AP Physics: 2-L Rocket Eggmission

Q-Focus:

1.  Q: What angle should we launch the rocket at?

A: Variant on how far you want the rocket to go, the wind, and how you want your rocket to come down on the egg.

2.  Q: How should we make our fins?

A:  We learned that there are many different ways to make one’s fins, and we decided to go with the angled ones to put a spin on our rocket.

3.  Q: How much water should we put in out rocket?

A:  Once again this depends on how far you want your rocket to go, and when we figured out that our egg protection was so weak we decided to minimalize our distance in preservation of the egg.  We ended up only using 150 ml and this was substantial to propel our rocket a whopping 33 meters.

Making our rocket:

Materials: duct tape, masking tape, epoxy glue, 5 2-liter soda bottles, plastic sheeting, sand, yarn, plastic bags, eggs, box cutter.

First, we epoxyed the fins onto the base bottle.

Then we cut off another tip of a bottle and packed sand into the front, adding some bubble-wrap to protect the egg at first, and then later adding yarn for cushion.

After throwing it to imitate flight and watching the epoxyed fins shatter off as it (lightly) touched the ground, we decided to tape the fins on in addition to the epoxy.

Soon after we developed a levitation device that would fit in the tip of the rocket, but later trashed it because of its obvious uselessness.

Physics of our rocket:

Pre-Launch: There are many forces acting on the rocket at this point, however the net force is zero as it is not accelerating in any direction.  Some of these forces include the amassed air pressure on the inside the bottle, which is greater that regular atmospheric pressure and is why it wants to escape, which is exerting a force on the plug and in turn the holding pin.  The holding pin is in turn exerting an equal and opposite force on the bottle, keeping it in equilibrium and resting on the rocket stand with obvious gravitational forces and normal forces.

Initial Launch:  The force of the pin has now been taken away and the rocket has blasted off of the rocket stand.  The pressurized air has pushed some of the water out and since the water has more mass than the air, the force is greater as it is being pushed out(>mass x an acceleration is going to be greater than <mass x an acceleration).  The sand in the tip of the rocket keeps it going straight and from flipping out of control as if there was no weight at the front of the rocket, the more massive rear with water still inside would want to go forward faster then the front end.  Gravity is still pushing down on the rocket, but the force of the water propelling the rocket is greater and causes it to go up.  It will also have a decent horizontal velocity by now, and as friction(in the form of air resistance) exerts a force on it backwards, the propulsion of the water on the rocket also overcomes this force.  The air flow will also be passing by and exerting a slight force on the underbelly of the rocket, pushing it slightly upwards as it rushes underneath.  At this point the rocket has some potential energy if there is still some water waiting to be thrusted out, but it is gaining both kinetic energy as the thrust is lost and gravitational potential energy as its altitude increases.

Main Flight:  The water fuel has now been used up and it has lost all of its force and in turn acceleration upwards(besides some air resistance pushing it upwards slightly) and is accelerating downwards at approximately 9.8 m/s squared.  The entire length of the flight before it reaches its peak altitude will consist of gaining gravitational potential energy and losing kinetic energy, while the length of flight after the peak will consist of gaining kinetic energy and a downward velocity and losing/using gravitational potential energy.  The fins, while stabilizing it in the initial launch, continue to stablilize it but with some additional effects as well.  Because of the angle of our fins along the bottle, the air flow/resistance pushed the rocket into spinning counter-clockwise.  This does add some air-resistance, but in turn along with the weight in the front keeps the rocket going very straight.  This method of fin usage also sacrifices the possibility of the rocket catching any drafts with its fins and gliding with some straight fins.  However my partner and I deemed this sacrifice worth it as it kept our rocket from being as affected by unwanted forces such as wind and it looked pretty sweet.

Collision:  The collision of the rocket with the ground was neither entirely elastic nor entirely inelastic.  The rocket bounced up off of the ground with a small percentage of its original velocity.  As for the forces acting upon the rocket, it is difficult for me to fathom what exactly they are.  I know that gravity is one of the forces acting upon it along with the normal force of the ground, but I believe that momentum somehow transfers into a force upon the ground however I do not know how to explain this transformation.  I believe that this transformation is best described by F = (m x (delta)v) / (delta)t.  Regardless, I am positive that the force at which the rocket hits the ground is proportional to the mass and velocity of the rocket.  The ground absorbs the majority of this force along with the crushed tip of our rocket and unfortunately the crushed egg inside of our rocket as well.  The potential energy is also entirely gone at this point and the kinetic energy Is completely absorbed through the ground and friction when it comes to a stop.

Egg Physics:

Unfortunately, our egg didn’t survive the impact of landing.  To attempt to prevent this, we tried many different techniques along the way to no avail.  Throughout the practice launches we tried cotton balls, bubblewrap, a Styrofoam ball encasement, and yarn.  All of these failed I think not because of anything wrong with the quality of cushion but the lack of quantity of it.  I also think that our minimal water and low angle caused it to land on its side more than expected and our cushion was primarily positioned for an impact directly into the nose cone.  I think that if I where to start over with another rocket, I would drastically increase the angle of my fins as to make it spin even faster and slow the rocket’s decent even further.  I would then have the egg chamber extended to a whole bottle instead of cutting some off or even using two and a half bottles.  Finally I would pack this newly enlarged chamber with cotton balls or yarn and add an easily collapsible nose-cone.  As for the physics concepts, the cushion serves the purpose of extending the time over which the impulse’s force is exerted upon the egg, allowing it to deal with smaller forces over time and survive the impact.

Results:

Our rocket barely made it 30 meters but it made it 30 meters nonetheless.  At 33m our rocket impacted and our egg died.  We launched it at 35 degrees with 150 ml of water.  As stated in my egg physics, I would have changed many things about our rocket and I think we had too much weight also.

Learning:

I learned a lot about physics throughout this project, primarily from watching how others attempted to make the best rocket.  When first receiving the project, I was very confident, but it was not until later that I realized how difficult keeping the egg alive would be.  I think that these hands-on types of projects are not only fun and a good excuse to hang out with friends but also very valuable and i look forward to next semester's boat project.