Designing a fast aerodynamic rocket

[Jeff Lassahn]

One thing to be aware of, OpenRocket doesn't really have much more knowledge of drag effects than you get from applying the suggestions in this thread. It's got some basic rules coded into it about how much drag you get from various nose and fin shapes, etc. but it's not got any magic fancy boundary layer CFD simulations or whatever that would give really accurate drag numbers.

There's sometimes a tendency to believe that twiddling details of your design in OpenRocket until it gives you the best reported coefficient of drag is the same as making an optimal real rocket. It's not, it only gets you kind of close.

Drag is notoriously hard to model with computers, much harder than lift, thrust, and so forth.

 

[Rocketeer123]

Do you suggest I use RockSim (I was thinking about buying it this would be one more reason to)

 

[dr wogz]

Totally disagree on the "no paint" idea. A mirror-finish paint job (like a Nathan build) is WAY less draggy than raw cardboard/FG/CF/whatever.

you have a point! was thinking glassine tubes, and paint adds weight. But I guess it is minimal at this point!

 

[neil_w]

With a Q-Jet or that Aerotech D24... Neil, what is the speed of balsa?

One way to find out! Balsa is probably not the right choice here if you want to keep the fins really thin to minimize drag (although maybe if papered they could survive, dunno).

 

[jqavins]

Do you suggest I use RockSim (I was thinking about buying it this would be one more reason to)

There are pros and cons, but drag calcuation is not among them.

 

[Rocketeer123]

So is there any way to see accurate drag effects of picking the shapes of nosecones and fins people advised me to do

 

[Igotnothing]

Performance is correlated to loss.

 

[jqavins]

So is there any way to see accurate drag effects of picking the shapes of nosecones and fins people advised me to do.

The only definitive way is to build a thing and put it in a wind tunnel. Short of that, drop tests, and you still have to build it before you can measure Cd. That's why (drag and other measurements) even giant aerospace companies put models in wind tunnels. And even then it's not perfect.

To do it predictively on your computer, for all practical purposes, no.

You can come a lot closer than OR and RS do by an analysys method called Computational Fluid Dynamics (CFD) which applys the analytical tecnique more generally known as Finite Element Analysis (FEA) to the forces and flow of air around an object (among other things). This used to take a supercomputer, but nowadays we have that much computing power on our desks, and indeed there are CFD programs available for hobby use, such as AeroCFD.

Even then, drag will be better, but not perfect. To improve it further, you need very accurate measurements of the surface texture, which are really, really hard to make.

Here's the good part of the bad news. Such a level of precision in your analysis doesn't matter. Variations in engine performance and weather will swamp out such small refinements of the simulation. So don't fret it. A drop test can be useful for some purposes, like trying to hit a particular altitude on the nose, not under or over, adjusting weight to get there. For your goal, just follow best practices and get what you get.

 

[Steve Shannon]

So is there any way to see accurate drag effects of picking the shapes of nosecones and fins people advised me to do

The drag coefficients of many shapes at various velocities have already been cataloged and graphed by NACA and its successor organization NASA. Also, look for a very good paper written by Gary Crowell that I believe was available through the NAR.

Google “NACA Airfoils” and also see this page: https://www.pdas.com/naca456.html

 

[Bat-mite]

Same rocket, same motor, launched five times, get five different results.

I saw two identical rockets drag-raced, same motor and everything. O motors, in fact. One had a tracker, one did not. The flyer assumed "mine will land next to hers." Both launched, hers was found, his was nowhere in sight. I don't think it was ever found.

 

[Rocketeer123]

I came across this NASA article about measuring drag in wing tunnels: https://www.grc.nasa.gov/www/k-12/rocket/dragdat.html

I'm no aerospace engineering so It's going to be hard to do this, but is my best bet to build a wind tunnel and test various nose cones measuring their drag coefficient

 

[jqavins]

The Van Milligan girls beat you to it. You can read the article here.

But remember, once again, the nose cone is but one contributor to the rocket's drag, and far from the biggest. The fin airfoil is much more important, and the body tube diameter is far, far more important yet. The finish quality is more important. The fillet quality ks more important, and the fillet size is probably just as important as the nose cone shape.

For the fin airfoil, consider a range of shapes from squared off to ideal. Simply rounding all the edges gets you a lot of the way, perhaps most of it. Rounding the leading and tip edges while tapering the trailing edge has you nearly all the way. The perfect shape (which I tried to describe above) gets you so little more that it will be lost in the noise when the effects of engine variation and weather come into play.

If fiddling in the margins and below the noise floor is fun, go for it. Otherwise it's really a waste of time. Use a long elliptical nose cone, design small, build light, round and taper the fins, finish smooth, and launch from a piston. All the rest won't make any practical difference in the end.

 

[boatgeek]

One more important factor--your fin alignment needs to be spot on so your fins aren't fighting each other. I'd put that below body tube diameter and fin airfoil, and maybe about even with fin thickness/shape/taper. It's definitely above body finish and fillet size/finish.

 

[jqavins]

Excellent point. I took that for granted, but I should not have.

And I just thought of another possible improvement. Some airliners use small winglets, sometimes in V shaped pairs, to reduce drag by reducing wing tip vortices. Might that be a good idea on a rocket's fins tips? Would it make a bigger difference than some of these other things?

 

[dr wogz]

And I just thought of another possible improvement. Some airliners use small winglets, sometimes in V shaped pairs, to reduce drag by reducing wing tip vortices. Might that be a good idea on a rocket's fins tips? Would it make a bigger difference than some of these other things?

aren't tip vortices the results of the two airstreams (upper wing surface / lower wing surface) meeting up, but at different velocities / pressures?

 

[GrouchoDuke]

aren't tip vortices the results of the two airstreams (upper wing surface / lower wing surface) meeting up, but at different velocities / pressures?

Yes. I wouldn't recommend winglets on a rocket.

 

[jqavins]

OK, I learned something today.

 

[dhbarr]

aren't tip vortices the results of the two airstreams (upper wing surface / lower wing surface) meeting up, but at different velocities / pressures?

Yes. I wouldn't recommend winglets on a rocket.

At nonzero AoA wouldn't this be a useful feature? I haven't really thought about it, thanks for the brainfood.

 

[GrouchoDuke]

At nonzero AoA wouldn't this be a useful feature? I haven't really thought about it, thanks for the brainfood.

I'm sure someone could do a PhD thesis on an edge case of rocket fin design where this could maybe be slightly beneficial. They're just not the right tool to throw at minimizing rocket drag though. If you're doing it right, your fins are flying at near zero AOA for the high drag portions of your flight. The extra weight, build complexity, landing damage risk, and varying airflow flow throughout a flight will make winglets more trouble than they're worth. On top of that, designing a proper winglet isn't a TLAR kind of thing.

That said, if someone wants to make a few test rockets & get some flight data on it, go for it.

 

[Rocketeer123]

I was wondering if everyone here majored in aerospace engineering or something similar. Everyone here seems so knowledgeable.

 

[Scott_650]

I was wondering if everyone here majored in aerospace engineering or something similar. Everyone here seems so knowledgeable.

Well, I can’t speak for anyone else but I’m no engineer of any kind. I have built a fair number of rockets, though I have little practical interest in performance - I rarely even round fins - the higher they fly the longer the walk. But I do have a solid background in math and problem solving plus I love an intellectual excursion like this. A smoothly finished minimum diameter 18 or 24 mm rocket with tapered/rounded thin G10 fins would be a fun project. One thing I’d be curious about would be going up a tube size and using a boat tail versus a straight min diameter. Trying for max altitude on the smallest motor sounds like my kind of goal more than just maxing out...

 

[David_Stack]

Maybe take a known 'proven design' that was designed for performance, already has the right-sized body tube, and start with that in the simulation program of your choice...

I'm thinking Estes Astron Sprint. BT-50 body tube, elliptical fins, elliptical nose cone. As designed it has an 18mm motor mount and a BT-50 - BT-20 boat tail, but you could omit the motor mount and the boat tail, install a thrust ring, and call it done... Or leave it as is, and use one of the 18mm composite D motors mentioned previously.

https://www.rocketryforum.com/threads/estes-astron-sprint-k-49-or-1249-gallery.19526/

 

[jqavins]

I was wondering if everyone here majored in aerospace engineering or something similar. Everyone here seems so knowledgeable.

Far from it. Note that I expressed a mere notion based on a single observation, and people more knowledgeable than I quickly said "Nope." Everything I've written in this thread before that has been common (probably not universal) knowledge in the hobby. And my hobby level knowledge is far outstripped by a real aerospace (the aero part) expert.

 

[Steve Shannon]

The drag coefficients of many shapes at various velocities have already been cataloged and graphed by NACA and its successor organization NASA. Also, look for a very good paper written by Gary Crowell that I believe was available through the NAR.

Google “NACA Airfoils” and also see this page: https://www.pdas.com/naca456.html

Here’s Gary’s article about nosecone shapes:
http://servidor.demec.ufpr.br/CFD/bibliografia/aerodinamica/Crowell_1996.pdf

 

[GrouchoDuke]

I was wondering if everyone here majored in aerospace engineering or something similar. Everyone here seems so knowledgeable.

Yeah, there are several aeros here.

Here’s Gary’s article about nosecone shapes:
http://servidor.demec.ufpr.br/CFD/bibliografia/aerodinamica/Crowell_1996.pdf

It's worth noting that not everyone who advertises a Von Karman nosecone is producing/selling nose cones with that profile. Also, current hobby-level rocket sims are better than they used to be, but they're definitely not perfect for figuring out various nose cone profile drag differences.