fin airfoils vs other shapes

[Rschub]

Everything is a compromise.

 

[Titan II]

And the downside is, it's draggy.

TP

On your run of the mill sport rocket, so what?

 

[rocket_troy]

On your run of the mill sport rocket, so what?

Fair'nuff

TP

 

[lr64]

Do you want more altitude for a given amount of noise, or more noise for a given altitude? For the latter criterion, mount the rocket on the rail upside down.

You can substitute "money spent" for "noise" in the sentence above. Or you can substitute "speed" for "altitude".

 

[kjhambrick]

Do you want more altitude for a given amount of noise, or more noise for a given altitude? For the latter criterion, mount the rocket on the rail upside down.

You can substitute "money spent" for "noise" in the sentence above. Or you can substitute "speed" for "altitude".

@lr64 / @rocket_troy --

There are different mission objectives for different people and varying launch site conditions.

Check out @Rschub's Level 2 Build Thread: Minnie-Magg drag mods for Level 2?

It ( still ) is an entertaining and educational read and IMO, he came up with an ingenious solution to meet his L2 mission criteria.

-- kjh

p.s. Knowing who posted the X-15 tail fins, I looked hard for a hinged spoiler in that wedge airfoil

 

[Rschub]

varying launch site conditions.

Lol, yes, we are a little tight. It's a 4-hour drive to anyplace open like URRG



And with a bit of modification, that "airfoil" might not be as bad as you think

 

[lr64]

@lr64 / @rocket_troy --

There are different mission objectives for different people and varying launch site conditions.

Check out @Rschub's Level 2 Build Thread: Minnie-Magg drag mods for Level 2?

It ( still ) is an entertaining and educational read and IMO, he came up with an ingenious solution to meet his L2 mission criteria.

-- kjh

p.s. Knowing who posted the X-15 tail fins, I looked hard for a hinged spoiler in that wedge airfoil

For cert flights when there are limitations on altitude, gratuitous drag makes sense. So they're an exception to what I was writing about.

Personally, if I was dictator, achieving a certain speed or altitude with one's own design and build would be an alternative to using a big motor for certification purposes, as it would demonstrate a certain level of skill and understanding.Say, Mach 1 for level 1.

 

[robopup]

Me, and anyone who's trying for performance or economy. At least below Mach 1.5 or maybe 2.

I think the part about this thread that confuses me is that "if you're not shaping your fins you're wasting money" is a strange hill to die on when there are so many other things that go into maximizing altitude/cost, if that's your thing, and so many other good reasons to fly rockets than to maximize that metric.

Hopefully everything you fly is minimum diameter. And only rockets which are the length of the motor+nosecone. And weight optimized. Integrated tailcone. Polished. Flown out of a tower. There are all sorts of things that go into flying high other than fin shape, and there are all sorts of reasons to fly rockets other than to go high.

 

[Rschub]

 

[lr64]

Some of my motivation comes from looking at the price of the larger motors. I have some swamp Yankee heritage, and maybe that's where the penny pinching comes from.

Shaped fins are fairly easy. I think that, for any particular rocket flier, there is some altitude they'd like to see, as a minimum. That 's why the rocket is right side up on the rail, even though that requires a much longer trip, possibly with bushwhacking, to retrieve it. Why not spend less to get to the same height? A rocket that's NOT minimum diameter can benefit even more from attention to aerodynamics than one that is. Part of the mission may include being big enough to see.

 

[ksaves2]

For cert flights when there are limitations on altitude, gratuitous drag makes sense. So they're an exception to what I was writing about.

Personally, if I was dictator, achieving a certain speed or altitude with one's own design and build would be an alternative to using a big motor for certification purposes, as it would demonstrate a certain level of skill and understanding.Say, Mach 1 for level 1.

How in the heck is one going to prove speed on an L1 flight besides simulations that might not be "that good" for a particular situation? Some flying sites, it's better to use an H in a heavier rocket for lower apogee, get the L1 cert in otherwise a "screamer rocket" would be lost. Nothing wrong with that.

If one wants to use "screamers" to cert better have electronics, trackers or the rocket will be lost. Especially if it's totally sight unseen. The deployment events might be nominal but if one doesn't see the rocket on descent, won't have a clue where to go to search without a tracker.
I had some totally out of sight flights (not on a cert flight) and for some reason the GPS tracker couldn't get a position out even though I blew the main chute up high (1000ft). I had no idea where the rocket went. I get one ping when the rocket is 75 feet above the ground, walk to that coordinate and there the rocket is!! Still have it though and have flown it many times thereafter and got it back every time.

I always sand my wood fins and for HPR, fiberglassed the smaller rockets with 1/8 oz. fine fiberglass on the fins and sometimes the cardboard bodytubes though it was a PITA as after the fins were glassed, would have to sand them again. Many times I glassed the fins before I epoxied them on the rocket.
I will qualify for larger rockets, I used heavier fiberglass cloth unless they were fiberglass fins and bodytubes to start with.

Have flown small rockets with up to an I-200 and some of my mixed I motors that lord knows what impulse they were. The mixed motors were only at a TRA launches where that was allowed. The NAR launches I only used certed motors and if pre-loaded, brought the motor bag to show the LSO but no one ever questioned me. I never tried anything nefarious either. Kurt

 

[lr64]

How in the heck is one going to prove speed on an L1 flight besides simulations that might not be "that good" for a particular situation? Some flying sites, it's better to use an H in a heavier rocket for lower apogee, get the L1 cert in otherwise a "screamer rocket" would be lost. Nothing wrong with that.

If one wants to use "screamers" to cert better have electronics, trackers or the rocket will be lost. Especially if it's totally sight unseen. The deployment events might be nominal but if one doesn't see the rocket on descent, won't have a clue where to go to search without a tracker.
I had some totally out of sight flights (not on a cert flight) and for some reason the GPS tracker couldn't get a position out even though I blew the main chute up high (1000ft). I had no idea where the rocket went. I get one ping when the rocket is 75 feet above the ground, walk to that coordinate and there the rocket is!! Still have it though and have flown it many times thereafter and got it back every time.

I always sand my wood fins and for HPR, fiberglassed the smaller rockets with 1/8 oz. fine fiberglass on the fins and sometimes the cardboard bodytubes though it was a PITA as after the fins were glassed, would have to sand them again. Many times I glassed the fins before I epoxied them on the rocket.
I will qualify for larger rockets, I used heavier fiberglass cloth unless they were fiberglass fins and bodytubes to start with.

Have flown small rockets with up to an I-200 and some of my mixed I motors that lord knows what impulse they were. The mixed motors were only at a TRA launches where that was allowed. The NAR launches I only used certed motors and if pre-loaded, brought the motor bag to show the LSO but no one ever questioned me. I never tried anything nefarious either. Kurt

I think you're right about the electronics and GPS. Miniature dual deploy could help, too. Successful use of these would demonstrate a certain level of mastery, of course.
A REALLY light rocket would be faster and have a lower apogee, too. If that was still too high, one could set up a small air brake to deploy at the instant the rocket began to decelerate.
As dictator, I would arrange for certain altimeters that had good accelerometers to be qualified for proof of speed. Or one could always launch another rocket with a microphone first and Iisten for the boom. ;-)

 

[SolarYellow]

You're talking about forcing people to do things they don't want to do, for no good reason.

I am OCD and want to make everything I build, to borrow a phrase from one of my mentors, "The nicest one of those I can figure out how to make." Between that and my easily-distracted (Ooh, that's a pretty idea!) focus and interest in many things rocketry-related causing me to work on a bunch of different things all at once, my build completion rate is as pathetic as one might imagine, but, man, when I get one done, it's going to be awesome....

Others just want to put something up in the air, and that makes them smile. If square fins make a person smile, fine. Mine will be at least rounded, and usually a lot better than that. Unless I'm using an ARF fin can to make that part of the rocket less OCDified.

I think what's driving this thread is you saying everyone else is wrong for not wanting to do it the way you want to do it. Well, you're wrong about them, right about you. Everyone doing it the way they want to, as long as it is safe, is the right way.

 

[lr64]

You're talking about forcing people to do things they don't want to do, for no good reason.

I am OCD and want to make everything I build, to borrow a phrase from one of my mentors, "The nicest one of those I can figure out how to make." Between that and my easily-distracted (Ooh, that's a pretty idea!) focus and interest in many things rocketry-related causing me to work on a bunch of different things all at once, my build completion rate is as pathetic as one might imagine, but, man, when I get one done, it's going to be awesome....

Others just want to put something up in the air, and that makes them smile. If square fins make a person smile, fine. Mine will be at least rounded, and usually a lot better than that. Unless I'm using an ARF fin can to make that part of the rocket less OCDified.

I think what's driving this thread is you saying everyone else is wrong for not wanting to do it the way you want to do it. Well, you're wrong about them, right about you. Everyone doing it the way they want to, as long as it is safe, is the right way.

I'm not talking about forcing anything. I'm advocating. Other people are free to go as ugly and/or inefficient as they want. We all claim to be interested in aerospace, where huge amounts of effort are devoted to efficiency.
---------
Incidentally, with an efficiency mindset, one might get something like the following rocket, which Openrocket thinks can achieve almost Mach 1.2, while staying under 3,000 feet. At least if you keep it light. If made heavier, it could get to almost 4000. Note that the motor only costs $10, at least in some places.

If 3,000 feet was too high, and the fins could stand the stress, a shorter delay could be used. Some taper in the fins might help with that.

Now all we need is a teensy altimeter and radio beacon.

 

[David_Stack]

Other people are free to go as ugly and/or inefficient as they want.

Rather judgmental don't you think?

From Merriam-Webster's definition of "Efficient":
1
: productive of desired effects
especially : capable of producing desired results with little or no waste (as of time or materials)

Note the use of the word "desired". The fact that someone's approach may not align with your desires does not mean that it is wrong to the individual who decides (desires) to prioritize their time or effort differently.

And yes, I do spend time attending to the leading and trailing edges of my fins, but I am not going to proselytize to those who don't.

 

[watheyak]

Late to the party, but I was excited to read this thread as my Balls project next year will have a biconvex profile milled from high-temp carbon plate. Radially tapered, even. Then the thread took a weird turn. Let me help it get even weirder.

@lr64 I find really strange that you're expending a huge amount of mental energy on airfoiling fins for a low power rocket, where the effort to airfoil a fin makes a minisule difference. Your whoosh-pop is (allegedly) 6 feet higher. Big whoop.

I also think your efficiency argument misguided. Black powder motors "ugly and inefficient" compared to APCP. (But the post burn smell is unbeatable. There should be a candle. I'd buy it.) I get that you're a penny pincher, but don't try and impose that preference on someone who gleefully spends their expendable income on their hobby. For me, APCP is money well spent. For efficiency's sake

Also, for credibility reasons, let me state that I love all rockets. Recently I've flown more low power than high power. I have also airfoiled many a balsa fin on low powered rockets. I love sanding and I'm great at it. But I will never airfoil a balsa fin again. It makes zero difference in a low power model rocket's flight, and makes them so delicate that they have a very short lifespan.

 

[lr64]

Actually, it was the high power rockets with square fins that amazed me the most. But there can be significant differences even with the black powder stuff. If we can believe Openrocket, a slightly modified Scout with an Estes C6 will go to 1,808 feet with square fins, 2160 with rounded ones, and 2360 with airfoiled ones. At 190 mps (close to the top speed of the square finned version, but not the others), the square finned version had 3.34 N of drag, the rounded 2.54, and the airfoil 2.54. Not sure how seriously to take these numbers, but I suspect they're in the ballpark.

Incidentally, on the Scout above, you could make the fins out of spruce instead of 8 lb balsa and add only 1.1 gram, plus a bit more nose weight, making it fly higher to boot. (I'm assuming an airfoil that's 5 percent thick measured parallel to the vertical axis.) Without much drag penalty, you could leave the trailing edge squared off and 1/64" thick. For very little weight, you could glue a 1/8" wide strip of basswood to the trailing edge before shaping, and end up with something substantially sturdier. Or you could add a 1/64" ply cap to the tip of the fin. Or paper the fins. Or use 3/4 ounce glass, just near the trailing edge.

If we put APCP in the Scout, things get a bit more interesting. With a square fin, a Quest D16 is supposed to get 1825 feet. Use an airfoiled fin and you can get 560 feet higher for more than $3 less with an Estes C6.

 

[Alan15578]

Square edged "airfoils" just look lazy and uncouth, and the edges will quickly wear through the paint. Still, there are some models where you want to increase the drag.

 

[lr64]

Square edged "airfoils" just look lazy and uncouth, and the edges will quickly wear through the paint. Still, there are some models where you want to increase the drag.

One could have a high performance rocket with a removable boat tail. For low performance, substitute an expanding cone.

 

[bjphoenix]

Actually, it was the high power rockets with square fins that amazed me the most. But there can be significant differences even with the black powder stuff. If we can believe Openrocket, a slightly modified Scout with an Estes C6 will go to 1,808 feet with square fins, 2160 with rounded ones, and 2360 with airfoiled ones.

If you're talking about the original Astron Scout that thing has so much fin sweep that it is hard to imagine that the fin profile is that significant, otoh those fins are like 3/16" or 1/4" thick.

 

[lr64]

If you're talking about the original Astron Scout that thing has so much fin sweep that it is hard to imagine that the fin profile is that significant, otoh those fins are like 3/16" or 1/4" thick.

The foil was one of the modifications. I challenge you to find a swept wing, supersonic airplane that has airfoils anywhere near as thick, in proportion.

 

[rfjustin]

The foil was one of the modifications. I challenge you to find a swept wing, supersonic airplane that has airfoils anywhere near as thick, in proportion.

Your continued attempts for intellectual flex, laced with your continued condescension, leaves me scratching me head as to the real point on this thread.

I challenge you to offer some actionable intel here, or any tips and techniques that one could employ in the high power/high performance realm. If you are not able to do that, this thread is rather fruitless.

 

[trialsguy]

If you really want a low flying rocket- don’t sand the balsa fins, leave them grainy and fuzzy. Then give them one light coat of rattle can spray paint, to get the balsa fuzz to stand up some more and stiffen. Launch it, and watch it ascend to about 1/4 the altitude that you’d imagine it could fly.

A buddy of mine, who does not like to think too deeply about things, and takes the greatest pleasure in “action”, and not “perfection” or results(?) - builds all his rockets this way. I am still amazed at how low he could get his rockets to fly.

 

[lr64]

Your continued attempts for intellectual flex, laced with your continued condescension, leaves me scratching me head as to the real point on this thread.

I challenge you to offer some actionable intel here, or any tips and techniques that one could employ in the high power/high performance realm. If you are not able to do that, this thread is rather fruitless.

"Flex"???? Not really. It would be silly when I've met a bunch of people smarter than I am. Please put up with what you call my intellectual flex the way I put up with the financial flex of others.

I'm interested in the engineering and design side of this hobby. Should I pretend I'm not? I was hoping for a bit of a technical discussion. There's been a bit of that, but it would be fun if there were more.

BTW, I concede that, for at least some slow rockets, real airfoils only help a little, other than by being stronger than thin wood fins and lighter than G10 fins.

As far as actionable intelligence, do you remember the first post?

 

[Alan15578]

If you really want a low flying rocket- don’t sand the balsa fins, leave them grainy and fuzzy. Then give them one light coat of rattle can spray paint, to get the balsa fuzz to stand up some more and stiffen. Launch it, and watch it ascend to about 1/4 the altitude that you’d imagine it could fly.

A buddy of mine, who does not like to think too deeply about things, and takes the greatest pleasure in “action”, and not “perfection” or results(?) - builds all his rockets this way. I am still amazed at how low he could get his rockets to fly.

My first rocket was like that. I had some finishing skills from building control line aircraft, but in my youthful rush to fly, I just hand brushed red dope over bare balsa. It was just as well, since it was destroyed by the ejection charge on the first flight.

The worst finish I ever saw was from the kid who just hand brushed layer after layer of sanding sealer until there there was no grain to be seen, never sanding anything.

I like the tech stuff, but without the craftsmanship to apply it, it can be pointless.

 

[lr64]

snip

I like the tech stuff, but without the craftsmanship to apply it, it can be pointless.

True enough, but I've seen some really marvelous workmanship from some really techie people. I've seen some appalling workmanship too, of course.

 

[bad_idea]

If I recall correctly, the fins need enough sweep to stay within the shock cone, if that's the right term for it. I think that might mean 63 degrees at Mach 2, though someone with more knowledge should correct me if I'm wrong.

Mach angle is arcsin(1/M), where M is the Mach number. For Mach 2, that is 30 degrees, which we generally interpret as a 60-degree sweep back. So you'd want a sweep back > 60 degrees for Mach 2.

I've lately started to wonder if the Mach angle is actually relevant to rocket fins, which would typically be too short to intersect the shock cone created by the nose of the rocket, unlike the situation for most supersonic airplanes. Do any of our aerodynamicist members have an informed take on this?

 

[dhurstell]

Those Reynolds numbers correspond to relatively small rockets. Somehow I missed that you wrote 2 inches, so my comment about big rockets was silly.

Friction drag can be lower if a substantial part of the flow is laminar. My number is based on multiplying the dynamic pressure times the area times the change in drag coefficients.

Sanding balsa into airfoils isn't very hard.

Edit: I calculated again and got 1.8 newtons again. What was the area of your fins?

Not ending up with a tiny piece of balsa from trying to sand in airfoils IS hard!

 

[Art Upton]

I've lately started to wonder if the Mach angle is actually relevant to rocket fins, which would typically be too short to intersect the shock cone created by the nose of the rocket, unlike the situation for most supersonic airplanes. Do any of our aerodynamicist members have an informed take on this?

@Chuck Rogers ?

 

[Chuck Rogers]

I've lately started to wonder if the Mach angle is actually relevant to rocket fins, which would typically be too short to intersect the shock cone created by the nose of the rocket, unlike the situation for most supersonic airplanes. Do any of our aerodynamicist members have an informed take on this?

It is not included in the RASAero II software. It does not need to be included because for most rocket configurations the nose cone is too far forward of the fins, combined with short fin spans, for this to have an effect on the fins.

Unlike the other rocket software packages, in RASAero II the fin planform effects from the Mach Cone Point of Influence from the first point of the fin at the root and the first point of the fin at the tip, and the 2-D and 3-D flow regions on the fin at supersonic flight Mach numbers, are all included in RASAero II using Missile DATCOM Methods.


Charles E. (Chuck) Rogers
Rogers Aeroscience