fin cross sections for Mach3+

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ghostfather

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I've been experimenting with 3D printed fins using symmetrical airfoils. Something other than flat plate materials for fins.
The NACA/Munk M-1 airfoil is fairly effective for subsonic flight.
m1-il_l.png

This kind of airfoil is great for subsonic flight, where air stays in laminaar flow along the surface of the fin, and reduces turbulence and therefore drag. It's has about a 6% thickness to length at a 30% chord, and makes a fairly strong fin. It also lets your rocket go higher (less drag). I've flown this kind of airfoil with success, printed from my 3D printer and reinforced with a layer of fiberglass.

While doing some research on supersonic profiles, I realized that laminar flow airfoils don't work because they produce a big shock wave at the leading edge. Current wisdom seems to prefer sharp knife edge leading and trailing edges. The double wedge airfoil used with the NIKE sounding rocket is an example, but many rocketeers use a trapezoidal cross section, sharp at the leading and trailing edges, flat in the middle. The other cross section used in supersonic research is the symmetric circular arc airfoil, which also has sharp leading and trailing edges. The idea is to reduce primary shock waves at the leading edge, and reduce the influence of secondary shock waves.

The symmetric circular arc airfoil got me thinking. What other shape do we use made from intersecting circular arcs? An Ogive nose cone!

An ogive nose cone isn't the most efficient shape for supersonic flight, but there are a number of other choices, like the Von Karman nose cone based on research in fluid mechanics. While different nose cones have advantages and disadvantages, the Von Karman profile is a good all-rounder, efficient at subsonic and upper supersonic speeds, somewhat less at transsonic speeds. You can see where this is going, huh?

Why not make a long thin supersonic fin with a Von Karman profile at front and back, like a Von Karman nose cone glued to a Von Karman tail cone, but then very long and thin? That might be pretty efficient.

Research on supersonic airplanes in the 70's seems to point to an optimal thickness of the airfoil between 4% and 6% of the length. Higher speeds favor the thinner airfoils at speeds higher than Mach2, so it'll be 4%.

So, are my hunches right? Feedback is welcome.
 
Already flew and designed supersonic airfoils for UTC SEDS 2017 USRC placed third. NASA wouldn't publicly comment on airfoil data I obtained when questioned at SEDS. They won't release an airfoil generator for data we had. We had professors b*tching how we obtained certain data points. These are a pain to manufacture. Make sure to flutter sim the tip as in reality that flutters first. I've done M1.5. We predicted 1.7-2.4. We hit a snag at interstage implosion L-1 multistage first HPR design. Thermally small scale print fins won't like Mach 3. You can't always CNC it at scales smaller than tomahawk fins. Had a university machinist with tomahawk fin experience, it takes a lot of jigs for metal. Certain sizes for 3.1cm diameter tube he screamed impossible tolerance for cnc no lie. A NASA contractor screamed impossible with 5 axi mill. Very few companies want business and fins are outrageous expensive when you get rejected enough. $170 + a fin not unheard of. Only certain printing works. Not FDM.
 
I've had compressible gas dynamics. I dropped my ego and emailed Dr. Sreenivas at UTC a hypersonics flow expert. You want a low wedge angle to reduce oblique shocks and Mach changes across shock. The double wedge is way easier to manufacture. (hello affordable!) You could try a bi-convex Von Karman or a power series for example. When you get a CFD you can compare Cd to Mach 20. A local uni would be more than helpful if you want virtual wind tunnel data before you mill a fin. Otherwise solidworks has crummy CFD compared to Linux hardcode or other softwares. Knowing the assumptions theoretically helps.
 
Why not make a long thin supersonic fin with a Von Karman profile at front and back, like a Von Karman nose cone glued to a Von Karman tail cone, but then very long and thin? That might be pretty efficient.

Been there...done that.

I did this back in 2011. Von Karmon fins.....drawing in first post. Knock yourself out & let us know how you do!
Machined......... there wasn't printing available then. Expensive & very time consuming.

https://www.rocketryforum.com/showt...ect-true-diamond-air-foiled-fins-Space-Cowboy!
 
Make sure to flutter sim the tip as in reality that flutters first.
I use FinSim which is tricky because it's made for flat fins with certain materials, not a fin that tapers from root to tip, and composite constructions are a guess as well. I just give it my best guess, and use a larger factor of safety.

Thermally small scale print fins won't like Mach 3.
I realize that a thermally printed fin would melt at supersonic speeds, but I'm leaning towards printing molds using high-temp PET or Nylon (something that will hold it's shape in my self-built autoclave), and laying in a few layers of CF, and vaccumn bagging the mold. Haven't decided on a core material yet, and I'm looking into a printer that can print larger pieces for molds. I'm limited to 300mm with my present printer.
I've also thought of using a cheap 2.5 axi CNC mill that I have access to for sheets of birch plywood, making half of the fins with the airfoil profile and using epoxy/CF core to glue the two halves together, and doing a few layers of CF on the outside, everything in a vaccumn bag. A tried and true way of constructing fins (CF around birch plywood core), except for the CNC induced airfoil. Might even work using CNC on aluminium plate, but I have no experience with bonding aluminium plates.

I have no access to fancy CNC mills, so I try to get by with what I have available. I'm just a hobbyist with a self-build rocket factory in the attic.

Thanks for the tips.
 
Make sure to flutter sim the tip as in reality that flutters first.
I use FinSim which is tricky because it's made for flat fins with certain materials, not a fin that tapers from root to tip, and composite constructions are a guess as well. I just give it my best guess, and use a larger factor of safety.

Thermally small scale print fins won't like Mach 3.
I realize that a thermally printed fin would melt at supersonic speeds, but I'm leaning towards printing molds using high-temp PET or Nylon (something that will hold it's shape in my self-built autoclave), and laying in a few layers of CF, and vaccumn bagging the mold. Haven't decided on a core material yet, and I'm looking into a printer that can print larger pieces for molds. I'm limited to 300mm with my present printer.
I've also thought of using a cheap 2.5 axi CNC mill that I have access to for sheets of birch plywood, making half of the fins with the airfoil profile and using epoxy/CF core to glue the two halves together, and doing a few layers of CF on the outside, everything in a vaccumn bag. A tried and true way of constructing fins (CF around birch plywood core), except for the CNC induced airfoil. Might even work using CNC on aluminium plate, but I have no experience with bonding aluminium plates.

I have no access to fancy CNC mills, so I try to get by with what I have available. I'm just a hobbyist with a self-build rocket factory in the attic.

Thanks for the tips.
 
I'm mostly trying to get feedback on whether my assumptions/guesses will result in an efficient fin cross section for Mach3+. I'm using comparative analysis of nose cone shapes, and hoping they extend to fin cross sections.

So, a conical nose cone is like the tried and true double wedge fin on a NIKE,
an ogive nose cone is like the symmetrical arc airfoil,
and the Von Karman nose cone is like the the cross section I'm considering.

Of these three nose cones, the Von Karman performs best at higher supersonic speeds, so I'm hoping that a Von Karman cross section on a fin might perform better than the other two airfoils.
Nose_cone_drag_comparison.png
 
Don't want to throw cold water on this discussion but seeing as the OP is from the Netherlands ITAR restrictions may come into play here, especially since he is talking about Mach 3. Might want to proceed with caution.


Tony

Specifically, ITAR [22 CFR 120-130]:

Covers military items or defense articles
Regulates goods and technology designed to kill or defend against death in a military setting
***Includes space-related technology because of application to missile technology***
Includes technical data related to defense articles and services
 
Don't want to throw cold water on this discussion but seeing as the OP is from the Netherlands ITAR restrictions may come into play here, especially since he is talking about Mach 3. Might want to proceed with caution.

:lol:

Pardon?!? What part of Mach 3 falls under ITAR? This is a HPR forum, I've flown above Mach 3 on an M motor, it's not unusual. Not exactly a military secret, either.

I believe you are mistaken about the applicability of ITAR regulations to this discussion.
 
:lol:

Pardon?!? What part of Mach 3 falls under ITAR? This is a HPR forum, I've flown above Mach 3 on an M motor, it's not unusual. Not exactly a military secret, either.

I believe you are mistaken about the applicability of ITAR regulations to this discussion.
If you read the regs on ITAR, one of the categories is:

"Launch Vehicles, Guided Missiles, Ballistic Missiles, Rockets, Torpedoes, Bombs and Mines"

A rocket is certainly covered under ITAR. Going Mach 3+ is unusual for a hobby rocket, in spite of your claim. It could been seen that giving you advice becomes helping a foreign national build a rocket that is as fast as a typical military missile. In today's crazy political environment, it seems prudent to make sure that there is not even the appearance of impropriety, even though ITAR technically only applies to companies doing business with the US government.

It's easy for you to laugh, you're not covered by the regs and won't have an FBI agent come knocking on your door. Several close friends work for companies that are covered specifically by ITAR regs and I can assure you they take compliance very seriously. My comments are based on my discussions with them and their experience.

Good luck on your endeavor,


Tony
 
I believe you are mistaken about the applicability of ITAR regulations to this discussion.

Hey dude. You are in Netherlands. They don't allow you to use military grade supersonic airfoils like I have in HPR for last year university project. Posting datapoints would put me jail or millions in fines by ITAR under US law if I talked technical coordinate details of my UTC university launch rocket. The airfoil coordinates relate to missile tech. They will not be discussed further.
 
Your first post ever relates to a very sensitive subject. You want a supersonic biconvex airfoil at Mach 3. Missiles share that application. So before anybody answers your questions further we need details of your "hobby" rocket. What is it a freaking suborbital sounding rocket??? Additionally AeroFinSim is copyrighted only usually available to US citizens. Most hobby rockets and sounding rockets get by with flat plates for that matter.
 
Tony,

Sorry to laugh, as I realize now you were serious. It's just that I've been building and flying and discussing rockets for years, and this is the first time anyone has brought up ITAR regulations.

If you read the regs on ITAR, one of the categories is:

"Launch Vehicles, Guided Missiles, Ballistic Missiles, Rockets, Torpedoes, Bombs and Mines"

A rocket is certainly covered under ITAR.

There is an article on the Tripoli site defining the limits of our hobby, and one of the key things that makes a hobby rocket different from anything remotely interesting to the military is that it has passive guidance (fins), and no active guidance (steerable fins, steerable nozzle, etc). Also a few other things like no explosive payload (nope, ejection charges don't count), not made of metal (or as little metal as possible), has to be recoverable (at least if the parachute deploys), etc. I'd argue that hobby rockets as defined by Tripoli rules are exempt from IFAR regulations.

I am a member of Tripoli, which is an international organization, I order rockets (with fins) and all kinds of rocket electronics that are shipped from the States, I can get the same Cesaroni and Aerotech motors available to you, and I've flown in several countries, including the United States, with rockets built overseas. The point I'm trying to make is that High Powered Rocketry is an international hobby.

Going Mach 3+ is unusual for a hobby rocket, in spite of your claim.
Might be unusual in the circle of friends you fly with, but I've hit Mach 3 with three different group projects. You might want to go to the Tripoli BALLS launch out in Black Rock Desert in September, they have lots of Mach3+ flights, though most stay somewhere between Mach 2 and 3. It takes a big motor (like an M or bigger) to go that fast, with a price card, so yeah, it is a bit limited to those with the financial resources and experience enough to build a rocket where the fins stay on at that speed. You generally only make those flights where you are have the ceiling (Black Rock allows flights to the Von Karman line, 100 km or about 61 mi), as rockets that go Mach 3 also tend to go very high.

It could been seen that giving you advice becomes helping a foreign national build a rocket that is as fast as a typic al military missile.
Ah, what makes you think I'm a foreign national? I happen to be an American living abroad. It's like me assuming you are an Italian national because your name is Tony. :wink: OK, you're half right, I have dual citizenship and two passports (USA and Netherlands). Yeah, that's legal.

I might point out that this rocketry forum, with all it's discussions af rockets, is on the public internet, therefore readable all over the world. Would you have helped me on the forum if you thought I were from Kansas?

even though ITAR technically only applies to companies doing business with the US government.
International Traffic in Arms Regulations applies to all "manufacturers, exporters, and brokers of defense articles, defense services or related technical data". I'm assuming you aren't an arms dealer or something (otherwise I wouldn't care anyway), and I'm not a company, and we aren't doing business, just talking about our hobby.

Summarizing: It's my opinion that talking about and exchanging information about High Powered Rockets does not fall under ITAR regulations. Neither does selling rocket kits, components, electronics or motors used in this hobby. Even talking about basic aeronautics taught at any aeronautical university around the word does not.

Tony, you're welcome to your own opinion, but I hope you'll continue to contribute to the forum. 'Nuf said.
 
Hey dude. You are in Netherlands. They don't allow you to use military grade supersonic airfoils like I have in HPR for last year university project.
Hey fellow dude. My son studied aeronautics and astronautics at Delft here in the Netherlands, and you'd be surprised at the technology they have access to, as well as one of the most advanced supersonic wind tunnels in Europe. I live 15 minutes from the HQ of the European Space Agency, the ones that launch the Arianne rockets. Lots of mil tech exists outside the States.

I'm also not asking for any military secrets or military airfoils, I'm asking for feedback on whether a Von Karman profile would make a good supersonic fin. The equation has been published in the 30's
dd0d54e6996d6d37784754ae7aaa19be5ca63285
y = R π θ − sin ⁡ ( 2 θ ) 2 + C sin 3 ⁡ θ {\displaystyle y={R \over {\sqrt {\pi }}}{\sqrt {\theta -{\sin(2\theta ) \over 2}+C\sin ^{3}\theta }}}
c64816bcf1415f471fff1ab1dc10340536e6faaf
Where:

C = 0 for LD-Haack (von karman)


You are welcome to not comment if you wish, but I hope you will get over it and contribute.
 
Your first post ever relates to a very sensitive subject. You want a supersonic biconvex airfoil at Mach 3. Missiles share that application. So before anybody answers your questions further we need details of your "hobby" rocket. What is it a freaking suborbital sounding rocket??? Additionally AeroFinSim is copyrighted only usually available to US citizens. Most hobby rockets and sounding rockets get by with flat plates for that matter.

Incorrect, AeroFinSim has no such restrictions. I bought it many years ago shortly before the guy that wrote it stoppd publishing and selling it. Also I know several fellow rocketeers in Europe that own a copy.
The general consensus on using flat plates as fin materials is one of cost and ease of construction. With the advent of 3D printers, it's possible to do more. That's something I'm busy with, and the reason I posted a question here on the forum.

Again, you are free not to comment, but I don't think you can speak for everyone, and forbid others to contribute to the discussion.
 
The Von Karman profile is fine to Mach 2. You trade performance in the high transonic region. It reduces the pressure wave drag to min. In fineness ratios exceeding 6:1 L\D you have a lower drag coefficient. For a fin application in an academic sense you need to have the lowest possible angle to reduce oblique shocks past Mach 1. Your real problem is you do not have a comparison of power series to Von Karman for Mach 2-3. As a mechanical engineering student I would create a 3D model then get a CFD analysis through university contacts because UTSI has a wind tunnel to Mach 4. You have to simulate the geometry either virtual or in reality to the exact Mach number for an accurate academic dicussion of which is better. It would be worthwhile to compare the data to a double wedge which is the most economical to produce.
 
Incorrect, AeroFinSim has no such restrictions. I bought it many years ago shortly before the guy that wrote it stoppd publishing and selling it. Also I know several fellow rocketeers in Europe that own a copy.
The general consensus on using flat plates as fin materials is one of cost and ease of construction.

When I downloaded AeroFinSim v4.5 last year for free through Dr.Cipolla's I had to prove US citizenship via email. Perhaps the paid for version did not have restrictions that was before my time in rocketry.


 
Ghostfather,

You might want to sharpen your observational skills. If you really knew anything about BALLS you'd have guessed my avatar was taken at BALLS. Most folks would also have guessed the rocket shown would be flown on an M or an N at BALLS. Your description of what it takes to go Mach 3+ is fairly condescending when you don't know anything about who you are addressing. It's also not a good way to impress folks when you are brand new to a forum.

I've been to BALLS many times and have flown over Mach 2 (on just a 54mm motor) on numerous occasions. I've probably also seen more flights at BALLS fail going over Mach 3+ than I've seen succeed. Even at BALLS, Mach 3+ is unusual as most flyers like to get their rockets back in one piece. I've flown many large motors (M-N) and have worked on a team that has launched several P motors. We have to take your word on who you say you are but there are lots of folks on this forum who know me as I post under my real name instead of anonymously.

As I mentioned in my reply, my opinion is formulated based on my discussions with friends who do work for companies that fall under ITAR regs, and I did mention they are written specifically for dealers and manufacturers.

You can laugh at me all you want, but based on what I know, I am following fairly sound advice from my perspective. You and others may think that's silly and even make fun of me but I'm happy to publicly stand by my decision. We each have to weigh the pros and cons of any public discussion. I simply stated that one might want to proceed with caution, which does not seem unreasonable to me.


Tony
 
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Short answer: no.

Still pretty short answer: VK nosecones are a 3D shape. Fin airfoils are much closer to 2D designs. (Not really 2D designs, but definitely not the same flow as a nosecone.) The optimum drag cross section will be different between the two.
 
I seem to remember a good treatment of supersonic airfoils somewhere in my reading. I suspect it was in this book:
xfundamentals-of-aerodynamics.jpg.pagespeed.ic.pwZ-ZJRc3F.jpg

https://www.google.com.au/url?sa=t&...DYNAMICS.PDF&usg=AOvVaw14G9kCPsPhDya2cGWWaROB

Just search on google and you will find a free pdf somewhere.

Really good book. Goes from basics to hypersonics in a really well-written and understandable style. I seem to remember the treatment of supersonic shock waves and hypersonics being really good.
 
Really good book. Goes from basics to hypersonics in a really well-written and understandable style. I seem to remember the treatment of supersonic shock waves and hypersonics being really good.

Thanks for the link, I've seen the book before, but never got around to studying it seriously because of the equations. He does explain some of the basic supersonic theory in clear conceptual terms, which is also useful. He also confirms what GrouchoDuke says, that fins are going to act differently than nose cones. Not unexpected in measurable terms, but I wasn't expecting a 1:1 relationship in performance.

I'm not so much interested in the calculations and wind tunnel optimizations and designing the optimal supersonic airfoil. I'm just looking to make a fin that is an improvement over the double wedge using some new construction techniques made possible by 3D printers. I've built a double wedge fin on a Nike Smoke model, and most of my supersonic fins are flat plates with sharp leading and trailing edges.

I may try to do some basic CFD analysis to compare the two shapes (double wedge vs Von Karman) to get a feel, but it's been a long time since I did CFD (for Soil Mechanics), and I'm not sure I remember enough from 30+ years ago to make the right assumptions in software. The comments from Andrew_ASC piqued my interest about theortical performance between Mach 2 and Mach 3, though I'm not expecting any surprises.
 
Don't be afraid to ask the engineering research staff at Delft for help. The Netherlands has to have equivalent CFD wind tunnels to Mach 25. They mechanical and aerospace university contacts there are extremely open minded to public research especially when you bring knowledge to student programs about rocketry. Ask for drag coefficients bring CAD models. Smile. The best they can do is help you make right assumptions and show you stuff you never knew. They may want to write a research article on the topic. The university helped me a lot as a engineer student. Perhaps they would be interested in your practical high power rocketry knowledge as student engineering teams undergrads get the university a lot of publicity when an international student rocket competition goes well. These groups usually want mentors as teams lack experience. Consider it more like mutual friendship. The worst they could say is no. It won't hurt to ask. They'll find your project fascinating if you catch them at the right time of year when research articles aren't due.

A few weeks of CFD work by them will help you decide which airfoils to use without even building them. If you are able to CFD without them then good on you. They can CFD multiple profiles in a Mach 2-3 environment where you lack drag data. They will have better software packages and more knowledge about subject to help. You may meet people that call supersonic flow trivial.
 
As a rough guess, I would tend to assume that there won't be a whole lot of difference in drag between various thin, pointy shapes for fin cross section at mach 2-3. You do want a very shallow angle wedge at the leading and trailing edge (to minimize flow turning), but then I would tend to think that for typical rocketry thickness to chord ratios, the exact profile wouldn't be very critical. A double wedge or trapezoidal profile might be a bit worse, since you will have the expansion fan coming off the point at the middle of the fins where the flow has to turn, but I don't have a great sense intuitively for what the magnitude of the difference would be between a pure double wedge and something like a biconvex.

As for thickness, thinner will always be better (assuming sufficient stiffness and strength to not flutter and fail) - those studies you mention are looking at lifting airfoils, where a non-negligible thickness actually improves L/D performance, but for well-aligned rocket fins, the intent is to be operating around a Cl of 0, and for that operating condition, thinner will always have less Cd, all else equal.

Also, all of this is public-domain, undergraduate level (and maybe masters level) compressible fluid dynamics stuff. It isn't even close to being something where ITAR would matter. Hell, even discussing guidance systems and turbopumps at an academic level wouldn't come into itar. Nothing we do in amateur rocketry is likely to ever fall under that umbrella.
 
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Thanks for the link, I've seen the book before, but never got around to studying it seriously because of the equations. He does explain some of the basic supersonic theory in clear conceptual terms, which is also useful.
If you start at the front of the book and work through you will end up with a good understanding of basic aerodynamics. It is very well written and thought out.
 
As a rough guess, I would tend to assume that there won't be a whole lot of difference in drag between various thin, pointy shapes for fin cross section at mach 2-3. You do want a very shallow angle wedge at the leading and trailing edge (to minimize flow turning), but then I would tend to think that for typical rocketry thickness to chord ratios, the exact profile wouldn't be very critical. A double wedge or trapezoidal profile might be a bit worse, since you will have the expansion fan coming off the point at the middle of the fins where the flow has to turn, but I don't have a great sense intuitively for what the magnitude of the difference would be between a pure double wedge and something like a biconvex.

I've gone with about a 4% thickness. The Von Karman is sort of a bi-convex, but not circular.

Attached are 3D models of fins for a 3D printer, for comparison:

dw-fin.stl - double wedge fin (supersonic)
vk-fin.stl - von karman profile fin (supersonic)
m1-fin.stl - NACA m1 airfoil (subsonic)


All are a factor 10x too large, so please scale them back before printing. They are meant to have a 300 mm root length and 100 mm span, about right for a 98mm air frame. They all have the same trapezoidal shape, and all have about 4% thickness. That means 12mm is the thickest part at the root, and 4mm thickest part at the fin tip.

The fins can also be scaled back for smaller rockets. I fly the m1 airfoil on a 54mm airframe after scaling it down another 50%.

View attachment dw-fin.stl

View attachment vk-fin.stl

View attachment m1-fin.stl
 
cjl,

The assertion that "Nothing we do in amateur rocketry is likely to ever fall under that umbrella." (ITAR and EAR) makes me think you haven't read the regs. Export can and often is transfer of information to a non-US national. Take a look at what is on the United States Munitions List (USML).

That something is well known abroad doesn't provide license for a US person to talk about it with internationals. Sorry. I didn't write the rules, but I have to abide by them.

Gerald
 
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cjl,

The assertion that "Nothing we do in amateur rocketry is likely to ever fall under that umbrella." (ITAR and EAR) makes me think you haven't read the regs. Export can and often is transfer of information to a non-US national. Take a look at what is on the United Stated Munitions List (USML).

That something is well known abroad doesn't provide license for a US person to talk about it with internationals. Sorry. I didn't write the rules, but I have to abide by them.

Gerald
Sad +1, that's why all the *nix crypto libraries were hosted outside the US for the longest, strangest time. It made no sense, but we needed to comply.
 
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