Up, Up, and Away - Day #1

Introduction/Motivation - Air Pressure

When talking about baseball, why does a curveball curve? Why does an airplane fly? The reasons can be found in Bernoulli's principle, which states that the faster a fluid moves the less pressure it exerts. Different air velocities are present on different parts of a curveball as well as on the different parts of an airplane. Bernoulli's principle tells us that these differences in velocity mean differences in pressure exist as well. On a curveball, the difference in pressure causes the ball to move sideways. Engineers use their understanding of pressure differences to make airplanes fly.

For a system with little change in height, Bernoulli's equation can be written:

P + (v²/2g) = constant

Where P is the pressure, v is the velocity and g is gravity. Because this equation is always constant for a system, if the velocity increases, the pressure must decrease!

Drag

The type of drag that we are most familiar with is form drag. This is the resistance (or pushing) sensation you feel when you walk into the wind. It is caused by all of the air molecules running into your body. Form drag is dependant on the shape of an object, the cross-sectional area of an object, and the speed of an object. In the case of drag, the cross-sectional area is the area of an object that is facing the direction of its movement. For example, if you hold your hand out of a car's window with your palm down, you do not feel much push from the wind. If you turn your hand right, so that your palm faces the direction the car is moving, the wind will push your hand back much harder. This increase in resistance (push) is due to the increase of the cross sectional area of your hand, not the overall size of your hand.

Engineers often consider drag in designing things like airplanes and cars. They try to design these things asstreamlined as possible. Streamlined means that the shape of the object, airplane or car can reduce the drag of the object (reduce the force opposing forward motion). This is why airplanes have rounded nose cones and why they pull up their landing gear after liftoff (to remove the wheels from being in the way and creating unnecessary drag). In this activity, we will demonstrate how shape and size affect the drag on something as simple as a piece of paper.

Thrust, Weight, and Control

How does a rocket make it all the way into space? It's not as easy as it looks. Rockets that fly into space are very heavy and require something powerful to "push" them. Engineers design rocket engines to do just that! The push that a rocket engine provides is called thrust. Do you remember which law helps us describe thrust? That's right, Newton's third law of motion states that for every action, there is an equal and opposite reaction. That means that as the rocket engine pushes hot gases (mass) out of the rocket, the rocket propels (pushes) forward into space. That forward push is called thrust.

In order to create a successful rocket for Tess, her engineering team must understand thrust and know how much force (push) it is creating. What do you think could affect the thrust of a rocket? Perhaps a bigger engine? Well, there are some other things that engineers can do to affect thrust as well.

How an engineer designs the shape of the rocket nozzle is very important to the performance and thrust of the rocket. The size and shape of the nozzle effects how fast the exhaust will leave the rocket as well as how much pressure it will have. Tess' engineering team must perform many complex mathematical calculations in order to design a nozzle that will create enough thrust to launch the rocket into space.

Today, we are going to design two paper rockets with different nozzle shapes. Although we will not be performing all the math calculations, we will still understand how the different sized nozzles affect thrust. And, we will use our own mouths as rocket engines! By blowing air out of our mouths, we can launch our paper rockets into the air. We are also going to put cotton balls on the end of our paper rockets. For what reason, do you think, the cotton is added? Well, engineers have to take safety into consideration when designing rockets as well, no matter what the size of the rocket. Today, with our small paper rockets, we will be taking safety into account by adding a soft cotton ball to the front of the rocket to protect its landing. Come on, let's go build some rockets!

The weight of a rocket is incredibly important to engineers who design them, and especially to you as Tess' engineering team. More weight means more energy is required to get the rocket off the ground. Engineers strive to make rockets as light as possible while still making them strong, and all as inexpensively as possible. Engineers cannot simply just remove all the weight from a rocket because it needs to be able to carry fuel, electronics, cargo and a structure to hold it all together. And, in Tess' case, she needs to transport satellites up to space to communicate with Maya. If the rocket structure is too light, it will not be strong enough to withstand the stresses of the launch. Engineers could use super strong and light materials, such as titanium, but titanium is very expensive. This means engineers must consider the tradeoffs between weight and cost and come to some affordable yet safe compromise between the weight of the rocket and the cost of the rocket.

Engineers also must be careful about which part of the rocket is heavier. They consider the weight distribution. Should they make the rocket heavier in the front, the back or equally heavy all around? Where does the cargo go? What might happen if the front of the rocket is much heavier than the back? Well, we will find out. Today we will attempt to answer these questions by making small paper rockets, called strawkets, and experimenting to see how weight affects their flight.

Challenge

Who can build a paper airplane that files the farthest?

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