Rocket Science 101

Wondering what to do with all those empty soda bottles? Of course you can recycle them, but we’ve got a much more fun idea. Turn them into rockets!

Materials:

• 2 2-liter soda bottles
• clay
• scissors
• duct tape
• fins (cardboard)

Rockets make fun toys, but if you can find a way to shoot them it’s literally a blast!  For those of you who live near us (San Diego area), you’re welcome to launch rockets with us on Oct. 21 (email us for details).  For the rest of you, we’re working on some ideas so stay tuned.  Or, if you come up with your own creative way to launch it, please share it with other viewers by posting a comment.

This year we will be launching our rockets at an angle with the objective of shooting them as far as possible.  It’s important to consider all of the manipulating variables ahead of time, because you most likely will not have enough launches to test all of them.  Here they are:

• angle
• volume of water
• air pressure
• design

Angle – Is there an angle that is always best for shooting projectiles the farthest, or does it change with different objects and circumstances?

Volume of Water – Water acts as a fuel.  By shooting out the bottom, it forces the rocket forward.  The more water that shoots out, the more thrust you’ll have; but it’ll also increase the overall mass of the rocket which slows it down.  Also, keep in mind that you need enough air (pressure) to push the water out.  Finding the right balance takes a lot of experimentation.

Air Pressure – If you filled the rocket entirely with water, it wouldn’t go anywhere.  It needs air pressure so that the air will expand and force the water out.  High air pressure pretty much means that there’s a lot of air crammed in a small space.  When the rocket is released at blast-off, the air immediately begins expanding and filling the fuel chamber.  How will pressure effect flight?

Rocket Design – You want a light-weight (low-mass) rocket that is also stable in flight.  One guideline for creating stability is to have the center of mass in front of the center of pressure.  To determine the center of mass, try balancing your rocket on a stick.  (This is the center of mass when the rocket is empty; but for the first part of it’s flight, the center of mass will be closet to the bottom, depending on the amount of water.)  You can move the center of mass forward by packing clay into the nosecone.

The center of pressure is harder to find, but it can be estimated by cutting out a two-dimensional drawing on cardstock and repeating the balancing act.  Placing fins at the rear of the rocket moves the center of pressure down, but too much fin will slow the rocket down.  Be careful to make each fin the same, place them evenly around the rocket, and line them up the same way.

There are so many variables to experiment with; be sure to only manipulate one of them at a time and to have controls (a baseline for comparing data).  If you are going to experiment with angles, trying 90 degrees (straight up) could be your control.  If you are experimenting with volume of water, running a trial with no water could be your control.  The thing we’re measuring (distance) is called the responding variable.

If you find yourself dreaming about rockets and can’t wait until the day of the launch, try this water rocket simulator by NASA. 

A little bit of research can give you quite a head start, but it doesn’t replace experience.  So prepare as best as you can for your first launch day, take notes of data and any special circumstances, share your notes and data and review those of others, and then start developing ideas for next year’s rocket.

Science terminology:

• manipulating variable
• responding variable
• control variable
• forces:
  • gravity
  • propulsion
  • resistance
• aerodynamics
• projectile motion
• mass
• center of mass
• pressure
• center of pressure