Physics Photo of the Week
JANUARY 27, 2012
Liquid Nitrogen Rocket
The Physics class last December made a "rocket"
using a pint-sized plastic bottle "fueled" by extremely cold
liquid nitrogen (about -200 deg C). The bottle was
partially filled with the liquid nitrogen and corked.
(Note: this is extremely
dangerous, the cork could be ejected in the hands of the
experimenter and blast the hands and face of the
experimenter with cryogenic fluid causing causing severe
"freezer burns"). The corked bottle,
partially filled with the liquid nitrogen, was placed in the
bottom of the 3-inch PVC pipe with the cork pointing
down.
When the corked liquid nitrogen is sitting in the capped
bottle, the liquid rapidly 
heats
up to ambient temperature, vaporizing the liquid
nitrogen. The pressure rapidly builds up to the point
where the internal pressure blows out the cork and the
remaining liquid. Because the massive liquid is forced
out of the bottle, Newton's third law dictates that an equal
and opposite force is produced on the bottle by the liquid,
ejecting the bottle from the pipe. The fog tail is
caused by the liquid nitrogen streaming through the air
condensing water vapor from the surrounding
environment. In the animation one can see the drops of
liquid nitrogen spewing radially outward from the path of
the rocket and falling to the ground. The outward
trajectories of the liquid droplets - perpendicular to the
path of the "rocket) - is presumably caused by the boiling
of the liquid in mid air once the pressure is
released. Notice also that there is considerable
discharge out of the open back end of the pipe. The
rocket is propelled even if there is no earth to push
against.
This experiment is rather anti-climatic - there is
hardly any noise compared with a combustion-propelled
rocket. The ejection of the liquid nitrogen from
pressing out the cork is quite silent. The Chemists at
WWC like to put a screw cap firmly on the plastic bottle,
place the bottle in a well-constructed metal waste can, and
wait. The rupturing of the plastic bottle creates a
satisfying bang!
Students performed the filming of this event while the
professor handled the corked LN2 bottle.
Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department. These photos feature interesting phenomena in the world around us. Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation. Atmospheric phenomena are especially welcome. Please send any photos to dcollins@warren-wilson.edu.
All photos and discussions are copyright by Donald Collins or by the person credited for the photo and/or discussion. These photos and discussions may be used for private individual use or educational use. Any commercial use without written permission of the photoprovider is forbidden.
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