4 The Trash Bag Sausage

The Trash Bag Sausage is a cylinder 50 inches (127 cm) tall and 20 inches (50 cm) across. The top of the balloon is closed with a straight seam like the bottom of a tube of toothpaste. This type of seam makes two points stick out like horns on either side of the balloon. The balloon is made by cutting off the ends of two trash bags and fixing them together using tape.

The bottom of the balloon is held open with a square frame made from plastic drinking straws and cardboard. Finally there is a simple cotton-ball burner mounted on wire stretched across the frame.

To make the Trash Bag Sausage fly best, you need to find the largest and thinnest brand of bags you can. As the thinnest bags are also usually the cheapest, it is well worth searching to find a good brand. See chapter 11 (page 215) for advice on finding the right kind of trash bags. After that, if you still cannot find very thin trash bags, the “What Next?” section at the end of this chapter (page 69) will tell you how to use slightly thicker ones.

You can see the balloon for many miles at night if you use kerosene or a similar fuel that produces a luminous flame. This is because the burner is mounted at the bottom of the balloon and it can be seen without the black envelope getting in the way.

The burner is quite small for a balloon as large as this, because the air must not get too hot. If the air does get very hot, the polyethylene sheet will soften and may even melt or shrink. But because the air is only warm, there is not much lift, so the balloon must be made as light as possible. This is why it is important to use very lightweight trash bags if you can.

A Bigger Balloon Is Always Better

Somewhere in every project in this book it says that to make the balloon fly better you should make it bigger. But why does a bigger balloon fly better? And is it always the case? You might think that although a bigger envelope has more air and so more lift, it also has more weight and this would cancel out the extra lift.

In fact, bigger balloons have a lot more lift, much more than the extra weight of a bigger envelope. This is because the volume of a three-dimensional shape grows faster than its surface area as the shape gets bigger. Because the lift depends on the volume of the shape, and the weight depends on the surface area, more lift is left over as the shape gets bigger. The best way to understand how this works is to look at an example.

Imagine you have two hot air balloons each shaped like a cube. One balloon has sides that are 1 meter along each edge, and the other has sides that are 2 meters along each edge. Both balloons have four tissue paper sides and a top, with the bottom face left open to allow the hot air in.

If you want to calculate the volume of each balloon, multiply the width by the depth by the height. As they are both cubes, this is easy.

The volume for the smaller balloon is 1 × 1 × 1 = 1 m³.

The volume for the larger balloon is 2 × 2 × 2 = 8 m³.

So the volume has increased eight times by making the sides twice as long. How much has the weight of the envelope increased?

The weight of the balloon is made up of the weight of paper needed to make a cube-shaped envelope.

That is just the area of paper you need to make five square faces multiplied by how much the paper weighs per square meter. (Assume each square meter of tissue paper weighs about 15 grams.) The area of a square face is easy to work out—you multiply the width by the height:

For the smaller balloon, the area of one side is 1 × 1 = 1 m².

The area for five square faces is 5 × 1 = 5 m².

So the smaller envelope weighs 15 × 5 = 75 grams.

For the larger balloon, the area of one side is 2 × 2 = 4 m².

The area for five square faces is 5 × 4 = 20 m².

So the larger envelope weighs 15 × 20 = 300grams.

Overall, this means that the volume of the larger balloon is eight times that of the smaller balloon, and so it has eight times the lift. The weight of the larger balloon is only four times bigger than the weight of the smaller balloon. Since the larger balloon has eight times the lift, it’s no surprise that the larger balloon flies better!

But things get even more interesting if we look at the lift available to carry a payload at the end of a flight as the burner flame gets smaller. If the air inside both balloons is heated to only 30°C above the surrounding air:

Smaller balloon lift from the heated air = 110.5 grams

Lift available for payload = lift - envelope weight = 110.5 - 75 = 35.5grams

Larger balloon lift from the heated air = 884.2 grams

Lift available for payload = 884.2 - 300 = 584.2 grams

So the larger balloon can lift a payload that is more than 16 times bigger. This means that the larger balloon will continue flying well even as the burner starts to run out.

If you make balloons bigger, they fly better. But what happens if you make a smaller balloon? Obviously they don’t fly as well because they have less lift. In fact, if you make a tissue paper balloon too small, it won’t fly at all.

How to Make the Trash Bag Sausage

Adult supervision required

Materials

2 lightweight plastic trash bags (the thinnest you can find—see chapter 11, page 215)

Thin clear tape, lightweight masking tape, or similar, 3/4 or 1 inch (19 mm or 25 mm) wide

14 jumbo plastic drinking straws, ¼-inch (6 mm) diameter

Empty cereal boxes (or other source of thin cardboard)

Thin, soft, iron wire

Cotton ball, about 2 inches (5 cm) in diameter

1 fluid ounce (30 mL) kerosene

Tools

Scissors

Ruler

2-inch (5 cm) ball of modeling clay

Pencil

Straight (dressmaker’s) pin

Pliers (with wire cutters)

Lighter or matches

What to Do If Things Go Wrong

If you make small holes in the plastic sheet or there are small gaps in the seams, see chapter 11 (page 213) to find out how to fix them.

If you put the tape in the wrong place, do not try to unpeel it, as you will just tear the plastic sheet. Complete the seam as best you can and put another length of tape over the gap.

How to Make the Envelope

Carefully cut the closed end off a plastic trash bag. Cut as close to the weld as you can. The weld is the melted line running straight across the bottom of the bag.

Open up the bag and flatten it out onto a table with one open end facing toward you.

Fold over ½ inch (13 mm) of the open end. Hold the folded-over edges in place with a short piece of tape, and secure the corner of the trash bag to the table with a second piece of tape.

Pull the other side of the open end to straighten the folded-over edges and hold them in place with a short piece of tape. Attach the corner of the trash bag to the table with another piece of tape.

Use two or three short pieces of tape to keep the edges folded over along the entire side.

Affix the free end of the roll of tape to the table next to one corner of the folded-over edge. Unroll a short length of tape and line it up over the seam, but don’t pull so hard that the tape stretches. Smooth the tape down onto the seam.

Keep the stuck-down length of tape in place with one hand and unroll another length of tape. Repeat until the whole folded-over seam is taped.

Lift the corner of the folded-over seam and cut the tape close to the side of the envelope with a pair of scissors. Fold any remaining stub of tape underneath the envelope.

Turn the envelope around so that the remaining open end is toward you.

Take a second trash bag and flatten it out with one of the open ends next to the open end of the envelope.

Slide the new trash bag into the open end of the envelope and position it so that there is a ½-inch (13 mm) overlap all the way along the edge.

Place a short length of tape across the seam to keep the two bags together and use a second short piece of tape to hold the end of the seam to the table.

Pull the other end of the seam to straighten it. Check that the overlap is still ½ inch (13 mm) all the way along the side and secure the end of the seam with two short pieces of tape as you did in the previous step.

Put three more small pieces of tape along the seam, making sure that the overlap is ½ inch (13 mm) all the way along the side.

Affix the free end of the roll of tape to the table next to one side of the seam. Unroll a length of tape, line it up over the seam, and smooth it down.

Keep the stuck-down length of tape in place with one hand and unroll another length of tape. Smooth the tape down over the seam. Repeat until the whole folded-over seam is taped over.

Lift the ends of the seam and cut the tape close to the side of the envelope with a pair of scissors. Fold any remaining stub of tape under the envelope.

Turn the envelope over and lay it down, carefully flattening out the seam so that the taped part lies flat on the table.

Check that the overlap is still ½ inch (13 mm) all the way along the side. Place a short length of tape across the seam at one end and use a second piece of tape to hold the end of the seam down to the table.

Pull the other end of the seam to straighten it. Check that the overlap is still ½ inch (13 mm) all the way along the side and secure the end of the seam with two short pieces of tape as you did in the previous step.

Put three more small pieces of tape along the seam, making sure that the overlap is ½ inch (13 mm) all the way along.

Affix the free end of the roll of tape to the table next to one side of the seam. Unroll a length of tape, line it up over the seam, and smooth it down.

Keep the stuck-down length of tape in place with one hand and unroll another length of tape. Smooth the tape down over the seam. Repeat until the whole folded-over seam is taped.

Lift the ends of the seam and trim the tape with a pair of scissors. Fold any remaining stub of tape under the envelope.
How to Make the Drinking Straw Frame

Lay the completed envelope flat on the table and measure across the opening at the bottom. You will need four plastic drinking straws exactly half the length measured in this step. This means you have to join two or three ordinary straws together to make a long straw.

Cut a sleeve 1 inch (25 mm) long from a spare straw.

Cut the sleeve along its length.

Slip two straws into this sleeve. Make sure both straws are pushed all the way into the middle so they touch.

Hold the sleeve tight and wrap a small piece of tape across the cut in the sleeve.

Put tape around the seams between the sleeve and straws. Join this double-length straw to another ordinary straw in the same way to make a triple-length straw. Make four of these long straws.

Cut the long straws so they are exactly half the length you measured in step 25.

Cut out two 1½-inch (38 mm) squares of thin cardboard from an empty cereal box or something similar.

Cut both squares across a diagonal line so that you have four right-angled triangles.

Use two short pieces of tape to attach one of the triangles to the end of one of the long straws as shown.

Attach a second straw to the other side of the cardboard triangle as shown.

Join all of the straws together using the cardboard triangles and tape to form a square frame.

Put one of the cardboard triangles on top of a piece of modeling clay and push a sharp pencil through the middle of the triangle to make a small hole (about 1/8 inch or 3 mm across). Make a second hole through the card triangle in the far corner (across the diagonal of the square frame).

Attach a corner of the frame to the open edge of the envelope using tape.

Repeat so that all four corners of the frame are attached to the envelope.
How to Make a Cotton-Ball Burner

Stretch a piece of wire diagonally across the frame. Cut the wire so that it is 6 inches (15 cm) longer than the distance across the corners of the frame.

Push a straight pin through a 2-inch (5 cm) cotton ball to make a hole.

Grip the cotton ball at least ½ inch (13 mm) away from the pin and gently pull the pin sideways to enlarge the hole. If you grip the cotton ball too close to the pin, the cotton fibers won’t slide over each other and the pin won’t move.

Feed the wire through the hole in the cotton ball. When the ball is in the middle, bend the wire in half, leaving the cotton ball on the bend.

Twist the wire together a few times close to the cotton ball.

Stretch out the wires on either side as shown.

Push one of the burner wires through the hole in the one of the cardboard triangles. Position the cotton ball in the middle of the frame and fold the burner wire down over the card triangle. Twist the end of the burner wire around itself a couple of times and cut off the excess.

Push the other burner wire through the cardboard triangle on the opposite side of the frame. Straighten the burner wire and make sure the frame is still square. Bend the second burner wire down over the card triangle, fold the burner wire around itself a couple of times, and cut off the excess.

The completed burner is in place.

What Next?

Check the envelope for any leaks by inflating it over an electric toaster or similar heater (see page 29). Take care not to scorch the cotton-ball burner or melt either the polyethylene envelope or the drinking straws. Look for gaps in the seams and repair them using small pieces of tape.

Once you have fixed any leaks, the balloon is complete and ready to fly. See chapter 2 (page 33) for instructions on flying a balloon with a burner.

If you can only find regular-thickness trash bags, you can get the Trash Bag Sausage to work if you use three trash bags to make the envelope rather than two. Complete the envelope as shown in this chapter but then repeat steps 10 to 24 to add the third trash bag. Then, continue on to add the base frame and burner. This larger balloon takes a while to fill and is still a bit heavy for the burner size, but it should fly.

Of course, if you do have very thin trash bags, an envelope made from three very thin trash bags will fly even better!