Finless Vent-channel Design?

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Alright, here is what the simulator in my head came up with so far, (A look of the rocket as it would appear from the exterior.)
I'm guessing something at or in-between these two extremes for "intake" sizes

And while we are on the subject what forum area would you suggest I put the build thread for this thing? Oddrocs, LPR, Scratch(Though kitbashed would be more accurate) or something else?

mads rocket 2.jpg


mads rocket.jpg
 
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Add a LOT of nose weight and airfoil the inside fins so that it spins in one direction. I have successfully flown a 2 and 1 fin rocket. But that was luck, this is more challenging.
 
fyrwrxz said:
Without throwing around much theory, look into some papers on inlet duct designs, esp the 'naca' duct which creates a low pressure area and uses existing (however transitory-our birds are in and out of laminar flow rather quickly-if ever) boundary layer pressure to assist in the 'ram air' principle indirectly. Think 'hood/brake scoops' in the glory days of muscle cars or Nascar. Ancillary to this may be the ring fin design problems approaching mach. Whole lotta stuff going on in the air column there-esp. shock wave reverberation and subsonic exit chaos showing as base drag. It would be hard (for me at least) to calculate what stability that may add.
Click to expand...

How funny, I was just discussing this very thing with the wife and kids around the dinner table tonight! :lol:
 
I'm still not willing to give it a thumbs up yet but the first design--long transition--looks better to me--. The torpedo analogy , from a pevious post, is a good one. That said hydrodynamics and aerodynamics differ a bit--water is a non compressible, so the flow differs a bit from areas of high preasure to low . I do however like the look--kinda retro--very 20ish. If you could make it work--I think you can--It deserves a retro /steam punk/copper/ brass ? look. I'm gonna follow this, hope you build it and see what happens---very cool !!:cheers:--H
 
Number 1 as HD said set up as Majordude said with a very long launch rod so it has as much velocity as possible at release. Its basically an under sized ring fin. Go for it.:dark:


Richard
 
I like it. One thing that may enhance stability (kind of a cheat, though) would be to extend the four "attachments" (essentially internal fins) along the length of the transition (sort of where you have the "lines" on your diagram.) This still leaves you with the "tubular" shape (and would allow launch from a launch tube, if you so desired.) Again increases the "effective" fin surface. Maybe paint them a contrasting color from you body tube to set them off a bit.

One thing I am NOT sure about (and maybe more experienced rocketeers can help out with.) Not sure how long to extend the ring fin in the fore-to-aft dimension. Too short and it won't have enough effect. To long and you may have the same issue as a tube fin (not a ring fin) where the resistance gets so high when they are too long that the air doesn't pass through them. I definitely wouldn't go any LONGER than pictured.

I may have missed your plan body tube sizes, the LARGER the outer or main body tube diameter and the SMALLER the inner lower body tube diameter (ideally minimum diameter for your engine) the better off you are gonna be.

Good Luck!
 
Since the ducts don't extend out into the airflow, the stability would depend on some method of directing the airflow inward into the ducts.

You could try actually extending the ducts outward.

You also might want to investigate the Coanda effect, which causes a redirection of a flowing gas as it flows past an orifice. The objective would be to direct more of the laminar flow of gas around the body tube into the ducting. Of course, you'd need to be sure of a laminar flow. Which means 1) laminar, and 2) a flow. Keep in mind that at the boundary of the gas and the body, there actually is no flow. You'll need to overcome that effect as well, in order to increase the amount of air flowing through the ducts, and thus their effectiveness. As for the laminar, you'd want to be sure that the nose cone you pick doesn't direct the oncoming air away from the body tube too hard, which would leave a partial vacuum on the side of the body. See Bournoulli's principle. I don't have specifics, but I'm guessing a long ogive or von Karmann nose cone would be among the best choices.

Also, as was mentioned, some ancillary stability like spinning and adjustments to the center of gravity, would be good.

Or, fly it up a string trailing behind a previously launched rocket. Or use a launch rod 1000' tall.
 
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BABAR said:
I like it. One thing that may enhance stability (kind of a cheat, though) would be to extend the four "attachments" (essentially internal fins) along the length of the transition (sort of where you have the "lines" on your diagram.) This still leaves you with the "tubular" shape (and would allow launch from a launch tube, if you so desired.) Again increases the "effective" fin surface. Maybe paint them a contrasting color from you body tube to set them off a bit.
Click to expand...

This was the idea all along, to have the internal fins run the length of the transition, but not have them extend past the outer diameter of the payload bay or ring fin.

I'd like to avoid spinning at all costs. I wouldnt be able to mount cameras on it without making myself sick, lol
I would also like to keep the entire rocket inside the outer diameter of the payload bay and nosecone.

If need be I can make the rocket longer than pictured. I'm entertaining ideas of kitbashing a QCC Explorer to make this thing.
 
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Definitely an Odd Roc when you start removing parts vital for stable flight from the true and trusted 3-4FNC formula. Big scoops to bring air flow to tiny internal fins, shoving the motor as far up the tube as possible, sickening amounts of stinkin’ performance robbin’ nose weight, stretching the length of the subject in a vain attempt to get some sort of workable CP-CG relationship, all are sure fire signs you are moving towards the Dark Side. A ping pong ball cut in half and fitted on a tube might make a real nice scoop.
 
DavidQ said:
Since the ducts don't extend out into the airflow, the stability would depend on some method of directing the airflow inward into the ducts.

You could try actually extending the ducts outward.

You also might want to investigate the Coanda effect, which causes a redirection of a flowing gas as it flows past an orifice. The objective would be to direct more of the laminar flow of gas around the body tube into the ducting. Of course, you'd need to be sure of a laminar flow. Which means 1) laminar, and 2) a flow. Keep in mind that at the boundary of the gas and the body, there actually is no flow. You'll need to overcome that effect as well, in order to increase the amount of air flowing through the ducts, and thus their effectiveness. As for the laminar, you'd want to be sure that the nose cone you pick doesn't direct the oncoming air away from the body tube too hard, which would leave a partial vacuum on the side of the body. See Bournoulli's principle. I don't have specifics, but I'm guessing a long ogive or von Karmann nose cone would be among the best choices.

Also, as was mentioned, some ancillary stability like spinning and adjustments to the center of gravity, would be good.

Or, fly it up a string trailing behind a previously launched rocket. Or use a launch rod 1000' tall.
Click to expand...

Flow entrainment might be the ticket - if the ducts empty out into the same space as the motor exhaust (which it looks like they do), you're going to get quite a bit of flow entrainment and will effectively create a suction pump which pulls air through the ducts.

Frankly, it looks like an interesting way to play with base drag, even if you had to stick with real fins...
 
Is there any relationship between "flow entrainment" and Krushnik effect? Always wondering whether you can get something for nothing. The Lil' Augie was CHAD staged with the sustainer well recessed within what was essentially a ring fin. Seems like with the current model suggested here you could do the same thing and place your engine nozzle INSIDE the ring. This would not only further augment the "flow entrainment" but also move CG slightly forward. Every little bit helps.

I also see people talking about the effectiveness of the little bitty internal fins. But once you achieve "entrained" flow through the ring it seems like the ring itself becomes a major stabilizing force.
 
krusty said:
If you extend the 'ring' portion down below the exit point of the motor, you may create more of a venturi effect and 'suck' air through the side vents....
Click to expand...

At that point it's starting to look a lot like the "Li'l Augie" and "Augie II" augmented-thrust designs from the 1960s.
 
AlexK said:
Flow entrainment might be the ticket - if the ducts empty out into the same space as the motor exhaust (which it looks like they do), you're going to get quite a bit of flow entrainment and will effectively create a suction pump which pulls air through the ducts.

Frankly, it looks like an interesting way to play with base drag, even if you had to stick with real fins...
Click to expand...

I've been kicking around the idea of a Big Bertha with a set of NACA ducts in the tube leading to long channels running down to the motor for thrust augmentation. This is that idea, but without the external fins.

I do hope this works--it'll be SO cool.....
 

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