Fins (Stability)

When a rocket is in flight, even a small gust of wind can cause the flight path to alter. Even a small alteration can cause the launch to be a failure, which is why when building rockets people ensure their fins are the proper size and shape. The job of the fins is to keep the rocket stable in flight so that it does not wobble and then crash. The rocket rotates about its center of gravity while in flight and the goal is to keep that center of gravity perfectly vertical so that it does not tilt left (coasting) or right (powered). The main characteristic of fins that effects the flight of a rocket is how far out they stick out from the base of the rocket, because that changes how much area is encountering air molecules and thus changes the aerodynamics of the rocket.

The idea is that the fins create more drag on the rocket, keeping the rocket stable since lift and drag are known as the restoring forces, as they have the ability to force the rocket back to the original flight path if it wobbles and alters its trajectory. For our rocket, we decided to go with a heavier material for the fins (metal) and have them stick out an average amount since we are launching a fairly large rocket with a powerful engine (D12-5), so it would have a large amount of thrust that we wanted to make sure would not destabilize the rocket while in the air. 

http://exploration.grc.nasa.gov/education/rocket/rktstab.html

Calculating Maximum Altitude

Today I began researching the force of thrust. Although I knew it was going to be a very difficult topic based on the phrase “its not rocket science,” I still believed I had a chance of understanding it. I have never been more wrong in my entire life. My goal was to be able to calculate the acceleration the rocket would ideally have and from there be able to use kinematics to calculate the max altitude it would achieve.

However, after setting up my F = ma equation and finding the different equations for the forces, I quickly discovered I was lacking too much information to even attempt to solve it for acceleration. I started out by plugging in the forces:

F = ma

a = F/m

where

m* = mass flow rate

Ve = exit velocity

Pe = exit pressure

Po = free stream pressure

Ae = area ratio from the throat to the exit

m = mass

g = gravitational constant (9.8m/s^2)

C = drag coefficent

d = density

V = the velocity of the rocket

A = reference area

Unfortunately since our rocket is just a hobby rocket, the only information that the company gave us about the rocket engine is a force vs time graph. So although we have an equation to calculate the acceleration, and all we would have to do is plug it into d = Vot + at^2 to find the maximum height it would reach, we cannot evaluate the equation and thus have no prediction of how high it will go.

http://www.grc.nasa.gov/WWW/k-12/airplane/rockth.html

http://exploration.grc.nasa.gov/education/rocket/drageq.html

Changing Mass (5/9/14)

So far this year in physics, we have only dealt with problems involving mass as a constant. However in many real world applications mass is constantly changing and thus must be treated as a variable. A system in which the mass is changing is known as a “Variable-Mass System.” The idea of a variable-mass system can be applied to rocketry with “The Rocket Equation”:

 

where

m = the mass of the rocket

dv/dt = the rate at which the velocity changes with respect to time

F = the external forces acting on the rocket

c = a constant velocity vector relative to the rocket

dm/dt = the rate at which the mass changes with respect to time

This equation allows us to calculate the effects of altering mass or the force produced by the engine.

Although our rocket is only a single stage and thus will not have a changing velocity, it is still interesting to take a look at how a multistage rocket would change the maximum altitude our rocket could achieve and how that effects the velocity.

I then encountered the Tsiolkovsky rocket equation, which relates the change in velocity to the final exhaust velocity and the initial and final masses:

where (delta)v = the change in velocity

 = the final exhaust velocity

 = the initial mass

 = the final mass

Click to access MIT16_07F09_Lec14.pdf

Parachutes (5/8/14)

Today I began exploring how parachutes work, because although it is Gabe’s learning objective, I was curious about how they work. The basic principle of parachutes is that they create a greater air resistance on the way down, slowing the decent and thus protecting the model rocket from getting destroyed when it lands.

 

 

The idea of a parachute is to increase the area parallel to the ground, forcing the object to encounter more air molecules and thus increasing the air resistance. The larger the parachute the greater the amount of air resistance, which is why model rockets only need small parachutes cause the objects are not as massive as a say a skydiver who need much larger parachutes.

 

https://van.physics.illinois.edu/qa/listing.php?id=2087

http://www.physicsclassroom.com/mmedia/newtlaws/sd.cfm

Aerodynamics (5/6/14)

Today I explored the basics of aerodynamics in preparation for the building of our rocket. Aerodynamics is the “study of forces and the resulting motion of objects through the air” (Smithsonian National Air and Space Museum). When an object is moving through the air it is experiences a type of friction called friction drag. Drag is caused by the friction between the object and the air molecules. On a surface moving through air there is a thin layer on the outside of the object called the “boundary layer,” where the drag occurs. 

 

After reading about the basics of drag I explored deeper how it affects the success of a rocket launch. The amount of drag that a rocket experiences depends mainly on the shape and size of the nose cone, the size of the body and how fast the rocket is moving. If a rocket is going slower than the speed of sound (1200 km/h)–which is is–then the best shape for a nose cone is round rather than a sharp point. 

As for the fins, the idea is that they cause a strong drag force, causing them to stay back and the nose of the rocket to go straight into the air, preventing it from wobbling. 

 

https://howthingsfly.si.edu/aerodynamics/friction-drag

http://www.sciencelearn.org.nz/Contexts/Rockets/Science-Ideas-and-Concepts/Rocket-aerodynamics

5/5/14

Today Gabe and I gathered the parts that we needed for the rocket from the rocketry supplies in the conceptual physics room. Our parts list so far consists of:

• Parachute

• -Diameter: 32.5cm
• -Mass: 2.5g

• Tube + Engine Mount

-Mass: 58g
-Length: 48.75cm
-Radius: 2cm

• Shock Cord

-Mass: 2.2g
-Length: 17.2cm

• Nose Cone

-Mass: 13g

• Engine (D12-5 Single Stage)

-Mass: 46.7g

 

After gathering our materials I began investigating the basics behind how rocket engines actually work. According to a NASA article on how rocket engines work, the basic principle is that the combustion of rocket fuel produces a downward force, and by Newton’s Third Law of Motion all forces must have equal and opposite reactions, therefore there is an upward, opposite force produced by the explosion of rocket fuel.

 

Click to access 153415main_Rockets_How_Rockets_Work.pdf

  http://uprepgabe.wordpress.com

Learning Objectives 2.0

5/2/14

After discussing our learning objectives with Ms. N, Gabe and I decided to rework our goals for the project. The most important thing we needed to do was to make our new goals much more specific. We each decided on six topics that we wanted to learn more about relating to rockets: how basic rockets work, parachutes, material sciences, multi stage rockets, changing masses and aerodynamics. We split the six learning objectives between the two of us so we could each focus on three of them and get a really in depth understanding of them.

We chose to do rockets because it is a very interesting topic that many basic physics classes do not talk about very often, and we also will get to put our knowledge into building something with a tangible result.

Our plan to present the knowledge we gained will be both with the final product (the rocket) as well as a research paper that we will present to the class.

Learning Goals

Gabe and I’s learning objective for this project is to explore the basics of rocketry while seeing how aerodynamics and air resistance impact the success of a rocket mission. We will demonstrate our knowledge of these topics by both building and launching a rocket as well as writing up a research paper based on our findings.