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At Cato 77 I was surprised to see my boat tail A SD with a launch lug clearly outperform Jay Calvert's pistoned minimum diameter model in terms of altitude. We had identical times of 67s, but my streamer was so tightly pleated that it fell much faster. We were both using A3-4Ts--the only sensible choice for this contest. I remember John Buscaglia of CMASS telling me that my rocket actually went higher than Jay's. John drove all the way to Connecticut to get some practice in before NARAM.
 
That's interesting...with a launch lug even! What was the diameter of your model (his was obviously ~.5")? Were there any obvious differences in finish (polished, naked, ...)?

Ya know, it would be fun to put it to the test with an altimeter. Min diam vs. boat tail, same weight, several flights each, same conditions. What's the answer?
 
Originally posted by illini
One request for clarification from DynaSoar: When you mention the area rule in your posts above are you referring strictly to supersonic rockets, or are you thinking that there's some benefit to the area rule for subsonic rockets as well?
Click to expand...

It applies to transonic, which I've seen defined as Mach 0.85 to Mach 1, and also as "an airflow like the speed of sound". My assumption is that the fluctuations in pressure due to vehicle shape will cause the airflow in some places to approach Mach despite lower velocity of the vehicle. BTW, Mach 0.85 at sea level is only 650 MPH.

I do theorize that it may have some benefit for subsonic also, in reducing the tendency for the airflow over the fins to spill outwards. I'm thinking a low pressure area caused by the boat tail might keep the airflow over the fins to remain more parallel to the major axis.

I definitely need to get AeroCFD. What I'd *really* like is plans for a decent wind tunnel.
 
Originally posted by JRThro
What I've gathered from all this, and I'm veering dangerously close to the topic of techniques here, is that I should buy TWO nose cones whenever I build a non-minimum-diameter rocket, and convert the second one into a boat tail.

That would seem to be a very simple way to get a reasonably optimum boat tail shape, without having to build one up from cardstock or something. PLUS it would avoid the dreaded sharp angle between the body tube and the boat tail.

DynaSoar's description of how he did it is what I have in mind here.
Click to expand...

I've made several just that way.

In fact PML's 2.1" through 7.5" boat tails (not tail cones) are made from their own nose cones.

Here's a picture of the motor/fin/boat tail assembly prior to inserting it in the body:
 
I seem to recall that Jay always used bare fins.

My rocket was covered with black Japanese tissue--I called it the black Ninja. Diameter was around 0.6 inches. As it had a balsa body tube, I doubt it weighed any more than Jay's model.

Looking at DeMar's model, it seems like he used a 26 degree taper for his boat tail--2:1 ratio. Anyone with a better interpretation of 2:1 5% boat tail?
 
Originally posted by DynaSoar
I do theorize that it may have some benefit for subsonic also, in reducing the tendency for the airflow over the fins to spill outwards. I'm thinking a low pressure area caused by the boat tail might keep the airflow over the fins to remain more parallel to the major axis.
Click to expand...

I'd take issue with calling it area rule in the subsonic case just because the area rule has to do with wave drag associated with trans/super-sonic flight. But I believe you are correct that the lower pressure in that region would at least partially have the effect you describe.
 
quote:
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Originally posted by Henry8minus1

After looking at Dr. Derek Brays power point slides that Bob pointed us to I wanted to bounce my understanding of what happens during the thrust phase. According to pages 34-35 of this presentation about base bleed I conclude that during thrust the base drag is reduced because of the pressure of the thrust. Do I understand this correctly?

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Much of Dr. Bray's power point presention on drag has to do with drag reduction on projectiles, not rockets. Once a projectile leaves the gun barrel, it can only slow down.

To increase the range you have two choices: increase the muzzle velocity or reduce the drag. You can increase the muzzle velocity by increasing the barrel length which is easy and is done on most modern artillery pieces, and/or by increasing the powder charge which increase the pressure in the barrel which dramatically reduces the barrel life-time so this is not done.

Since drag increases as velocity squared, unless the drag coefficient is reduced substantially, you get little addded range by increasing the muzzle velocity, so drag reduction is critical.

Adding a 0.5 calibre boattail typically reduces the drag coefficient by ~50%, but you still have a base vacuum that is generating a lot of drag. Base bleed is the burning of a solid propellant to generate gas to fill in the vacuum and thereby reduce the base vacuum and therefore the base drag. The gas evolution rate is relatively low and it adds no thrust to the projectile. Modern artillery shells with a boattail and base bleed have a drag coefficient that is 80% lower than the old standard flat bottomed projectile.

These drag reduction techniques apply equally well to rockets, however the overall cd for the rocket includes the drag of the fins which can be substantial, so a simple cd based on the rocket diameter will not decrease the overall drag as much as you might expect.

Also there are big differences between subsonic and supersonic fins. Low drag fins designed for use below M=0.7 are relatively thick with chords up to 0.2 or so. Low drag fins for M>1 are thin with chords < 5%.

quote:
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Taking this into account then a boat tail rocket with a larger diameter would be less advantageous during thrust than a minimum diameter rocket because both would reduce drag from the thrust, but the boat tail would have a larger cross sectional area. So the boat tail rocket would have to make up for this during the coast phase if it were to perform better. Is this a correct deduction, or am I understanding this wrong?
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No necessarily. If the Cd bt * Area for the boattailed rocket is lower than the Cd md * Area of the minimum diameter rocket, then the boattail wins.

I think you will find that after doing the math, the boat tail wins every time, both on boost and coast.

also to JRThro

You are correct. Recessing the rear centering ring on a rocket increases the base drag, and additionally creates a recirculation zone for the hot exhaust plume to cook the base of the rocket. I don't understand why some designs have this terrible feature. It's better aerodynamically to have the motor tube extend from a flush rear ring since it acts as a psuedo boattail. It does however move the CG rearward, so you don't want to get carried away with the projection.

Bob Krech
 
I measured the Black Ninja. The diameter is 0.66 inches. It is 12.3 inches long--rather long for A SD. The weight with the nose cone and shock cord but no streamer is 3.7g--probably a little less than the typical A SD model. My ASP Duration 13 weighs 4.4g. The nose cone is bare contest balsa, sanded smooth with fine sandpaper in a lathe.

The previous year I won A SD with a BT-20 boat tail model with a more efficient streamer--it did 88 seconds early in the day--landing behind tall trees. The 2nd place model had a time of 87 seconds.
 
I crunched out a few numbers for comparison with min diameter models.

First, inner and outer diameters of common tubes:

Code: 
BT-5:  ID - 0.515", OD - 0.541"
BT-20: ID - 0.71", OD - 0.736"
BT-50: ID - 0.95", OD - 0.976"

Assuming the lower diameter of the boat tail is the inner diameter of the tube, here are the upper diameters for 9-degree boat tails at 3 calibers (assuming you rolled your own tube as Zack Lau did):

Code: 
Caliber:     0.5       1           1.5
BT-5:        0.559     0.612       0.675
BT-20:       0.771     0.844       0.931
BT-50:       1.032     1.129       1.246

Here is the percent area increase this represents relative to the outer diameter of the standard tube:

Code: 
Caliber:    0.5         1         1.5
BT-5:       6.9%        27.9%     55.9%
BT-20:      9.8%        31.4%	  60.1%
BT-50:      11.7%       33.8%	  63.0%

Bottom line: If you do a 1/2 caliber boat tail at 9 degrees, the area increase is pretty moderate for the tubes here. If the Cd reduction is better than 12% - which is likely - then you have reduced the overall drag by using the boat tail, even though the area has increased. No wonder Zack Lau's SD model did so well.
 
Excellent link. Thanks! I'm printing that puppy out and going over it today.
 
Originally posted by illini
I crunched out a few numbers for comparison with min diameter models.
Click to expand...
illini,
Thanks for the detailed analysis.

To all,
I am a bit confused about this thread. Are we talking about transition sections at mid body, boat-tail "cowling" at base of rocket, or both?

Thank you.
 
Originally posted by Mike_BAR

To all,
I am a bit confused about this thread. Are we talking about transition sections at mid body, boat-tail "cowling" at base of rocket, or both?

Thank you.
Click to expand...

Mike, we are talking about a boat tail at the base of the rocket as a means of reducing base drag.
 
Originally posted by illini
(snip)
Assuming the lower diameter of the boat tail is the inner diameter of the tube, here are the upper diameters for 9-degree boat tails at 3 calibers (assuming you rolled your own tube as Zack Lau did)
(snip)
Click to expand...
Illini,

OK, I now follow the general discussion in a better light.

Back to your calculations, I am confused by the 3-caliper figure. Where does that fit into the equation? Your data points are for the 0.5, 1.0 and 1.5-caliper samples.

Is the shape of the conical boat-tail based on a 9-degree slope, and a length of 3-calipers?

Again, thanks again for grinding out these numbers.
 
Sorry. What I should have said was "at 3 different calibers" meaning that 0.5, 1.0, and 1.5 were the three calibers I calculated. So 3 boat tails were considered, each with 9 degree slope: 0.5, 1.0, and 1.5 calibers.
 
Originally posted by Mike_BAR
illini,
Thanks for the detailed analysis.

To all,
I am a bit confused about this thread. Are we talking about transition sections at mid body, boat-tail "cowling" at base of rocket, or both?

Thank you.
Click to expand...

GENERALLY SPEAKING FORWARD FACING COWLINGS CAN BE STEEPER THAN REARWARD. THE OPTIMUM SUBSONIC SHAPE IS A TEARDROP AND THE OPTIMUM TRANSSONIC SHAPE IS AN OGIVE.

MY PRIOR RULE OF THUMB POSTING WAS HALF ANGLES. THAT IS, THE ANGLE OF EACH SIDE OF A SHROUD.

AND NO I AM NOT SHOUTING.

JERRY
 
Originally posted by Zack Lau
At Cato 77 I was surprised to see my boat tail A SD with a launch lug clearly outperform Jay Calvert's pistoned minimum diameter model in terms of altitude.
Click to expand...

I think I saw this in action today. I flew the rocket with the boat tail that I posted the picture of. Although it's 7 feet long, it's extremely light, being all Estes parts. I put an F21-7 in it and it went up, and up, and up. It didn't go very fast, but it just kept soaring up. It was still going when the ejection went off. It was so light (7.8 oz. before the motor) that I thought it would have lost far more momentum. It was almost as if it had some lift, but probably more just a lack of base drag after burn out as compared to most other things I flew.
 
This is just too cool. I'm half tempted to buy an altimeter just to test this.
 
Originally posted by illini
This is just too cool. I'm half tempted to buy an altimeter just to test this.
Click to expand...

I'm not tempted to do that, but I am tempted to build two as-identical-as-I-can-make-them (i.e., not very) models, one with a boat tail and one without, and then fly them repeatedly and record the results.

Now, should I build two identical models and add a boat tail to one of them, or build the to-be-boattailled model shorter than the non-boattailed model by the length of the boat tail, so the overall lengths are the same?

illini and everyone else, this IS a great topic!

:)
 
Here's a suggestion: Build 3 models as similarly as possible:

1) Use Totally Tubular T-20+ with a 1/2 caliber boat tail down to engine diameter (note that this is the correct tube size for a 9 degree boat tail in the table above).

2) Use Totally Tubular T-20+ without a boat tail

3) Use BT-20 to build an equivalent min diameter model.

Make the tubes all the same length. The 1/2 caliber boat tail added to model 1 isn't adding much skin friction...no big deal that it is longer (certainly within the noise for this test). Make all 3 the same weight. When flying, weigh the motors first to get them as close to each other as possible in weight. Ensure that the rod angle is 0 degrees (not sure how to do this, by the way). As best as possible, try to record atmospheric conditions for each flight...pressure and temperature. Anything else I'm missing? The test will be noisy and you probably won't get enough flights for statistical significance, but hopefully the qualitative trends will emerge.
 
Ya know, I woulda thought the previous suggestion would have been a good one for a test. But then I RockSim'd the Pratt Super Six (which uses a T20+ tube) with and without a 1/2 caliber 9 degree boat tail and it made no difference. According to RockSim, the base drag of the Super Six is only about 5% of the overall drag, so even though RockSim (and the equations in the Pantalos paper) predicted a 23% reduction in base drag, it just didn't matter. Even if RockSim and Pantalos are off by a factor of 2, it still doesn't matter.
 
One thing I have always wondered about drag reduction using boattails is, does the actual exhaust plume of the motor have something to do with the amount of drag reduction via a boattail.


Also Lets say you recess the motor perhaps 1/8" into the engine tube and then drill some small diameter holes around the engine tube....If you assume that the airflow is laminar until it hits the fins where it becomes tubulent flow...

What I am suggesting is there may be some simple way to increase the boattail effect because we are basically working here with pressure differences.....
 
Originally posted by shockwaveriderz
One thing I have always wondered about drag reduction using boattails is, does the actual exhaust plume of the motor have something to do with the amount of drag reduction via a boattail.
Click to expand...

That's a good question and one I've been thinking about too. Both the Pantalos and John DeMar papers took data in a windtunnel. Great, highly controlled, but obviously no plume. Certainly during thrusting conditions that test data doesn't fly (pun intended). What about coast with a smoke delay being exhausted from the motor? According to the Brays powerpoint link, "base bleed" is a good way to reduce base drag...is the smoke delay enough to create some kind of base bleed effect?


Also Lets say you recess the motor perhaps 1/8" into the engine tube and then drill some small diameter holes around the engine tube....If you assume that the airflow is laminar until it hits the fins where it becomes tubulent flow...

What I am suggesting is there may be some simple way to increase the boattail effect because we are basically working here with pressure differences.....
Click to expand...

I'm not sure if what you're suggesting would work, but I think what you're after is a way to create the base bleed effect, no? Again, not to slam Pantalos and DeMar (hey...its infinitely more data than I've collected on this), but I do wonder how much flight conditions would affect their conclusions?
 
Originally posted by illini
Ya know, I woulda thought the previous suggestion would have been a good one for a test. But then I RockSim'd the Pratt Super Six (which uses a T20+ tube) with and without a 1/2 caliber 9 degree boat tail and it made no difference. According to RockSim, the base drag of the Super Six is only about 5% of the overall drag, so even though RockSim (and the equations in the Pantalos paper) predicted a 23% reduction in base drag, it just didn't matter. Even if RockSim and Pantalos are off by a factor of 2, it still doesn't matter.
Click to expand...

T20+ telescopes over T20, right?
Not much of a diameter difference.

Something with a big flat butt would probably show a difference, especially if the original design called for a recessed aft ring.

T60 (1.6") body and 18mm motor tube. That's a difference. Slap a cored plastic nose (ogive; we're taking subsonic) on the bottom of a Big Bertha-like bird. Don't even control for the extra weight; make it "conservative". Just have the motor tubes hanging down the same distance.
 
Yup, I agree. What I was trying to compute was a reasonable 1/2 caliber boat tail with 9 degree slope for some kind of controlled test. Frankly, we have some conflicting pieces of information that I'm not sure how to reconcile.

Brays: 1/2 caliber boat tail with 9 degree slope is optimal for subsonic. Anything longer is diminishing returns.

Pantalos: 5 - 7 degree slope is optimal. Doesn't mention boat tail length. But notes that theory is:

Base drag with boat tail = base drag without boat tail * (base diameter / body tube diameter)^3

The book "Supersonic and Subsonic CTOL and VTOL Airplane Design" by Gerald Corning says that the above equation is the result of wind tunnel tests, not derived strictly from theory.

Barnes McCormick (Book: "Aerodynamics, Aeronautics, and Flight Mechanics"): Quoting - "...the shape of the base affects the flow over the rest of the body ahead of it, so the base is viewed as simply an integral part of the overall pressure drag." In other words, he punted.

So, just to summarize - one piece of information suggests boat tail length is important. Another suggests the overall area reduction is the key. Together they recommend boat tails from 5 to 9 degrees. But a 1/2 caliber boat tail isn't offering much area reduction at only 5 - 9 degrees. Would seem you need something longer. Next, all of the above is for power-off conditions. What about during thrust and coast...now how much benefit is there?
 
Barnes McCormick (Book: "Aerodynamics, Aeronautics, and Flight Mechanics"): Quoting - "...the shape of the base affects the flow over the rest of the body ahead of it, so the base is viewed as simply an integral part of the overall pressure drag." In other words, he punted.
Click to expand...

No, I don't think he punted. He just was not clear. These comments are correct but only apply to subsonic flow.

By defination in supersonic flow, aerodynamic disturbances can not be propagated forward because they are acoustical in nature and propagate at the speed of sound. As an acoustic wave, they can't pass through the shock front.

To get a good picture of what streamling and base drag is all about check out figure 23 and 24 in Dr. Bray's presentation

https://www.rmcs.cranfield.ac.uk/aeroxtra/e338drag.ppt#23

https://www.rmcs.cranfield.ac.uk/aeroxtra/e338drag.ppt#24

Figure 29 breaks up the drag coefficient into skin friction and base drag.

https://www.rmcs.cranfield.ac.uk/aeroxtra/e338drag.ppt#29

Figure 20 give an idea of the nose cone effect on drag in the supersonic regime of flight.

https://www.rmcs.cranfield.ac.uk/aeroxtra/e338drag.ppt#20

Figure 37 give the complete picture of drag components as a function of Mach number

https://www.rmcs.cranfield.ac.uk/aeroxtra/e338drag.ppt#37

Bob Krech
 
Oops! You're right...he was clearer than that. I inadvertently left out the part of his comment where he clarified he was referring to subsonic flow. And to everyone else, Bob's comment regarding the difference in disturbance propagation between subsonic and supersonic flow is worth noting. Basically what this means is that in subsonic flow the shape of the boat tail affects the flow ahead of the boat tail, while in supersonic flow this is *not* true.
 
Originally posted by illini
Oops! You're right...he was clearer than that. I inadvertently left out the part of his comment where he clarified he was referring to subsonic flow. And to everyone else, Bob's comment regarding the difference in disturbance propagation between subsonic and supersonic flow is worth noting. Basically what this means is that in subsonic flow the shape of the boat tail affects the flow ahead of the boat tail, while in supersonic flow this is *not* true.
Click to expand...

Is it true that in subsonic flow the shape of the boat tail DOES affect the flow ahead of the boat tail, or that it CAN affect it?
 
Originally posted by JRThro
Is it true that in subsonic flow the shape of the boat tail DOES affect the flow ahead of the boat tail, or that it CAN affect it?
Click to expand...

Not just can. It does. What Bob said above and what Barnes McCormick said (although I still think he punted...;) ) is effectively that. You can't just stick a boat tail on a subsonic vehicle without considering the upstream affects. More mathematically, there are 3 characteristic disturbance velocities in a flow of speed u: u + a, u, and u - a, where a is the speed of sound and u < a if we're subsonic. For subsonic flow, that last characteristic (u-a) is negative (pointing upstream) while the other two are positive (pointing downstream). For supersonic flow, all three characteristics are positive and pointing downstream.
 

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