Aerodynamic drag of body tube joints

Not sure this is the right subforum for this topic, but it is kinda simulation related.

While commenting on cable cutters, I got to thinking about one of their advertised benefits: Less aero drag because of one body tube joint, not two or three. Has anybody quantified this? For most hobby rockets, I think the difference between one joint and two is negligible. Perhaps OR, RS, and RA software can be tricked to into modeling a body joint gap/discontinuity and generating a change in Cd, but I am not sure. Anybody tried it? Wind tunnel test? CFD?

I do not believe any of the commercially available sim programs would simulate this. The caveat would be if there is a difference in body tube diameter. Even then, it is doubtful it would do so accurately with the minute diameter change.

I think if you could quantify the dimensions of the gap between the airframe sections and ran it through a wind tunnel or CFD program versus a contiguous airframe you would see a difference. How much I cannot say.

Your post made me think about using "ghost" transitions in OR. I wondered if using two transitions would be something that might answer your question so I gave it a shot. I pulled up Aerotech's stock Arreaux to try this on. Keeping a separate file for the control, I inserted two transitions at the joint. The first transition was 0.25mm long and transitioned from 1.9" to 1.8". The second transition was also 0.25mm. long and transitioned from 1.8" back to 1.9". The masses of the transitions were also overridden to 0 since they're parts with mass and a gap is not. Doing this, the transitions should somewhat simulate an imperfect body tube joint without adding mass to alter the flight statistics that way.

Both rockets were simulated with a G64 in identical conditions and the results were that the rocket with the transitions flew 70' lower than the control and had a max velocity 4 mph slower. I'll post the .ork files so that others can improve this idea or disprove it entirely. Either one is just as valuable. Enjoy. I know I did.

Works Cited:
This thread for Aerotech body tube dimensions: https://www.rocketryforum.com/showthread.php?70990-aerotech-tube-dimensions

Edit: I discovered that, for some reason, I was not allowed to override the mass of the transitions so they do add about 0.17 grams. That could account for the differences.

View attachment Aerotech Arreaux TRANSITIONS.ork

View attachment Aerotech Arreaux CONTROL.ork

HeavyLight said:

Your post made me think about using "ghost" transitions in OR. I wondered if using two transitions would be something that might answer your question so I gave it a shot. I pulled up Aerotech's stock Arreaux to try this on. Keeping a separate file for the control, I inserted two transitions at the joint. The first transition was 0.25mm long and transitioned from 1.9" to 1.8". The second transition was also 0.25mm. long and transitioned from 1.8" back to 1.9". The masses of the transitions were also overridden to 0 since they're parts with mass and a gap is not. Doing this, the transitions should somewhat simulate an imperfect body tube joint without adding mass to alter the flight statistics that way.

Both rockets were simulated with a G64 in identical conditions and the results were that the rocket with the transitions flew 70' lower than the control and had a max velocity 4 mph slower. I'll post the .ork files so that others can improve this idea or disprove it entirely. Either one is just as valuable. Enjoy. I know I did.

Works Cited:
This thread for Aerotech body tube dimensions: https://www.rocketryforum.com/showthread.php?70990-aerotech-tube-dimensions

Edit: I discovered that, for some reason, I was not allowed to override the mass of the transitions so they do add about 0.17 grams. That could account for the differences.

Click to expand...

Hey, good work HeavyLight! You are a man of action and generated some objective data for discussion!

I looked at your files. You overrride the total mass equally, so the 0.17g transition mass does not come into play. I futzed around with the sims and reached the same conclusion as you.

Even more basic, I simply looked at the drag analysis of both designs. The Transition model is generating more drag than the Control model, mostly in Pressure Drag. I was expecting more Friction Drag? Hmm.

So, it seems OR can be tricked into simulating the drag of body tube joints, with an altitude loss of about 3%. Seems plausible.

Exactly, you have to use a "trick" to get it to simulate the joint. It will not do it directly.

markkoelsch said:

Exactly, you have to use a "trick" to get it to simulate the joint. It will not do it directly.

Click to expand...

I am OK with that. The capability is there. The "trick" may eventually turn into a "standard feature" of the software.

Pretty sure that the simulated drag would be greatly overestimated. OR would be assuming that the air actually flows down and back up the transitions, in reality this would not be happening. I think you'll find the air inside the joint will pretty much travels with the rocket. There will just be some disturbance to the flow of air beside the rocket.

Buckeye said:

I am OK with that. The capability is there. The "trick" may eventually turn into a "standard feature" of the software.

Click to expand...

The standard trick seems to have too many possible variations- quality of the joint type issues. How do you account for kerf variation in the sim? Neat idea, but likely a waste of effort. I say this to mean that it will be a small portion of the drag- likely small enough that the effect will be negligible in comparison to other effects. Also, small effect when compared to the allowed variation in motors.

markkoelsch said:

The standard trick seems to have too many possible variations- quality of the joint type issues. How do you account for kerf variation in the sim? Neat idea, but likely a waste of effort. I say this to mean that it will be a small portion of the drag- likely small enough that the effect will be negligible in comparison to other effects. Also, small effect when compared to the allowed variation in motors.

Click to expand...

Of course. All simulation parameters pale in comparison to motor errors and variation.

I am interested only in Cd variation in this thread. The cable cutter and GLR Mariah shotgun advocates think there is a drag savings in their designs. I would like to know how much and if it can be simulated. HeavyLight simulated some drag changes and SpaceManMat thinks they are overestimated. Good discussion so far.

Pretty sure the only way to find out would be wind tunnel testing, unless of course there has been a previous study on this.

There are plenty of other areas that OR could be improved on to get better estimates, suspect this is one of the least important. Having just typed that I've come up with an alternative work around you might want to try.

Add a small length of body tube at the join (remove the length added from the actual BT of course) Make it fairly short, at a guess say 2 x the body tube wall thickness. Change the Finnish on this tube to be "unfinished". This will add drag for a rough surface at the join. If someone can come up with some real life data on joins then you could vary the length till it matches.

SpaceManMat said:

Add a small length of body tube at the join (remove the length added from the actual BT of course) Make it fairly short, at a guess say 2 x the body tube thickness. Change the Finnish on this tube to be "unfinished". This will add drag for a rough surface at the join. If someone can come up with some real life data on joins then you could vary the length till it matches.

Click to expand...

That is an excellent idea and I would think it would simulate the joint effects more accurately then the transitions.

So the next question becomes, how much drag do you get from unfilled spirals? Is it enough to make up for the extra weight of the filling material? Can you adjust the finish to simulate unfilled spirals verses filled ones?