The Weight of Paint - Part 3 - Surface Texture: Drag Reduction Vs. Added Mass

Click here for Part 1.

So far, we've established that painting a model rocket can add significant weight. We've also established that added weight can have a negative impact on the altitude we can expect from a given model. Comparing simulated flights between our heavier, painted Estes Monarch with the lighter, unpainted Monarch showed this difference.

Then, in Part 2, we took a detour to discuss aerodynamic drag - the resistance our rockets encounter from the surrounding air. By comparing flight simulations of our smaller, 3-finned Estes Monarch with the larger, 4-finned Big Bertha, we saw that, even though a lighter rocket will usually fly higher than a heavier one, drag is significant enough a force to change that; the larger, draggier Big Bertha simply cannot catch up to our finished, painted Monarch, even if we change the weight of the Bertha so that it matches that of the 25% lighter, unpainted Monarch we've been looking at.

What about two rockets of the same design - exactly the same size and shape - but one is heavier and one is lighter? One is painted, and the other isn't?

All things being equal, a lighter model rocket will usually fly higher than a heavier one. We saw that in our flight simulations from Part 1 of this series. The exception to this is if the rocket is far too light - below the optimal mass for its size and shape. A featherweight rocket may not have enough inertia to overcome the drag holding it back. Remember our comparison of throwing a bowling ball, a baseball, and a foam rubber ball up into the air.

But since most model rockets are probably heavier than they need to be even before building, you probably won't encounter that problem*.

But when you paint a rocket, all things are no longer equal.

* * *

When you use OpenRocket to design a rocket or build a simulation of an existing design, you start with the nose cone, and add parts one by one. All parts will have default settings - such as weight and center of gravity - depending on what they're made of and how thick they are.

The default setting of each part for surface texture is "Regular paint," and is followed by the number 60 and the symbol μm.

This represents the average roughness height of the surface, and is measured in micrometres, or microns. A micron is 0.001 millimeter - very tiny. Here's a photo of a carbon fiber filament - only six microns in diameter, compared to a human hair, about 50 μm in diameter.

So, in OpenRocket, if you change nothing about the surface texture, the assumption is that the rocket is painted, and that the tiny bumps in the paint job are, on average, 60 μm high.

Rough surface texture creates more turbulent airflow over the rocket, which in turn increases drag. Another thing increases drag dramatically - the velocity of flight. It might make sense that drag goes up as speed goes up. If you stick your hand out a car window while traveling at 20 miles per hour, the force on your hand is very light, while at 60 miles per hour, the wind pushes hard on your hand.

But here's the thing: while drag from  laminar flow increases proportionally with airspeed, drag from turbulent flow increases as a square of the increase in velocity. That means if you double the speed of the rocket, drag goes up four times. If you triple the rocket's speed, drag increases nine times. You can see the need to decrease the amount of turbulence the rocket experiences. That's what a smooth paint job can do - decrease turbulent airflow.

We've seen how a heavy paint job adds weight, and how added weight reduces altitude. But to see how the surface texture of paint affects drag, we need to factor that into our simulation.

Returning to the unpainted Monarch simulation, I click on the nose cone and select "Unfinished," then click the box that says "Set for all."

Now all components of the simulated rocket - the nose cone, body, fins, and even launch lug - have a simulated surface texture with an average height of 150 μm.

Here, we should note something. A lot of things in OpenRocket are approximations, and this is a good example. Obviously, all unfinished materials don't have the same texture. An unfinished plastic nose cone is smoother than a balsa cone, with its exposed wood grain, and a set of raw balsa fins are rougher than an unpainted paper body tube with a slick coating. But I think this is good enough to illustrate the principle we're talking about here.

Also, the altitudes are approximate. You might find you get a different result each time you run a simulation. We'll go into why another time.

How do I know how good my paint job is? I don't have a means of measuring the surface roughness in microns. But I know that my paint job isn't polished, and the paint didn't go on as smoothly this time as I've managed in the past. There's a bit of visible texture.

Here are a few closeups.

There are little bumps. But, they're smooth bumps, and I've seen and done far worse. So, I'm calling this "Regular paint." It's a guess, but, I think, a fair one.

Our original simulations from Part 1 showed the lighter Monarch beating the altitude of the heavier Monarch by a significant margin: 89 feet higher on an Estes C6-5 motor, and 42 feet higher on a D12-5.

The lighter Monarch results

The heavier Monarch results

The fact that the margin narrows with the D12-5 can be attributed to the fact we discussed before, that drag increases exponentially with velocity. On the D12, the both rockets fly nearly 100 miles per hour faster than on the C6. The lighter rocket still wins, but the drag force is significant enough to narrow the gap.

Now that we've got both the mass and the surface texture adjusted for the Monarch before paint, let's run a new flight simulation. Remember, the unpainted rocket weighs 57.4 grams, and the smoother, painted rocket weighs 75.8 grams - over 32% heavier.

Here are the results of the new flight simulation:

With a rougher, unfinished surface texture, the unpainted Monarch has lost some altitude. It still beats the heavier Monarch on a C6-5 motor - this time by 53 feet. But on the D12-5 motor, the smoother, painted rocket now wins by 27 feet!

Why is this? Again, look at the maximum velocity of the models. On a D12 motor, the rocket flies 94 miles per hour faster than on a C6-5 motor. The increased drag at higher speeds works to stop the rocket short of the heavier Monarch's peak altitude.

What if we got a better paint job on the Monarch?  What if our paint job was good enough we felt comfortable calling it "Smooth paint," with an average surface roughness of only 20 microns?

Now how high does the rocket fly?

By decreasing turbulent airflow even further with a smooth paint job, we increase our altitude even further, nearly catching up with a C6 motor, and breaking 1,100 feet with the D.

What if we carefully polish the finish on the paint job, and get it nearly perfect, with a mirror-smooth finish of only 2 microns in average height?

Now we've increased our altitude on this rocket even more.

But we still haven't caught up to the lighter rocket flying on a C6-5 motor. That paint job is just a little too heavy.

Let's look at the flip side to all this. What if we could paint the rocket nice and smooth, and the paint weighed absolutely nothing? Let's take our "unpainted" Monarch, give it an imaginary paint job with no added mass, and polish it smooth. What kind of altitude could we expect from a lighter but smoother rocket?

Now we've gained some serious altitude. By keeping the rocket light and making it as smooth as we can, we're giving the rocket the advantage in overcoming both gravity and aerodynamic drag. We've taken this little sport rocket and pushed it to perform its best.

Of course, paint doesn't weigh nothing. We're always going to add some mass. But we can try to minimize that. The Monarch isn't a high-performance model, with its goofy, oversized, 1/8-inch-thick fins, but that doesn't mean we can't make the most of it.

OK, here's one more scenario...

What if we painted the rocket with real paint - and, as happened in the case of my rocket, the paint went on a bit heavy - but instead of a nice, smooth paint job, we got something less than desirable? Sometimes, a paint can will have chunks of pigment settled in it, and you'll end up with a paint job that's really spiky and rough.

The paint came out of this can like it was Silly String.

Rough paint - neither pretty nor aerodynamically advantageous

When my friend Chad moved across the country, he gave me his rockets. I picked one up, and it actually hurt - the texture was like little needles. The surface was as if it were covered in tiny claws  for grasping at the air as the rocket flew.

What if that were the case with our painted Monarch? Let's select a "Rough" finish, with an average height of 500 microns.

Let's check our altitude now...

Here we see how paint can be a hindrance, if done badly. Not only will the rocket not look nice, it won't fly as high.

So, if you want to get the most performance from a model rocket, the key is to build light, and make the surface smooth.

* * *

I build most of my rockets to look nice. I take a few steps in the hopes that they'll perform a little better, but I don't always try to maximize their altitude. That's certainly true of the Monarch.

This all started with a simple question in my mind: How much weight am I adding by painting the rocket? But now that I see the results of our simulations, something's bugging me.

The painted Monarch can't match the unpainted one on a C motor - only on a D - even by reducing the drag and making the paint as smooth as possible. But the Monarch was never meant to fly on D motors. The only reason mine can is that I set the kit motor mount aside and upgraded to a larger mount. What if I want to paint the rocket, but I also want it to fly as high as it can?

The key is to paint lighter and maybe smoother. How much lighter? And can it be done? And how can it be done? I've promised to answer these questions the last two posts. Next time, we'll actually look into it.

Click here for Part 4.

*There are some exceptions to this. Model rocket have been made from things like Styrofoam, pool noodles, and even Mylar balloons. These rockets are so light they don't coast very much after motor burnout. Such model rockets are some of the few which would probably benefit from a little added weight. But, then, such rockets are meant to fly to relatively low altitudes, even on higher thrust motors.

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The Weight of Paint - Part 2 - So Why Paint?

Click here for Part 1.

In the last post, we saw how much weight paint can add to a model rocket, and how that weight might affect the rocket's flight. The Estes Monarch I recently built has a total weight of 75.8 grams - 25% of which is just paint and primer. And an OpenRocket simulation of flights of both the painted and unpainted rockets showed a significant loss of altitude in the heavier, painted version.

So, this begs the question: Why paint rockets at all? There are two good reasons.

Appearance and Variety

It may seem obvious - the main reason we paint rockets to make them look nice. As Chris Michielssen rightly pointed out in the comments section of the last post, not every rocketeer is concerned with maximizing altitude. A small loss in altitude is worth it to get a nice-looking rocket.

Some people actually prefer a rocket with a slower liftoff and lower apogee. They enjoy the spectacle of a dramatic launch, and want to see the whole flight. A high-flying rocket can be hard to spot when it's at altitude, and for some people, that's not as fun. A larger, lower-flying rocket is often referred to as low and slow. The Estes Big Bertha is one of these.

Some people do "fly naked," as it's called - they enjoy building and flying rockets, but skip the step of painting. As a result, they're able to finish rockets more quickly, and their rockets are certainly lighter as a result.

Others will build the rocket and wait to paint until they've flown the rocket once. The rocket must "earn its paint." They want to see how well it flies and how little damage it will accrue before taking pains to give the rocket a nice finish. I have a hard time doing this, though. If the rocket gets some minor damage in flight, I know I'll have a hard time giving it the finish I like, and I want the rocket to look great - at least once.

For me, building a rocket is part science project, part art project. I do want the rocket to perform well - as well as it can for its design. But I also want it to be pretty. A model rocket or high power rocket is a beautiful, sculptural object. Since it spends more time on a shelf at home than it does flying, I want it to make the place look nice. It adds an unexpected decorative element to my home.

And since I am on a mission to spark an interest in rocketry in people who may never have even heard of the hobby, I like having something to show off. If someone comes over for dinner, or to watch a movie, I can say, "Hey, come here. I want to show you something cool." I open the door to my Rocket Room, and they see this:

...a fleet of beautiful, colorfully-painted rockets. This always inspires questions and gives me a chance to talk about the work I've done on them. A shelf full of unpainted rockets might not have the same effect.

Most of the kits you buy are sport models - not competition rockets - so you can only expect to get so much altitude from them. The Monarch I've chosen to focus these blog posts on is certainly not designed to be a high flier. But even if you're building an Estes kit, perhaps you want to see how much performance you can get from it, even if it's not really a "performance rocket."

Based on the information from the previous post, you might then think that it would be better to save wait and not paint the rocket. But this brings us to our second benefit of painting a model rocket: drag reduction.

Let's take a brief detour and discuss aerodynamic drag.

What Is Drag?

Try this little experiment. Take a sheet of paper, and crumple it loosely into a ball, like this.

Now, sit on the floor, and see how high you can throw it upwards. Try to hit the ceiling.

I certainly couldn't. But now take the ball and wad it much tighter, like this.

Try throwing it now. This time, I hit the ceiling, no problem. The paper weighs the same, so the effect of gravity on it is the same. But the first time, it was difficult to throw. In fact, if the paper had weighed even less, it would have been even harder to throw. The difference between the two throws is the air - a force known as drag.

Drag is an aerodynamic force acting opposite the direction of a moving object. It's also sometimes called air resistance or wind resistance. As a rocket flies upward at high speed, the surrounding air exerts a force to slow it down. It may not seem like it should have much of an effect, but the drag force can be quite significant on a model rocket.

There are several types of drag working against a rocket as it flies.

Pressure drag, also called form drag, is the result of high air pressure at the forward or front end, and low pressure on the aft or back end. A rocket speeding through the air will compress the molecules of air in front of it, causing an area of high pressure on the nose cone, the leading edges of the fins, the leading edge of the launch lug, etc. When the rocket passes through the air, behind the rocket - and on the trailing edges of the fins, launch lug, etc., is a partial void or vacuum - an area of low pressure. Once the rocket has passed a region of the air, the air flow separates from the rocket, and the molecules rush to fill in the empty space behind it, causing turbulence and drag. Pressure drag has to do largely with the shape of the rocket - the shape of the nose cone, whether the fins are left square or streamlined by the rocketeer - as well as the size of the rocket. A fatter rocket will have more pressure drag than a skinny one, because the wider diameter means that there's more area of high pressure at the front and low pressure at the aft.

Pressure or form drag - from Learn To Fly

What stopped our loose paper ball from hitting the ceiling in our experiment was, largely, pressure drag.

Induced drag is caused by the fins, and it is due to the lift force. Lift, on model rocket fins, works to correct the rocket in flight and keep it stable. If something causes the rocket to waver or wobble in flight, the fins will experience high pressure on one side and low pressure on the other. That's lift, and the pressure will cause the rocket to rotate back around its Center of Gravity (CG) and straighten in out again. Lift acts perpendicular to the rocket's fins, and it's necessary to keep the rocket stable. But with lift comes drag - at a 90 degree angle to the lift force. Again, high pressure on the leading side of the fin and low pressure on the trailing side of the fin, while it corrects the rocket's trajectory, also causes increased drag.

Lift and drag - in this case, on an airplane wing. Image from The Recreational Aircraft Association of New Zealand wiki.

Skin friction drag has to do with the air flowing over the entire surface of the rocket. Smooth airflow - known as laminar flow - creates less air friction than rough or turbulent flow. The layer of air right next to the rocket is called the boundary layer, and the viscosity of the air plays a large role here. A laminar boundary layer is nice and smooth, whereas a turbulent boundary layer is full of swirls and eddies. In laminar flow, the layers of air slide easily over one another.

An example of laminar flow on a Mercedes Benz in a wind tunnel

Not so in turbulent airflow. The layers swirl and mix together chaotically.

Flow will always go from laminar to turbulent.

The plume from a candle going from laminar to turbulent flow. From Wikimedia Commons.

This is especially true at higher velocities. The faster the rocket flies, the more turbulent airflow the rocket will experience. The Handbook of Model Rocketry describes an experiment to illustrate this. Turn on your kitchen faucet. If you turn the water on only a little, it comes out in a smooth stream. If you increase the flow of water, the flow is smooth for a bit, but by the time it hits the basin of the sink, it has changed from laminar to turbulent flow. And if you turn the water on full blast, nearly the whole stream will be turbulent.

Skin friction drag can be pretty significant, and if you want to fly higher, you want to reduce the amount of turbulent airflow over the body of the rocket. The boundary layer will always transition from smooth to turbulent somewhere along the rocket, but the trick is to try to make the effects of turbulent flow less dramatic.

* * *

You've probably had the experience of riding in a car with your hand out the window. If you face your palm flat into the wind, it will push hard on your hand. But if you flatten your palm out and hold it parallel to the ground, like an airplane, there's much less wind resistance on your hand.

If you then tilt your hand slightly upward, the air will push your hand up toward the top of the car. Tilt your hand downward, and your hand goes down. As a kid, I used to let my hand move up and down in waves like that while riding in the passenger's seat of the car.

From 1000 Awesome Things

The strong force you feel on your hand with your palm open to the wind - that's pressure or form drag. It lessens dramatically when you point your hand directly into the wind like an airplane. The force moving your hand up and down as you tip it up or down, making waves - that's lift! ("Lift" doesn't mean upwards, by the way. Even when your hand is tilted downwards and the air pushes your hand toward the ground, that's still the lift force.) While your hand is moving up or down, you feel increased air pressure pushing your hand backwards - that's induced drag.

And the feeling of the wind on the hairs on the back of your hand - that's friction drag.

* * *

So, when we launch a rocket, we're not just fighting gravity to achieve altitude. We're also fighting drag. A lighter model rocket will tend to fly higher than a heavier one, because most models are probably heavier than they need to be to achieve the highest altitude possible for their profile, on the motors with which they'll fly. But mass alone isn't the only determining factor in altitude.

Let's look at an example of a rocket which weighs the same as our painted Monarch - the Estes Big Bertha.

The Monarch's fins are reminiscent of the Bertha. But the Bertha is larger - over an inch taller, larger in diameter, and with four fins instead of three. The Bertha's fins are also slightly larger.

My Big Bertha weighs almost exactly the same as my Monarch. It may be a gram or two heavier, but they're pretty close. But I didn't paint my Bertha as heavily as I did the Monarch. If I created a simulation of both rockets, identical in weight, how would they differ in performance?

Let's do a comparison. Like I did with the Monarch, I'll simulate flights with both the Estes C6-5 and D12-5 motors.

First, let's review the flight simulations of the heavier, painted Monarch.

599 feet on a C6-5, and 1054 feet on a D12-5.

Here are the results of the Big Bertha simulation flights.

Despite the fact that both rockets are of the same mass, the Bertha can't match the altitude of the Monarch. The reason is drag.

With a larger diameter and four fins instead of three, the Bertha has more pressure drag. And with the increased surface area, due to the rocket being over an inch longer, plus the larger diameter, the fourth fin, and the fact that the fins are larger than those on the Monarch, the Bertha experiences more skin friction drag.

What if we make the Bertha "unpainted," and as light as the unpainted Monarch?

Here are the results of the simulations on the "unpainted" Bertha.

Even weighing 25% less, the unpainted Bertha flies a bit higher, but simply has too much drag to compete with our sleeker but heavier Monarch. In this example, the Bertha is our loosely crumpled ball of paper, and the Monarch is our tightly crumpled ball.

What Does All This Mean For Our Rocket?

In the previous post, we ran simulations of our lighter, unpainted rocket, and our heavier, painted one. The painted Monarch was over 32% heavier than the unpainted one, and the simulations showed a dramatic decrease in performance for the painted rocket - nearly 100 feet on a C6-5 motor.

We've already run a simulation on the painted and unpainted rockets, so we already know which will perform better, right? Pressure drag depends on the size and shape of the rocket, and skin friction drag increases with exposed surface area, so isn't lighter just better?

Well, when I built the simulation, there was one important aspect I left out: surface texture. Paint can make a rocket smoother, and a smoother rocket will have less skin friction drag.

How will this difference affect the performance between the two rockets? And, can you paint a rocket while still keeping it lightweight? We'll examine these questions in our next post.

Click here for Part 3.

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