OpenRocket Needs Surface Roughness of Painted Surfaces

[H. Craig Miller]

We're looking into adding additional Component finish settings; the current selections are rough, unfinished, regular paint, smooth, and polished. The thought is that the selection names would be changed to better describe what the surface roughness is like. For example, regular paint could be changed, and additional selections added, so that the new selections would be flat paint, gloss paint, and high gloss paint. The problem is determining what the surface roughness is for different types of paint so that OpenRocket can use that data to calculate the correct Cd.

Does anyone have or know of where to get a chart (or any data source) showing the surface roughness of different types of paint?

 

[Handeman]

I don't know where to find that, but is there really a need for that many different surface types? Is there really that much difference between them and does that amount of granularity really matter considering the wide range of other variables like motor thrust? Seems like you're looking at the trees and not the forest.
Just my opinion...
It just seems like there might be other areas to look at that could provide a greater improvement to accuracy then finish smoothness, like base drag. Seems like we shouldn't have to use the base drag hack to get accurate sims.

 

[Hobie1dog]

an existing article referencing OR and this subject:
http://rocketn00b.blogspot.com/2016/11/the-weight-of-paint-part-3-surface.html

 

[Handeman]

an existing article referencing OR and this subject:
http://rocketn00b.blogspot.com/2016/11/the-weight-of-paint-part-3-surface.html

That is interesting and seems very theoretical, but I've left blue painter tape loosely stuck to the fins when I've launched my HPR and after 3000 ft and +250 mph, the tape is still stuck to the rocket after the flight. That indicates to me that if the Latimer flow can't blow a piece of low tack blue painters tape off when going +250 mph, the paint surface finish shouldn't have much impact on the actual altitude the rocket reaches either. Just my observations...

 

[rharshberger]

I don't know where to find that, but is there really a need for that many different surface types? Is there really that much difference between them and does that amount of granularity really matter considering the wide range of other variables like motor thrust? Seems like you're looking at the trees and not the forest.
Just my opinion...
It just seems like there might be other areas to look at that could provide a greater improvement to accuracy then finish smoothness, like base drag. Seems like we shouldn't have to use the base drag hack to get accurate sims.

Maybe just a slider in the "roughness" section, it centers on whatever the mid level Cd currently is, but by moving the slider the altitude can be "dialed" in to the actual flight numbers (not that Cd is the only thing affecting altitude). Finish smoothness iirc are measured in microns....maybe....

 

[H. Craig Miller]

Does anyone have a copy of "Topics in Advanced Model Rocketry" by Gordon K Mandell, George J Caporaso, William P Bengan, by any chance?

 

[SiboVG]

Does anyone have a copy of "Topics in Advanced Model Rocketry" by Gordon K Mandell, George J Caporaso, William P Bengan, by any chance?

+1, would be interested in this as well, or a suggestion on a similar book.

 

[Josh Anderson]

Does anyone have a copy of "Topics in Advanced Model Rocketry" by Gordon K Mandell, George J Caporaso, William P Bengan, by any chance?

I've got a copy of it. Can look something up in it if you need

 

[H. Craig Miller]

I've got a copy of it. Can look something up in it if you need

That would be very helpful, I'll send you a private message.

 

[steveh.jae]

Does anyone have a copy of "Topics in Advanced Model Rocketry" by Gordon K Mandell, George J Caporaso, William P Bengan, by any chance?

There may be an electronic copy of this available

 

[Neutronium95]

I typically don't touch the surface roughness when simulating a new design. I typically just use it as a way to tweak performance so that the simulation matches the real life data better.

 

[Bill S]

I saw this article years ago, from the Rocketnoob, that I thought was interesting... concerns surface texture and altitude.

Weight of paint, part 3

Weight of paint, part 4

 

[Banzai88]

I typically don't touch the surface roughness when simulating a new design. I typically just use it as a way to tweak performance so that the simulation matches the real life data better.

Same. It's one of the last things that I mess with, and my sims are routinely within 3%, many times within 1%.

 

[Handeman]

Same. It's one of the last things that I mess with, and my sims are routinely within 3%, many times within 1%.

For one motor, or a range? My last rocket in OR was <1% for the I1299N, >3% for the I327DM, but >10% for the I180W.

What do you tweek to get it close over the full range of motors flown?

 

[Banzai88]

For one motor, or a range? My last rocket in OR was <1% for the I1299N, >3% for the I327DM, but >10% for the I180W.

What do you tweek to get it close over the full range of motors flown?

Full range about 3%, the 1% window is rocket/motor specific and until I fly a range of them, there's not really a way to predict other than data correlation. The other thing that I find is that AT reds and whites are SUPER consistent across any motor diameter, and anything smoky or sparky an order of magnitude less so, which also complicates things somewhat. This season I'm going to fly a lot of Super Thunders in 54 and 75mm, so we'll see how those go!

I fly a LOT of reds, so on any given sim I tend to tweak to my core of motors that I like for a particular rocket and accept that there will be greater variability outside that 'tweak zone'. If you chase the rabbit on this, you'll be as mad as a hatter.

Regardless, 3% is probably within the noise of allowed motor variability. The other source of variability that I see in my data sets is the reporting of any pair of flight computers within a specific rocket. My pairs of RRC3s usually report within several hundred feet of each other, my single pair of SLCFs report within 50-60 feet of each other, my gen 1 and 3 AltusMinis are also within 50-60 feet of each other, and my gen2s are always within 10 feet of each other.

So, sampling size and data collection source both seem to matter!

ETA: I just looked at my flight records for the AT I180W as it pinged something in my memory. I see now that I have flown 5 of them across 2 rockets, and have a LOT of variability in that particular motor.