The Physics of Zorp Rockets

What is a Zorp Rocket made of?

A Zorp Rocket is made from two pieces of thin film plastic heat sealed together. The plastic is about a thousandth of an inch thick, but very strong. On one side of the plastic is an even thinner layer of polyethylene. The polyethylene sides of two pieces can melt together when heated. The other side of the plastic is sprayed with a microscopic layer of aluminum to make the rocket shiny. A small lump of foam is sealed in at the tip.

How is a Zorp Rocket made?

Zorp Rockets are made on the same machines that make Mylar balloons. A wide roll of the thin film plastic is mounted on the machine and drawn into it. The film is folded over so that the polyethylene sides are touching. The machine holds a metal plate with a ridge shaped like the seam around the edge of a Zorp Rocket before you wrinkle it. The machine heats up the plate until it is hot enough to burn your finger. The plate is pushed onto the folded film so that the ridge touches the film and seals the two sides together. Another ridge that is even hotter is around the ridge that makes the seal. This outer ridge melts right through the plastic film to cut the new Zorp Rocket free. The plate is pulled back and some more film is pulled into the machine to make the next rocket.

Why do I have to wrinkle a Zorp Rocket?

A new Zorp Rocket is flat. If you try to launch it, it will flutter to the ground rather than glide. It is not strong enough to hold its shape. When it has wrinkles it is stiffer and can hold its shape. This is just like a corrugated metal roof. The metal is made wavy so it will be harder to bend across the waves. A wrinkled Zorp Rocket is wavy in all directions at irregular intervals. This gives it strength in all directions and keeps it from going back to being flat. The pop when it comes out of the launch tube sets the wrinkles to hold the right shape for flying.

How much does a Zorp Rocket weigh?

The short answer is “almost nothing”. The real answer is about 1.3 grams or .045 ounces. This is about half as much as a penny. What is interesting is that the 10 cubic inches of air inside a Zorp Rocket weighs 0.007 ounces. This is about 16% of the weight of the rocket. Thus 14% of the momentum of a Zorp Rocket in flight comes from the air inside the rocket.

How fast does a Zorp Rocket go?

The fun answer is “faster than the blink of an eye”. A Zorp Rocket has been measured going 65 ft/sec (about 45 miles per hour) as it left the launch tube. What is amazing is that it is accelerated to this speed in just 5 inches or about 0.00008 miles. The formula for achieving velocity V after accelerating at A over distance D is V² = 2×A×D. Solving for A gives A = V²/(2×D). Using feet per second gives A = 65²/(2× 5/12) or 5,070 ft/sec². The standard acceleration of gravity is 32.2 ft/sec² or one G. Thus a Zorp Rocket can accelerate at over 150 G’s. Compare this to the 3 G maximum acceleration of the space shuttle during launch. A trained fighter pilot wearing a G suit can take up to 9 G’s before blacking out. 150 G’s can be experienced in a very bad car crash.

Why does a Zorp Rocket go so fast?

It goes fast because it is so light compared to the force you can apply to it with your breath. A Zorp Rocket weighs about 1.3 grams or .045 ounces. We can use Newton’s second law of motion to calculate the force needed to accelerate a Zorp Rocket at the 5,070 ft/sec² we calculated above. The formula for Newton’s second law of motion is F = M×A where F is the force in pounds, M is the mass in slugs, and A is the acceleration in ft/sec². Notice that the force is in pounds and the mass is in slugs not pounds. It is common to use pound for the unit of mass that exerts one pound force at the surface of the earth, but a pound is really a unit of force not mass. An object’s mass is the same no matter what planet it is on, but its weight in pounds depends on the strength of gravity. The scientific unit of mass is a slug, which weighs 32.17 pounds. A Zorp Rocket weighing .045 ounces thus has a mass of about 0.000087 slugs. Plugging the numbers into the formula we get F = 0.000087×5070 or about 0.44 pounds. This force is applied by blowing on the launch tube creating air pressure inside the rocket. The force is equal to the area of the rocket opening times the air pressure. The launch tube has a diameter of 1 inch and thus the area of the rocket opening is π×0.5² or about 0.8 square inches. (The area of a circle is pi times the radius squared.) You only have to keep the launch tube pressurized to 0.55 pounds per square inch to apply .44 pounds of force accelerating the Zorp Rocket at 150 G’s to 45 miles per hour in 5 inches.

Why doesn’t it hurt when a Zorp Rocket hits you?

A Zorp Rocket at maximum launch speed hurts just as much as a penny dropped from 6 feet. This was experimentally determined using the bare tummy of the boy on the front of the Zorp package. The pain of stopping an object depends on the amount of skin, the pressure applied to the skin, and the duration of the pressure. The amount of skin affected by a penny on edge is less than the amount of skin affected by a Zorp Rocket strike. Thus a penny stimulates fewer pain sensors, but it exerts a higher pressure on the skin for a given force. The force depends on the momentum of an object and its softness. A softer object slows down over a longer time thus applying less pressure for the same momentum. The momentum of an object is its velocity times its mass. A penny weighs 0.09 ounces and is moving at about 20 ft/sec after a 6 ft fall. Thus the momentum of the penny is about 1.8 oz-ft/sec when it hits the tummy. The Zorp Rocket is moving at 65 ft/sec and has a mass of about 0.045 ounces plus 0.007 ounces of air inside so it has a momentum of 0.052×65 = 3.4 oz-ft/sec at tummy strike. However a Zorp Rocket is softer than a penny. When it hits you it collapses, slowing down over more time than a hard penny. The foam tip of a Zorp Rocket spreads the force over a larger area lowering the pressure. Thus the pain is approximately the same even though the Zorp Rocket has almost twice the momentum. Usually a Zorp Rocket slows significantly before it hits someone so it has much less momentum and exerts much less pressure.

Why does a Zorp Rocket slow down and glide?

Once a Zorp Rocket is launched it has to push air out of the way to keep moving. Air resistance goes up as the square of the speed. When the Zorp Rocket is moving fast the air slows it down quickly because it is very light. Once it has flown about 10 feet it has slowed down to a speed where there is still some air resistance but not much. It starts gliding at a steady speed. As it goes forward it drops a little to keep up its speed. It is using gravitational potential energy to overcome the slight air resistance.

Why does a Zorp Rocket have a lump in the tip?

To glide smoothly the Zorp Rocket has to be heavier in the front. This keeps the nose down so that it can convert its gravitational potential energy into forward motion. Without the weight it would turn up, stop in mid air, and tumble to the ground.

[ Home ] [ Up ] [ Products/Zorp/PhysicsOfZorpRockets.PDF ] [ The Physics of Zorp Rockets ]