Sunday, November 13, 2016

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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  1. Hi Daniel,
    Great articles! Again, send these to Apogee. Talk to Tim first and see if he is interested. You certainly cover the subjects in great detail.

  2. Daniel-- You forgot the obligatory "Don't try this at home" disclaimer. Now you're going to be responsible for hundreds of thousands of arms torn off by the air pressure when they stick their arm out of a moving auto. For shame! LOL!

    1. I'd say it's safe, unless the car is moving at transonic speed. Then I'd keep the hands inside and the windows rolled up, for safety.