Tapered Body tubes

[zway2b]

I've built several model kits with my kids for school and we have done well in the altitude sessions using conventional modeling technics but now we want to design our own rocket. We want to design with altitude in mind for A/B engines and C/D engines.

My question is why do we not see any tapered body tubes when most of the wind tunnel test seem to indicate that the optimum design to reduce Cd is a Tapered tube? Is the data faulty? Two reports I have seen show the Drag on a rocket with a 1.8X 20:1 tube as half of conventional tubes. I am going to build one and compare for myself, but I am curious why this obvious advantage of design is not being marketed.

Thanks Dave

[CPMcGraw]

I've built several model kits with my kids for school and we have done well in the altitude sessions using conventional modeling technics but now we want to design our own rocket. We want to design with altitude in mind for A/B engines and C/D engines.

My question is why do we not see any tapered body tubes when most of the wind tunnel test seem to indicate that the optimum design to reduce Cd is a Tapered tube? Is the data faulty? Two reports I have seen show the Drag on a rocket with a 1.8X 20:1 tube as half of conventional tubes. I am going to build one and compare for myself, but I am curious why this obvious advantage of design is not being marketed.

Thanks Dave

From a commercial point-of-view, you're looking at mandrels with one specific use (or, one customer only), meaning it will be more expensive to produce. The more-common straight-sided tube can be sold to dozens (if not hundreds) of customers for more than one purpose (ie candy containers, model rockets, or electrical insulators using the same identical part), only requiring a single investment in tooling. Each tapered mandrel has to be sized not only for the start and stop diameters, but a specific length, and as such, only one unique item can be made from it.

Not to say it couldn't be done, nor that there's no company doing it. Just that it represents a more expensive component for the model rocket kit manufacturer to purchase compared to a standard tube, and that the final price to the customer (you and me) might be out of our wallet range or sensible justification.

[Solomoriah]

Building conical rockets or rocket parts using 110# cardstock isn't too hard. Look at the old Centuri Vulcan (not exactly a low drag design though) or the tailcone on the classic Estes Sprint.

The current Estes Bullpup has a plastic tailcone, though you really need a razor saw to build that one (so I might decide against it for classroom use).

[zway2b]

Building conical rockets or rocket parts using 110# cardstock isn't too hard. Look at the old Centuri Vulcan (not exactly a low drag design though) or the tailcone on the classic Estes Sprint.

The current Estes Bullpup has a plastic tailcone, though you really need a razor saw to build that one (so I might decide against it for classroom use).



I just started building paper models and I do have access to some heavy coated paper, mainly 60 to 110 lb stock. I also have some mylar I might give a try to. I do want t,o give this body style a try. I tried to post a picture but the the format was wrong even thought it is only a 6K file but if you look at John S. DeMar's "Model Rocket Drag Analysis" report On the NAR R&D list # NAR 52094. It is body model # 12. http://web.syr.edu/~smdemar/rocketdrag.html



Thanks for your help Dave

[Solomoriah]

Looking at the page you mention, I see what you mean. I'd modify the design somewhat to make it easier to build; probably around 2" of BT-60 and a Bertha nose cone, mated to around 10" or so of BT-20 for the lower sustainer, then wrap a long paper transition from the lower edge of the BT-60 to the tail of the rocket. Attaching fins will be a challenge, though... probably I'd slot the transition and attach the fins directly to the BT-20 tube. Elliptical fins like those on the Estes Sprint would be perfect for this model, I expect.

Or you could make the transition come up a bit short of the tail of the rocket, exposing some body tube for fin attachment. That might make the drag go up a bit though, since you'd lose the boattail effect.

[Solomoriah]

... now I'm going to have to build one. dang.

[Gus]

My question is why do we not see any tapered body tubes when most of the wind tunnel test seem to indicate that the optimum design to reduce Cd is a Tapered tube?
For a design where the frontal cross sectional area is mandated, like in egg-lofting, tapering back to the diameter of the minimum motor size makes overall sense. But in designs where the only size requirement is a cross sectional area large enough to accomodate a specific motor, increasing the nosecone cross sectional area to accomodate a taper behind it yields more of a drag penalty than a benefit.



RockSim (http://www.apogeerockets.com/other_rocksim_features.asp) is a good way to play around with various designs to see their effect on oveall drag.

[Royatl]

I've built several model kits with my kids for school and we have done well in the altitude sessions using conventional modeling technics but now we want to design our own rocket. We want to design with altitude in mind for A/B engines and C/D engines.

My question is why do we not see any tapered body tubes when most of the wind tunnel test seem to indicate that the optimum design to reduce Cd is a Tapered tube? Is the data faulty? Two reports I have seen show the Drag on a rocket with a 1.8X 20:1 tube as half of conventional tubes. I am going to build one and compare for myself, but I am curious why this obvious advantage of design is not being marketed.

Thanks Dave


Tapered rockets are used in specialized areas, such as egg-lofting, where some advantage can be gained by tapering from the diameter of the egg capsule (1.8-2.0" typical) to the diameter of the motor. The taper is usually custom made, precluding commercial parts. Many years ago, Apogee Components sold competition egglofters using pre-printed heavy cardstock shrouds. I still have three of those rockets. You can find the plans of similar egglofters around the net. Possibly over at George Gassaway's site you might find the Three-minute Egg or something similar.

[zway2b]

Thanks Roy I'll give that a try.

Dave

[A Fish Named Wallyum]

There are several plans at www.nar.org :cool:

[zway2b]

Gus, That's what I would expect also, but according to this research paper not so. It must be an error in the altitude prediction software. They never did test the results empiracally but it must be wrong since this is 10 years old. The report indicates that even with an 80% larger nose cone area than a standard 18mm rocket that is should achive an additional 30%+ altitude because the Cd is half a traditional body style.

I think I'll have to try it.

Thanks Dave

[Solomoriah]

I seem to recall reading that, for subsonic rockets, a parabolic nose cone has the least drag; so using a parabolic cone with a mostly tapered rocket should yield very good performance.

Look at the performance of the old Estes Sprint... it's a real nice flier even though it's as thick as an Alpha, and the boattail is supposedly the reason why.

A Semroc part # BNC-50KP cone:

http://www.semroc.com/Store/ProdImages/nosecones/bnc-50kp.jpg
(hopefully everyone can see the picture)

mated to a short length of BT-50, 1.5" or so, with a 9" length of BT-20 below it, and a long transition tapered to the tail, would be a nice looking rocket. With the transition rolled that tight, I might even attach Sprint-like fins directly to the transition (it's not as though it will ever see a D engine). If I put about 0.75" of the BT-20 into the BT-50, that would yield a rocket with an overall length of about 12.5".

Dang, I just have to build one now.

... but where to put the launch lug? Standoff? Embedded in a fin? Hmm. Don't want to screw up the aerodynamics of it, but I can't justify building a launch tower just for one rocket...

[CPMcGraw]

...Dang, I just have to build one now.

... but where to put the launch lug? Standoff? Embedded in a fin? Hmm. Don't want to screw up the aerodynamics of it, but I can't justify building a launch tower just for one rocket...

Piston launcher, anyone???

[CPMcGraw]

...mated to a short length of BT-50, 1.5" or so, with a 9" length of BT-20 below it, and a long transition tapered to the tail, would be a nice looking rocket. With the transition rolled that tight, I might even attach Sprint-like fins directly to the transition (it's not as though it will ever see a D engine)...

You mean, like this???

Length: 12.98"
Diameter: 0.976" (BT-50)
Fin Span: 4.24"
Weight: 0.59 oz

C6-7......1900'......Dv 3.5 FPS......22" launch guide length

[rocketguy101]

The report indicates that even with an 80% larger nose cone area than a standard 18mm rocket that is should achive an additional 30%+ altitude because the Cd is half a traditional body style.

Somewhere I have seen a diagram comparing the drag of teardrop compared to a sphere (or round wire in another case) and for the same amount of drag force, the teardrop was considerably thicker than the wire or sphere.

This isn't what I was thinking of, but this NASA page (http://www.lerc.nasa.gov/WWW/K-12/airplane/shaped.html) has some data for students.

What you might run into, though, is you could be increasing frictional drag because you have more surface area "wetted" by the airflow. As mentioned earlier, if you have a required payload diameter that is larger than your motor, then tapering or boat-tailing would help. See this (http://www.princeton.edu/~asmits/Bicycle_web/blunt.html) page for that discussion.

[Solomoriah]

Craig, two words... thanks and WOW! 1,900' on a C6-7...

yowee. i'd never see it again...

I had almost decided to go back to the Bertha-based model, but this is a pretty cool design too.

I may build both. For the Bertha-based unit, I'm thinking possibly as much as 3" of BT-60 with maybe 7"-8" of BT-20 sticking out (plus whatever goes inside the BT-60, an inch or so probably) and a transition that comes down to within an inch of the end of the BT-20. I can use an engine hook with that design, which I prefer. I'd slot the transition and insert the leading edge of the fins into the slots. As a kitbasher, I'd probably start with a Baby Bertha and one of those Wal-Mart Launchables packs (Sizzler and Star Dart). The Sizzler body tube is a bit less than 9" and would work well as the tail end of the unit.

... and I'd paint it either green or slate gray and call it the Rocket Propelled Goony. :D

I might cut the tail end of the nose cone off completely, glue it into the tube and arrange for the entire thing to break apart just above the transition. This would give me a large space for the parachute, but require only 3 or 4 sheets of wadding (told you I was cheap) stuffed into the BT-20. Kevlar through the centering rings and tied around the BT-20 under the transition, tied to elastic or Estes rubber band attached inside the BT-60 using an Estes-type folded paper anchor.

[Solomoriah]

... ooh. but if I do that last bit, I'll need a coupler connecting the centering rings for stiffness.

[rocketguy101]

Somewhere I have seen a diagram comparing the drag of teardrop compared to a sphere (or round wire in another case) and for the same amount of drag force, the teardrop was considerably thicker than the wire or sphere.


this (http://www.aerospaceweb.org/question/aerodynamics/q0094b.shtml) still isn't the image I was looking for, but its closer.

[zway2b]

That is exactly what I was thinking and am going to try. I might use BT20 and BT55 as my sizes though.

Keep me posted.

Dave

[Rocketking]

A look at the NASA educational website offers the equations related to model rocket flight.

If I remember correctly, when the diameter of the rocket increases, the drag is SQUARED!

That's the reason a minimum diameter BT20 rocket will go nearly out of sight on a B engine!

Try whatever, but I'm not sure you will be heading for "warm fuzzies" Solo & Dave.

[zway2b]

Thanks Glen, my free altitude prediction software shows the same results. Increasing the nose cone to 1.35 inches on a .736 tapered body goes only about 80% as high as a .736 nose cone. Even with the assumption that the Cd is half on the tapered as opposed to the straight body style.

I'm guessing this is why the assumptions from the original report I sighted are flawed.

Thanks for the advice and the links.

Dave

[Solomoriah]

Dunno. I just downloaded the trial of Rocksim, and it says my Rocket Propelled Goony should make it to 1201' in altitude on a C6-7; that's the design with the Bertha cone.

Minimum diameter rockets like the Estes Sizzler claim 1600' in maximum altitude on the same engine; the BT-50 version Craig simulated claimed 1900'. The gain in drag from the larger frontal area may be offset by the reduction in drag from the boattail.

There is a prototype car I saw a few months back in Popular Mechanics that claims the lowest drag coefficient possible for a passenger car; it is rather boxy, with a truncated tapered tail end, similar in principle to this design. They claimed it was based on a fat little fish that happened to be very slippery in the water. I'll have to dig through the back issues and find that one.

... never mind, my Google-fu comes through:

http://www.worldcarfans.com/news.cfm?newsid=2050607.004/country/gcf

Proof that it doesn't have to be skinny to be slippery.

[Royatl]

Somewhere I have seen a diagram comparing the drag of teardrop compared to a sphere (or round wire in another case) and for the same amount of drag force, the teardrop was considerably thicker than the wire or sphere.



You are probably thinking about the following diagram from Doug Malewicki's Estes TR-10 report.

[rocketguy101]

You are probably thinking about the following diagram from Doug Malewicki's Estes TR-10 report.
That presents the same idea, but the figure is in Shapiro's "Shape and Flow" (figure 72). I scanned it--not the best quality, but the paperback book is nearly 30 years old, and I didn't want to get aggressive trying to flatten it in my scanner. This is an excellent reference, and is available on Amazon (probably others too).

I posted a second figure, showing the relation of the forces acting on the teardrop. I wonder if Rocksim takes these into account?

[zway2b]

Saw a report on the web that Rockketsim assumes the area of the nose cone is flat so no shape consideration is really considered. This may be old and corrected in the lastest version.

I still want to true both designs with roughty the same length and weight to see the results.

Built a couple of paper models tonight. The Lemanski Dart and the Fliskit Tour De Duece. They turned out pretty well but no where near competition class. I thought I might design a eggloft style Dart to compare with the original to see if this concept has any merit. I think using a transition style body tube with a motor mount and a parabolic nose cone might be pretty easy. Can you use a nose cone shaped from styrofoam or is that asking for trouble? Excuse me I am very new at this.

Thanks Dave.

[rocketguy101]

Can you use a nose cone shaped from styrofoam or is that asking for trouble? Excuse me I am very new at this.

Thanks Dave.

Many folks make nose cones from foam--the type used for insulation that you buy from Home Depot, etc. It is either blue or pink. Search on this forum for threads about it--I am not positive there are any here, but I know there are some over at The Rocketry Forum. You might peruse through Rocket Team Vatsaas' (http://www.vatsaas.org/rtv/) website--they make entire rockets from foam.

[Solomoriah]

There's an article in Model Rocketry Magazine (one of the last four I think) that describes using thinned epoxy to finish styrofoam. The original article was aimed a boost gliders, but the technique should apply just as well to a nose cone.

[zway2b]

Ok so I have my Lemanski Dart completed. I will weigh it on the scale at work tommorrow. It is a .736 body tube roughly 10 inches tall. I also have completed the template for a modified Lemanski Dart using the same fin style, launch lug, with a tapered body tube. The original dart has a conical nose cone.

I want the test to be as good a representation as I can so here are some questions.

I think I need to use a spherical nose cone on both models to maximize the Cd reduction. Correct?
I also think I should allow the original to weigh proportionately less because it has a smaller diameter body tube and nose cone. Or would it be better to have them weigh the same?

I am going to launch with A8-3's first flight and then use the best B sized motor based on altitude prediction from weight and diameter.

Sound right?

Thanks Dave

[rocketguy101]

zway2b, I am not clear on what you are trying to test. I don't believe a minimum diameter rocket will have less drag than a larger-diameter rocket w/ a taper. There could be some exceptions--I recommend you go to Estes' Educator Site (http://www.esteseducator.com/cfusion/publications.cfm) and download TR-10 (Altitude Prediction). In the first part, there is discussion of "ballistic coefficient" which may answer your question about how much the two rockets should weigh--if you are trying to compare the two shapes directly, I believe you need to make the betas equal .

Drag coefficient isn't your only consideration--frontal area plays a part in the drag calculation.

Where your tapered design will shine is (as mentioned before) where your rocket has a diameter dictated by a payload: egglofting, parachute duration, etc. The payload diameter is (usually) larger than the motor diameter, and you would gain by having a taper from the payload diamter to the engine diameter. If you are building a purely altitude bird, then (IMHO) a minimum-diameter bird to fit the motor is the way to go.

Just my $0.02

[zway2b]

Rocketguy101. Thanks for the reply. I am going for purely altitude with a 18mm engine. All the data does suggest that a minimum diameter body tube with competition style nose cone and fin are the best alternatives. I found one reference "sited early on in the thread" that stated a tapered body tube using a 32.5mm nose cone with a 18mm engine block would out perform a traditional 18mm body style.

I realise the additional surface area is a factor in the altitude equation, and I have not crunched the numbers manually but none of the altitude prediction software that I have does not take into account different nose cone shapes or tapered bodies.

Looking at it purely from a mathematical model it doesn't pan out. The 3.2 times additional nose cone area will completely wipe out the benefit of the Cd being halved, but this report predicts that the tapered body will acheive a higher altitude. Sounds like "an internet truth" but I want to test it.

Thanks Dave

[rocketguy101]

Sounds like "an internet truth" but I want to test it.

Thanks Dave
Cool! Mythbusters YORF style! Keep us posted.

[zway2b]

Ok I have a couple of test models almost complete. I still need to do a stability test on both and try to get the weight right proportionally before I test but here they are. I hope this image comes through.

Dave

[A Fish Named Wallyum]

Ok I have a couple of test models almost complete. I still need to do a stability test on both and try to get the weight right proportionally before I test but here they are. I hope this image comes through.

Dave

The one on the right looks like the Semroc Mark II from the neck up. :D