Another finless rocket

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BABAR

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Question: If a ringtail rocket seems prone to weathercocking, what is the remedy? More weight in the nose? Less? Canards?
great question. Lots of options, some more practical than others.

adding nose weight will make rocket MORE stable rather than less, with critical cavea: can the motor still get the rocket to sufficient velocity by the end of the rod or rail with that extra mass? You can end up chasing your tail, literally and figuratively, add nose weight, need more thrust, means bigger motor, means more tail weight, so now need MORE nose weight, and so forth. Less nose weight will make the rocket less stable, and while lower weight will INCREASE launch velocity, the FASTER the rocket goes the less weathercocking you get, so if you CAN decrease nose weight and still maintain adequate stability it’s a win,

something often easy to do but often ignored is simply to change the length of the rocket. For CG determinations, the effect of each mass component is multiplied by the distance from the center of mass. So making the rocket LONGER, such as lengthening the body tube and keeping the same mass, moves CG FORWARD,due to both the mass of the nose cone moved forward AND the mass of the extra length of body tube. The price is a heavier rocket (although depending on how heavy your paint is or if you fiberglass your rockets, a length of cardboard tubing doesn’t add much) and a bit more surface drag. Cost is also structural stability, the longer the rocket the more stress on the rocket, hence the SuperRoc competition category, some formula of altitude x length. Super long rockets tend to bend.

The converse is also true, if you find your rockets tend to weathercock too much, consider shortening the rocket as long as you have room for your recovery system and adequate wadding. This will also LIGHTEN your rocket and get it off the rail faster. Really fat bottomed rockets like Big Daddy and Fat Boy had something called “base drag” which I really don’t understand but often let’s them get away with much smaller fins than you would otherwise expect. Dropping your number of fins struts from 4 to 3 will also reduce stability and is usually structurally do-able.

I never like adding weight to the tail of a rocket (unless it is for trimming a boost glide), yes it will reduce stability but also decreases speed off of the rod or the rail, again if your motor is up to it yes it is an option but it may cost you in terms of performance (although in SOME cases heavier rockets fly higher on the same motor.)

I don’t like canards for intentionally reducing stability. I think they can be great for cosmetic effects, but I can’t see putting them in just to adjust stability. Usually I think it’s the other way around, “Hey, I want to add canards cuz they look cool, how much additional nose weight do I need to add or tail fin surface area to add so the rocket REMAINS stable?”

what kind of winds are you usually encountering?

if winds are 5 mph or less, weathercocking may not be much of a problem.

oh, and SPINNING your rocket with a fin tab or an airfoiled fin strut will also reduce weathercocking, although it will cost some altitude. Given you fly small fields, that may be a plus in your case.

good article on that here

 
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Sooner Boomer

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I believe that tube and ring fin rockets are less prone to weathercocking than standard orthogonal fin rockets, relative to the surface area of the fins.
Gee, I hope that's correct! We'll find out in less than 12 hours or so. We get one launch per month. Saturday is it. Winds start out around 5 and may gust up to 15 by noon. This is a special weekend. We actually get to launch twice, Sat. and Sunday. Got some students working on their Argonia Cup Challange (GO BULLDOGS!). Sunday's winds start where Sat's left off, and climb from there. Pics if I fly it.
 

KenECoyote

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My mindsim agrees with Babar that tube and ring fin rockets are less prone to weathercocking. This is imo because of the rounded side profile. Taking an extreme example, a sideways wind would hit a flat fin pretty squarely and affect it more than the rounded side of a tube or ring fin. Picture in your mind the rocket flipped on it's side and launched up...the flat parts would have huge drag/be affected by wind more compared to the rounded parts. :)
 

Sooner Boomer

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Tried my rocket today on a D12-3. Somewhat high winds and gusts. Looks like they *do* weathercock. At least the one flight I got in headed into the wind. I was using an Estes 24" chute, and it blew out several shroud lines, so I didn't fly it again. There was a very small amount of carbon/burn to the inside of the lower tube. Sorry, no video. I'll try to take pics later, worn out now.
 

KenECoyote

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Tried my rocket today on a D12-3. Somewhat high winds and gusts. Looks like they *do* weathercock. At least the one flight I got in headed into the wind. I was using an Estes 24" chute, and it blew out several shroud lines, so I didn't fly it again. There was a very small amount of carbon/burn to the inside of the lower tube. Sorry, no video. I'll try to take pics later, worn out now.
I haven't studied weathercocking much, but I believe a stable rocket should actually weathercock because it is designed to follow straight into the airflow...even if the airflow changes.
 

dhbarr

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I haven't studied weathercocking much, but I believe a stable rocket should actually weathercock because it is designed to follow straight into the airflow...even if the airflow changes.
Think of when the nose is in the crossbreeze vs. when both the nose and the fins vs. when only the fins are still in that crossbreeze. Anyhoo, the solution to overstability is to just go faster 🙃
 

KenECoyote

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Think of when the nose is in the crossbreeze vs. when both the nose and the fins vs. when only the fins are still in that crossbreeze. Anyhoo, the solution to overstability is to just go faster 🙃
Yes! I think I earlier mentioned bigger motor on a heavier rocket... velocity combined with more mass in motion should help to lessen wind current influence. :p
 

Sooner Boomer

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Think of when the nose is in the crossbreeze vs. when both the nose and the fins vs. when only the fins are still in that crossbreeze. Anyhoo, the solution to overstability is to just go faster 🙃
I flew low power stuff Saturday. Everything I flew turned into the wind. Everybody else flew high power. Their stuff was off the pad so fast the wind was wondering where it went (little or no weathercocking). My stuff was a lot easier to recover, though. ;)
 

Sooner Boomer

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Inside the aft cylinder...

aftcyl.jpg


It looks squished or distorted - it's not. That myst be the camera lens.
 

Sooner Boomer

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I wish I had a smoke generator of some kind. I'd like to do a static test and see how much is actually entrained into the aft cylinder. Obviously, it would vary from actual flight.

I think I'm going to rebuild this design. I'm going to use some sort of positive motor retention, and extend the front section an inch or two; maybe put a better parachute in it.
 

Sooner Boomer

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I looked back at the first of this thread. Over 5 years ago! Wow! Sorry it took so long. A bunch of bad stuff happened between then and now; I stepped away from rocketry for a while. The fellowship/friendship of the local group brought me back into activity.
 

KenECoyote

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I looked back at the first of this thread. Over 5 years ago! Wow! Sorry it took so long. A bunch of bad stuff happened between then and now; I stepped away from rocketry for a while. The fellowship/friendship of the local group brought me back into activity.
Same thing happened to me! Actually things got so bad for me that it eventually got to the point that I came back for the comfort and actually getting something done "rightish" in my life! :p

Regardless, I hope all is better for you/us!
 

Sooner Boomer

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After having flown this successfully, there are some small improvements that can be made.
1. I've cut the bottom off of the nosecone and glued in a bit more weight, and a loop of nylon cord. This makes more room for the parachute.
2. The rocket really could use positive motor retention. Tape works, but it's a pain trying to reach up inside the tube to insert/remove it. I'd have to remove the cone, but how to attach a clip? The cone is just under 2" long, less than the length of a motor. The cone is easy to remove. I guess I could cut a regular motor hook off (to make it shorter), secure it by wrapping in cord and epoxy, then putting a cone back on. Would also have to cut a slit or other way for the hook to flex to allow install/removal of engine.

 

KenECoyote

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Even with a tapered end, you can add a motor hook...just cut out a clearance strip for the clip to be able to pull up enough to clear the motor casing (I took this off the Quest Tomahawk build instructions and made a simple "[" shaped hook using the spring steel rib from an old windshield wiper):
IMG_20210405_192618847.jpg
 

Mike Haberer

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After having flown this successfully, there are some small improvements that can be made.
1. I've cut the bottom off of the nosecone and glued in a bit more weight, and a loop of nylon cord. This makes more room for the parachute.
2. The rocket really could use positive motor retention. Tape works, but it's a pain trying to reach up inside the tube to insert/remove it. I'd have to remove the cone, but how to attach a clip? The cone is just under 2" long, less than the length of a motor. The cone is easy to remove. I guess I could cut a regular motor hook off (to make it shorter), secure it by wrapping in cord and epoxy, then putting a cone back on. Would also have to cut a slit or other way for the hook to flex to allow install/removal of engine.

If this is 18mm or 24mm, don't use a hook, use a Rocketarium retainer. Screw on, screw off. I only use hooks on 13mm anymore.
 
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