Aerodynamics of tail cones

what are the aerodynamic properties of different types of tail cones? would an sears-haack(von karmen) type of ogive work better than a conical boat tail for increasing altitude and reducing drag? many people who make high performance rockets make tail cones to add to the back of their rocket, and as far is i've seen, are usually conical.

i was told that conical boat tails are the absolute worst for reducing drag, and sometimes go so far as to increase drag on your rocket. well i'm a big fan of cold hard facts, so i'd like some experts to back this up. what's the deal with tail cones?

tuxxi said:

what are the aerodynamic properties of different types of tail cones? would an sears-haack(von karmen) type of ogive work better than a conical boat tail for increasing altitude and reducing drag? many people who make high performance rockets make tail cones to add to the back of their rocket, and as far is i've seen, are usually conical.

i was told that conical boat tails are the absolute worst for reducing drag, and sometimes go so far as to increase drag on your rocket. well i'm a big fan of cold hard facts, so i'd like some experts to back this up. what's the deal with tail cones?

Click to expand...

Typically, subsonic rockets always benefit drag-wise from tailcones, but past a certain turning angle they start to instead contribute to drag in the supersonic regime.

Tune in in a few weeks; hopefully I'll have data on various steepnesses of blended-conical tailcones at Mach numbers up to ~1.6.

check out the old Estes tech report, TR-11 Dr. Gregorek discusses boat-tailing on pg 35. He says the ogive is a little more efficient, but conicals are more common due to ease of construction.

from what I've read, you start getting flow seperation with transition angles greater than 8 degrees. lot easier to make a paper cone than a paper ogive.
rex

rocketguy101 said:

.... the ogive is a little more efficient, but conicals are more common due to ease of construction.

Click to expand...

That is correct, and summarizes the most important advantages of each.

The trick to a good subsonic boattail is to avoid a sharp, hard corner in the external contours. For absolute minimum drag, the boattail should include a lenghtwise zone where the diameter gently transitions from the full diameter of the body tube to the tapering aft surface of the boattail; from a side view there should be smooth curve that is tangent to the body diameter and curves until it is tangent to the boattail. The curve should not be too abrupt but should blend between the end tangencies over a length of, say, one half to one BT diameter. (More length is probably not necessary, much shorter starts to become a hard corner.) The final turn-in angle of the boattail should not be much steeper than around ten degrees or the airflow might follow around "the perfect curve" and then still separate (become turbulent and draggy) due to an adverse pressure gradient on a steep tailcone surface.

After you get past the curved transition zone, a simple conical shape (from there back to the aft edge of the tailcone) will work just about as well as anything. On a practical level, the drag differences between a simple conical shape and some other exotic contour will be quite small. Here is one place where I would recommend saving a little build time and putting the effort into a good curved transition and then use a simple conical shape behind it.

It is much much easier to fabricate a simple conic tailcone shape and add it to the rear edge of a body tube. That is the first, second, and third reason why most folks choose that option.

As a practical note, worrying about creating an optimized tailcone (again, for subsonic flight) should be a bit further down your list behind some other key features:

1---Select an elliptical nose cone shape with an exposed length-to-diam ratio of 1 to 1.5 to 2. Much shorter starts getting draggy again. Much longer only adds wetted surface area back into your design. The NC must have a smooth surface; when polished you should be able to "see yourself" in the reflection. Literally.

2---The joint from the NC to the BT should be invisible to the airflow. You should be able to scrape your fingernail down the side of the rocket (gently) without being able to feel any joint at the base of the exposed surface of the NC. Yes, this is tough, and requires a special set of fairly advanced building/finishing techniques. Some people dodge this issue by permanently attaching the NC and making a separation joint somewhere at mid-body (where the effects of any airflow transition to turbulent flow will be reduced).

3---Eliminate launch lugs, rail clips, rail buttons, and any other protruberances. Launch from a tower launcher.

4---Optimize (or semi-optimize) the rocket configuration and longitudinal mass distribution, where possible, to move the overall loaded c.g. as far forward as possible. This may call for forward location of payloads, retention devices to hold recovery components forward (during boost thrust), and even adding empty BT length ahead of the MMT to move some of the rocket's mass forward. This mass distribution feature is to minimize fin size, as in the next step

5---Do the homework, run the Barrowman eqns, find the optimum fin size to keep your rocket design barely stable. This will typically be a stability margin of 1.0 to 1.5. If the margin is much less than that you may be gambling a little, if the margin is a little beyond you will start wasting drag. Design your fins with an aspect ratio of around 1 to 2 for best subsonic aero; less than this starts to lose aerodynamic effectiveness, more than this gets you a fin that is long, narrow, and more easily breakable. Select a fin root location at the rear edge of your full-diameter, ahead of the start of the boattail transition.

6---Design (and build) the proper fins. Yes, elliptical fin planforms are pretty, and work best. However, a simple trapezoidal fin design has straight leading and trailing edges and is easier to build. A taper ratio (that is, the ratio of tip chord to root chord) of around 0.5 will achieve some 95% of the aerodynamic efficiency of the elliptical shape. Fin thickness should taper from the root to the tip in the same proportion to the chord (this really is important). Fin leading edges should be rounded (for subsonic flow), it is OK to leave the sides of the fin flat from around the 20% chord line back to the 50-60-70% chord line, then the fin thickness should taper back to almost a knife edge at the trailing edge. Tip shape should be left square, or sanded to a sharp edge, but should not be rounded. The choice between three fins or four is probably a tough one, and depends on your level of craftsmanship and finishing as much as anything else.

7---Attach the fins carefully, aligning them with the rocket's longitudinal axis carefully. It would probably be worth it to build (or buy) a fin jig to perform this step. The root of the fin should fit well against the BT; sand and/or remake components until you achieve this. DO NOT try to compensate for crappy root fit with mass quantities of glue. Prep the BT surface with light sanding to give the adhesive something to grip. For initial construction, use only enough glue to make the fins adhere, come back afterwards with a thin top-coat of glue to reinforce the joint, do not add gobs of root fillet material as you have seen posted here on TRF (I have not seen one yet that was sized or contoured correctly).

Somewhere around the bottom of this list is where a good boattail design ranks.

Supersonic design is a little different. Transonic design is a whole lot more complicated than both of these put together. A transonic design is, in actuality, the category where about 99% of the "supersonic" designs you see online end up falling, but everyone likes to fantasize about their own baby being supersonic and fails to account for the biggest part of the flight profile which is still subsonic or barely transonic.

I hope some of this helps? (Because a bunch of this is going to p*** off some of our TRF members.)

powderburner said:

That is correct, and summarizes the most important advantages of each.

The trick to a good subsonic boattail is to avoid a sharp, hard corner in the external contours. For absolute minimum drag, the boattail should include a lenghtwise zone where the diameter gently transitions from the full diameter of the body tube to the tapering aft surface of the boattail; from a side view there should be smooth curve that is tangent to the body diameter and curves until it is tangent to the boattail. The curve should not be too abrupt but should blend between the end tangencies over a length of, say, one half to one BT diameter. (More length is probably not necessary, much shorter starts to become a hard corner.) The final turn-in angle of the boattail should not be much steeper than around ten degrees or the airflow might follow around "the perfect curve" and then still separate (become turbulent and draggy) due to an adverse pressure gradient on a steep tailcone surface.

It is much much easier to fabricate a simple conic tailcone shape and add it to the rear edge of a body tube. That is the first, second, and third reason why most folks choose that option.

As a practical note, worrying about creating an optimized tailcone (again, for subsonic flight) should be a bit further down your list behind some other key features:

1---Select an elliptical nose cone shape with an exposed length-to-diam ratio of 1 to 1.5 to 2. Much shorter starts getting draggy again. Much longer only adds wetted surface area back into your design. The NC must have a smooth surface; when polished you should be able to "see yourself" in the reflection. Literally.

2---The joint from the NC to the BT should be invisible to the airflow. You should be able to scrape your fingernail down the side of the rocket (gently) without being able to feel any joint at the base of the exposed surface of the NC. Yes, this is tough, and requires a special set of fairly advanced building/finishing techniques. Some people dodge this issue by permanently attaching the NC and making a separation joint somewhere at mid-body.

3---Eliminate launch lugs, rail clips, rail buttons, and any other protruberances. Launch from a tower launcher.

4---Optimize (or semi-optimize) the rocket configuration and longitudinal mass distribution, where possible, to move the overall loaded c.g. as far forward as possible. This may call for forward location of payloads, retention devices to hold recovery components forward (during boost thrust), and even adding empty BT length ahead of the MMT to move some of the rocket's mass forward. This mass distribution feature is to minimize fin size, as in the next step

5---Do the homework, run the Barrowman eqns, find the optimum fin size to keep your rocket design barely stable. This will typically be a stability margin of 1.0 to 1.5. If the margin is much less than that you may be gambling a little, if the margin is a little beyond you will start wasting drag. Select a fin root location at the rear edge of your full-diameter, ahead of the start of the boattail transition.

6---Design (and build) the proper fins. Yes, elliptical fin planforms are pretty, and work best. However, a simple trapezoidal fin design has straight leading and trailing edges and is easier to build. A taper ratio (that is, the ratio of tip chord to root chord) of around 0.5 will achieve some 95% of the aerodynamic efficiency of the elliptical shape. Fin thickness should taper from the root to the tip in the same proportion to the chord (this really is important). Fin leading edges should be rounded (for subsonic flow), it is OK to leave the sides of the fin flat from around the 20% chord line back to the 50-60-70% chord line, then the fin thickness should taper back to almost a knife edge at the trailing edge. Tip shape should be left square, or sanded to a sharp edge, but should not be rounded. The choice between three fins or four is probably a tough one, and depends on your level of craftsmanship and finishing as much as anything else.

7---Attach the fins carefully, aligning them with the rocket's longitudinal axis carefully. It would probably be worth it to build (or buy) a fin jig to perform this step. The root of the fin should fit well against the BT; sand and/or remake components until you achieve this. DO NOT try to compensate for crappy root fit with mass quantities of glue. Prep the BT surface with light sanding to give the adhesive something to grip. For initial construction, use only enough glue to make the fins adhere, come back afterwards with a thin top-coat of glue to reinforce the joint, do not add gobs of root fillet material as you have seen posted here on TRF (I have not seen one that was sized or contoured correctly yet).

Somewhere around the bottom of this list is where your boattail ranks.

Supersonic design is a little different. Transonic design is a whole lot more complicated than both of these put together. A transonic design is, in actuality, the category where about 99% of the "supersonic" designs you see online end up falling, but everyone likes to fantasize about their own baby being supersonic and fails to account for the biggest part of the flight profile which is still subsonic or barely transonic.

I hope some of this helps? (Because a bunch of this is going to p*** off a lot of our TRF members.)

Click to expand...

+1 on all this! Excellent!

MIL-HDBK-762(MI)​

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• 
  [*=left]In subsonic flight, the lower the diameter ratio, the greater the reduction in base drag. Think football.
  [*=left]In transonic flight, reducing the diameter rato below 0.4 increase base drag.
  [*=left]ln supersonic there is no advantage in reducing the diameter ratio below 0.7.
  [*=left]Cones have lower drag than olgives.
  [*=left]In transomic and supersonic flows, increasing skin friction with lengrh limits the useful length of a boat tail.

There were a couple of articles in Model Rocketry magazine (July 1971 and August 1971) with theory and wind tunnel tests of boat-tails.

powderburner said:

That is correct, and summarizes the most important advantages of each...

Click to expand...

Powderburner,

Really nice summary. But I'd like to add the caveat that with regard to model rocket aerodynamics, the more you learn the more you learn how little we really know. I am not an aero-guy (my term for someone actually trained in rocketry aerodynamics). But I hang out with a lot of them on the U.S. Spacemodeling team. And the primary thing I've learned from them is that the simplest of questions, like "what shape nosecone is best" or "do boattails help" or "what fin shape is best" have no clear answers.

For those unfamiliar with why this would be, it has to do with how a fluid like air moves over a surface at various speeds. A more viscous fluid will behave differently from a less viscous fluid and each particular fluid has particular characteristics at different speeds. The interaction between a fluid's viscosity and inertia is described by a dimensionless number termed a "Reynolds number". For most of model rocketry operating at subsonic speeds, we are dealing with very low Reynolds numbers. And the reality is that we have far less useful information about rocketry at low Reynolds numbers than we do for higher numbers (professional and military folks haven't had much use for rockets that go slow and not very high).

As a complete aside, the military used to feel the same way about airplanes that went slow. Then, in the 70's, some very bright folks at MIT started playing with human powered flight, and ultra-low Reynolds numbers. That eventually led to aircraft that were incredibly efficient, could stay airborne at very low speeds for very long periods of time, which eventually resulted in much of what we see today in drone technology. Unfortunately it's a bit more difficult to conceive of uses for very slow rockets.

All that said, and not to imply in any way that Powderburner is incorrect (if we differ it is most likely because we are referencing rockets moving at different speeds), here's my take on his points:

1. For nosecones at model rocket type speeds (A thru D), consensus among my aero friends is that 2:1 ogive shape is best, but of very little overall difference between types at these speeds, and diameter is way more important than anything else. As for how smooth is good enough, Bob Parks maintains that anything smoother than what is produced by 400 grit sandpaper is unnecessary (again, having to do with the viscosity of air at various speeds) and too smooth may actually be detrimental (think turbulators like golfballs).

2. True, any skin surface irregularity may trip a surface out of laminar flow, where skin friction is usually 1/3 or less than turbulent flow. So, the optimal surface for altitude is unibody nosecone/body tube, minimum diameter (hence no boattail), no protuberances, with rear ejection. I can honestly say that I, personally, have never seen a rear-ejection high power model. But I imagine they must get flown at Black Rock at times.

3. Yes, eliminate anything that sticks out from body surfaces. For A thru C model rockets the common wisdom is that a launch lug can contribute to almost 1/3 of total drag on your rocket. In addition to (or instead of) using a tower launcher you can also use a piston.

4. True, for the most part. But there is always a tradeoff in altitude between adding noseweight and reducing fin size. Similarly there is a tradeoff between increased skin drag of lengthening a body tube and decreasing fin size. And the whole issue devolves into hideous complexity when you start talking about anything other than very small angles of attack. But Powderburner's admonition to try to move noseweight as far forward as possible and reduce fin surface as much as possible is the maxim I live by.

5. Again, you can find lots of configurations where this might not be true but without incredibly advanced analysis there is no way to know so this plan is solid for most shapes and is what I would go with.

6. I will differ from Powderburner on the question of fins. I have never been able to get any of my aerodynamicist friends to even remotely agree on what fin shape is best from a purely theoretical standpoint, let alone with regard to what can actually be built. So let me just say that virtually no one on the international competition scene uses elliptical fins. Almost all use some form of clipped delta. As for sanding a planform into fins (teardrop shape fore to aft), almost none of us do this because unless the planform is created by a machine, you are as likely to sand in discontinuities between fins that will result in more drag than the planform will reduce. So most of us just sand very thin fiberglass fins flat, then round the leading and trailing edges. As for decreasing fin thickness from root to tip, again, almost impossible to do consistently by hand so very thin uniform thickness is used. Also, fins of varying thickness (fore to aft, or root to tip) also make using a fin jig almost impossible (see item 7). The bigger tradeoff for competition model rockets is overall (uniform) fin thickness versus stiffness. You want thin to reduce drag but thick enough to avoid flutter. As for how many fins, for altitude, we all fly with 3.

7. The biggest issue by far with regard to fin drag is putting on perfectly aligned fins. Way more important than anything else you might consider in fin drag. A fin jig is absolutely necessary and it is necessary to attach each fin individually using the same jig. Jigs which allow you to attach multiple fins simultaneously sacrifice accuracy for speed. Nothing else offered for sale even comes close to an Art Rose fin jig for accuracy, but they are very expensive. I also agree with Powderburner that minimalistic smooth fillets are critical for drag reduction. How far forward of the aft end of the rocket fins should be places is a matter of debate (again, a ridiculously simple question that can get the aero guys going for hours).


A few concluding notes. First, be aware that virtually all of the aerodynamics research regarding drag comes from wind tunnel research so virtually none of it accounts for the aerodynamics occurring from a thrusting motor. This is particularly important when you think about boattails. Bob Parks has pointed out that in instances where your motor size is less than body tube size (like we fly in FAI altitude events, 18mm body tube, 10 or 13 mm motor), CFD analysis suggests a protruding motor will act very similarly to a boattail in reducing base drag, even without actually adding the boattail. But, again, it's almost impossible to say exactly what will happen with a particular models base drag during motor thrust.

Again, kudos to Powderburner for a great discussion of boattails and their relative influence on overall drag. The complexities of the factors involved in increasing altitude in model rockets are really fascinating. The questions are simple but the answers are really, really hard.

Thanks for the kind words, Gus. I would try to take a bow but there's a good chance I would hit my head on something when I bent over, so.....

Gus is correct in all the places where he points out the subtle effects of Rn (Reynolds number).

I generally assume that for purposes of model rocketry, BG and RG birds are solidly outside (below) the range of even "low Rn" data, and down into the world of flying insects; there are some really bizarre aerodynamics that take place here. Flat-plate wing airfoil sections (and fin airfoils) are the rule here, with the possible exception of rounding some leading edges for a little drag reduction.

If you want to prove it to yourself, build two of whatever-your-BG-design-is and build one with an airfoiled wing and one with a flat sheet wing (be careful to ballast so overall weights are matched, to eliminate that effect) and glide-test them both. You will typically have more variation in flight time due to launch/release conditions or due to model-to-model variations in warped balsa than due to wing airfoil differences.

I assume that we can begin to apply "standard" aerodynamics to some classes of competition models, especially altitude rockets, because they generally have significantly higher flight speeds (at least for part of the ascent). If you are going to build with balsa or basswood fins (that is, fins with much of any significant thickness) then IMO there is a small advantage to be gained by airfoiling. Mind you, it is not going to cut the drag in half or anything radical, but for altitude competitions some of these guys would kill their own mother to consistently get an extra two-three-four feet of altitude. If you are building with wafer-thin fiberglass fins then there just isn't much room to be able to create any kind of airfoil.

The tapered-thickness-from-root-to-tip-thing is a nod to the aerodynamic effect on fins with tapered planforms where reduced tip chord with the same thickness effectively gets you an increased thickness/chord ratio outboard, which promotes stall conditions on the outboard portion of the planform. How much this effect actually occurrs in model rockets is probably questionable, and would only accomplish a small reduction in a potential drag that was also relatively small in the first place, but I thought we were talking here about squeezing out the absolute best performance, so I kept it in my list.

I agree that trapezoidal fin planforms are probably best overall; easier to make, work nearly as well as elliptical, and can be built with simple jigs in fairly repeatable and consistent airfoil shape. (I thought I said that earlier, maybe I wasn't very clear.) I believe that a decent airfoil job on all fins will still give lower overall drag than leaving them flat. I used to get what I felt were good results by making about twice as many as needed and selecting what I thought were the "best" three or four.

Elliptical fins may be pretty (I have always loved the looks of the Spitfire) but they are a royal pain in the arse to make.

Trimming with added ballast to achieve maximum altitude can definitely deliver some performance advantages. I am not sure, however, that a lot of modelers have the analytical/engineering discipline to make this work well. From what I remember of talking to competitors (all those years ago, when I was also competing), we did not have personal computers, the theory of mass trimming was mostly that (theory), and most modelers could not wade through all the hand calcs and/or did not have a really firm grasp of what they were doing, and probably ended up hurting their rocket's performance instead of improving (if their designs were even light enough in the first place to benefit from mass trimming). Hopefully, this bunch today is more computer savvy and could write an EXCEL spreadsheet to run a few trade studies?