Drag question

I'm in the middle of designing/tweaking/planning/dinking around with a 54mm minimum diameter rocket. I've run into a question concerning drag that I don't know the answer to.........so the obvious thing is to post it here!

Here's the thing: I've been planning on using "thinwall" glass tubing for this. I've also been trying to design this rocket to be optimized around my Loki 54/2800 case (though it will fly CTI and others as well). I found that the thrust ring on Loki cases is just a bit wider diameter than the OD of the thinwall tubing. Which means that there would be a small lip and subsequent "bump" in the airflow right at the tail end of the airframe.

So, the question is: would any of the gains of lesser drag from a thinner airframe (smaller OD than the "regular" stuff) be negated by that thrust ring hanging over the edge? Or to put it another way, would a thicker wall airframe (which would have more drag due to a larger OD) and NO thrust ring sticking out over the edge be more efficient aerodynamically?

To be sure, we are talking minutiae here - the OD of the two airframes in question is very similar, and that thrust ring sticks out just a small bit with the thinwall - but still. I'm more interested in the theory than I am absolute performance. It seems to me that little edge created by the thrust ring sticking out would cause a relatively large amount of turbulence/drag compared to a totally smooth but slightly larger diameter airframe.

Oh, and by the way, I do have a CTI 54mm tailcone rear closure, which fits PERFECTLY to the OD of the thinwall tubing. Sweet!

Thanks for any comments you might have,
s6

First, how are you retaining the motor?

I wonder if you could grind the thrust ring a bit to get the diameters to match?




Mark Koelsch
Sent from my iPhone using Rocketry Forum

You should draw your design with the correct weight of parts. If you are optimizing it for that beast of a motor, you may find the slightly heavier reg. tube might be the way to go & actually coast higher.

You must first find optimum weight before you will know if you need to add any with the thin wall or even the reg.

Last year for a drag race with 3 grain CTI white, we built a super lightweight rocket with a cast tailcone. 3in diameter Comp 3. cardboard. All the other entries were glass kits. All entries had to fit the "NASCAR" template.
Ours weighed only 4.3lbs while the others averaged 8-9lbs....we were beat by 1800 to 2400 ft! That extra weight carried them farther.

You have to optimize for a particular thrust profile if you are really trying to "dial" it in.

Yes you can grind the thrust ring, I have on several occasions, it's a bear if made from stainless though. I finally took a belt sander and clamped it in a vice... Dremel...nope.

That tail cone will give you an additional 80-100 ft per thousand!

I, too, have wondered about the "Kamm Effect" on MD birds. I love tailcones so I've cheated and done a 7:1 'shoulder' on thin wall with the 'bumps' trimmed off an Estes retainer I haven't flown yet. I also beveled the trailing edge (hey-it's plastic!). I wondered if the base drag benefits by this after burnout. It's only 29mm but I like to experiment any way I can and if I lose it without a tracker, it's all on me. I'll find out at Springfest in Jean next year. Certainly as Jim points out optimizing for mass is crucial to maximum altitude.

Mark - I'd be friction fitting for this rocket, with the shock cord attaching to the forward closure so even if the motor did pop loose it wouldn't be going anywhere.

Jim - totally understood about optimum mass and will be taking that into account. That said, for this thought-experiment, I'm trying to isolate just the drag issue.

So, with that in mind, let me rephrase the question:
* A given pair of rockets with equal mass, the same fins, the same nosecone profile, length, etc.
* One of the rockets uses a standard thickness fibreglass airframe (and therefore a slightly larger OD) and the thrust ring OD is less than the OD of the airframe.
* The other rocket uses a "thinwall" glass airframe (and therefore a slightly smaller OD), but the thrust ring OD is larger and therefore it sticks out and forms a small lip at the tail edge of the rocket.
* Which rocket has more drag and/or negatively-affecting turbulence?

The aerodynamics that I don't fully understand are those that are happening right at the tail end of the rocket. I do know that tailcone has a dramatic impact, so I've got to think that a small lip would also be a big factor. But would it be enough that a thicker diameter airframe would actually be better?

s6

Stealth, apologies if I derail this thread for a moment, but you mention that you are friction fitting a Loki 54/2800 into a minimum diameter case. This is pretty much exactly what I did with my Blackhawk 54 a few days ago

Anyways, what exactly are your plans for attaching the shock cord to the forward enclosure? Are you simply removing the thumb screw and screwing in an eye bolt to the forward enclosure? Or does Loki make forward enclosures with attachments? I considered screwing an eye bolt into the forward enclosure, but I was worried about the threads/hole being too small and ripping the screw out. Maybe it might be fine though.

Any description of your plans to do this would be appreciated

I've taken my fair share of aerodynamics/fluid dynamics classes, so I feel moderately qualified to comment on the theory here. The difference in frontal area (for base drag) and wetted area (for friction drag) are going to be approximately zero, so I don't think you'd see a difference in drag if you stuck the two (sans motor) into a wind tunnel. However, with the motor case forming a small ridge in the aft, you would create a non-smooth discontinuity in the body. (I'm supposing that there is no discontinuity between the nosecone and the air frame on both cases. Correct me if I am wrong.) I would say that non-smooth ridge is going to contribute more to the drag than the delta in airframe diameter. That all said, unless it's more than 1/16th of an inch (arbitrary), I really doubt either will have a significant impact on the drag.

I am no expert, but how close are the fins to end of the tube?

As the airflow passes the fins it creates non-laminar airflow and if they are close to the end of the tube, the lip on the thin wall tubing with the Loki Case may not make a difference as there is a disruption of laminar airflow.

Just a thought.

Non-flush rivets with their heads protruding from the surface in the tail cone area, are often seen on large aircraft. The idea is that the protruding head does not add significantly to drag in the tail cone, and is cheaper to manufacture. Why? Well, the idea is that the turbulent airflow off of the rivet head occupies a bit more airstream than the uninterrupted flow does in the critical zone where air has to rush back in to fill the void of the departing airframe.

jmattingly13 said:

I've taken my fair share of aerodynamics/fluid dynamics classes, so I feel moderately qualified to comment on the theory here. The difference in frontal area (for base drag) and wetted area (for friction drag) are going to be approximately zero, so I don't think you'd see a difference in drag if you stuck the two (sans motor) into a wind tunnel. However, with the motor case forming a small ridge in the aft, you would create a non-smooth discontinuity in the body. (I'm supposing that there is no discontinuity between the nosecone and the air frame on both cases. Correct me if I am wrong.) I would say that non-smooth ridge is going to contribute more to the drag than the delta in airframe diameter. That all said, unless it's more than 1/16th of an inch (arbitrary), I really doubt either will have a significant impact on the drag.

Click to expand...

I agree with jmattingly. I would guess that the flow trip due to the retainer would be slightly more draggy than the additional wetted drag from a larger airframe. I don't think any of our hobby drag predictors would capture this? Probably need a high-res wind tunnel or CFD.