Abnormal Through the Wall Fins

[GrossApproximator]

Hi all. When you have a rocket that has forward fins, but you want to give the forward fins through-the-wall mounts, how do you do it? If you have the forward fins the usual fin tabs, wouldn't they snag on your recovery gear? For thick wall tubes, can you simply make the fin tabe the same thickness as the tube? And, finally, for you big high-power projects, what would you do then?

[rockets4kids]

Forward fins have a negative effect on stability, and for model rockets they are purely for show. Just surface mount them, and if you are worried about strength, just use an epoxy fillet.

[GrossApproximator]

Forward fins have a negative effect on stability, and for model rockets they are purely for show. Just surface mount them, and if you are worried about strength, just use an epoxy fillet.

I've done that in the past. I was wondering if there is another method used in high power rocketry-- 'cuz even though I'm just getting into mid-power now, I've got my eye on the big birds

[rockets4kids]

There is less "show" and more function in HPR.

That said, if you really want forward fins, and you really need them to be TTW, I can thing of two options:

1. If you can keep all of your recovery gear above the forward fins, I can't see any reason why you would have any problems with just a small section of smaller tube for the TTW mount.

2. If you want the forward fins ahead of the recovery gear, just use a slightly narrower tube than the BT long enough to hold all of your laundry.

[GrossApproximator]

There is less "show" and more function in HPR.

Precisely what I dislike about HPR. But anyways, good suggestions. I'll think about 'em.

[jungmann1]

Some of the Semroc SLS birds use thru the wall fins where the tabs are just as long as the airframe tube thickness. The Jaguar is one that I know is like that. I, too, would be worried about anything protruding into the recovery area. Unless the fin tabs are extending into the airframe to the point where they contact some other structural part, the tab being longer than the tube wall is kind of pointless anyway, I would think. Rockets4kids was on to something, I think, when he suggested a smaller, inner tube to hold the recovery gear. That would keep the clearance you want and give the forward fin tabs something to attach to. Go for it, if you have room! John.

[GrossApproximator]

Believe it or not, I used Rockets4Kids' idea already. It works well, and makes your rocket a little stronger and stiffer (I hate the "squishy" feeling of LPR tubes).

Edit: Is that Rockets4Kids' or Rockets4Kids's ? My grammar's a little rusty.

[Peter Olivola]

What does that mean?

Precisely what I dislike about HPR. But anyways, good suggestions. I'll think about 'em.

[JStarStar]

There is less "show" and more function in HPR.

That said, if you really want forward fins, and you really need them to be TTW, I can thing of two options:

1. If you can keep all of your recovery gear above the forward fins, I can't see any reason why you would have any problems with just a small section of smaller tube for the TTW mount.

2. If you want the forward fins ahead of the recovery gear, just use a slightly narrower tube than the BT long enough to hold all of your laundry.

IMO inserting smaller tubes inside your main airframe is a risky proposition -- when the rocket takes off it is going to undergo some pretty hefty G forces during the peak-thrust period of motor burn, and it is not uncommon for chutes, shock cords, etc etc to slide downward during this phase.

And if you have an interior tube inside the main airframe, you are setting up a whole mechanism by which the laundry can get snagged on ejection.

As you note if you did it, you would have to figure out some way (a screen or baffle or something) to ensure the recovery laundry does not slide down farther than the narrow interior tube.

As others have noted too, in model rocketry without active guidance systems, forward fins at best are ornamental fixture with negative CP implications, whose presence has to be compensated for in terms of stability (usually with more nose weight).

It does look cool and of course it is necessary for some scale models, but it's a design tradeoff.

You could do a very simple version of TTW mounting by cutting slots in the BT and making the standoff section on the fin template very thin -- the thickness of the tube itself, so probably about 1/32 or so.

Then run glue around the root edge of the fin, insert in the slot, then apply fillets to the root edges after it dries.

That would secure the fins to the BT more firmly than normal surface gluing, but not raise potential laundry-snagging protrusions inside the tube.

[rockets4kids]

IMO inserting smaller tubes inside your main airframe is a risky proposition -- when the rocket takes off it is going to undergo some pretty hefty G forces during the peak-thrust period of motor burn, and it is not uncommon for chutes, shock cords, etc etc to slide downward during this phase.

This is easily enough solved with a nub at the lower end, or a baffle, or a piston. Personally, I am a big fan of pistons with a stop to limit their travel down the body tube.

[GrossApproximator]

What does that mean?

All the high power rockets I see look very similar to one another. Nose cone; tube; fins. Some are big, some are small, and some are honkin' huge. Some have simple paint jobs while others have more complex paint jobs. However, other than the honkin' huge rockets, I've never seen a high power rocket that I really liked the look of. Since rockets spend 99.99% of their lives on the ground, I want to see a rocket that looks so wild that you can't take your eyes off of it--even if the rocket only ever flies once or twice.

[GrossApproximator]

This is easily enough solved with a nub at the lower end, or a baffle, or a piston. Personally, I am a big fan of pistons with a stop to limit their travel down the body tube.

In the past, I've built a rocket that uses 3 tubes in this configuration: the main airframe is the outer tube, there is an inner tube that runs the entire length of the main airframe (with a small gap near the top to accomodate the nose cone's shoulder), and the inner-most tube is the motor mount. I usually use 2 rings to center the middile tube inside the main airframe, and 2 more centering rings to center the motor mount in the middle tube. Strips of wood are used as necessary to reinforce the rings, as any TTW fin tabs will not mount directly to the motor mount tube.

[cavecentral]

My guesses are:

1) Use a payload section. Attach fins there. No recovery inside.

2) Use something like a motor mount anbout 1/2" smaller diameter to allow fin tabs to go TTW all the way to the inner tube and keep the recover in the inner tube. You need to leave room for a nosecone shoulder.

[Peter Olivola]

Then I take it you've never built and flown your own HPR?

All the high power rockets I see look very similar to one another. Nose cone; tube; fins. Some are big, some are small, and some are honkin' huge. Some have simple paint jobs while others have more complex paint jobs. However, other than the honkin' huge rockets, I've never seen a high power rocket that I really liked the look of. Since rockets spend 99.99% of their lives on the ground, I want to see a rocket that looks so wild that you can't take your eyes off of it--even if the rocket only ever flies once or twice.

[rockets4kids]

All the high power rockets I see look very similar to one another. Nose cone; tube; fins. Some are big, some are small, and some are honkin' huge. Some have simple paint jobs while others have more complex paint jobs. However, other than the honkin' huge rockets, I've never seen a high power rocket that I really liked the look of. Since rockets spend 99.99% of their lives on the ground, I want to see a rocket that looks so wild that you can't take your eyes off of it--even if the rocket only ever flies once or twice.

These guys do some interesting HPR builds:

http://www.vatsaas.org/rtv/arsenal/arsenal.aspx

[T-Rex]

Here is my answer to the problem. A chute cannon is the most common name I believe. Using a smaller tube inside the main tube, it runs from the top of the motor mount to above the forward fins, giving them TTW ability.

A true chute cannon, I believe runs to just below the nose cone, and would be even smaller in diameter. I may still do that in this case.

[Peter Olivola]

That's a variation on an extended stuffer tube and a perfectly good way to solve the problem.

Here is my answer to the problem. A chute cannon is the most common name I believe. Using a smaller tube inside the main tube, it runs from the top of the motor mount to above the forward fins, giving them TTW ability.

A true chute cannon, I believe runs to just below the nose cone, and would be even smaller in diameter. I may still do that in this case.

[jeffgeraci]

GrossApproximator:

You should consider making it an open-body rocket. It will allow you to mount TTW fins at ANY point on the rocket. I did it with my Cobra

Check it out:

http://www.rocketryforum.com/showthread.php?t=23141

[GrossApproximator]

Then I take it you've never built and flown your own HPR?

No, I never have. I would like to, but if I can't make a cool one that is safe, then I'll just stick with low and mid power stuff. I've seen high power rockets go unstable, and it's a little nerve racking.

[GrossApproximator]

GrossApproximator:

You should consider making it an open-body rocket. It will allow you to mount TTW fins at ANY point on the rocket. I did it with my Cobra

Check it out:

http://www.rocketryforum.com/showthread.php?t=23141

Cool. But how does the "open-body" concept work? I can't really tell from the pictures.
. . . I sure hope that I'm not just missing something obvious.

[GrossApproximator]

These guys do some interesting HPR builds:

http://www.vatsaas.org/rtv/arsenal/arsenal.aspx

Neat! I especially like the RAF Basset.

[jeffgeraci]

Cool. But how does the "open-body" concept work? I can't really tell from the pictures.
. . . I sure hope that I'm not just missing something obvious.

It's simply a "thick" double-walled rocket, and the motors are mounted inside the walls. The chute is packed in the walls too, and a tail cone can be used as the nose cone. Only drawback is less payload space and motors must be clustered (minimum of 3) Forgot about that part, sorry.

[Peter Olivola]

I've seen just as many LPR/MPR rockets go unstable. Understanding why and how to make them stable is a fundamental challenge of the hobby. There's no difference between LPR/MPR/HPR on the subject of stability.

No, I never have. I would like to, but if I can't make a cool one that is safe, then I'll just stick with low and mid power stuff. I've seen high power rockets go unstable, and it's a little nerve racking.

[rockets4kids]

There's no difference between LPR/MPR/HPR on the subject of stability.

True, but the feeling you get in your stomach when you see an HPR rocket go unstable off the pad is *quite* a bit different than an LPR one!

[DavieRockets]

The PML Matrix has 3 thin forward fins on the main body tube and 6 small canard fins on the forward body tube. PML partially routes through the wall of this Quantum Tube PVC tubing giving a tight slot that indexes the fins nicely. Here are some shots of my build showing how I scratch up the bottom of the slot for better adhesion. Also shots of fillet making using West Systems Epoxy thickened with colloidal silica.

If you have a router or table saw slotting setup you can use this method too,

[kelltym88]

There is less "show" and more function in HPR.

Precisely what I dislike about HPR. But anyways, good suggestions. I'll think about 'em.

All the high power rockets I see look very similar to one another. Nose cone; tube; fins. Some are big, some are small, and some are honkin' huge. Some have simple paint jobs while others have more complex paint jobs. However, other than the honkin' huge rockets, I've never seen a high power rocket that I really liked the look of. Since rockets spend 99.99% of their lives on the ground, I want to see a rocket that looks so wild that you can't take your eyes off of it--even if the rocket only ever flies once or twice.

I respectfully disagree...... All 38MM except the Bomber(white and red one), it is 2x29MM with a central 54MM......


As for the original question: I 'm not sure what the problem is. For an HPR, the shape of the fins doesn't interfere with the recovery. If you intend to make them TTW, you just cut fins to the shape you want, but include the tab in your cut. You want a forward swept fin that is 4", just add whatever the distance is from the BT to the MMT, (say 1/2") and cut your fins 4 1/2". Slit your BT and with your MMT inserted adjust the fin tabs so your fins sit flush with the body tube.

[Peter Olivola]

That's what the safety code is designed to mitigate. You do follow the safety code, don't you? But on balance, for you, you're right and HPR isn't something you should be scaring yourself with.

True, but the feeling you get in your stomach when you see an HPR rocket go unstable off the pad is *quite* a bit different than an LPR one!

[rockets4kids]

That's what the safety code is designed to mitigate. You do follow the safety code, don't you? But on balance, for you, you're right and HPR isn't something you should be scaring yourself with.

I was referring to some of the fiberglass HPR rockets flown by others that I have seen go unstable. Personally, I have never flown either fiberglass or HPR.

Like the OP, I have little interest in re-inventing the wheel. Kits do nothing for me, and even copying other designs provides little reward. I derive enjoyment from creating things which don't look like they should fly, but do.

I should point out that I do not build randomly. I've spent extensive time designing and building from the simulator, and I've taken simulated designs about as far as possible.

When venturing beyond the simulator I work in an iterative process gradually pushing the design and paying careful attention to how the rocket flies. I build as small and as light as possible during this phase to minimize any damage that may occur if the rocket goes unstable. Once I am happy with a design I gradually scale it up. Even my largest models are made almost entirely of paper and only minimal use of wood.

I generally fly rockets that do not require an FAA wavier, which means less than 1500 grams on the pad and 125 grams propellant. I do hope at some point to scale up with larger motors and composite materials, but at the moment I do not feel as if this is limiting me in any way.

[jeffgeraci]

Like the OP, I have little interest in re-inventing the wheel. Kits do nothing for me, and even copying other designs provides little reward.

I have to agree wholeheartedly with this statement. I'd much rather sit down with a blank sheet of paper and some crayons, as opposed to coloring in a coloring book where the pictures have already been drawn for you. I'm not bashing kits, they're an EXCELLENT way to learn the build process, they're just not for me.

[Coop]

All the high power rockets I see look very similar to one another. Nose cone; tube; fins.

...And TTW canards which help lock in the ballast.

Later!

--Coop