ready to do it... :roll:

Later and hope this helps! OL JR

 

[luke strawwalker]

Actually, I use a template generator to create a bevel template. I happen to like this one.

To get FSI-style parallel bevels at the two ends of the tube, first I use the Fin Wrapper tool in VCP to create a two-fin wrapping guide for the size of tube that I am using. I use it to mark two lines on the tube that are 180° apart.

Then I create the bevel template, sizing it so that the two ends just meet when it is wrapped around the tube. I wrap it around the tube and use a strip of tape to hold the two edges together, Then I rotate it on the tube until the joint between the two edges is right over one of the lines, and the tip of the template is even with the top of the tube. I mark the tube for the bevel, and then slip the template off.

I turn the template around and slip it back onto the tube, so that the tip is on the other end. I rotate the template around so that the joint between the edges is even with the other line that I marked, 180° away from the first line. Again, the tip of the bevel is even with the top of the tube. I mark the outline for the other bevel cut.

I put a new blade in my hobby knife and lightly trace the cut line for one of the bevels with it. If I have anything that will fit into the tube and support it, I will slip it in while I am cutting. Otherwise I just very carefully cut the first bevel using multiple light passes with the knife. I repeat the process for the other end. Finally, I sand the bevels lightly by stroking them on a sheet of 320 grit sandpaper placed on my bench. I use care to only sand just enough to clean up the cut edges. If you do too much sanding, you can unintentionally change the angle of the cut.

After filling and sanding the spirals smooth on the outside and inside of the tube, I coat the inside with thin CA and then sand it smooth. Occasionally I need to do this a second time in order to get the inside of the tube completely smooth.

MarkII

You can also just use a "mitre box" and a razor saw to cut the tube off... I've done it both ways...

(a mitre box is a three-sided (two sides and a bottom) box with slots cut in it, usually at 45 degrees, into which you place the object you want to cut (the tube) and put your saw blade into the groove made by the slots to hold your saw blade at the prescribed angle as you saw through the material... they sell them at Hobby Lobby sized for the X-acto razor saws (which they sell as well) but you can make your own with any angle you want from lumber or use a regular carpenter's miter box from the lumberyard if your saw will fit it.

Good luck! OL JR

 

[Pem Tech]

How do you guys cut those wonderful angle cuts on those tubes?

Andrew From Texas

Andrew,
Here is a direct link to our King Kraken Destruction manual. Clicky

The file contains all the information you will need to cut our specialty scuplted tube fins.


Here is the Rocksim file showing how we used the Levinson Method to simulate the tube fins. Clicky


Hope this helps..

 

[shrox]

Andrew,
Here is a direct link to our King Kraken Destruction manual. Clicky

The file contains all the information you will need to cut our specialty scuplted tube fins.


Here is the Rocksim file showing how we used the Levinson Method to simulate the tube fins. Clicky


Hope this helps..

Now tell him how to cut ping pong balls...

 

[RandyT0001]

Now tell him how to cut ping pong balls...

Well, with feet set about shoulder width apart, I stand about 2.5 meters back from the target, pull my Mk 1 hand phaser off the velcro, set the power level to, oh, about 7, and "shooting from the hip", I "eyeball" the aim as I slice the ball, typically starting above the ball and slicing in a downward motion. Be carefull and make sure you release that power activiation button as soon as you finish slicing 'cuz you can end up cutting the table in half also.

:neener:

How do you do it?

:duck: