Water Waiver

A team of scientists and engineers at UBC in Vancouver, Canada is developing rockets and systems to collect weather data up to 20,000'. The unguided rockets will be launched from buoys out in the middle of the Pacific Ocean, in almost any weather, to improve weather forcasting. This year we are developing the rockets to reliably achieve 20,000' and fit in as small diameter launch tube as possible. We received a 20,000' waiver to launch at a lake in BC. Sorry, we're not allowed to share our waiver with hobby rocketeers. This summer I built about 30 rockets to test various fin designs. Most of the rockets were tested on G through J engines at HPR meets in Oregon and Canada. We're still waiting for our special Cesaroni K-1,000 engines to arrive, to achieve the goal altitude.

So far, we successfully sent a weather measuring sonde to over 9,000' at the Lethbridge, AB meet this year, out of a 4" launch tube, on a J engine. We also built and used a floating launch pad for our lake launches. Ever try to track a high altitude rocket from a rocking boat? We quickly found that we would lose most of the rockets we launched at the lake. Ever try to add enough floatation to a a minimum diameter rocket for a water recovery? We have photos and movies of many launches, including water launches.

We have tried every kind of fins, anyone could think of, to make the rockets fit in small launch tubes. Concentric circular (tube) fins have proven to be reliable and can be surprisingly small in diameter. We are concerned about their drag at supersonic speeds, however. Anyone know where to find information about that?

I am back home in Texas, continuing developement using Cesaroni G engines in 38mm minimum diameter rockets, launching out of a 3" launch tube. Mike Dennett of Cesaroni has been very helpful, making special engines for us, and sharing his vast rocketry engineering experience.

I have learned a great deal about launching out of tubes, minimizing drag, circular fins, folding fins, and minimizing fin size. If anyone has any specific questions, don't hesitate to ask. I am making new rockets every week, testing various designs.

Launching out of launch tubes requires no lugs, so I am achieving some amazing altitudes. If you're interested in our project, below is my latest "in house" memo. If there is enogh interest, I'll continue to post developement progress.

************************************************************

After trying every radical approach in Vancouver, I am working on more conservative/subtle improvements. I tried moving the circular fin down a couple inches, but it was not strong enough down there. I am able to move the circular fin down about 3/4", and still have excellent strength. That has the same effect as moving the engine's weight up 3/4", which moves the center of gravity up about 1/2".

I am using long, narrow, radial fins to support the circular fin. I agree with Catherine that this will help broadside center of pressure significantly.

Another subtle improvement is using carbon fiber for the fins. The carbon weighs about half of what fiberglass or aluminum does. It is an obvious way to move the center of gravity up a little... more on my light weight test rockets.

Using the engine casing as the bottom of the rocket body helps a lot. It can move the center of gravity up significantly on your full sized rockets.

One way to decrease drag is to make the rockets less than minimum diameter. Normally, a minimum diameter rocket uses the engine OD as the body tube's ID, with a small clearance. For example: Our 54mm rockets are 57.5mm diameter. Since we plan to use the engine casing as the bottom rocket body, it's not too hard to make the rocket engine be the largest OD of the rocket body. So our 54mm rockets can be just 54mm diameter. Mike already told us it is no problem making the top of the engine include an addapter to attach the small upper body tube directly. So there won't be anything at the transition that makes the rocket larger diameter. The fins can be glued directly to the engine casing to eliminate the fin ring at the bottom. The drag at the bottom of the rocket is 12% less with a 54mm OD, since the area of the end of the body is proportional to the square of the diameter.

Reducing drag allows a lighter optimum weight... so reducing drag will keep us from having to add weight to our rockets to get them to coast high enough.

My second test rocket here has all of these improvements. I made the upper 3/4 of the rocket out of smaller, lighter, 1+1/4" OD, spiral wound, carbon tubing. This moves the center of pressure and center of gravity down a lot. I glued the fins directly to the engine casing, and used the top of the fins to connect to the upper body tube. This rocket should weigh half of what my first one weighs, not counting the engine and recovery system. It should accellerate much faster, attain a much higher maximum speed at a much higher altitude, and coast really high. This will push a circular fin to a faster speed than we've ever tested.

My third rocket will test making the circular fin taller. That makes it have much more broadside stability. Mechanically, it lets me have more of it extend below the engine. This will move the center of pressure way down on the rocket, allowing me to make the nose cone very light. I tapered the thickness of the circular fin so the bottom of it is very light.

I'm still making different sizes of tubing out of fiberglass and carbon fiber. I decided it is easier to make tubing to fit a store bought nose cone, than to make custom nose cones.

Originally posted by MarkStull
...We have tried every kind of fins, anyone could think of, to make the rockets fit in small launch tubes. Concentric circular (tube) fins have proven to be reliable and can be surprisingly small in diameter. We are concerned about their drag at supersonic speeds, however. Anyone know where to find information about that?...

Click to expand...

You might see if you can find a copy of MIL-HDBK-762(MI), DESIGN OF AERODYNAMICALLY STABILIZED FREE ROCKETS. This was available on-line, but I can't find the link, sorry. It discusses ring fins at super-sonic velocities.

Keep posting Mark! I find this extremely fascinating! I'm also very interested in tube fins and ring fins so, I'd be very interested to hear how your experimentation goes.

mark:

C.E.Brazzel, J.H.Henderson,and C.J.Craft
Longitudinal Stability Characteristics of a Series of Ringtail-Body Combinations at MACH Numbers of 0.8-4.5
Report RD-TR-65-7
US Army Missile Command
Redstone Arsenal,AL 1970

Available from Defense Technical Information Center(DTIC)
https://www.dtic.mil


C.Wayne Dahlke
Aerodynamics of Longitudinally Supported Ringtail Configurations
Report RD-TM-71-6
US ARmy MIssile Command
Redstone Arsenal,AL 1971

A friend has this he had to do a FOIA request to get it............

C.Wayne Dahlke, J.C Craft
Experimental Aerodynamic Investigation of Configuration Shape for RingTail Missiles at Mach Numbers from 0.5 to 1.3
Report RD-TM-72-5
AD Number ADA039676
1972
Available from NASA CASI
https://www.sti.nasa.gov/RECONselect.html


for MIL-HDBK-762(MI), DESIGN OF AERODYNAMICALLY STABILIZED FREE ROCKETS

https://131.82.253.19/quicksearch/
and then type: MIL-HDBK-762 in the document ID field and you can download this as a PDF file.....
https://131.82.253.19/eAccess/index.cfm?ident_number=71236

shockie B).............

Dear Mr. Brown,

I use the terms: tube, ring, and circular fins to mean the same thing. They definitely work great for getting a rocket to fit in a small launch tube... about 1/3 less overall diameter than the narrowest radial fins, in my experience. I usually support them with 4 tall, narrow radial fins, that continue up the rocket to near the center of pressure. These radial supports contribute to stability. Circular fins are easy to make, and align. I usually make them out of fiberglass or carbon tubing. I do grind and sand an airfoil into the outside of them. Sometimes I lay them up with a tapering number of plys. They need to be fairly stiff at their top 1/3, so they hold their shape.

I learned to attach the circular fins to the radial supports with a high strength rubbery glue, like Loctite Handyman's or Plumber's adhesive. The circular fin does flex and vibrate enough to crack a more brittle glue. I have yet to determine what the optimum hight to diameter ratio is best. If you make them more than 2/3 as tall as thier diameter, they look kind of clunky.

The neat thing is being able to launch out of a tube with no draggy lugs. Ordinary, thin walled PVC pipe works great as a launch tube. Be sure to find a piece that's still round. I put a reducer tee at the bottom of the launch tube and block off the bottom, so the exhaust blows out to the side. I also add a use a hose clamp to attach a piece of tin to the exhaust hole, to deflect the blast away from the ground a bit. Don't glue the tee on the end of the tube. You'll need to remove it to load the rocket, and run the ignition wires.

Tall skinny rockets work best. A shorter rocket will lean more in the tube, sometimes yielding a brief wobble in the first 100' of flight. This can make the rocket go off vertical, as much as 5 degrees for every degree of lean in the tube. We have tried using wires to center the top of the rocket in the tube. They work, but add drag.

I have a movie of a leaning rocket launch. It did a half an oscillation: The rocket come out the tube leaning slightly one way. It straightens itself out very quickly, but continues to yaw until it is leaning the other way even more. Then it goes straight that way. The next flight with the same rocket in the same tube went perfectly vertical.

Be sure to allow enough clearance between the fins and the launch tube, so the rocket can lean without binding. I allow more than 0.1" clearance (0.05" on each side) of the circular fin. The radial supports have to taper a bit toward the top, so the rocket can lean.

Mark,

This may sound really dumb, but is it possible to use a piston launcher in conjunction with or instead of your tube? I don't even know if a piston is even doable with larger rockets, but I'd imagine with a high enough initial impulse and the right materials for the piston, it would be. If it is, you might be able to do away with the draggy ring fin and launch tube altogether (also doesn't require a launch lug or any other drag-inducing attachments either).

Forgive me if this is dumb, but it's past my bedtime and I was just churning this topic through my head.

Jon

Dear KermieD,

What is a piston launcher? Is this something that is light weight? with no exposed moving parts? that will work on a rocking buoy? in a salt environment? remotely? with huge waves occasionally splashing over it? in any weather? to 20,000'?

Weight is a huge issue on our buoys. We are hoping to get 400 rockets/launch tubes on a buoy that's about the size of a ski boat. One of our team went out to sea with a Canadian buoy tender to learn about buoy maintenance. Maintenance and replenishment costs a fortune, and is very dangerous.

We plan to remotely launch one rocket per day. The entire rocket will be lost and sink, each flight. There is no recovery system, other than a small chute on the sonde, which is also lost. It will cost $1,000 to $2,000 per launch. About half of that goes for the buoy tender. We're comparing our costs to a manned weather ship, that costs about $1,000,000 per year. We have many engineering and funding problems yet to solve.

One of our team is working out the systems and algorithm to get the rockets to launch just as the buoy rocks through level.

We'll have some photos on our web site soon. The water launches are way cool. Hopefully, we'll be launching the K engined rockets off the floating launch pad by the end of this month. We're still waiting for the engines. Our engines are being developed in conjunction with Cesaroni's new 54mm line.

The weather is your real issue with a piston launcher, but if used in conjunction with your tube, it just might work. A piston launcher is basically a tube sealed at the bottom that is the exact diameter of your body tube. It has about 3 dowels on the outside of it to hold the rocket on top and the ignition wires are wired up from the bottom through a hole that is then sealed around the wires. The exhaust gases fill the tube and actually then push up on the rocket. It's sort of like shooting your rocket off of a spud cannon. It's used in altitude events in contest flying and is reported to add up to 10% to altitude.

It has no moving parts and would not add to the diameter needed for each flight. The weight would be dependent upon the materials you'd have to use to make sure that the gases expelled the rocket upwards instead of the walls of the tube outwards.

A possible low-tech solution to your vertical launch problem could be (again, just tossing out goofy ideas here that may have no practical application) a mercury switch of sorts with 4 contacts laid out in a square on a very slightly concave surface. The blob of mercury would have to touch all 4 contacts to trigger the launch, thus ensuring that it's level along both an x and a y axis. It's not perfect, but it's cheap. This of course would rely on virtually instantaneous ignition, which, in my experience, ain't that easy to accomplish!

Jon

Dear KermieD,

Thanks for the idea. I love new ideas. Unfortunately the sonde wouldn't survive the tremendous accelleration of a piston launch to 20,000'. Even spreading the accelleration out for the two seconds of our K-1,000 engine burn, we're right at the limit.

I finally started launching test rockets, here in Texas. So far, I've launched 3, and lost 1. I'm learning more about circular fins with every launch. It is always windy at my hill top launch pad. I don't mind, because our final rockets will have to launch in almost any wind. I just bought a longer (9') launch tube to help get more vertical launches in the wind.

I launched my third rocket, T3C, Sunday. It had a circular fin that was almost twice as tall as the earlier rockets. It extended way below the bottom of the rocket body. It looked way over stable, especially with the small diameter upper body tube, made out of relatively heavy fiberglass. It was unstable, tumbling end over end. This was a hugely important test for us. This proves to me that making the circular fin taller doesn't help at all. Niether does extending it below the rocket body. The effect of the circular fin must all be in the leading edge. The radial support fins must be doing a lot of the work, so making them extend up the rocket body is important.

My "less than minimum diameter" rockets have turned out plenty strong. I have no way to measure how much less drag they have.

Making a 38mm rocket launch out of a 3" tube, without adding weight to the nose, is a lofty goal.

Cheers,

That seems very odd to me that the longer ring fin would cause the rocket to become unstable. If I'm remembering my physics correctly, the ring fin simply adds a correcting moment when any aspect other than "head on" is presented in the direction of travel. It would seem that more surface area would produce a greater correcting moment. Does anyone have any explanation that would satisfy my inquiring mind?

Dear Mr. Brown,

I'm not saying that making the circular fin taller decreased its effectiveness. It just didn't help... which defys logic to some degree for me too. I would expect the taller fin would definitely help broadside CP... and yet the rocket tumbled.

I can surmise that once the angle of attack of the flying surfaces of the circular fin is great enough to become stalled, the rocket already has enough tumbling yaw momentum to continue tumbling. The circular fin would tend to stall at around 7 degrees, depending on the thickness and type of airfoil, while the broadside profile doesn't increase that much until the angle of attack is several times that... leaving a stability gap, for a tall circular fin, between the stall angle and the point where broadside profile comes into play.

We've known all along that circular fins tend to work more like a wing. Tall, narrow, radial fins suffer tremendously from the wing tip effect, defeating their effectiveness. Circular fins have no wing tip, making them very effective... as long as the angle of attack is kept below the stall angle.

Another way of looking at it: The leading centimeter or so of the circular fin grabs the air, like the upper surface of a wing, and sends it parallel to the rocket. Making the circular fin taller doesn't help, because the air is already moving parallel to it when it gets to the trailing part. Also any incremental improvement in stability is countered by the added weight of the taller fin moving the CG down.

When you're talking about circular fins and radial fins and you use the word taller, are you using the word in the same context? For circular fins, I assume that taller means in the direction parallel to the airframe. However, in the below quote, I get the impression that by tall radial fins you mean the direction perpendicular to the airframe.

Originally posted by MarkStull
Tall, narrow, radial fins suffer tremendously from the wing tip effect, defeating their effectiveness.

Click to expand...

Am I misunderstanding your usage of the terminology?

Were you able to check your nozzles after launch to make sure you didn't get vecored thrust issues? Maybe i'm way off base here (and of course it wouldn't be the first time!), but it seems to me that an airfoil moving at launch speeds would be very difficult to stall.

Also, the lift produced by an airfoil would be perpendicular to the rear of the rocket (either in or out, depending on how you did the airfoil) and since you're using a ring fin, it seems that if you stalled the airfoil on one side of the body, the corresponding section of airfoil opposite of the stall would compensate, bringing the rocket back to vertical (or wherever the CP relationship works out to based on wind, etc). Or am I way off and the airfoil isn't asymmetric?

Another thought is that, if your fing fin is extending too far back, that you may be inviting the Krushnik (sp?) effect, either with the fin itself or with the airflow coming through the interior of the ring fin.

Again, I may be way off base on both thoughts, and I find this fascinating, so please educate me!

It is hard to form opinions without all the rocket specs: length, fin dimensions, weight, CP, CG, motor used, etc. But of course, anything I came up with would just be opinion. Is your rocket really "less than minimum diameter"? I'd like to have the specs to put them into Rocksim.

My random questions/comments:

You mention leaning in the tube. I'd think that this wouldn't be desireable and would increase binding. You also mentioned that short rockets lean more than long ones? I don't understand this comment. Maybe some sort of fall-away support in the front of the rocket would help to keep the rocket centered.

I'm not sure the Krushnik effect would be an issue since there is airflow between the ring and the body tube. As I remember, ducting air into the motor and body tube reduces this effect considerably. Could there be issues similar to the Krusnik effect but related to using a tube? Have you thought of using some sort of sabot below the rocket?

Making a circular fin *too* long could certainly reduce its effectiveness, either by causing overstability or moving the CG backward. With high winds, overstability would not be desireable. I'm not sure that having more lateral area on the ring-fin wouldn't help.

Finally, it seems to me that being on a moving buoy will make the angle at launch fairly random and will have a big effect on the altitude achieved.

PS - Take my comments/questions as an attempt to understand and not criticize. I'm suffering from a killer cold and my head feels like it's full of cotton. Clear answers to my questions may very well be in the previous posts!

Dear Dick,

Yes, my rocket really is less than minimum diameter. The largest body diameter of the entire body is 38mm... the engine casing. There is no body tube or anything other than the fins that is larger. The fins are glued directly to the engine casing, and continue up the rocket, above the engine to make a connection to the upper body tube. The flight log with the dimentions is below. I'll try to give you enough to sim it out. Sorry about the mix of metric and inches. Rocket has no lugs.

As far as shorter rockets leaning more in the tube... you must not be feeling well... you'll figure it out. Yes, we have considered every kind of support. The rocket fins need to have enough clearance to not bind when the rocket is leaning with the side of the nose cone against the inside of the launch tube.

You're right... The Kruchnik effect does not come into play. Yes, we've considered sabots.

The rockets will be launched from the buoys at the instant it rocks through vertical. Designing the algorithm, systems, and sensors to accomplish this is a major project, being developed by another member of our team at UBC.

FLIGHT LOG
Rocket T3C

Circular fin is 70mm D by a wopping 97mm tall. 72mm of it extends below the rocket body (which is really the engine casing).
The four 0.6" wide radial support fins start 1/4" below the bottom of the engine casing and measure 5+1/4" up to where they start to taper. They taper for 1+3/4" to where they blend into the upper body tube.
Rocket is less than minimum diameter: 38mm OD for the bottom 4+1/2" (not counting the 3/8" long reload flange... which was ground down so it did not exceed 38mm OD)
Above the engine casing, the rocket tapers gently for the next 2" down to the...
Upper body tube is 28mm OD fiberglass 27+1/2" long measured from the top of the taper.
The nose cone extends 4+1/2" above the body tube. It is a very light plastic one.
Overall length 41+1/4"
Added nose weight: none
Total launch weight 442g
CG: 11" above the bottom of the circular fin.
Estimated CG with the recovery system crushed from launch accelleration: 10+1/4"
Est. CP: 8+1/2"
Engine: Cesaroni G 69
Recovery fuse: 9 seconds

Reason:
To test very tall circular fin. To test extending more of the circular fin below the rocket body. To test home made electric match igniter.

Flight:
Winds were about 15mph. Rocket was unstable, tumbling end over end. Home made igniter worked fine... took about 1+1/2 seconds to ignite.

Recovery:
Recovered on the ground. No damage.

Conclusions:
This rocket looked overly stable, with the bulky, tall, circular fin so low. I was shocked that it wasn't stable. Making the circular fin taller doesn't help at all. Nor does moving it down below the engine. The circular fin's leading edge must do almost all of the work. JB Weld is not weakened by the hot engine casing. It's not too hard to make an electric match igniter. Less than minimum diameter attachments are plenty strong. Getting a 38mm rocket to fit in a 3" launch tube, without adding weight to the nose, is a lofty goal.

For what it's worth, I stuffed your specs into a Rsim model. The ring fins were modeled using a technique devised by Bruce Levinson and published on the Apogee site. I was just curious what this would show. I got a CP (static) that is ~3.8 - 5.3 inches behind the CG. Could overstability have caused the problems given the high winds you were launching in? I dunno. Dynamic stability is too much for me.

The estimation of a bouy rocking is indeed challenging. And, given the variability of ignition times in high power motors, it seems like it would even more difficult to coordinate the launch with the bouy's attitude. Please keep us informed as the project progresses.

Maybe some work needs to be done on stabilizing the buoy? for example you could use pieze electric motion sensors to determine remotely what the atittude of the buoy is right prior to launch and it the sea was choppy, the buoy could have have 4 small rotary motors in each axis that you could start up to stabilize the platform......that way regardless of the weather and water conditions you could get a stright level launch..... heck if the buoy also had a wind speed and direction sensor attached you couls also perhaps have a tiltable launch system that could tilt to take advatnge of the wind........


I agree with rstaff3 as it sounds like this model was way over stable and as it hit the airstream , the wind force was such that it just was oscillating back and forth until it became unstable....

heres some additional info gleamed from RMR posts about ring tail model rocket:

Regarding the ring tail code, I took data from MIL-HDBK-762, that
states that a ringtail has twice the force moment as a rectangular
cruciform fin of the same chord length and span. (This also
apparently matches with data in Hoerner's fluid dynamics books.) Of
course this has *not* been verified by model flight tests - R&D
project for anyone interested.


So, let me attempt to re-iterate: I simulate a ring tail rocket by
calculating the CP as if it were a normal four-finned rocket (with 90 deg between fins). The four fins are rectangular, with the chord dimension being the "height" of the ring piece, and the span being the radius of the ring minus the radius of the body tube. Such a calculation of CP is then overly conservative, because the ring tail is actually twice a good as that. Is that what you're saying?

We've also played around a bit with ring-tails, and they are fun! The stability of a ring-tailed missile is much, much better than that of a fin-tailed missile with the same general dimensions. In other words, a ring-tailed model with a ring diameter of 3" and a ring chord of 1.5" is much more efficient than a fin-tailed missile with four fins of 1.5" and an overall fin span of ??.

Stability of ring-tails increases rapidly with increasing outside diameter of the ring, and also increases rapidly as the chord of the ring is increased up to 1/2 the ring diameter. There is little if any increase to be gained by making the ring chord greater than 1/2 the ring diameter.

Hmmm, Hoerner's Ring Wing equation can be used to work out a 'ring tail' type of configuration too...

For a ring tail, I've used the information in MIL-HDBK-762 that says that a ring tail is about twice as effective as a rectangular
cruciform tail with the ring tail chord = the rectangular fin chord,
and the ring tail diameter = the cruciform tip-tip span. If Hoerner's
data agrees with '762, then we'd get similar (CN)t for both of the
following examples:


______
| |
|______|__________ 4 fins, s = 1, a = b = 2,
|
|________________ d = 1
| |
|______|

______
| |
| |__________ ringtail, a = 2, tail dia = 3
| |
| |_________ d = 1
| |
|______|


For the ringtail I'd first assume that the 'ring' body interference is
negligible (as long as the ring diameter is >> the body dia), and
that we can ignore the ring supporting structure. With these
assumptions then,

(CN)frt = (CN)trt = pi * d/c (rt = ring tail)


So if this is going to match what the '762 says, then for the above
examples, the following should be true: (also assuming that the
fin/ring Xf's are similar)

(CN)trt = 2 * (CN)t


scratching that out, for that example, I get 4.7 ~= 4.4

fudge in some body-ring interference & that gets way too close for a coincidence..

Normal Force = N
Surface Area of Ring Fin = S
Diameter of Ring Fin =D
Fin Span = r
Root Cord = b

N = (32 * (S/D)^2) / (1 + Sqrt( 1 + (r/b)^2))
Fin Cp location = 0.75 * b


The general rule appears to be that a ring fin of diameter X and chord Y will give twice the aerodynamic effect as four fins of semi-span X/2 and chord Y -- that is, it's twice as effective as a set of conventional fins with the same projection.

shockwaveriderz, dude, I know where to go next time I need a bit of research done

I know its off topic but I'm really interested in how their buoy work is going. A small buoy in choppy water appears to have as random a motion as you can expect, and at a fairly fast frequency. Maybe a long boom below could server to damp the motion? Sensing the attitude and motion may be easier than reacting to it, whether it be control motors, or timing the ignition of a rocket motor. I'm now rambling about things I don't know that much about. It's been many moons since my control systems course.

Dear Dick,

Thanks for taking the time to sim out my rocket. No way could an over stable rocket cause instability. Please note that the CG measurements were from the bottom of the circular fin, not the bottom of the rocket body. I think this may explain why your sim showed over stable. Take your time and reread the specs I wrote down to make sure you entered everything right. My measuring was not done with your sim in mind.

I would put you in touch with the grad student who is working on the buoy launch systems, but she is so busy tying to get it done, that she doesn't have time to write about her progress. You'll have to wait until its complete, and read it. Yes, we've thought of all the things y'all have suggested, and many, many more. Aparently, the wave motion is more predictable than you might think. The Cesaroni engines do ignite pretty instantaiously reliably. The buoy will have a few weather instruments, including an anemometer. So there will be a set limit on wind speed for launches, and the rockets can be launched between gusts, and even in the trof between swells. We'll have to test to see how high a wind we can launch in, and still get an acceptable altitude.

To the unnamed author of the post with all the ring fin data... Thanks very much for your input. I think I just proved your statement about a cord more than half the diameter doesn't help. I just cut all my cords down to that yesterday. It helps me picture the true stability, with all that excess gone.

I'm starting to wonder if extending the radial support fins up the rocket body really helps. For the first couple months, I argued against it. I felt that any wind the extensions would catch would be better caught by the circular fin. Looking at the rocket broadside, they would seem to help. I only started extending them up in the last few weeks. I have a movie of one of our water launches that, in stop action, shows the rocket momentarily leaning more than the stall angle. So I started considering it. It was doing a half of an oscillation in the first second of flight at the time. (That rocket did not have a circular fin.)

I think this is one of those things I'll just have to test. Our final rockets will accellerate really hard out of the launch tube (about 30 Gs as I recall), so the relative wind over the fins (angle of attack) should be reasonable by the time the fins exit the tube. It would seem that the yaw inertia of the rocket would be enough to hold course until the angle of attack is less than the stall angele. Even if broadside stability should not be a factor, I've learned it is necessary for the rocket to be stable with the circular fin stalled... especially since we plan to launch in winds that hobby rocketeers wouldn't contemplate. It is pretty important that we be able to launch in storms.

I have the feeling my "as small diameter as possible" circular fins are being influenced by the boundary layer of the rocket body above it, or other things. This is why actual testing is so important to our project. It is fun playing with computer sims, but we won't know for sure until we test various configurations in strong winds.

I'm glad I got y'all thinking, anyway.

Cheers,

One tool that might be helpful (or might not). Apogee has a program out there that's called AeroCFD. It's an airflow simulator and might be just the thing you need to determine where the boundary layer becomes an issue. The question is whether it will simulate ring fins for you or not.

Dear Mr Brown,

By taller, I always mean parallel to the airframe. Since I'm fitting a 38mm rocket in just a 3" launch tube, fins cannot stick out more than 0.7". So trying to make fins more effective, the only way I can go is taller. But a tall, narrow radial fin is all wing tip.

Dear KermieD,

My circular fin can stall because it is so small. It is just less than twice the body diamteter.

There are a few dynamics that come into play as the angle of attack increases with circular fins.

I am only putting an airfoil in the outside of the circular fin. This makes the leeward side of it more effective, but the leeward side ends up in the "shaddow" of the rocket body above it when the angle of attack increases to the point where the air flowing down the leeward side of the rocket body becomes turbulent.

The windward side of the circular fin has the advantage of the "ground effect". The inner surface of the circular fin is very hard to stall because the rocket body is there to keep the flow parallel.

So, as the angle of attack increases, different things come and go affecting stability. Rough guestimations:
Up to about 7 degrees angle of attack, the entire circle is flying unstalled.
Above that, a large portion of the leeward side is in the shaddow turbulence, so total fin effectiveness is about half.
Around 12 degrees, the rest of the leeward part of the circular fin begins to stall.
Around 45 degrees, the broadside effective drag starts to come into play, and the windward side might even stall.

I think our final rockets will not exceed 12 degrees angle of attack in any wind situation.

The nozzle looks fine. These G engined test rockets are not going all that fast as they exit the launch tube, although my new longer launch tube should help.

The krushnik effect was discussed earlier. That is moot now that I'm not extending any fins below the engine nozzle.

Dear Senor Stull,

I did follow your specs in my sim. My CP's (Rsim and Barrowman) are affected my the way I simmed the ring, which the program does not handle directly. This is only an approximation, and as I said, this is a static CP only.

I have never experienced instability in an overstable rocket in high winds, but this had been reported so I believe it is possible.

I don't relly need to converse with anyone on the buoy issue...just curious.

Anyway, thanks for 'stirring the pot'...in a good way, of course.

Mr Stull,

I have a small idea for you buoy turbulence problem. If you were to mount another platform above the existing one, using shocks or other springlike mechanisms to elevate the 2nd platform, you might be able to virtually eliminate the turbulence from the water swells.
Using a series of shocks (or other spring like mechanisms), when the original platform is lifted at one end from the start of a swell, the shock will compensate for it, and keep the 2nd platform level. the more shocks you have, the more stability it will create as the swell moves through or past the entire platform.
You would just have to experiment with spring stiffness, and maybe hydraulics (or pneumatics) to keep the spring from bouncing the 2nd platform around.
This is all theory.
Hope it helps!

Rich