Japanese Water Jetpack | Popular Science

Japanese Water Jetpack

The physics of a human bottle rocket

Most years in my physics courses we construct water bottle rockets as a class project; but this stunt takes bottle rocketry to new levels. We never considered launching actual people!

Water bottle rockets illustrate a multitude of relevant physics principles, and they're fun to build too. You can construct them using plastic soda bottles as a fuselage, plastic or balsa wood fins, and some PVC piping for a launch pad. Then all you need is water and (compressed) air for rocket fuel. There are a multitude of websites that explain how to build simple water bottle rockets. Just make sure you pay attention to the safety precautions.

As far as the physics goes, we could talk about the potential energy stored in the bottles after they are pressurized, the fluid dynamics of the expelled air/water fuel mixture, air drag while in flight, conservation of momentum during the launch phase, and the kinematics of projectile motion. However, what's most striking is the relatively large amount of thrust you can generate with this simple set-up. It may surprise you to learn that the world-record launch for a water bottle rocket is just a hair over 2,000 feet. This means that the rocket in the video left the launch pad at a speed of over 200 miles per hour! In the video it appears that they use 25 two-liter bottles to launch the "pilot" a horizontal distance of approximately 25 or 30 meters from the launch point. He appears to achieve a maximum height of up to 10 meters.

We won't go through all of the calculations here, but applying some kinematics and Newton's laws of motion (and knowing that the thrust for a water bottle rocket occurs over a time period of about 0.1 to 0.2 seconds) we can reasonably approximate the acceleration and thrust force acting on the pilot during the time the water/air mixture is being expelled from the rockets. We find that during this brief period, our prospective astronaut is experiencing forces of up to 2,000 pounds and an acceleration in the neighborhood of 10 _g_s! No wonder he looks so shaken up afterwards.

Now who says the Japanese space program isn't alive and well?

Adam Weiner is the author of Don't Try This at Home! The Physics of Hollywood Movies.


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