This lesson is based on California's Middle School Integrated Model of NGSS.
NGSS Performance Expectation (PE): (MS-ETS1-1) Define the criteria and constraints of a design problem with sufficient precision to ensure a successful solution, taking into account relevant scientific principles and potential impacts on people and the natural environment that may limit possible solutions.
Science and Engineering Practice SP1 Asking Questions and Defining Problems. Students are required to build a hot air balloon from tissue paper and glue sticks that can stay aloft and fly across the school's campus.
Disciplinary Core Ideas (DCI): ETS1.A Defining and Delimitating Engineering Problems - The more precisely a design task's criteria and constraints can be defined, the more likely it is that the designed solution will be successful. Specification of constraints includes consideration of scientific principles and other relevant knowledge that are likely to limit possible solutions.
Crosscutting Concepts (CCC) Influence of Science, Engineering, and Technology on Society and the Natural World - The uses of technologies and any limitations on their use are driven by individual or societal needs, desires, and values; by the findings of scientific research; and by differences in such factors as climate, natural resources, and economic conditions.
The principle of hot air rising will be harnessed to create a tissue paper hot air ballon. The molecules within hot air are moving faster than the molecules within cold air. These after moving hot air particles have more kinetic energy and are thus spaced further apart. This causes hot air to be less dense than cold air. A column of hot air will rise above a column of cold air. These same principle can be observed with hot and cold water in the ocean.
By trapping the hot air in a balloon the students will be able to harness this principle and create a craft that can rise in the air. The teacher will have to decide if the students should follow a set plans to build a hot air balloon (1 week) or allow the students to free build a hot air balloon of their own design (2 weeks). If you have the time to allow for free building, it will probably be necessary to allow your students time to build a second prototype once their first balloon design fails or needs to be improved. I would also recommend requiring your students to submit a 'design application' outlining how they intend to construct their balloon.
The basis of this hot air balloon launch rig is a turkey fryer. I waited until Thanksgiving to purchase mine at a local hardware store. This particular fryer has a shut off button that must be held closed when the flame is lit. If the button is released the flame shuts off - nice feature to have around students.
I then went to the heating and cooling section of big box hardware store (Home Depot or Lowes) and starting looking at round heating duct reducers. These metal ducts are used in homes to force air into the many rooms from a central point. I found a ring that fit over the top of the turkey fryer, then gradually found fittings that reduced the opening to about five inches. It took some playing around with different rings until I got what I wanted.
I then covered the whole thing with aluminum coated bubble wrap insulation. This insulation is meant to save any little hands that decide to touch the rig when it is still hot. For that reason I am the only person that is allowed to touch the rig when it is lit. Next year I'm planning on attaching some handles to the side of the aluminum column to make it easier to manipulate and reduce the chance of a burn. CAUTION: The rig does get VERY hot.
There are two ways to approach this activity. One is to provide a pattern for your students to follow, which can be traced onto tissue paper and assembled. The other method involves skipping the pattern and allowing your students to engineer a balloon of their own design without any sort of directions to follow. I've done both, following a pattern is quicker and guarantees a successful flight, but having students design their own balloon gives them a crash course in the engineering design principles covered in NGSS. Yes, designing a balloon from scratch will take longer and there will be many failed launches. But the real learning occurs when they analyze what isn't working, and use that to determine how to modify their design.
If you plan on having your students design their own balloon I would suggest scheduling two rounds of building (ten days to two weeks). During the first round the kids will probably build cylinders that will not have enough lift potential to fly. During the second round they will figure out how to build a larger cavity for hot air.
This year I only had a week to dedicate to hot air balloons so I chose to provide the pattern. If you were to allow your students to design their own balloon I would set aside about two weeks of time.
The pattern I used can be found at Super Hot Air Balloons. To make the pattern (see picture below) I trace the outline onto butcher paper and provide one pattern to each group. If you wanted to increase the complexity you could simply provide the measurements and have the students reproduce their own pattern.
This pattern can then be traced onto tissue paper to make six balloon sections. These six section will then be glue together using glue sticks to form a complete balloon. Overlap the edges of each pattern by about an inch and attach with a liberal amount of glue from a glue stick. Start at the top and work your way to the bottom of the balloon, pulling the edges together as you glue. Start by attaching two sections together, then add the third, fourth, fifth, and sixth section until the six sections form a round balloon. It's okay to pull and crinkle the edges to get them to come together. You may need to show the kids the technique at first, but once they see the method they are able to build on their own.
The top of the balloon will have an open hole that should be tied shut with fishing line. It is beneficial to include a loop in this fishing line in order to hang the balloon while it is inflating. The bottom opening will need to be rolled open like a shirt sleeve. This rolling action provides a grabbing point when inflating the balloon and creates just the right amount of counter weight to keep the bottom of the ballon pointed downward during flight. If this counter weight was not built into the balloon the ballon could roll over during flight and all the hot air would escape from the inverted bottom (now facing upward) and propel the balloon downward. I have heard that some people attach string to the bottom of the balloon to serve as a counterweight, but in my experience the rolled bottom opening is sufficient.
At my school we purchased tissue paper in 3 foot squares. It becomes necessary to attach two squares together (3" X 6" rectangle) in order to make one section of balloon, in order to trace the patten on it. If your students plan carefully they are able to alternate color and have a checker pattern of alternating tissue paper or stripes.
The secret to a successful launch is to close any possible air leaks inadvertently made during construction. To identify these open seams we hang the balloon from the ceiling in the classroom and fill the balloon with a hair dryer. I have also use a hot air gun (used to strip paint). Careful examination will reveal any gaps in the seams. It is more important to concentrate on any leaks at the top of the balloon then at the bottom of the balloon. Tears can be repaired with small pieces of tissue paper glued over the rip. Make sure you keep the balloon as light as possible, if the balloon is too heavy it may not fly to its highest potential.
To launch the balloons pick a calm day, preferably in the morning when the air is cool. A hot air balloon will fly higher if the surrounding air is cooler. I tell my kids that if it is too windy we will have to "scrub" any flights and try again tomorrow. Scrubbing a flight means to cancel -- I try to use vocabulary that is commonly used with flight vehicles.
During the actual launch I have one student in charge of the propane cylinder and open flame. The turkey fryer I use has a button that must be held down to activate the flow of propane gas. If the button is released the propane shuts off, which is a nice safety feature when I am working around kids. Another student holds an old broom stick with a hook on the end of it to position the balloon over the launch rig (reason for the loop on the fishing line at the top of the balloon). The hot air coming out of the launch rig is VERY hot, so I position myself at the bottom of the ballon and hold it over the launch rig. When the balloon is sufficiently inflated the broom stick can be detached and when the balloon had developed sufficient lift, the ballon can be released.
If you can get the balloon to a sufficient height and into any strong cross winds the balloon can drift several hundred miles and never return. I'm not concerned about littering, the balloon is made from tissue paper and at the first moment of rain it will disintegrate into nothing.
For this activity the student's measure the hot air balloon's altitude and time the balloon's flight. They record this data in a chart of their own design in their Science Interactive Notebook. Altitude is determined with an easy-too-make device designed by NASA Education - NASA Altitude Tracker.
Each group must record the altitude and flight time of the other balloons. This also has the added benefit of keeping the entire class engaged while the balloons are being launched. I position theses other groups at 15 and 30 meters, rotating the groups between these two points. Theses two distances are critical to the NASA Altitude Tracker being able to calculate accurate altitudes. A stop watch on the student's smart phones is used to measure flight time.
Student Work Sample